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EFFECT OF SOIL PROPERTIES ON SORPTION AND MOBIUTY
OF CADMIUM IN SELECTED ARIDISOLS AND ANDISOLS
by
Saud Sebayle AL-Harbi
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF SOIL, WATER AND ENVIRONMENTAL SCIENCE
In Partial FuIfiUment of the Requvements
For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1999
UMI Number 9957937
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UMI Microfomi9957937
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2
THE UNIVERSITY OF ARIZONA «
GRADUATE COLLEGE
As members of the Final Examination Committee, we certify that we have
read the dissertation prepared by
entitled
Saud Sebayle Al-Harbi
Effect of Soil Properties on Sorption and Mobility of Cadmium
in Selected Aridisols and Andisols
and recommend that it be accepted as fulfilling the dissertation
requirement for the Degree of
David ae.drlcksX^^,
Doctor of Philosophy
10-it'll
Date
"•tnrtch Bohn
Date
tT<'}
Silvertooth Jefferv
Date
Peter Ffolliot
^ Oc^ f(fqq
Vicent Lopes
f ig
Date
Date
Final approval and acceptance of this dissertation is contingent upon
the candidate's submission of the final copy of the dissertation to the
Graduate College.
I hereby certify that I have read this dissertation prepared under my
direction and recommend that it be accepted as fulfilling the dissertation
requirement.
David Hendricks
Dissertation Director
Date
3
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fiilfiUment of requirements for an
advanced degree at the University of Arizona and is deposited in the university library
to be made available to borrowers under rules of the library.
Brief quotations from this dissertation are allowable without special permission
provided that accurate acknowledgment of source is made. Requests for permission for
extended quotation from or reproduction of this manuscript in whole or part may be
granted by the head of the major department or the Dean of the Graduate College which in
his or her judgement the proposed use of the material is in the interests of scholarship. In
all other instances, however, permission must be obtained from the author.
Signed:
4
ACKNOWLEDGMENTS
All praise and thanks to Allah, the Master of the Day of Judgment, without His help
I would have not finished this work.
I wish to express mysincere appreciation and deep gratitude to Dr. David Hendricks,
Chairman of my dissertation committee for his invaluable assistance, guidance throughout
the program,and also for his timely correction of this manuscript.Without his assistance and
material support, the completion of this work would have been impossible.
I am especially grateful to Dr. Hinrich Bohn for his guidance and assistance during
the preparation of the dissertation. It is my pleasure to acknowledge him for his professional
and personal involvement in my academic life.
I am also grateful to my dissertation committee members: Dr. Jefi&ey Silvertooth,
Dr. Peter Ffolliot and Dr. Vincent Lopes for their kind advice and comments.
Further thanks goes to Dr. Bolton K. who helped me in surface complexation
modeling and to Dr. Leckie J. who provided me with the latest version of the HYDRAQL
program.
Financial support fi'om KingSaud University, Saudi Arabia which was used to cover
my school and personal expenses during my studies here in USA is highly acknowledge.
Finally, my heartfelt appreciation to my wifeSarah and my sons Wa-el and Fiesal for
their love, support, encouragement and company which made my life easier and helped me
concentrate on the mission of my stay at the UniversiQr of Arizona.
DEDICATION
I dedicate this work to my parents, my wife Sarah, and my sons Wa-el,
and Fiesal for thcff love, encouragement, and endless support.
TABLE OF CONTENTS
Page
LIST OF TABLES
LIST OF ILLUSTRATIONS
ABSTRACT
CHAPTER I: INTRODUCTION
CHAPTER 2: REVIEW OF LITERATURE
Effect of Soil pH on Cadmium Sorption
Effect of Ionic Strength on Cadmium Sorption
Effect of Soil Organic Matter on Cadmium Sorption ...
Effect of CEC on Cadmium Sorption
Effect of Calcium on Cadmium Sorption
Effect of Hydroxides on Cadmium Sorption
Cadmium Mobility
Solid Activi^ Coefficients of Cadmium Adsorbed
Chemistry of Cadmium
CHAPTER 3: MATERIALS AND ANALYTICAL METHODS
Soils
Criteria for Selection of Soils
General Description of Soils
Preparation of Soils
Methods of Soil Characterization
Texture
pH and pH-Adjustment
Cation Exchange Capacity
Iron and Aluminum Oxides
Water Soluble Cation and Anions
Adsorption Isotherms
Cadmium Analysis
Total Dissolved Cadmium
Cadmium Activity Measurement
Cadmium Mobility Study
Preparation of the Soil Plates
Soil TLC Procedures
Solid Activity Coefficients of Cadmium Adsorbed
8
7
TABLE OF CONTENTS-continued
Speciation and Modeling Adsorbed Cadmium
47
CHAPTER 4: THEOR TICAL CONSIDERAHONS
48
Modeling Approach
Sorption of Cadmium
Sorption Isotherms
Langmuir Isotherm
Freundlich Isotherm
CHAPTER 5: RESULTS AND DISCUSSION
Soil Characteristics
Adsorption Isotherms
Effect of pH on Cadmium Sorption
Cadmium Activities in Soils
Cadmium Mobility in Soils
Modeling of Cadmium Sorption on Soils
Solid Activity Coefficients of Cadmium Adsorbed
SAC of Cd Adsorbed (Unadjusted pH and varied Cd loads)
SAC of Cd Adsorbed (adjusted pH and fixed Cd load)
CHAPTER 6: SUMMARY AND CONCLUSIONS
Summary
Conclusions
REFERENCES
48
50
50
51
52
53
53
54
90
94
102
116
136
136
141
150
150
154
157
8
LIST OF TABLES
Table
Page
I.
Parameters used in the surface complexation model
49
2a.
General characteristics of soils used in the study
53
2b.
General characteristics of soils used in the study
54
3.
Percentages of sorbed cadmium onto the soils
76
4.
Langmuir and Freundlich constants for cadmium adsorption
77
5.
Simple correlation coefficients (r) between adsorption constants and soil
characteristics
87
6.
Water soluble anions and cations in the equilibrium solution
89
7.
Batch adsorption experimental results compared to nonelectrostatic and
diffuse
layer models predictions using HYDRAQL; [Cd]p= 1.14 xlO"*lS!l
8.
Cadmium activities in soils measured by ion selective electrode (ISE)
9.
Cadmium activities in soils at different pH values
101
10.
A summary of regression model constants for estimating cadmium activity
in soils from soil pH
101
II.
Cadmium mobility in soils
103
12.
Rf values for the mobility of cadmium in the soils studied
104
13.
Simple correlation coefficients (r) between Rf and soil characteristics
and the adsorption constants
106
Cadmium distribution coefficient (Kd) and retardation factor (Red)
values of the soils studied
110
Simple correlation coefficients (r) between cadmium retardation
factor (Red) and soil characteristics
Ill
14.
15.
95
9
LIST OF TABLES-continued
Table
16.
Page
A siunmaiy of best step-wise regression models (1, and 2) found
to estimate retardation factor of cadmium (Red) in soils studied
113
Concentrations of the humic acid ([HjA]^ and [H BJ ) and hydrous ferric
oxides ([FeOOHJ-r weak and [FeOOH]x strong) in the soils studied
117
Aqueous Cd^* activities, ion activity products LAP of Cd(0H)2, soil mole
fractions X, saturation index (lAP/Ksp), solid activity coefficients, g'
and SAC of adsorbed cadmium at five levels of [Cd]^ added (ppm)
and at their natural pH
139
Aqueous Cd^"" activities, ion activity products lAP of Cd(0H)2, soil mole
fractions X, saturation index (lAP/Ksp), solid activity coefficients, g' and
SAC of adsorbed cadmium at five levels of [CdJ^ added (ppm) and at their
natural pH
140
19.
pH measurements at equilibrium
141
20a.
Aqueous Cd'* activities, acd2+, ion activity products JAP, soil mole
fractions X, saturation index (lAP/Ksp), solid activity coefficients, g' and
SAC of adsorbed cadmium at the highest [Cd]-p added
146
Aqueous Cd*^ activities, acji^, ion activity products lAP, soil mole
fractions X, saturation index (lAP/Ksp), solid activity coefficients, g'and
SAC of adsorbed cadmium at the highest [Cd]^ added
147
Simple correlation coefficients (r) between solid activity coefficients of
cadmium and soil characteristics
148
17.
18a.
18b.
20b.
21.
J
T
10
LIST OF ^LUSTRATIONS
Figure
Page
1.
Cadmium sorption isotherm (soil I)
55
2.
Cadmium sorption isotherm (soil 2)
56
3.
Cadmium sorption isotherm (soil 4)
57
4.
Cadmium sorption isotherm (soil 6)
58
5.
Cadmium sorption isotherm (soil 10)
59
6.
Cadmium sorption isotherm (soil U)
60
7.
Cadmium sorption isotherm (soil 12)
61
8.
Cadmium sorption isotherm (soil 13)
62
9.
Cadmium sorption isotherm (soil 14)
63
10.
Cadmium sorption isotherm (soil 15)
64
11.
Cadmium sorption isotherm (soil 16)
65
12.
Cadmium sorption isotherm (soil 17)
66
13.
Cadmium sorption isotherm (soil 18)
67
14.
Cadmium sorption isotherm (soil 19)
68
15.
Langmuir plot of cadmiimi sorption isotherm (soil 1)
70
16.
Langmuir plot of cadmium sorption isotherm (soil 4)
71
17.
Langmuir plot of cadmium sorption isotherm (soil 6)
72
18.
Langmuir plot of cadmium sorption isotherm (soil 11)
73
19.
Langmuir plot of cadmium sorption isotherm (soil 16)
74
20.
Langmuir plot of cadmium sorption isotherm (soil 17)
75
11
LIST OF ILLUSTRATIONS-continued
Figure
Page
21.
Freundlich plot of cadmium sorption isotherm (soil 1)
81
22.
Freundlich plot of cadmium sorption isotherm (soil 4)
82
23.
Freundlich plot of cadmium sorption isotherm (soil 6)
83
24.
Freundlich plot of cadmium sorption isotherm (soil 11)
84
25.
Freundlich plot of cadmium sorption isotherm (soil 16)
85
26.
Freundlich plot of cadmium sorption isotherm (soil 17)
86
27.
Relationships between the amount of Cd adsorbed at the highest [Cd]^
added (12.8 ppm) and the soil pH for soils (1, 2,4, 6, and 10)
92
28 .
Relationships between the amount of Cd adsorbed at the highest [Cdjy added
(12.8 ppm) and the soil pH for soils (11, 12, 13, 14,15,16, 17,18, and 19) ... 93
29.
Cd^* activities as a function of pH for soils (1,2,4, 6. and 10)
96
30.
Cd-^activities as a function of pH for soils 11,12, 13,14, 15,16,17, 18,
and 19
97
31.
Solubility diagram for the Cd-soil equilibrium for soils (1,2,4,6. and 10) ... 98
32.
Solubility diagram for the Cd-soil equilibrium for soils 11,12,13,14,15,16,
17,18, and 19
99
33.
Cd^^activities as a function of pH for fourteen soils
100
34a.
Distance (cm) moved by cadmium over a 10 cm soil TLC
105
34b.
Red (Transport retardation factor for cadmium) and Rf (cm) versus
sand (%)
107
35.
Log Red (transport retardation factor for cadmium) and pH for two different
amounts of hydrous ferric oxides in a soil/water system
112
36.
Log Red (transport retardation factor for cadmium) versus Feo-Fep(%)
in soils
114
12
LIST OF ILLUSTRATIONS-continued
Figure
Page
37.
Log Red (transport retardation factor for cadmium) versus Feo (%) in soils ..115
38.
Measured and predicted Cd adsorption onto soil (1) using the surface
complexadon model
119
Measured and predicted Cd adsorption onto soil (2) using the surface
complexation model
120
Measured and predicted Cd adsorption onto soil (4) using the surface
complexation model
121
Measured and predicted Cd adsorption onto soil (6) using the surface
complexation model
122
Measured and predicted Cd adsorption onto soil (10) using the surface
complexation model
123
Measured and predicted Cd adsorption onto soil (11) using the surface
complexation model
124
Measured and predicted Cd adsorption onto soil (12) using the surface
complexation model
125
Measured and predicted Cd adsorption onto soil (13) using the surface
complexation model
126
Measured and predicted Cd adsorption onto soil (14) using the surface
complexation model
127
Measured and predicted Cd adsorption onto soil (15) using the surface
complexation model
128
Measured and predicted Cd adsorption onto soil (16) using the surface
complexation model
129
Measured and predicted Cd adsorption onto soil (17) using the surface
complexation model
130
Measured and predicted Cd adsorption onto soil (18) using the surface
complexation model
131
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
13
LIST OF ILLUSTRATIONS-continued
Figure
51.
Page
Measured and predicted Cd adsorption onto soil (19) using the surface
complexation model
132
Cadmium speciation for the nonelectrostatic and the diffuse layer models,
where [H,A] = 9.70x10'^ M,
=4.30X10'^ M, and
[FeOOHlx = 1.03X10-^ M
1
134
Cadmiimi speciation for the nonelectrostatic and the diffuse layer models,
where [HiAJx = 1.66x10"^ M, [HjBJx = 7.44x10"^ M, and
[FeOOHlT =3.76x10-^ M
135
54.
lAP of Cd(0H)2 as a function of soil pH
143
55.
lAP of Cd(0H)2 as a function of soil pH
144
52.
T
53.
14
ABSTRACT
Concern over environmental quali^ has generated interest in the chemistry of Cd in
soils. When Cd sorption and the influence of soil characteristics on the process are better
understood, Cd contamination of ground water and plant availabili^ may be assessed more
accurately.
Therefore, a series of experiments, including laboratory batch studies, soil thin layer
chromatography studies and selective ion electrode procedures were conducted to evaluate
the effect of soil properties on Cd sorption and mobility by fourteen different Aridisols and
Andisols having a range of chemical properties. In addition, surface complexation models
were used to model the amount of Cd adsorbed onto each of the humic acid and hydrous
ferric oxide surfaces. Sorption isotherms were obtained using batch experiments by in
which 25 ml of solution containing a total of 0.8, 1.6, 3.2, 6.4, and 12.8 ppm of Cd'"
to 0.5 g were added to soil samples.
The results of the batch experiments indicate that in all the soils used the Cd sorption
is best described by the Freundlich sorption isotherm. The maxima soil sorption capacities
were significantly correlated with the free iron oxides. The Cd activities in the soils varied
from 10"^^ to 10"^ *^ M . It increased with increasing total Cd added and were inversely
related to the soil pH .The Cd mobiliQr retardation factor (Rf) obtained from soil thin layer
chromatography ranged from 0.25 to 0.95. It showed that Cd was slightly mobile in 64 %,
moderately mobile in 29 % and very mobile in 7 % of the soils.The Cd Rf indicates that Cd
mobiliQr would decrease with increasing amounts of iron oxide fractions; silt % and
exchangeable Mg^" in the soils.
15
The non electrostatic and diffuse layer models results indicate that humic materials
are an important factor in Cd sorption at pH values greater than 3 and hydrous ferric oxide
surfaces are important at pH values greater than 7. The ion activity^ products of Cd(0H)2
ranged from 2.3x10*" M to 5.6x10"'® M, while the solid activity coefiQcients of adsorbed
Cd(SACcd2J ranged from 3.9x10"® to 4.6. The SAQdiH-values were significantly correlated
yet negative with silt %, CEC, Aid, Alo, O.C and iron fractions.
16
CHAPTER 1
INTRODUCTION
Cadmium is an especially toxic heavy metal and is very mobile in soil. It deserves
special attention due to its zootoxicity and relatively large mobility with respect to plant
uptake and leaching. In unpolluted, uncultivated soils, Cd concentration is largely governed
by the amount of Cd in the parent material and the pedogenic processes of soil formation. In
cultivated soils, a significant amount of the element can be introduced by anthropogenic
pathways such as the use of fertilizers or waste products as amendments. In particular
phosphate fertilizers have been shown to be one of the sources that contributed to increasing
levels of Cd in cultivated soils (Tiller, 1989; and Alhaji et al.; 1993). Studies of cadmium
levels in plants grown on contaminated soils indicate that plants will readily absorb cadmium
from soil (John et al., 1972b).
Heavy metals are natural components of soil. In addition, they are introduced into
the ecosystem by the disposal of waste resulting from the manufacture and use of materials
containing heavy metals. Soil is contaminated by material from the air and by direct
depositon of pollutants. The accumulation of contaminants is aided by the capability of soil
to bind these contaminants by clay minerals and organic substances.
Most cadmium used in the world is obtained as a by-product from the smelting of
zinc, lead, or copper. Cadmium has a number of industrial applications, but it is used mostly
in metal plating, pigments, batteries, and plastics. John et al., (1972a) found that cadmium
contamination of the environment has been attributed to smelting and electroplating of
metals, combustion of lubricating oils, and vulcanization for rubber of automobile tires.
17
Cadmium is a naturally occurring element in the earth's crust Pure cadmium is a soft
silver-white metal, but this form is not common in the environment Rather, cadmium is
most often encountered in combination with other elements such as o^Qrgen (cadmium
oxide), chlorine (cadmium chloride), or sulfur (cadmium sulfide). Low levels of cadmium
are found in most waters and its typical concentrations in groundwater and drinking water
is one ^ig/L or less (Meranger et al., 1981, Page 1981), which may be present as the free
cadmium ion or adsorbed to suspended particulate matter.
Major sources of cadmium emissions into the air include burning of fossil fuels
(coal, oil, and gasoline) and incineration of municipal wastes. The usual form of cadmium
in ambient air is cadmium oxide, adsorbed to suspended particulate matter of varying sizes.
Typical atmospheric concentrations range from about 1 to 5 ng/m3 in rural areas to between
5 and 40 ng/m3 in urban areas, while levels of500 ng/m3 or higher may occur in the vicinity
of specific industries such as nonferrous metal smelters, although such levels are now less
common because of the increased use of emission controls (EPA, 1981).
Typical topsoil levels of cadmium range from about 100 to 1000 Hg/kg, with an
average value of about 260 ^g/kg (Carey 1979). Cadmium in soils and dusts may be in the
form of the oxide or as ionic complexes with other cations, anions, and organic soil
constituents. Cadmium may be present in soil as free cadmium compounds or in solution as
the Cd'*'^ ion dissolved in soil water. It may also be held to soil minerals or organic
constituents by cation exchange, in which case it is not readily leached from the soil by
rainwater. Two major pathways by which soil becomes contaminated with cadmium are
municipal sludge land spreading and deposition of airborne cadmium (Yost 1983).
18
Cadmium is of a greatest concem inagricultural soils. It is not required for the growth
and development of either plants or animals and can be toxic to both. It is loosely held by soil
constituents and is readily available to plants. Thus, increased concentrations in soils result
in increased concentrations incrops. Soil particlescontaining boundcadmium may be eroded
into air or water, which results in the spread of cadmium to the environment. Contamination
of topsoil may be indirectly responsible for the greatest human exposures to cadmium,
through uptake of soil cadmium by plants and tobacco (EPA, 198Sa). Very high cadmium
levels in playgrounds and other areas can cause health risks for small children by oral
ingestion. A special case of concem over cadmium in drinking water arises when the water
is soft (lacking calcium and magnesium) and has low alkalinity. Such water may have a pH
as low as 6, or even 5, and tends to dissolve cadmium fix)m water lines and from soft solder
used in connecting water lines (EPA 1981).
If heavy metal contamination exceeds the binding capabili^ of the soil, there is a
danger that heavy metal compounds can flow with percolation (seepage) into the
groundwater. Contamination of soil by heavy metals is particularly problematic because they
are not degraded in the soil. Heavy metals in soil cannot be permanently eliminated, but at
best they can be locally reduced by a redistribution in the ecosystem or removed from
circulation byimmobilization. Several processes tend to keep the concentrations of cadmium
low in groundwater. These include sorption by mineral matter and clay, binding by himiic
substances, precipitation as cadmium sulfide in the presence of sulfide, and precipitation as
the carbonate at relatively high alkalinities.
Sorption can be described as the process in which cadmium entering the soil becomes
19
physically or chemically bonded to the colloidal stirfaces, therefore causing a net decrease
in its concentration in the solution phase. Sposito (1986) distinguished between adsorption
and precipitation, in which adsorption is an accumulation of matter at the interface between
an aqueous solution and a solid adsorbent with the development of a two-dimensional
molecular arrangement, while precipitation is the growth of a solid phase exhibiting a
primitive molecular unit that repeats itself in three dimensions. When the nature of cadmium
adsorption and the influence of soil characteristics on the process are better understood,
cadmium contamination of ground water and plant availability may be assessed more
accurately (Johnetal., 1972a).
Concern over environmental quality has generated interest in the chemistry of
cadmium in soils. As with other metals, the chemistry of Cd can quantitatively be described
as affected by (1) the specific adsorption or exchange adsorption at a given mineral interface,
(2) the precipitation of sparingly soluble compounds of which they are a constituent, and (3)
the formation of relatively stable complex ions or chelates which result from interaction
with soil organic matter (Lagerwerff, 1972).
Several important factors controlling the distribution of Cd between soil and solutes
have been identified. The concentration of competing cations such as Ca or other heavy
metals, pH and organic matter content significantly affect the Cd sorption equilibrium
(Christensen, 1987).
The Objectives of This Study Were
1) To study the effect of soil chemical and physical properties on Cd sorption and mobiliQr
by different Aridisols and Andisols.
20
2) To study the relationship between soil solution pH and Cd^^ activity.
3) To study cadmium mobility in different soils by means of thin layerchromatography (soil
TLC).
4) To measure the solid activity coefficients (SAC) of cadmium adsorbed.
5) To model Cd sorption on hydrous ferric oxide surfaces and humic acid using
HYDRAQL, a chemical equilibrium program.
21
CHAPTER 2
REVIEW OF LITERATURE
Effect of Soil pH on Cadmium Sorption
Several studies have found that the sorption of heavy metal cations increases as pH
increases (Boeidioldetal,.1991,BastaandTabatabai, 1992, and Naiduetal., 1994). Liming
of a soil usually affects the availability of heavy metals to plants (Singh 1971 and Maclean,
1976). Raising the pH of a soil solution can lead to the formation of hydrolysis products that
have differentafSnity for permanent charge and other exchange sites. Also it can change the
exchange nature by hydrolyzing or precipitating aluminum ions which occupy the exchange
sites of acid soils, hence creating more exchange sites and decreasing the fraction of sites
occupied by the strongly held Al^* ion (Cavallaro and McBride, 1980).
In general, when the pH decreases there is a decrease in Cd and Zn sorption caused
by increasing H" and Al"^ concentration in the solution, which compete with Cd and Zn for
ion sorption sites on the soil sorbate surfaces and by decreasing the negative charge of the
sorbate (Garcia-Miragaya and Page, 1978). Another cause of pH effects is the formation of
hydroxy metal complexes MCOH)"" at pH above 6, which are preferentially adsorbed
(Wilkens et al., 1992). Hatch et al., (1988) found that, although dissolved Cd in the soil
solutions is larger at low pH, the extent to which Cd uptake by plants increases with
decreasing pH in soil will be less than expected based on solution concentrations alone.
During vegetation growth absorption of Cd by plants is suppressed at low pH by increased
competition between Cd and proteins and the same tendency can be expected inthe presence
of competing cations in solution such as Zn (Boekhold et al., 1993).
22
Anderson et al., (1988) found that the pH of the system was the most highly
correlated characteristic with the distribution coefficient Kd of Zn, Ni, Cd and Co. The
charge characteristics of oxide and clay surfaces are related to their zero point charge (zpc)
and to the pH of the soil suspension (EL-Swaify, 1976). Kuo and Baker (1980) reported that
Cd sorption was markedly increased as the pH increased &om 4 to 5. They also added that
since the sorption of Cu, Zn and Cd occurs at a pH well below the ZPC of soil, the sorption
can proceed even in the presence of electrostatic repulsion between the positively charged
surfaces and the metal cations. Specific bonding between metal ions and the surfaces of the
soil particles would be required for such a sorption process (Forbes et al.,1976).
Christensen (1984) found that the sorption capacities of soils are increased about
three times for each increase in pH of one unit in the pH interval 4 to 7. He concluded that
pH is the most critical factor governing the distribution of Cd between soil and solute. Soil
pH appears to be one of the mostsignificant soil properties that determines Cd sorption, with
decreasing solubility associated with increasing pH (Cavallaro and McBride, 1980; Elliot,
1983; Basta and Tabatabai, 1992 and Boekhold et al., 1993). Cadmium sorption proved to
be very sensitive to pH with each unit increase in pH resulting in twice as much sorption of
Cd. Similar results have also been reported by others (Boekhold et al., 1993).
Temminghoff et al., (1995) reported that when pH decreases one unit Cd sorption is
reduced by about 75%. McDufRe et al., (1976a) found that trace Cd was extensively
adsorbed by riversediments at pH of 7.5, but completely released at a pH of 3.0. An increase
in negative surface charge ( pH dependent charge) as pH increase reflects a corresponding
increase in availability of adsorption sites and with less competition firom IT could be a
23
factor in the increased Cd sorption at higher pH (Garcia-Miragaya and Page,1978). Farrah
and Pickering (1977) investigated the effect of pH change and the presence of ligands on Cd
and Pb sorption by clay minerals and found that sorption of these metals increases as pH
increases until a threshold pH value is exceeded with ligands present having the threshold
point shifted to higher pH values.
Buchter et al., (1989) reported that by studying the correlation of the Freundlich Kd
and n retention parameters with soils and elements, the relationships between soil properties
and retention parameters can be used to estimate retention parameters when retention data
for a particular element and soil type are lacking, but soil property data are available. They
also showed that Co, Ni, Zn, and Cd are sufficiently similar that these elements can be
grouped together and an estimated n value of any one of them could be estimated from soil
pH data using the regression Equation which they developed.
Effect of Ionic Strength on Cadmium Sorption
An increase in ionic strength causes a decrease in Cd sorption as a result of a decrease
in its activity coefficient, an increase of an inorganic complexation and an increase in Ca
competition (Tremminghoff et al., 1995). Ionic strength of soil solutions range from < 0.005
M in the soils of tropics (Curtin et al., 1991) to > 0.10 M in less weathered soil from
temperate climates (Edmeades et al., 1985). The effect of ionic strength on anion and cation
adsorption in variable charge soils varies with pH through its effect on the electrostatic
potential in the plane of adsorption (Barrow, 1984, Barrowand Ellis, 1986). Garcia-Miragaya
and Page (1978) examined the influence of ionic strength and complex formation on the
sorption of trace amoimts of Cd by Na-montmorillonite. They showed that increasing the
24
ionic strength and the levels of Cd-complexing ligands decreased Cd retention in at least a
qualitatively explainable manner.
Miragaya and Page (1976) reported that as a result of Ca competition the Cd
adsorption, was reduced by 60-80% when CaG^03)2 was used compared with NaNOj as
background electrolyte. Further, they reported that a maximum reduction of Cd adsorption
of 60% and 25% was observed for the effect of ionic strength between 0.003 and 0.3 M for
Ca(N03)2 and NaNOj as background electrolytes respectively. Escrig and Morell (1998)
found that increasing the background CaClj concentration from 5x10"^ to 5x10'^ M in a
cadmium sorption study, resulted in a decrease of cadmium adsorption by soils. The
cadmium adsorption capacities were reduced by 63% to 70% with tenfold increase in
concentration of salt matrix (CaClj) and that may be due to the competition of the index
cation with cadmium, for soil adsorption sites as well as to the chloride complex formation
which will increase the mobility of metal (Escrig and Morell,1998).
Increasing the ionic strength increases the net surface negative charge by decreasing
the positive charge below the PZC and by increasing the negative charge above the PZC
(Boian et al., 1986). According to Barrow (1986), if increasing ionic strength of salt
solution decrease cation adsorption, then this implies that increasing the salt concentration
is causing the potential in the plane of adsorption to be more positiveand this woulddecrease
Cd sorption.
Schindler et al., (1987) found that adsorption of Cd, Cu and Pb by kaolinite increases
with increasing pH and with decreasing ionic strength. Moreover, they suggested that both
effects can be explained by a model assuming two kinds of binding sites:1) weakly acidic
25
groups XH that account for ion exchange, and 2) ampholytic surface hydroxyl which form
inner sphere complexes with divalent metal ions. Miragaya and Page (1976), summarized
the possible causes for Cd sorption reduction as ionic strength increases to: (1) competition
between Cd and other cations in the electrolytes for clay sites; (2) decrease in Cd activity in
the solution; and (3) "formation of uncharged ion pars and/or uncharged and negatively
charged complexes of Cd with the anion (ligand) of electrolyte."
Effect of Soil Organic Matter on Cadmium Sorption
One of the most important properties of soil organic matter is its ability to exchange
ions which is very high compared with most mineral components. Humic polymeric chains
may dissociate depending on the pH of the system. An important feature of the chemical
structure of these humic chains is the ability to form complexes involving chemical
bonds (specific sorption) which is different firom non-specific adsorption of cations in
double layer (Van Dijlc,1971).
Johnston (1992) reported that in acid grassland soils, organic matter appeared to be
more important than soil pH in the retention of Cd. A scaled sorption model for Cd was
suggested by Van der zee and Van Riemsdijk (1987) that accounts explicitly for effects of
both pH and organic carbon content, O.C (g/g%) of the soil. Boekhold al., (1992) reported
that using a scaled sorption model ( q = K*OC(ir)*^ C"), where OC is the organic carbon
content (g/kg) ,and IT is the proton activity in soil solution (mole/L), variability of OC and
pH were explicitly accounted for when calculating Cd &om HN03-extractable Cd contents.
They concluded that organic matter content and pH are the most important soil properties
that control and regulate Cd sorption in the studied soil.
26
By studying the distribution coefficient of Cd, Co, Ni, and Zn in soils, Anderson et
al., (1988) found the soil organic matter content to have little influence on the distribution
of metals. However, they observed a weak correlation between total organic carbon and
distribution of Cd and Ni, but there was no significance between supposedly more reactive
hemic substances and any of the metals. Further they reported that the extracted organic
matter from the soil during the batch adsorption experiments had no influence on metal
distribution except a slight correlation (r =" 0.24, p<0.01) between soluble carbon
concentration and Kd for Cd element. For sixty four different soils Christensen (1989a)
found that both organic matter content and pH of the soil is attributed to the variability of the
sorption parameter (K).
Wilkens and Loch (1992) showed that sorption of Cd and Zn on both iron oxide and
organic matter content, is strongly pH dependent However, the adsorption of Cd and Zn on
iron oxides appears to be of minor importance below pH 5 while organic matter content is
the most importance binding component of Cd and Zn. Elliott and Denney (1982) reported
that as the soil solution becomes acidic, the influence of organic ligands on heavy metal
uptake diminishes as a result of decreasing ability of the ligands to bind the metals.
The amount of orgam'c matter in soils affects many physico-chemical properties.
Yuan et al., (1997) conducted a study where they treated two agricultural soils, to partially
oxidize organic matter and to decease soil pH for evaluating the effects of acidification and
organic matter oxidation on trace metal sorption onto soils. They observed that when a soil
with a pH value of 6.74 and organic carbon content of 46.9 g/kg lost 11% of its organic
matter due to oxidation, the reduction on its original sorption capacity was 97,72 and 62 %
27
for Cu, Zn and Cd, respectively, while the corresponding values caused by pH decrease of
one unit were 32,16, and 29% respectively.
Sorption of heavy metals is very important in regulating the solubility of these metals
in soil solution as well as their uptake by plants. He and Singh (1993) pointed out that the
dry matter yields of ryegrass were not affected with increasing levels of organic matter
addition to the soils, but the concentration of Cd in each clipping decreased with increasing
levels of organic matter addition.
EfTect of CEC on Cadmium Sorption
The buffered cation exchange capacities (CEC) of soils are commonly used to
estimate their capacity to adsorb heavy metals and retain these metals against plant uptake.
However, soils with similar buffered CEC values often show very different abilities to
adsorb metals (Cavallaro and McBride, 1978). The maximimi quantity of Cd adsorbed in
soils was strongly correlated with CEC and organic matter content for 10 alkaline surface
soils from Italy, (Levi-Menzies el al., 1976), suggesting that Cd ions are retained by ion
exchange and complexation.
Sanchez-Martin et al., (1993) found that the distribution coefficient at 15 and 30
^g/L equilibrium concentration were related to cation exchange capacity of the soils. Buchter
et al., (1989) reported that soils with both high pH and high CEC retained greater amounts
of cation species than did low pH values and low CEC soils. A High sorption capacity or
bonding energy seems to be favored by high CEC values or high levels of organic matter
(White and Chaney, 1980; and Elliot etal., 1986). Kou and Baker (1980) observed that
CEC was more important than organic matter in the sorption of Cd.The most important soil
28
properties controlling both Cd and Zn adsorption in tropical soils was the CEC (Hanafi et
al, 1998).
Plant uptake of Cd from soils conelate negatively with the tendency of these soils to
adsorb Cd from solution, which is related to soil CEC (VfcBride et al, 1981). Amending
soils with organic matter increase the CEC ofsoils, and astrong negativecorrelation between
metal solubility or plant uptake is commonly observed (McBride et al, 1981). He and Singh
(1993) reported that the CEC of three soils with varying textures increased linearly with
increasing levels of organic matter from addition of peat to the soils. The increase in CEC
may be responsible for the increase in the soil's ability to adsorb cations.
Haghiri (1974) concluded that a reduction of plant uptake of Cd with increasing
levels of organic matter addition was predominantly due to the effectof increasing soil CEC.
Using several acid tropical soils from Borneo Island, Hanafi and Sjiola (1998) confirmed
that the CEC of the soil was the most important factor in influencing the adsorption of Cd
andZn.
Effect of Calcium on Cadmium Sorption
Sorption of Cd by mne acidic agricultural soils from the north-eastem United Sates
correlated best with exchangeable Ca, and to a lesser extent with total exchangeable bases,
soil pH and NH40ac (pH 7) CEC (McBride et al., 1981). Hendrickson and Corey (1981)
showed an increase in selectivity coefficients K^Ca for increasing a Ca concentrations for
Ca-saturated soils. This dependence on relative metal concentrations is apparently due to the
heterogeneous composition of complexing sites in most soil systems and competition among
the various cations present for these sites (Hendrickson and Corey, 1981).
29
Competition between Cd and Cadiminished Cd adsorption by 80% in Ca-electrolyte
as compared with a Na-eiectrolyte at an ionic strength of 0.03M (Boekhold et al,. 1993). The
effect of Ca competition alone can be examined according to Christensen (1984), if the
results of cadmium adsorption studies are "normalized" with respect to the activity of firee
metal Cd^". Escrig and Morell (1998) found that a tenfold increase in calcium concentration
reduced the Cd adsorption capacity approximately by one third, while the Ca'^ seemingly
does not appear a significant competitor with Zn^*for soil adsorption site.
Naidu et al., (1994) found that Cd adsorption was approximately doubled when Na
rather than Ca used as index cation in adsorption study. Naidu et al.,(1994) reported that
when they studied Cd adsorption at(NaNOj and Ca(N0])2) electrolyte solutions of thesame
inoic strength (0.03 M), the Cd adsorption in the presence of Na was at least 2-4 times more
than that observed in Ca solution. They concluded that this effect of indexcation may be due
to (1) increased competition between Ca^" and Cd for soil sorption sites, (2) the effect of
divalent cation on the pH of the soil solution, 0.1-0.2 units lower in the presence of Ca*", and
(3) greater specificity for divalent cations and the effect of divalent cations on the thickness
of the double layer and greater specificity overall.
Effect of Hydroxides on Cadmium Sorption
Metal adsorption onto oxide surfaces is an important processes affecting trace metal
transport in natural systems. At pH values above the zero point of charge (zpc) an oxide
surface has a negative charge and therefore cation adsorption prevails and at pH values
below zpc an oxide surface has a positive change so anion adsorption prevails. According
to Schindlerand Stunmi(1989)deprotonating surface hydrm^lsexhibitLewis base behavior
30
and adsorption of catk>ns as a result understood as competitive complex formation involving
one or two surface hydroxyls. Bolton and Evans (1996) indicate that by using surface
complexation modeling, humic sur&ces accounted for adsorption at pH values starting at
about 3.5 and that hydrous ferric oxide surfaces accounted for adsorption at pH values
greater than seven.
John (1972) reported that Cd sorption by 30 soils from British Columbia, Canada,
was related to the total A1 and Fe, and soluble A1 contents, but not to cation exchange
capacity, organic matter or clay contents. Alloway et al., (1985) reported from multiple
regression analysis, that pH was one of the key factors, together with organic matter and
hydrous oxide content controlling the adsorption of Cd by 22 different soils. Scokat et al.,
(1983) reported that Cd and Zn level increased with depth in an acidic soil as compared to
a neutral sandy soQ as a result of adsorption on clays and free oxides.
After studying Ni, Zn, and Cd uptake by goethite, Brummer et al., (1988) showed
that adsorption of these heavy metals increased with pH. reaction time and temperature.
Moreover, they added that pH dependent relative difiiision rates (Ni, Zn, and Cd) is
influenced by ionic radius and the conclusion was that the reactions of heavy metals with
goethite involves (0 adsorption of metals on external sur&ces, (ii) solid-state diffusion of
metals from external to internal binding sites, and (iii) metal binding and fixation at positions
inside the goethite particles. The most important mechanism to account for the binding of
trace metals by oxides is specific adsorption which is strongly pH dependent and usually
happens between pH 3-8 (Barrow, 1986a).
In another study, (Zachara et al., 1992a), indicated that there are particle-particle
31
interactions between positively charged hydroxyl sites on the Fe oxides and exchange sites
on layer silicates that appeared to block access of Cd to these sites on the layer silicates. By
applying a multisite model to Cd sorption in a different range of pH and at ionic strengths
of 0.1 and 0.01M, Cowan et al (1992), concluded that the complexation of Cd with hydroxyl
sites produced a strong pH dependency in Cd sorption and was responsible for all of the Cd
sorption at ionic strength of 0.10 M and at pH above 6.S.
On the basis of hydrous oxide cation adsorption behavior, there is one common soil
component thought to play an important role in heavy metal immobilization. The abiliQr of
hydrous oxide surfaces to accumulate cations despite an unfavorable surface charge,
indicating that a specific adsorption mecham'sm exists (Forbes et al., 1976). Competitive
adsorption of metals on amorphous Fe oxyhydroxides has shown that there are specific
binding sites for Cd (Benjamin and Leckie, 1981). Adsorption of Cd was greatest when
organo-clay fractions separated from a sil^ loam soil contained high quantities of organic
matter, Fe and Al hydroxides (Levy and Francis, 1976).
Cadmium Mobility
Minimizing the threat of soil and ground water pollution by accurately predicting the
movement and cycling of hazardous contaminants is an ultimate goal. Hershaft (1972)
suggested that the soil is the ultimate receptacle of most solid and liquid wastes. After
studying the movement of some trace elements including Cd in 11 soils,(Korte et al., 197S)
reported that the soil texture, surface area, percentage of iron oxides, and pH provide the
most useful information forestimating an element's migration.Scokatetal., (1983) reported
that when the pH value dropped below 6, the mobility of Cd increases as compared with a
32
higher pH value. Moreover, they added that the availabili^ of heavy metals is lower in
natural loamy soils than in sandy acidic soils due to precipitation of carbonates and
phosphates in the former one.
Metals may occur in the soil solution in different forms, each having a different
mobility and toxici^. Hydrolysis species of Cd do not contribute to total Cd in acidic soils
(Lindsay, 1979). Studies done on the chemistry of metals in forest soils of South Sweden
have shown a strong correlation between soil solution pH and total Cd (Berggren,1992).
Fuller (1977) established a classification according to which cadmium should be
fairly mobile in soils of pH 4.6 to 6.6 and moderately mobile in those between 6.7-7.8 pH
units. Sanchez-Cmazano et al., (1993) studied the mobility of Cd in various natural soils by
soil thin-layer chromatography and found that, the ratio of distance moved by cadmium to
the one moved by the developer, Rf, range between 0.14 and 1.0 for soils with a mean of
0.64. Their conclusion was that Cd was slightly mobile in 27%, moderately mobile in 14%,
mobile in 41% and highly mobile in 18% of the soils studied. The pH, sum of the
exchangeable bases, exchangeable Ca and Mg, CEC, and clay content have a significant
effect on cadmium mobility^ in soils (Sanchez-Cmazano et al., 1993).
Scokart et al., (1983) reported that the results of complex trace metal (Cd and Zn)
interactions with soil components show a higher accumulation capacity^ for loamy soil than
for sandy soil even though they have a similar CEC. The addition of organic-rich wastes
such as municipal sludges and industrial residues can provide enough organic matter to the
soil to bind other toxic heavy metals such as Cd, Zn, and Pb (Elliott and Denney, 1982).
Lund et al., (1976) suggest that heavy metals below sewage disposal sites can move
33
downward through the soil column as soluble metal-organic complexes.
Elliott and Denney (1982) observed that metal-binding organic substances in waste
water or sludge designed for land use may affect the fate of toxic metals. They showed that
Cd adsorption in the presence of soluble organic ligands, ammonium acetate (Ac), oxalate
(Ox), nitrilotriacetate (NTA), and EDTA was modified based on a competition for the metal
ions between soil metal-binding sites and added soluble organic materials. In addition they
added that when the ligand bond was weaker than metal-soil such as Ox-Cd, adsorption was
not effected. However, for ligands capable of strongly binding Cd (NTA and EDTA) out of
competing for soil sites, adsorption was reduced due to the formation of non adsorbing
complexes.
Kirkham (1977)suggests avoiding the use of effluent containing chelating agents for
irrigation. The importance of organic ligands on the mobility of metals in soil environments
should be minimal for acidic solutions and weakly complexing ligands. (Elliott and Denney,
1982).
Solid Activity Coefficients of Cadmium Adsorbed
The ion activity product (lAP) is another concept to describe the relationship
between the amount of cadmium adsorbed by soil and the concentration of cadmium in soil
solution. Hendrickson and Corey (1981) found that the lAP is usually less than the
solubility^ products (Ksp) of pure substances and varies &om soil to soil. The standard solid
activity for the solid phase is assigned to be unlQr according to thermodynamic definition.
The mole fraction (x) for pure solids approaches 1 and the contribution to the chemical
potential of the solid from the free energy of mixing and the potential free energy is equal
34
to zero; however, in natural systems the mole fraction is expected to be different from
unity (x = I) as in the case for pure solid phase due to isomoiphous substitution of foreign
constituent ions in the crystalline lattice (Wayne, 1998).
According to (Lowe, 1973), if the isomorphous substitution results in no
concentration gradient within the lattice, the emerging solid phase is a homogenous solid
solution. If the constituent ion of interest is part of the major component of the solid phase
the lAF which is an indicator of the saturation of the solid solution will only differ slightly
from the Ksp from the pure mineral phase. If the constituent ion of interest is part of the
minor component of the solid phase, the resulting lAP at saturation can be substantiality less
than predicted from Ksp of the pure mineral phase. One of the reasons for the difference
between an lAP in soil solutions and the Ksp of the pure substance is due to adsorption and
nonequilibrium in the soil (Bohn, 1983).
Bohn and Bohn, (1986) indicated that the chemical potential (activity) and aqueous
solubilities of solid compounds depend on their concentration in the solid phase, and that
approximate solid activitiescan be calculated from measurements of the ion activity product
in the aqueoussoil solution, with the assimiptions thata solid's surface composition remains
constant and a partial equilibrium is attained.
Bohn and Bohn, (1987) proposed the solid activity^ coefScient to express the
chemical potential for nonideal solid mixtures:
lAPi = g,X,Kspi
(I)
where g is the activi^ coefficient of the solid i in the solid solution or the solid phase, X is
the mole fraction of the solid i, and Ksp is the solubiliQr product of the pure solid.
35
Bohn and Bohn, (1987) showed that Eq.(l) can also be modified to give a more useful
coefficient by combining g and Ksp to get g':
lAP, = g •JQ
(2)
They pointed out that the g and Ksp values are unique to the ion, Ksp is independent of the
soil, and its value is usually uncertain in the chemical literature. This makes the g'
coefficient more predictable since the g' value depends only on soil measurements. Further,
they recommended using the solid activity coefficient concept i.e. Eq. (1), and (2) to
evaluate adsorption in soil. They mentioned that there are four reasons for applying such a
concept to soil-water reactions." (1) soil minerals and surfaces are O^'and OH' matrices with
mostly Si"*, Fe^" and Al^"^, in the interstices, (2) crystals should more easily accommodate
small amounts of defects such as foreign ions compared to large amounts, (3) the chemical
composition of soil surfaces is more homogeneous than the soil as a whole, and (4) assuming
homogeneity for trace metals throughout the soil have to be better than for major elements".
The solid activity coefficients, g, derived from Eq.(l), should improve the accuracy of
models used particularly for predicting the fate of trace elements in the soil solution,
groundwater, and other natural systems.
Chemistiy of Cadmium:
The oxidation states of cadmium in a soil is limited to Cd^^ As seen from the
following reaction:
Cd^* + 2e- - Cd(c) log K" = 43.64
pe = -6.82 -t- '/ilogCd--
(3)
logCd^" - 13.64 +2 pe
(4)
36
Equation (3) shows that a 1 M Cd*" in solution requires a redox potential of pe = -6.82
before Cd(c) can form. And since Cd*^ activity in soil is near 10'^ M, pe <-10.3 would be
required (Eq.l). Since this pe range is below the stability field of water for pH values
below 10.3, Cd(c) is not expected to be present in soils. Because there are no other oxidation
states of cadmium that are stable in the redox range of soil, and chemistry of cadmiimi in
soils is limited to Cd^^ minerals and complexes (Lindsay,1979).
Street et al., (1978) found Cd*^ activities to be about 10'^ M in the pH range of 6
to 7.5. Their findings are summarized by the following reaction:
soil-Cd«Cd-'
log K" = -7.00
Cadmium in aqueous solution has a pronounced tendency to form soluble complexes with
both organic and inorganic ligands. In natural water and industrial waste water systems are
complexes formed by combination with hydroxide, chloride and to some extent ammonia
(Weber and Posselt. 1974).
The equilibria pertaining to hydroxy comple.xes of cadmium can be presented in
terms of the stepwise formation of these species;
Cd^*
+ OH-
« CdOBT; k,
CdOir + OH-
«. Cd(0H)2;
Cd(0H)2 + OH-
» HCdOi- + HjO; kj
HCdCi- + OH-
« CdO,^- + H^O; k^
Where the log(lO) values ofthe stepwise formationconstants ki, kj, k3,and k* at zero ionic
strength and 25°C are 4.16,4.23,0.83, and -0.32 respectively (Weber and Posselt. 1974).
In most environmental circumstances the solubiliQr of cadmium is governed by hydroxide
37
or carbonate. The corresponding soiubili^ product expressions are
Ksp=[Cd-*][OHf =2.2x10'^
(5)
BCsp = [Cd-*][C032-] =5.2xl0-'2
(6)
Solubility calculations using these solubility products (Eq. 5 and 6), for example
log[Cd^'"](^ = 14.34 - 2 pH, are good only to about pH 8, beyond which the contributions
of hydroxy-complexes must be considered. In the absent of carbonate in the pH range 7-12,
the mass balance for soluble cadmium in terms of formation constants is given by:
C = [Cd^1 (1 + p,[OH] + PjtOHf + PJCOH-]^ )
(7)
where C is the total Cd (moles/I) and substitution of the solubility producing, Eq. 5 yields
an expression for C which is independent of cadmium concentration as long as the system
remains saturated with respect to solid cadmium hydroxide (Weber and Posselt. 1974). In
this situation C is equivalent to die solubility (S).
s=
(1 + P,[OH ] -I- P,[OHf + PjfOH-r)
[0H-]2
Where Pi= k,,
k,-kiand P3= kj-kj-kj
In aqueous systems having both hydroxide and carbonate, the ratio of the
concentrations and the corresponding solubility products of these ions will determine which
of the corresponding Ksp values applies to the particular case. Division of the two solubility
products gives the ratio R:
Where is R =
0.00423
38
The R is a suitable parameter for corporation of systems conditions. If R > 0.00423, Eq.(S)
applies and if R < 0.00423, S is calculated in accordance with carbonate solubility products
Eq.(6). IfRisjust equal 0.00423, either relationship holds (Weber and Posselt. 1974).
39
CHAPTERS
MATERIALS AND ANALYTICAL METHODS
Soils
Criteria for Selection of Soils
Fourteen soils were employed in this research. The soils were selected by the
following criteria: (1) A varying range of organic matter to show its effect on cadmium
sorption and mobility, (2) a varying range of pH (4.9-8.8) to examine the effect of natural
pH on cadmium sorption and mobility, (3) a wide range of cation exchange capacity (4.1-99
cmol/kg) to study its effect on cadmium sorption and mobility, (4) a significant deviation of
bases saturation, (5 ) a wide range of iron and aluminum oxides to examines its effect on
cadmium sorption and mobility.
General Description of Soils
Soil samples used in this study were taken from two locations Arizona and Hawaii.
The classification and locations of the soils are as follows: (I) Pima clay loam (fine-silty,
mixed, thermic Typic Torrifluvent, located near Marana, AZ., (2) Casa Grande sandy loam
(fine-loamy, mixed, hyperthermic Typic Natrargid), located near Casa Grande, AZ., (4)
Gilman loam {coarse-loamy, mixed (calcareous), hyperthermic Typic Torrifluvent}, located
near Gila Bend, AZ., (6) Gadsen clay (fine, montmorillonitic, hyperthermic Vertic
Haplustoll), located near Yuma, AZ., (10) Superstition sand(sandy, mixed, hyperthermic
Typic Calciorthid), located near Yuma, AZ. These soils represent most of the major
agriculture soils in the lower Sonora Desert (Hendricks et al., 1985). (11) Kawaihae (Silt
loam)medial, isohyperthennic Ustollic Camborthids, (12) Maile (clay loam) hydrous.
40
isomestic Typic Hydrudands,(14) Amalu (loam) hydrous, isomesic Typic Hydrudands, (17)
Puaulu (Sandy loam) medial over hydrous, isomesic typic Hydrudands, and (18) Puaulu (silt
loam) medial over hydrous, isomesic typic Hydrudands all located on the Kohala peninsula
on the Island of Hawaii. The Andisols &om Arizona includes (13) Sandy loam, Entic
(Hapludands), (15) a Cindery, amorphic (ferrihydrite), frigid Typic Ustivitrands, and (16) a
Cindery, amorphic (allophane/imogolite), mesic Typic Ustivitrands located near Flagstaff
and (19) Silt loam, Typic Haplocryands located west of Springerville.
Preparation of Soils
The soils samples were provided by Dr. Hendricks and were stored in 200 ml
polyethylene bottles. The fine earth (< 2mm) of the soils were used for the sorption
experiment, and all chemical and physical analyses. For the cadmium mobility experiment,
the soil samples were further ground in a mortarand pestleand subsequently sieved through
a I60-^m nylon mesh to obtain samples with a small and nearly homogeneous particle size.
Methods of Soil Characterization
Texture
Most of the texture analysis for these soils were done by the USDA Natural
Resources Conservation Service (unpublished data). Soils with no particle size distribution
data available were determined by the pipet method after removing the cementingagent and
dispersing by sodium pyrophosphate (Kilmer and Alexander, 1949).
pH and pH-Adjustment
pHwas measured potentiometrically in a 1:1 liquids and soil suspensions by using
a combination pH-electrode. The readings were carried out while stirring the suspensions
41
slightly. Adjustment of pH in the experiment was accomplished by adding a small amount of
acid 0.1 M HNO3 or base 0.1 M NaOH to the soil suspension and equilibrating for 24 to 48
hours before adding cadmium to carry out the sorption isotherm study. Adjustment of pH
over more than 0.3 units required 10-12 hours to approach equilibrium, while adjustment
within 0.1 approached equilibrium in 4-6 hours. Reaching complete pH equilibrium required
between 24-48 hours with occasionally agitation. The pH was determined to the second
decimal, and ± 0.03 pH units where accepted as correct pH, although a better precision was
sought. For accuracy the pH-meter was calibrated with pH 4 and 10 buflFers every half hour
during use.
Cation Exchange Capacity
Cation exchange capacity for soil samples (1,2,4,6 and 10) were reported by Unruh
et al., (1994). For soil samples 15 and 16 CEC were reported by Chen (1989). Soil samples
(11-14, 17-19) CEC were reported by the USDA Natural Resources Conservation Service
(unpublished data). Most of the samples were analyzed for a second time by the displacement
method, after washing using leaching tubes with buffered NaOAc at pH 8.2 with Na as the
index cation. Additionally, NH40Ac extractable bases, namely Ca, Mg, K, and Na, were
measured employing atomic absorption spectrophotometer and flame emission analyses using
an IL Vklio12 atomic absorption spectrophotometer. The chemical and physicalcompositions
of the soils are given in Tables 2a and 2b.
Iron and Aluminum Oxides
The extractable iron and alummum contents of the soQ were determmed according to
(Blakemore et al., 1987). The poorfy^ crystalline, Feo and Ab of Fe and Al mmerals were
42
extracted, using acid ammonium oxalate in the dark. For the total oxides, hydroxide and
ojQrhydroxides, (Fed and Aid) the soils were extracted using ditionite-citrate-bicarbonate
(DCB) buffer. Sodium pyrophosphate was used to extract the organically bound of Fe and
Al, (Fep and Alp). The concentration of the inorganic poorly crystalline Fe fraction, FeoFep was calculated from the different between poorly crystalline and organically bound
oxides (Evans and Wilson, 1985).
Water Soluble Cation and Anions
Five grams of the soil (in triplicate) and 25 ml of distilled water were placed in 50-ml
polypropylene centrifuge tubes and shaken for 24 hours. Immediately after shaking, the tubes
were centrifliged for 30 minutes and the aliquot of the supernatant was analyzed for total
cations and anions, namely; bicarbonate, chloride, fluoride, nitrite, bromide, nitrate,
phosphate, sulfate, iron, aluminum, calcium, magnesium,sodiimi, and potassium. Inductivity
coupled plasma (ICP) emission spectroscopy was used to measure the concentrations of
soluble cations in soil solution. The anions in the extracts were also analyzed using the
Dionex 2020i ion chromatograph.
Adsorption Isotherms
Preliminary kinetic studies were conducted to determine the appropriate cadmium
concentrations and the suitable equilibration time to be carried out for the main study. A 0.5
g of soil was mixed with 25 ml background electrolyte solution of 0.025 M NaNO}
containing 3200 ^g/L of Cd'" in 50 mL polypropylene centrifuge tube was shaken on an
automated shaker at a temperature of 25 ± 2°C for various time periods (6,12, and 24 hr).
Immediately after shaking, the tubes were centrifliged for 30 minutes and an aliquot of the
43
supernatant analyzed for total cadmium using an atomic absorption spectrophotometer.
Whenever some cloudiness was present the solutions were filtered through a 0.4S ^m
membrane filter prior to analyses.
The preliminary studies showed that the adsorption reached a steady state in 24
hours, and therefore, the equilibrium time of 24 hr was used in all the experiments.
Adsorption isotherms were obtained by batch experiment at a solid:solution ratio of 1:50 or
O.S g soil and 25 ml of solution. A 25 ml of 0.025 M NaNOs background electrolyte
solution containing 0.8, 1.6, 3.2, 6.4, and 12.8 ppm of Cd^^ were added to 0.5 g of soil
samples (air dried and passed through 2mm nylon mesh) in 50 mL polyethylene cenOrifuge
tubes and shaken at room temperature (25 ± 2°C) for 24 hr. All experiments were carried out
in triplicates plus a blank reference.
Cadmium Analvaia
Total Dissolved Cadmium
Immediately after shaking, the tubes were centrifliged for 30 minutes and an aliquot
of the supernatant was analyzed for Cd using an atomic absorption spectrophotometer.
Whenever some cloudiness was present, the solutions were filtered through a 0.45 ^m
membrane filter prior to carrying out the analyses.
Cadmium Activity Measurement
A selective ion electrode procedure was employed for the determination of the
concentration of firee ionic Cd, (Cd~^), in the soil solution. The instrument used to measure
Cd^" activity in both soils and standards solution consisted of a cadmium electrode (Orion
Model944800) withsingle-junction referenceelectrode (Orion Model 900100), a reference
44
filling soiutron 90001 (Orion), pH-meter connector, and a magnetic stirrer. A set of CdCNOj),
standard solutions ranging fi'om lO'^M to lO"* M total Cd were made up in a background
solutnn of0.025 MNaN03. Cadmium ion activities in the standard solutions were measured
by ion selective electrode (ISE), and a standard curve of electrode potential (mV) versus Cd"'
activity (M) was obtained. Each time before measuring the Cd"* activities in soQ solutions,
the CdCNOj), standard solutions were measured and a plot of electrode potential (mV) vs.
Cd"* activity was made. The assumption was that the Cd"^ activity was equal to the total Cd
concentration in the standard solutions. In measuring Cd~^ activities of a soil solution about
23 mL of the soil extract that had been filtered through a 0.45 nm membrane filter was
placed in 50 mL beakers and used for the activity measurement.
Durmg the Cd~* activity measurements the soil solutions were stirred contentiously
to insure equal ion distribution as suggested by the manufacture (Orion reasearch,1990). The
electrode potential (mV) stabilized quickly for Cd"* concentrations greater than 10"' M but
more slowly for lower Cd'* concentrations. Because ofthis slow stabilization the mV readings
tended to drift at the lower concentration and as a result, the readings were taken from the
instruments at about 5-10 minutes after the electrodes were immersed in the solutions.
For, quality measurements, after measuring the standard solutions and between each
sample measurement, the electrode was cleaned by polishing the electrode with a cleanmg
strip and then by immersing it into 0.007 N NHOj until the electrode potential equilibrated
to a baseline reading of -90 mV as suggested by Orion Research (1990). Also the reference
filling solution was always mamtained at the same level in the electrode m order to obtam
constant level and elunmte its mfluence hi the measurement.
45
Cadmium Mobility Study
Preparation of the Soil Plates:
The Cadmium mobility was studied by means of soil thin layer chromatography(soil
TLC). The Soil samples were ground in a mortar and sieved through a 160-nm nylon mesh
sieve to obtainsamples with a small and nearly homogeneous particle size. A soil water slurry
of solidrsolution ratio of 1;2 (7.5 g of soil and 15 ml of distilled water) was then prepared.
The plates then were coated with water slurry ofthe soil (0.5 mm thickness) over each of the
20 X 5 cm plates wfth aid of an ordinary TLC applicator. The selected plates, which have an
even soil thickness distribution, were then air dried at room temperature and relative humidity
of 70% in a desiccating chamber.
Soil TLC Procedures
The plates were marked with two horizontal Imes at different distances from the base
(3 and 13 cm) so that the standard development distance of 10 cm was used in all the plates.
A 5 |iL aliquot of of cadmium nitrate solution (CdCNOj), 0.1 M) was applied as spot on the
base line of triplicate TLC plates in a single application with the aide of a micro pipette. The
drops were placed at a distance of 3 cm from the bottom of the plates.
The loaded plates were then developed by ascending chromatographic tank containing
distilled water as solvent. The development was done for 1-3 hour depending on soil sample
textures. Then, the plates were washed for 13 cm from the baseline and allowed to dry at
room temperature. The dried plates were sprayed with 0.05 % dithizone m CCl, in order to
detect Cd as orange spots of Cd-dithizone.
Cadmium mobili^ was measured visual^ as the frontal Rf value (noobility) using the
46
following relationship:
^ Fm
•^^Fd
Where Fm is frontal distance moved by the metal and Fd is the one moved by the developer
solution.
Solid Activity Coeflicients of Cadmium Adsorbed
[on activity products for cadmium complexes {Cd(OH),} were calculated form the
activity measurement ofCd'" usmg the Cadmium Selective Electrode (Orion Model 944800)
as described in the activity measurement section and from the pH electrode for OH'.
Cadmium hydroxide activity products was calculated according to the following:
(Cd(OH),J = (Cd^nCOH-)-
(8)
The solid activity coefficient (SAC) was calculated according
to (Bohn and Bohn, 1986) as:
lAP
= x:^
(»'
where lAP is the ion activity product calculated from Equation (8), Ksp is the solubility
product of Cd(0H)2, X is the mole fraction of the soil, and estimated from
X= (total cadmium - soluble cadmium )/30. The units of total and soluble Cd is mole/kg soil,
and 30 that refers to 30 moles ofoxides/Kg as calculated by Bohn (1983). The Ksp value
used to calculate lAP of Cd was 10*'^^^ (Lindsay, 1979). Moreover, the solid activity
coefficKnt g', was calculated as follows:
g'=SAC-Ksp
(10)
47
where the single activity coefficients for Cd~' were calculated according to (Bohn 1992) as
follows:
^Cd(0H)2 solid
SACcd
(SACQH XQH)"
Separating the OH' into
^
(11)
and H' and assuming the O^' activity in solid phase as:
=1
Then, from Eq.ll, the solid activity of CdCOH), is:
^Cd(0m2 solid ~ SACc, Xcd(aO)2(SACHXH)=
~ SACqj X<;J (SACH XH)*
and the mean solid activity coefficient for SACcjz-
_
SACcd- = SAC =
(12)
cations in CdCOH), is:
IAPcd(OH)2
xCd (xH)
Speciation and Modeling Adsorbed Cadmium
The computer speciation program, HYDRAQL, (Papelis et al., 1988) was used to
model the amount of Cd adsorbed onto each of the humk: and hydrous ferric oxide surfaces.
The method used to estonate humic and ferric oxide sur&ces and its properties are given in
the Theoretical Considerations Section.
48
CHAPTER 4
THEORETICAL CONSffiERATIONS
Modeling Approach
The adsorption isotherms aUows for the determination of Cd adsorption maxima, but
they are not mechanistic and therefore can not be used to predict adsorption at different pH
values or for soils with different properties (Bolton and Evans, 1996). Thermodynamically
based surface complexation models can be used to accurately predict the adsorption of metals
on difierent humic materials, phyllosQUcate clays, and hydrous oxides (Bolton et al, 1996;
Schindler et al., 1987; and Dzombak and Morel, 1986).
Complexation of cadmium with organic colloids was modeled using a discrete
functional group model in which it was assumed that there were two unique diprotic
complexation sites, H;A and H^B, associated with the humic materials (Bolton et al.,1996).
The total concentrations of [H:A]T and [HIBJT were estimated using the following
Equation;
[H2L]T=
Msoii-OC-THA-0. 50-1. 724
(14)
where [H,L]x is the total concentration of either [HiAJt or [HiBIt in M, M„a is the mass
of the soil (g), OC is the organic carbon content (g/kg), THA is the total humic acidity
(mole/g) of HiL (Table 1) and V is the volume of solution (ml). The values of 0.5 and 1.724
are included in the Equation to account for the diprotic nature of the acid and to convert &om
carbon to humic acid, respective^ (Bolton et al., 1996).
In order to model Cd adsorption on hydrous ferric oxide surges the diflRise layer
49
Table I. Parameters used in the surface complexation model
Humic acid*
THA(H,A) = 2.9x10'^ molg'
THA(H,B) = 1.3x10*^ mol g'
Logp
'^FT+HA=^2H'+A--
H,A
H,A
-4.00
-9.32
'=^Fr+HB=^2H-+B=-
HoB
H,B
-7.43
-16.66
FT + CdHA'
HjA + Cd"
-1.29
H,A + Cd-*
— 2H* + CdA°
-5.92
H,B + Cd-^
'^2H-+CdB''
-4.39
H,B + Cd-^
— 3ir + CdH.,B-
-3.72
Hydrous ferric oxides'*
Ns(strong) = 0.005 mol mol Fe"';
Ns (weak) = 0.2 mol mol Fe '
5Fe-0H,/
'5=^ ^Fe-OH"
+ H*
-7.29
=Fe-OH° + H'
—
=Fe-0-
+ H-
-8.93
=Fe-OH°
— =Fe-0-Cd*
+ Fr
Cd--
-2.9 (strong)
0.47 (weak)
"All values taken from Bolton et al.(1996).
''All values taken from Dzombak and Morel (1990).
model of Dzomi3ak and Morel (1990) was used. The intrinsic dissociation reactions for
these sur&ce sites are written m the form;
=Fe-OH,' ^ =Fe-OH°
+
sFe-OH°
+
=Fe-0-
where""and ^K^'^are the mtrinsic dissoc^onconstants (Table I) and =Fe- represents
the presence of iron at the surface ofthe hydrous ferric oxide.
50
The complexation reaction for the surface can be described by;
HFe-OH" + Cd-" ^ HFe-O-Cd^-+ fT;
where
""
"" is the intrinsic complexation constant. Dzombak and Motel (1990)
distinguished between the two types of surface sites which they refer to as strongly binding
and weakly binding (Table I).
The total concentration of adsorption sites associated with hydrous ferric oxide,
[FCOOHJT, were calculated according to Bolton et al.,
n:-r»r»tn
[FeCX)H]T
(1996) from:
(Feo - Fep) • M soil > N.
(15)
Where (Feo-Fep) is the inorganic poorly crystalline Fe fraction, N, is the site density (mol
mol Fe"') of the strong or weak sites (Table 1) and MWp^ is the molecular weight of Fe. The
specific surface area, S, of the hydrous ferric oxide was assumed to be 600 m^g'' (Dzombak
and Morel 1990).
Sorption of Cadmium
The distribution of cadmium between the solid phase and the solute in the soil
environment is governed by sorption processes. The term sorption may include several
mechanisms such as ion exchange, surface induced precipitation reactions, formation of
chemical bonds on the solid surface, and adsorption into mineral lattices. Experimentally,
these mechanisms can not be separated for a complex sorbent matrix such as soil, and
therefore, their overall contribution is described as sorption.
Sorption Isotherms
The equilibrium cadmium content of the soil versus the equilibrium content of the
51
solute is called a sorption isotherm and is used for expressing the partition of cadmium.
Several theoretical and empirical approaches exist for modeling of sorption isotherms. The
two isotherms which are widely used to describe the sorption behavior are the Equations by
Langmuir and Freundlich.
Langmuir Isotherm
The Langmuir isotherm was developed by Langmuir (1918) to describe the
adsorption of gases by solids. The Langmuir isotherm is derived from kinetic considerations
of the sorption process and assumes formation of a monolayer on uniform sorption sites
(constant free energy and no interaction between sorbed ions).
The general form of the Langmuir isotherm is:
S=
k-b-c
l + k-c
The linear form:
C
—=
s
1 +—
1 *0
b*k b
The parameters are:
s - amount of metal sorbed per unit of solid.
c = equilibrium concentration of metal.
k = sorption parameter related to the energy of sorption.
b = maximum sorption capaci^ per unit of solid.
The Langmuir isotherm characteristically exhibits saturation of the sorbent at high
solute concentration, corresponding to the maximum sorption capacity as expressed by the
parameter b. If two types of sorption sites exist on the sorbent surface, the Langmuir
52
Equation may be extended and add another right side term with different k and b parameters
(Syers et al.,1973). For adsorption data fitted to the linear form of the Langmuir Equation,
a plot of the ratio C/S against C should yield a straight line with slope of (b)'' and intercept
of (kb)*'. And if one takes the slope over the intercept value one obtains directly k.
Freundlich Isotherm
The Freundlich Equation is the oldest of the nonlinear sorption Equations and has
been used widely to describe solid retention by soils (Sposito 1984). The Freundlich
isotherm is an empirical relation of the form of (s = k • c"").
The linear form of the Freundlich isotherm as follows:
logs = logkdH—logc
n
The parameters are; s = amount sorbed per unit of solid, c = equilibrium concentration,
kd = sorption distribution coefRcient. n = is an indicator of sorption capacity.
The value of n is restricted to 0 < n < 1 and when found to be unity the isotherm
becomes linear, allowing the description of adsorption in the form of a distribution
coefficient. The theoretical derivation of the Freundlich isotherms may be obtained by
assuming that the fi-ee energy of adsorption decreases with increasing surface coverage, due
to surface heterogeneity or interactions between sorbed ions. The Freundlich isotherm can
be derived theoretically by considering the heterogeneous nature of adsorption sites
(Spositio, 1984). One limitation of the Freundlich isotherm is that, like the linear isotherm
model, it does not imply a maximum quantiQr of adsorption. Freimdlich isothemis are
constructed by plotting log s against log c. The values of sorption parameters k and n can be
obtained from the intercept and the slope of the plot respectively.
53
CHAPTERS
RESULTS AND DISCUSSION
This chapter presents the experimental results and the correspondmg discussion.
Soil Characteristics
The tburteen soils investigated in this research are characterized by the parameters
given in Tables 2a and 2b. The soils range in particle size distribution and texture from sand
to siky clay.They exhibited a range of pH values from strongly acidic (pH of 4.9) to strongty
alkaline (pH of 8.8), and a wide range of cation exchange capacities from very low of 4.1
cmol/kg to very high of 99.0 cmol/kg, and a wide range of organic carbon contents from 0.6
g/kg to 103.0 g/kg. The exchangeable cations m these soils ranges in the following: 0.2-14.6
cmol/kg for calcium, 0.1-8.0 cmol/kg for magnesium,0.4-3.4 cmol/kg for sodium and 1-2.6
cmol/kg for potassium.
Table 2a. General characteristics of soils used in the study.
sand silt clay pH
Mg
CEC Ca
Soil
Cmol/Kg
%
1:1
l.Typic Torrifluvent
22
51
27
8.4
23.7 lO.O 1.8
64
1.4
2.Typic Natrargid
15
21
8.2
10.9 8.0
60
4.Typic Torrifluvent
2.0
24
16
8.4
10.6 6.8
18
6.Vertic Haplustoll
51
31
8.2
25.2 14.6 3.4
84
7
0.6
lO.Typic Cdciorthid
9
8.8
2.2
4.1
38
2.2
1 l.UstoUic amborthids
44
18
6.3
16.0 2.8
l2.Typic Hydrudands 84
0.5
13
3
4.9
99.0 0.2
20
13.Entic Hapludands
64
6.9
33.7 11.6 2.4
16
34
1.0
l4.Typic Hydrudands
53
13
7.0
62.5 4.2
3.3
IS.Typic Ustivitrands
73
20
7
7.4
18.6 5.5
1.7
16.Typic Ustivitrands 94
5
7.8
6.3
l.O
1
0.5
n.Typic Hydrudands 70
5.8
1.5
27
3
6.9
1.4
IS.Typic Hydrudands 55
39
6
4.9
21.3 2.5
0.4
19.TvDtc I^Dbcrvands 34
54 12
6.1
22.3 5.3
Na
K
8.0
0.9
1.4
6.0
0.9
1.4
1.5
0.3
0.3
0.6
0.7
0.1
2.3
O.l
1.9
1.5
0.7
1.2
0.3
1.0
0.3
2.6
1.3
0.5
0.2
0.1
0.2
1.2
54
Table 2b. General characteristics of soils used in the study.
O.C Feo-Fep Feo
Fep Fed
Soil
g/kg
l.Typic Torrifluvent
2.Typic Natrargid
4.Typic Torrifluvent
6.Vertic Haplustoll
lO.Typic Calciorthid
ll.Ustollic Camborthids
12.Typic Hydrudands
13.Entic Hapludands
H.Typic Hydrudands
15.Typic Ustivitrands
l6.Typic Ustivitrands
l7.Typic Hydrudands
IS.Typic Hydrudands
19.Typic Haplocryands
3.6
1.6
1.0
2.8
0.6
15.9
103.0
53.0
24.2
16.9
3.1
3.2
39.1
22.9
1.6
3.6
2.8
2.1
4.4
7.8
7.1
20.1
28.0
29.0
11.6
ll.l
7.1
11.0
1.7
3.7
2.8
2.3
4.4
8.8
10.1
21.1
31.0
29.7
12.2
13.1
15.1
11.9
0.1
0.1
0.1
0.2
0.1
1.0
3.0
1.0
3.0
0.8
0.6
2.0
8.0
0.9
2.9
2.7
3.8
2.8
5.4
51.0
16.0
34.9
33.0
27.0
14.0
ll.O
14.0
34.3
Alo
Alp
Aid
3.4
3.2
2.2
3.6
1.5
8.4
81.6
8.5
17.0
5.2
5.7
8.6
13.0
20.2
0.4
3.7
2.1
3.0
1.2
2.0
31.0
2.3
6.0
2.3
3.3
l.O
9.0
4.2
3.0
3.1
2.3
2.2
2.9
8.0
36.0
6.2
8.4
3.1
5.5
3.0
9.0
8.7
The fractions of iron oxides in these soils were in the following ranges: The poorly
crystalline (oxalate extractable), Feo, ranged &om 1.7 to 31.0 g/kg; the organically bound
(pyrophosphate extractable), Fep, ranged &om 0.1 to 8.0 g/kg and the total oxides,
hydroxides and oxyhydroxides (ditionite-citrate-bicarbonate extractable). Fed, ranged from
2.7 to 51.0 g/kg. The fraction of aluminum oxides were also in the following ranges: The
poorly crystalline, Alo, ranged from 1.5 to 81.6 g/kg; the organically bound. Alp, ranged
from 0.4 to 31.0 g/kg and the total oxides, hydroxides and oxyhydroxides. Aid, ranged from
2.2 to 36.0 g/kg.
Adsorption Isotherms
The adsorption isotherms expressing the partitioning of cadmium between the soil
and the dissolved phase are shown for soils 1,2,4,6, and soils 10-19 in (Fig. 1-14). The
sorption isotherms are slightly curvilinear indicating that soils sorption capacity^ decreased
55
0.8
0.7
0.6
r
Freundlich isotherm fit
Langmuir isotherm fit
•^0.5
E
I 0.4
s
30.3
a
T
0.2
0.1
0.2
0.4
0.6
0.8
Cd in solution (mg/L)
Figure 1. Cadmium sorption isotherm (soil 1).
1
1.2
1.4
56
0.8
0.7
Freundlich isotherm fit
0.6
Langmuir isotherm fit
SO.5
I 0.4
8
30.3
0.2
0.1
0
1
2
3
Cd in solution (mg/L)
Figure 2. Cadmium sorption isotherm (soil 2).
4
5
6
57
0.8
Freundlich isotherm fit
Langmuir isotherm fit
0.4
0.3 0.2
0
0
0.5
1
1.5
2
2.5
Cd in solution (mg/L)
Figure 3. Cadmium sorption isotherm (soil 4).
3
3.5
4
58
0.8
n7 —
Freundlich isotherm fit
0.6 —
—
l0 5 E
O
Langmuir isotherm fit
,/
^
10.4w
8
3 0.3 -
o
0.2 —
0.1
0
^
0
0.2
0.4
0.6
0.8
Cd in solution (mg/L)
Figure 4. Cadmium sorption isotherm (soil 6).
1
1.2
1.4
59
0.8
Freundlich isotherm fit
0.6 —
Langmuir isotherm fit
0.5
0.4
7? 0.3
0.2
0
0
0.5
1
1.5
2
2.5
Cd in solution (mg/L)
Figure 5. Cadmium sorption isotherm (soil 10).
3
3.5
4
60
0.8
07 —
0.6 :o.5 -
Freundlich isotherm fit
Langmuir isotherm fit
^- O
-^
0.4 —
o 0.3 -
^
'
0.2
Ti -•
0 —^
0.2
0.4
0.6
0.8
Cd in solution (mg/L)
Figure 6. Cadmium sorption isotherm (soil 11).
1
1.2
1.4
61
0.8
Freundlich isotherm fit
Langmuir isotherm fit
0.5
0.4
7? 0.3
0.2
0.1
0
0
1
2
Cd in solution (mg/L)
Figure 7. Cadmium sorption isotherm (soil 12).
3
62
0.8
0.7
Freundlich isotherm fit
0.6
Langmuir isotherm fit
fO.5
•o 0.4 —
s
3 0.3
0.2 —
0
- . 6
0.1
0.2
0.3
Cd in solution (mg/L)
Figure 8. Cadmium sorption isotherm (soil 13).
0.4
0.5
0.6
63
0.8
0.7
i
\
Freundlich isotherm fit
T1
0.6 4_
i
~r(
lo.5 i
\
Langmuir isotherm fit
I
y
!
i
/ '
I 0.4
S
30.3
0.2
-r1
_L
O.l
T
1
yex"
/"
i
1
0
0.1
0.2
0.3
Cd in solution (mg/L)
Figure 9. Cadmium sorption isotherm (soil 14).
0.4
0.5
0.6
64
0.8
0.7 T
0.6
t
Freundlich isotherm fit
Langmuir isotherm fit
10.5
E
I 0.4
1
gO.3
,'0
0.2
;0
O.l
73
0
0.1
0.2
0.3
Cd in solution (mg/L)
Figure 10. Cadmium sorption isotherm (soil IS).
0.4
0.5
0.6
65
0.8
0.7
Freundlich isotherm fit
0.6
Langmuir isotherm fit
1 0.4
7? 0.3
0.2
0
0.2
0.4
0.6
0.8
Cd in solution (mg/L)
Figure 11. Cadmium sorption isotherm (soil 16).
1
12
1.4
66
0.8
0.7
Freundlich isotherm fit
0.6
Langmuir isotherm fit
•^0
o» 5
E
0.4 —
30.3
0.2
0.1
6/
0
0.5
1.5
2
2.5
Cd in solution (mg/L)
Figure 12. Cadmium sorption isotherm (soil 17).
3.5
67
0.8
0.7
Freundlich isotherm fit
0.6
Langmuir isotherm fit
10.4
0.2
I
0
0
0.1
0.2
0.4
0.3
Cd in solution (mg/L)
Figure 13. Cadmium sorption isotherm (soil 18).
0.5
0.6
68
0.8
0.7
Freundlich isotherm fit
0.6
Langmuir isotherm fit
0.5
0.4
0.3
0.2
0.1
0
0.1
0.2
0.3
Cd in solution (mg/L)
Figure 14. Cadmium sorption isotherm (soil 19).
0.4
0.5
0.6
69
relatively for increasing solute concentrations. The percentages of sorbed cadmium onto the
soils as shown in (Table 3), varied from soil to soil. The percentage of sorbed cadmium over
the ranges of the total cadmium added (0.8,1.6,3.2,6.4, and 12.8 ppm) to the soils were in
following ranges: soil 1 (93 to 95%), soil 2 (56.3 to 65%), soil 4 (75.8 to 95%), soil 6 ( 93.8
to 96.3%), soil 10 (74.2 to 83.8), soil 11 (93 to 97.5%), soil 12 (70.7 to 93.8%), soil 13 (95.7
to 98.8%), soil 14 (97 to 97.5%), soU 15 (90 to 96.8%), soil 16 (97 to 98.8%), soil 17 (82
to 98.8%),soil 18( 95.9 to 97.5%), and soil 19 (95.8 to 98.8%). The Langmuir equation can
be linearized by;
C
1
1
—=
H *0
s b*k b
s = soil equilibrium concentration, mg Cd/g soil.
c = solute equilibrium concentration, mg Cd/1
k = sorption parameter related to the energy of sorption, 1/g.
b = maximum sorption capacity, mg/g.
The Langmuir and Freundlich parameters of Cd sorption are given in (Table 4). The
maximum sorption capacity of the soils ranged from 0.59 to 3.84 (mg/g), and for the
sorption parameters k from 0.23 to 3.24 1/g. Figures (15-20) present some of the Langmuir
plots of Cd sorption isotherm of the soils studied namely soil 1, 4, 6, 11, 16, and 17. The
linearized fit of soil I, exhibited a correlation coefiGcient (r) 0.95 and a maximum sorption
capaci^ of 1.67 mg Cd/g soil corresponding to 1.48 cmol/kg soil or approximately 6.3% of
the CEC of thesoil. For soil 2, the linearized fit exhibited a correlation coefficient (r) of 0.99
and a maximum sorption capaciQr of 0.67 mg Cd/g soil corresponding to 0.58 cmol/kg or
6—
^5
%
"
S3 —
a
I
82 ^
3
I
aJ —
1
-r-
0a
0
02
0.4
0.6
Cd in solution (mg/L)
0.8
Figure IS. Langmuir plot of cadmium sorption isotherm (soil 1).
0
0.5
1
1.5
2
Cd in solut'on (mg/L)
2.5
Figure 16. Langmuir plot of cadmium sorption isotherm (soil 4).
3
3.5
0
0
0.2
0.4
0.6
Cd in solution (mg/L)
0.8
Figure 17. Langmuir plot of cadmium sorption isotherm (soil 6).
1
0
0
02
0.4
0.6
Cd in solution (mg/L)
0.8
Figure 18. Langmuir plot of cadmium sorption isotherm (soil 11).
1
74
Cd in solution (mg/L)
Figure 19. Langmuir plot of cadmium sorption isotherm (soil 16).
0.5
1
1.5
2
2.5
Cd in solution (mg/L)
Figure 20. Langmuir plot of cadmium sorption isotherm (soil 17).
3
3.5
Table 3. Percentages of sorfaed cadmium onto the soils.
[Cd]x added
0.8
1.6
Soil
1.Typic Torrifluvent
2.Typic Natrargid
4.Typic Torrifluvent
6.Vertic Haplustoll
lO.Typic Calciorthid
11.Ustollic Camborthids
12.Typic Hydrudands
13.Entic Hapludands
M.Typic Hydrudands
IS.Typic Ustivitrands
16.Typic Ustivitrands
l7.Typic Hydrudands
IS.Typic Hydrudands
19.Typic Haplocryands
3.2
6.4
12.8
93.9
64.2
81.3
95.0
80.8
96.4
75.6
98.1
97.3
97.3
94.8
92.2
96.6
97.5
93.0
56.3
75.8
93.8
74.2
93.0
70.7
95 7
97 0
973
90.6
82.0
95.9
95.8
Cd sorbed (%)
95.0
65.0
95.0
96.3
83.8
97.5
93.8
98.8
97.5
98.8
97.5
98.8
97.5
98.8
95.6
70.6
88.8
95.6
83.8
96.9
89.4
97.5
97.4
98.1
95.0
97.1
96.9
98.1
94.7
69.4
85.3
96.9
78.8
95.3
82.5
97.8
97.5
97.8
95.3
95.3
96.9
97.8
approximately 5.5% of the CEC of the soil, for soil 4, the linearized fit exhibited a correlation
coefficient (r) of 0.93 and a maximum sorption capacity of 0.67 mg Cd/g soil corresponding
to 0.59 cmol/kg or approximately 5.6% of the CEC of the soU, for soil 6, the linearized fit
exhibited a correlation coefiScient (r) of 0.91 and a maximum sorption capacity of 1.33 mg
Cd/g soil corresponding to 1.18 cmol/kg or approximately 4.7% of the CEC of the soil, for
soil 10, the linearized fit exhibited a correlation coefficient (r) of 0.93 and a maximum
sorption capacity of 1.09 mg Cd/g soil corresponding to 0.97 cmol/kg or approximately
23.7% of the CEC of the sofl. The linearized fit of soil 11, exhibited a correlation coefiScient
(r) of 0.94 and a maximum sorption capacity of 0.99 mg Cd^g soil corresponding to 0.88
cmol/kg or approximately 5.5% of the CEC of the soil, for soil 12, the linearized fit exhibited
a correlation coeflBcient (r) of 0.84 and a maximum sorption capacity of 0.63 mg Cd/g soil
77
Table 4. Langmuir and Freundlich constants for cadmium adsorption.
Langmuir constants
Freundlich constants
Sou
Kd Kdl.5
\r)
'(r)
K(l/g) b (mg/g)
n
1/g
l.Typic Torrifluvent
2.Typic Natrargid
0.60
0.20
1.67
0.67
0.95
0.99
0.68
0.10
0.64
0.09
0.87
0.85
1.00
0.99
4.Typic Torrifluvent
0.71
0.67
0.93
0.23
0.19
0.60
0.99
6.Vertic Haplustoll
1.01
1.33
0.91
0.78
0.73
0.83
0.99
lO.Typic Calciorthid
0.23
1.09
0.93
0.19
0.18
0.82
1.00
11.Ustollic Camborthids
1.67
0.99
0.94
0,71
0.64
0.74
0.99
12.Tvpic Hydrudands
0.54
0.63
0.84
0.20
0.17
0.57
1.00
13.Entic Hapludands
2.81
1.02
0.92
1.06
0.95
0.72
0.98
H.Typic Hydrudands
0.51
3.84
0.94
1.62
1.59
0.95
1.00
IS.Typic Ustivitrands
2.14
1.36
0.81
1.31
1.20
0.78
1.00
16.Typic Ustivitrands
1.50
0.90
0.95
0.55
0.48
0.69
0.99
n.Typic Hydrudands
2.98
0.59
0.99
0.37
0.30
0.49
1.00
IS.Typic Hydrudands
0.97
1.80
0.93
1.10
1.04
0.86
1.00
19.Typic Haplocr}'ands
3.24
0.95
0.98
1.01
0.90
0.71
1.00
'Significant at the 0.01 probability level.
corresponding to 0 56 cmol/kg or approximately 0.6% of the CEC of the soil. Moreover, for
soil 13, the linearized fit exhibited a correlation coefficient (r) of 0.92 and a maximum
sorption capacity of 1.02 mg Cd/g soil corresponding to 0.90 cmol/kg or approximate^ 2.7%
of the CEC of the soil, for soil 14, the linearized fit exhibited a correlation coefficient (r)
of 0.94 and a maximum sorption capacity of 3.84 mg Cd/g soil corresponding to 3.41
cmol/kg or approximately 5.4% of the CEC of the soil, for soil 15, the linearized fit
exhibited a correlation coefficient (r) of 0.81 and a maximum sorption capacity of 1.36 mg
Cd/g soil corresponding to 1.21 cmol/kg or approximately 6.5% of the CEC of the soil.
The correlation coefficient (r) for soil 16 was 0.95 and the maximum sorption capacity
78
was 0.90 mg Cd/g soil corresponding to 0.80 cmol/kg or approximately 12.7% of the CEC
of the soil, for soil 17, the linearized fit exhibited a correlation coefiRcient (r) of 0.99 and a
maximum sorption capacity of 0.59 mg Cd/g soil corresponding to 0.53 cmol/kg or
approximately 9% of the CEC of the soil, for soil 18, the linearized fit exhibited a correlation
coefficient (r) of 0.93 and a maximum sorption capacity of 1.80 mg Cd/g soil corresponding
to 1.6 cmol/kg or approximately 7.5% of the CEC of the soil, and for soil 19, the linearized
fit exhibited a correlation coefficient (r) of 0.98 and a maximum sorption capacity of 0.95 mg
Cd/g soil corresponding to 0.84 cmol/kg or approximately 3.8% of the CEC of the soil.
In addition, the k-values were estimated to be 0.60, 0.20, 0.71, 1.01, 0.23,1.67, 0.54, 2.8,
0.51,2.14, 1.50,2.98,0.97 and 3.241/g for soil 1,2,4,6,10,11,12, 13, 14, 15,16, 17, 18
and 19 respectively.
The Freundlich equation is the oldest of the nonlinear sorption equations and has been
used widely to describe metal retention by soils (Sposito 1984). The Freundlich equation can
be linearized by;
1
log s = log kd —log c
n
The parameters are;
S = soil equilibrium concentration in mg Cd/g soil.
kd = distribution coefficient in l/g soil.
n = sorption parameter, dimensionless.
As seen fi-om (Table 4) the value of the distribution coefficients of Cd sorbed in the soils
ranged fi-om 0.10 to 1.62 (l/g) and the parameter n, ranged fi-om 0.49 to 0.87. The linearized
79
As seen from (Table 4) the value ofthe distribution coefficients of Cdsorbed in the soils
ranged from 0.10 to 1.62 (1/g) and the parameter n, ranged from 0.49 to 0.87. The linearized
Freundlich isotherm equation exhibited a very significant (p< 0.01) correlation coefficient
(r) ranging form 0.98 to 1. The n values which are a measure of the nonlinear weight in the
Freundlich equation were less than unity for all soils studied suggesting a non linear
relationship between the mass of the element in the solid and the solution phases. Having
a value less than one, also means that at higher and higher Cd concentrations added, the soils
exhibited difficulty in sorbing additional Cd or that the specific binding sites become filled
or remaining sites are less attractive to the additional Cd added. The n-values were 0.87,
0.85,0.60,0.83 ,0.82,0.74,0.57, 0.72, 0.95,0.76, 0.69,0.49, 0.86, and 0.71 for soils 1,
2,4,6,10,11,12,13,14, 15,16, 17,18, and 19 respectively.
Figures (21-26) present some of the Freundlich linearized plots of Cd sorption
isotherms of some of the soils studied, namely soil 1, 4, 6, 11,16, and 17. A high value of
the distribution coefficient, kd, indicates that Cd has been retained by the solid through
sorption reactions, while a low value of kd indicates that most of the added Cd remains in
solution where it is available for transportation and biological activities.
Thekd-values were0.60, 0.10, 0.23, 0.78, 0.19, 0.71, 0.20,1.06,1.62,1.31,0.55,
0.37, l.l, and 1.0 I/g for soils 1, 2, 4, 6, 10, 11, 12, 13, 14, 15, 16, 17, 18, and 19
respectively. Soils 2, 4, and 17 have the lowest kd values of 0.10, 0.23 and 0.37 (1/g)
which is an indication of low sorption capaciQ^. The corresponding maximum sorption
capacity of these soils also were the lowest and were 0.67,0.67 and 0.59 mg/g for soils 2,4,
and 17 respectively indicating a good agreement with the kd values in which most of the
80
added Cd remains in solution. The Freundbch equation showed a better fit for Cd sorption
data than the Langmuir equation for all soils studied (Table 4 and Fig. 15-26).
When the sorption data of Cd were plotted (Fig.1-14) according to both equations
Langmuir and Freundlich, the predicted sorption values varied form being underestimated to
being overestimated depending on the soils. The Lai^muir isotherm severely underpredicted
Cd sorption in soils 1,6, 11, 13,14, 15,16,18, and 19, while overpredicted Cd sorption in
soils 2 and 12. On the other hand, the Freundlich isotherm predictions of Cd sorption were
in good agreement with the measured data over the entire region of the concentrations used
in this study. In order to determine the influence ofsoil propertieson the Cd sorption process,
the single correlation coefiScients between the Langmuir sorption constants b (mg/g) and k
(l/g); and the FreundUch sorption constants n, kd, kd L5(at 1.5 ppm equilibrium concentration)
and soil properties were obtained (Table 5).
The maxima soil sorption capacity b ( mg/g ) was significantly correlated with
inorganic poorly crystalline iron (Feo-Fep), and the total oxalate extractable ion (Feo)
at the probability level of (p < 0.05); and with silt %, and organically bound iron (Fep) soil
contents at (p < 0.1). The sand % was significantly correlated though negative with b (mg/g)
at (p < 0.1). The parameter k (I/g) values were significantly correlated with total iron oxide
(Fed) and with Feo-Fep at (p < 0.05) and (p < 0.1) respectively. Moreover, K (l/g) was
negatively correlated with exchangeable Na~ at p < 0.1. The n values were found to be
significantly correlated with the sUt and clay soil contents at (p < 0.01) and with exchangeable
Ca^*, Na", and K* at the (p< 0.1). The values of n were also negatively correlated with
sand % and Alo soil content at (p < 0.1).
81
,/-
!
-0.4 —
j
~
B'
-0.6 —
"a
S -0.8 —
V5
U"'
_
M)
,/•
-12—
!
•X
'
i
I
-1.4 m
-1.6
-1.4
-0.9
-0.4
LogC (mg/L)
0.1
Figure 21. Freundlich plot of cadmium sorption isotherm (soil 1).
Figure 22. Freundlich plot of cadmium sorption isotherm (soil 4).
83
-0.2
-0.4
-0.6
f-0.8
Vi
t
-1.2
-1.4
-1.6
-1.6
-1.1
-0.6
-0.1
LogC (mg/L)
Figure 23. Freundlich plot of cadmium sorption isotherm (soil 6).
0.4
84
•02-r
"T
-0.4 —
-0.6 —
S-0.8 —
VI
00
2 -1
T
t
-1.2 —
-1.4
-1.6 —
-1.5
-1
-0.5
LogC (mg/L)
Figure 24. Freundlich plot of cadmium sorption isotherm (soil 11).
0.5
85
-0.2 ^
"T
-0.4 —
-0.6 —
1-0.8
5«
^
SO
2-1-^
-1.2
-1.4
-1.6 —
-2
-1.5
-1
-0.5
LogC (mg/L)
Figure 2S. Freundiich plot of cadmium sorption isotherm (soil 16).
0.5
00
9
-1.6
-2
-1.5
-1
-0.5
LogC (mgA.)
0
Figure 26. Freundlich plot of cadmium sorption isotherm (soil 17).
0.5
87
Thekdandkdl.5 values both were significantly correlated with silt%, Feo-Fep; Feoat
(p < 0.01); with Fed at (p < 0.05); and with Fep at (p < 0.1). The sand % was also
significantly correlated with both kd and kdl.5 at (p < 0.05), though negative.
Table 5. Simple correlation coefficients (r) between adsorption constants and soil
characteristics.
Adsorption constant
Langmuir
Freundlich
sou property
Kd(l/g) ^Kd,^(l/g)
b (mg/g) K(I/g)
n
-0.49»»
-0.49**
-0.37*
-0.19
-0.40*
sand
%
0.42*
0.34
0.35*
0.62***
0.61**^
0.17
-0.21
0.44*
0.06
0.1
-0.04
-0.29
0.23
-0.28
-0.27
CEC
0,32
-0.21
-0.03
0.19
0.20
Ca
0.06
0.01
0.36*
0.17
0.17
0.02
0.03
0.26
0.30
0.29
Na
0.11
-0.37*
0.35*
-0.05
-0.03
K
0.20
0.11
0.37*
0.29
0.29
O.C
0.13
-0.00
-0.16
0.19
0.19
0.5
0.39*
0.14
0.75*»*
0.74»»*
0.56»»
0.36*
0.16
0.79***
OJi***
0.35*
-0.04
0.11
0.35*
0.36*
Fed
0.23
0.52»*
0.02
0.5S**
0.58»»
A10
-0.08
-0.10
-0.36*
-0.13
-0.13
A1 p
-0.07
-0.26
-0.27
-0.17
-0.16
Aid
-0.08
-0.14
-0.32
-0.13
-0.13
silt
clay
1:1
pH
cmol/kg
Mg
Feo-Fep
Feo
Fep
g/kg
*» **. ***significant at the 0.10,0.05,0.01 probabiliQr levels, respectively.
The CEC showed a positive and relatively weak correlation (r - 0.32) with the
maximum sorption capaciQr suggesting that Cd sorption will increase with increasing soil
88
the nonsiginificant correlations between these constants and b (mg/g), suggested that the
sorption of Cd by soils could be more dependent on the nature of organic mater and the
mineralogy of clay than on their contents.
Despite all of the circumstantial evidence that organic matter is critical in controlling
metal solubility, metal adsorption experiments on soils often fail to show a strong correlation
between organic matter content and metal solubih'ty or metal adsorption capacity. One of the
reasons is that treating the orgam'c matter content as another variable ignores the
heterogeneous nature of soil organic matter (Basta et al.,1993). Moreover, the amounts of
iron hydrous oxides which could be associated with clay and organic matter were found to
be significantly correlated with the maximum soil sorption capacity of the soils studied.
The extent to which the A1 oxides to undergo hydrolysis is less than for the Fe oxides
at the given conditions. The small negative correlation found between b, and the extractable
A1 fractions in soils may be due to the presence of exchangeable A1 on the soil sorbing sites
and which will block Cd sorption by these soil or due to its competitive nature with Cd.
Exchangeable A1 would be important in strongly acid soils imposing a competition
with Cd for sorbing sites. Iron and Aluminum oxides can be easily hydrated under most soil
conditions. Thus the surface of these oxides and hydroxyl groups are capable of forming
complexes with Cd. The higher correlation between the maximum sorption capacity and the
noncrystalline (Feo, Feo-Fep) iron fraction confirm the importance of iron oxides in Cd
sorption. A1 oxide on the other hand, were surprisingly not significantly correlated with b
and were negative. The soluble A1 values (Table 6) were much higher in all soils than the
soluble Fe ^ch may indicate the competition of Al^"^ element with Cd for the sorbing sites.
89
In addition, in order to determine the significance of combinations of soil properties with b
(mg/g), multiple regression analysis was obtained using the SPSS statistical program. The
result showed that the best model to explain most of the variation on b included the
combinations of Feo (%), clay (%), and exchangeable calcium (cmol/kg). The model was
significant at p < 0.01 with a correlation value of (r = 0.82). The b (mg/g) can be predicted
according to the following:
b (mg/g) = -0.27 + 0.77 Feo + 0.11 Clay - 0.15 Ca
Table 6 Water soluble anions and cations in the equilibrium solution.
Soil HC03 CI
N03 S04
Ca
Fe
A1
Mg
F
No.
91.50 7.03
1
Na
K
ppm*
5.59
1.16
4.27
1.70
1.40
6.50
0.75
6.96 4.50
2
183.00 7.39
1.26
3.01
5.16
1.05
0.70
3.27
1.20 10.50 15.5
4
335.50 5.32
0.88
4.63
5.63
1.05
0.45
9.15
1.33 11.50 5.50
6
320.25 7.01
1.16
1.85 18,60 1.15
0.20
9.10
1.51 12.90 4.30
10
305.00 1.34
0.20
0.30
2.22
1.20
3.30 15.00
1.20
7.50 4.50
11
106.75 3.43
0.62
0.61
3.91
1.10
1.40
0.50
0.33
3.50 2.06
12
106.75 4.38
0.47
1.04
3.50
0.80
5.00
1.12
0.47
2.90 0.93
13
91.50 4.82
0.59
0.86
3.50
1.00
0.30
2.22
0.96
0.28 1.54
14
152.50 2.69
0.20
0.30
1.86
0.15
2.30
0.98
0.68
1.39 1.05
15
167.75 4.48
0.20
0.30
5.84
1.80
3.50
1.50
1.12
1.80 1.86
16
45.75 2.32
0.31
0.48
2.03
1.20
0.30
2.40
0.82
0.27 1.10
17
61.00 3.11
0.44
0.51
3.24
0.70
1.40
0.98
0.76
4.50 0.55
18
137.25 2.85
0.20
0.30
1.87
1.00
2.40
1.35
0.79
1.59 0.63
19
61.00 2.18
0.99
0.52
2.44
1.00
2.00
1.55
0.86
0.86 0.83
'Average of three values
90
The amount of noncrystalline or poorly crystalline (Feo) components, even when
present in small amounts can make a great contribution to the physical and chemical
properties of soils due to their high CEC, high surface area and high reactivity (Fey and
LeRoux 1977). In general, the simple correlation between sorption constants b, k, kd, and
n values and soil properties, showed that the iron oxide fractions to be an important factor
on Cd sorption. In fact, the hydrous ferric oxides were significantly correlated with the
maximum soil sorption capacity and therefore a hydrous ferric oxides complexation model
was used to predict Cd sorption in soils.
Effect of pH on Cadmium Sorption
The amount of Cd sorbed (batch adsorption study) at different pH values in the
ranges of (5-8) for all soils studied is given in (Table 7). In addition, the amount of Cd
sorbed at these pH values are plotted in (Fig. 27) for soils 1,2,4,6, and 10, and in (Fig. 28)
for soils 11-19. Cadmium sorption in soils at pH values of 5,6,7, and 8, was investigated
at a fixed concentration of dissolved Cd of 12.8 ppm. For all the soils investigated, the
amounts of Cd sorbed increased with increasing pH (Fig. 27 and 28). It increased over the
entire pH range although the magnitude of increase differed from soil to soil.
In soils 1,2,6,11,13,14, lS,and 18, the increased pH resulted in a small percentage
increase in Cd sorption, while in soils 4, 10, 12, 16, 17, and 19 larger increments were
observed. The change in amount of Cd sorbed with increasing pH were notably less at
pH > 7 for most of the soils. In the highly Cd retentive soils, pH had only a small effect on
the sorption soil capacity since most of the Cd was sorbed regardless of the pH level (soil
1,13,14, and 15).
91
Table 7. Batch adsorption experimental results compared to nonelectrostatic and diffuse
layer models prec ictions using HYDRAQL; fCdlr = I.14xl0-^M.
"Nonelectrostatic and
diffuse layer models
'Batch Adsorption Data
predictions of Cd Adsorbed
Cd adsorbed (%)
Soil
pH
l.Typic Torrifluvent
2.Typic Natrargid
4.Typic Torrifluvent
6.Vertic Haplustoll
lO.Typic Calciorthid
1 l.Ustollic Camborthids
12.Typic Hydrudands
13.Entic Hapludands
14.Typic Hydrudands
IS.Typic Ustivitrands
16.Typic Ustivitrands
l7.Typic Hydrudands
IS.Typic Hydrudands
19.Typic Haplocryands
'Average of three values
5
6
7
8
92.2
85.2
18.0
84.4
49.6
88.3
12.5
95.3
89.5
96.1
39.8
14.5
75.9
58.6
93.1
93.8
67.2
95.4
72.4
97.9
60.9
98.9
99.0
97.3
72.8
88.4
96.6
97.8
97.0
95.8
98.0
98.8
76.9
97.1
90,0
98.9
98.8
98.8
78.0
96.3
98.8
99.5
98.8 11.6 19.0
97.9 5.9 13.5
98.8 4.0 9.8
99.2 9.4 16.6
90.4 2.7 9.9
97.0 37.1 53.8
90.6 79.4 87.2
98.6 66.7 81.4
98.6 48.3 72.9
98.2 40.0 68.3
94.1 11.6 29.7
96.7 11.8 29.4
98.8 59.1 72.5
99.5 46.0 63.6
5
6
7
8
32.1
35.4
30.0
32.3
35.1
67.1
89.7
88.1
86.2
85.0
58.3
57.5
78.9
74.9
66.2
84.3
77.6
72.0
87.5
95.5
96.8
99.0
99.3
99.4
97.4
97.1
95.6
97.4
The soils that showed a small or insignificant increase over the range of pH used
showed a relatively high maximum sorption capacity compared to the other soils (Table 4).
The amount of Cd sorbed increased by 1, 8,49, 11, 22,10, 48, 4, 9, 1, 33, 74, 20
and 39% for an increase ofonepH unit &ompH 5 to pH 6 for soils 1, 2, 4, 6,10, 11,12,
13,14,15,16,17,18, and 19 respectively. Precipitation ofCd is unlikely throughout the pH
range used according to the Cd(0H)2 solubility product, but precipitation of calcium
carbonate in the Aridic soils namely 1,2,4,6 and 10, may have occurred at about pH 8. In
these soils where calcium carbonateprecipitated, Cd ions may be occluded orcoprecipitated
and hence removed by mecham'sms not considered as adsorption processes.
92
TOO
600
soil 1
:500 -
ra
E
Io«0
^
M
soil 2
•ora
soil 4
iaooi
<0
soil 6
200
soil 10
100
4.5
5.5
6.5
7.5
PH
Figure 27. Relationships between the amount of Cd adsorbed at
the hi^est [Cd]T added (12.8 ppm) and tfie soil pH for
soils (1,2,4,6,and 10).
8.5
93
TOO
soil 11
aoo soil 12
•ssoo
O)
soil 13
E
^400
soil 14
M
"S3OO
soil 15
O
E
"200
soil 16
100 soil 17
4.5
5.5
6.5
7.5
8.5
soil 18
PH
Figure 28. Relationships between the amount of Cd adsorbed at
the highest [Cd]T added (12.8 ppm) and the soil pH
forsoUs (II, 12,13,14,15,16,17,18, and 19).
soil 19
94
At low pH values, the decrease insorption capacity with respect to Cd may have been
affected by increased competition from calcium, magnesium, sodium, and potassiimi
released by the soil upon the additionof acid for adjusting pH. The amounts adsorbed at pH
S were the lowest for soil 12, adsorbing only 12.5 percent of the total added Cd, and this
could be due to the competition between Cd and A1 for the sorbing sites since this soil has
the highest amount of soluble A1 (Table 6). The increase in a soil's sorption capacity^ upon
increased pH may be explained as a decrease in competition by
for sorption sites or
increased sorption of Cd monohydroxy complexes at increased pH values. Moreover, the
reason for the increase in Cd sorption with increasing soil pH, is the increase in soil CEC
especially of those with colloids dominated by pH-dependent charges where deprotonation
of the surface bound H" on the soil exchange will increase with the increasing pH values and
resulting in higher CEC.
The results on the effect of pH on soil sorption capacity of Cd showed that pH must
be tightly controlled in studies on Cd sorption. A small change in pH can cause a dramatic
change in the sorption capaciQr of the soil, especially at pH below 6. Moreover, the results
indicate that the binding and migration of Cd in soils are very sensitive to the pH of the soil
environment and may be effectively controlled by increasing the non-acid conditions and
maintaining a pH value of greater than 6.
Cadmium Activities in Soib
The Cd activities which were measured by ISE in all soils studied at their original pH
and the adjusted pH are given in (Table 8 and 9 ). The Cd'*^ activities measured in the soils
studied varied slightly, ranging from
to 10"* *^ M with an over all average of
95
10'^"' M at the natural pH of the soils. The
activities increased with increasing total
Cd added. The cadmium activities at constant [Cd]^ added of 12.8 ppm, and at different
levels of pH (5,6, 7, and 8), for the same soils ranged from 10 * * to 10"*" M with an over
all average of 10'®
M, and were inversely related to the soil pH (Fig. 29-33).
Table 8. Cadmium activities in soils measured by ion selective electrode (ISE).
[Cd]
0.8
12.8
6.4
1.6
3.2
Soil
'Aqueous Cd2+ activity (M)
l.Typic Torrifluvent
1.5E-07 1.8E-07
1.5E-07
1.5E-07
l.OE-06
2,Typic Natrargid
3.3E-07 3.4E.07
5.0E-07
7.6E-07
7.1E-06
4.Typic Torrifluvent
6.3E-07 1.6E-07
8.3E-07
1.5E-06
3.2E-06
6.Vertic Haplustoll
6.2E-08 1.9E-07
2.2E-07
5.4E-07
l.lE-06
lO.Typic Calciorthid
2.2E.07 1.4E-06
1.6E-06
2.1E-06
1.4E-05
11.Ustollic Camborthids
2.0E-07 5.5E-07
7.9E.07
9.3E-07
2.5E-06
12.Typic Hydrudands
4.4E-07 1.3E-06
6.2E-06
5.3E-06
8.5E-06
n.Entic Hapludands
5.3E-08 4.7E-08
6.1E-08
1.2E-07
1.5E-05
14.Typic Hydrudands
5.3E-08 8.7E-08
1.3E-07
1.4E-07
2.6E-07
IS.Typic Ustivitrands
8.9E-08 6.2E-08
1.7E-07
7.6E-07
8.5E-07
16.Typic Ustivitrands
3.6E-08 5.3E-07
1.1E.06
3.1E-06
6.1E-06
17.Typic Hydrudands
1.3E-06 I.7E-06
2.2E-06
5.7E-06
1.3E-05
IS.Typic Hydrudands
1.6E-06 1.6E-06
1.9E-06
3.7E-06
l.lE-05
19.Typic Haplocryands
l.lE-06 1.2E-06
1.4E-06
2.0E-06
l.lE-05
The relationship between the logarithm of the Cd^"^ activi^ and soil pH was highly
significant (p > 0.01), with correlation coefficients (R^), ranging from 0.79 to 0.99 (Table
10). The decrease in Cd^"" activi^r with increasing pH is due to the fact the at higher pH a
96
Figure 29. Cd^~ activities as a function of pH for soils (1,2,4,6, and 10).
97
-3
soil 11
soil 12
-5 -
soil 13
+•
<N
y,-6 -
soil 14
-7 -
soil 15
so
soil 16
-8 -
soil 17
-9
4.5
5.5
6
6.5
pH
7.5
Figure 30. Cd^~ activities as a function of pH for soils
(11,12,13,14,15,16,17,18andl9).
8
8.5
soil 18
soil 19
98
I
-4 ^
CdcOHb
soil 1
-2 -
0 -
soil 2
^ 2-
soil 4
4i
-i
soil 6
6 -
soil 10
8 -
10
4
4.5
5
5.5
6
6.5
7
PH
7.5
8
8.5
9
9.5
10
Figure 31. Solubility diagram for the Cd-soil equilibrium for soils (1,2,4,6,10).
soil 11
soil 12
Cd(OH):
-2 -
soil 13
0 -
soil 14
2-
soil 15
6 -
soil 16
8 -
soil 17
10
4
4.5
5
5.5
6
6.5
7
pH
7.5
8
8.5
9
Figure 32 Solubility diagram for the Cd-soil equilibrium for
soUsdl, 12, 13, 14, 15,16, 17, 18, 19).
9.5
10
soil 18
soil 19
100
-2
Log Cd^ = -4.46X10-3.0.92pH
^ -i-4 —
+-5 —
CM
•o
o
1
-6-^
-7 4-
-8 —
4.5
5.5
6
6.5
7.5
PH
Figure 33. Cd^~ activities as a function of pH for fourteen soils.
8
8.5
101
Table 9. Cadmium activities in soils at diflferent pH values.
5
6
7
pH
'Aqueous Cd2+ activity (M)
Soil
I .Typic Torrifluvent
Z.Typic Natrargid
•.Typic Torrifluvent
S.Vertic Haplustoll
lO.Typic Calciorthid
I l.Ustolltc Camborthids
12.Typic Hydrudands
l3.Entic Hapludands
14.Typic Hydrudands
IS.Typic Ustivitrands
I6Typic Ustivitrands
IT.Typic Hydrudands
IS.Typic Hydrudands
I9.Typic Haplociyands
'Average of three values.
8.1E-06
1.4E-05
9.1E-05
1.3E-05
3.8E-0S
1.2E-05
9.IE-05
3.0E-06
I.lE-05
4.4E-06
6.5E-05
7.9E-05
2.7E-05
4.4E-05
4.4E-06
3.IE-06
3JE-05
4.8E-06
1.5E-05
4.6E.07
8.SE-06
1.0E.06
I.OE-06
2.7E-06
2.0E-05
1.3E-06
2.1E-06
2.1E-06
8.9E-07
2.9E-06
2.7E-07
1.3E-07
I.4E-05
8.0E-08
5.3E-08
2.8E.07
4.4E-07
1.2E.06
1.4E-05
6.9E-07
4.1E-08
3.2E-08
8
1.4E-08
1.8E-08
6.2E-09
I.6E-08
3.6E-06
2.3E-08
6.2E-08
5.3E-08
2.0E-07
4.4E-08
2.7E-07
8.9E-09
1.2E-08
6.2E-09
Table 10. A summary of regression model constants for estimating cadmium activity insoils
from soil pH.
Soil No.
Constants (A)
Xcoefficients (B)
•R0.87
I
0.79
2
0.95
4
0.96
6
0.91
10
0.95
11
0.88
12
0.99
13
0.91
14
0.82
15
0.83
16
0.94
17
0.97
18
0.98
19
•Significant at the 0.01 probability level.
0.26
0.26
-3.66
-0.48
2.85
0.71
-1.70
2.55
2.38
1.94
0.32
-1.82
-1.30
-2.25
0.90
0.87
1.46
1.03
0.31
0.89
1.17
0.58
0.56
0.63
0.73
1.21
1.18
1.34
102
higher portion of Cd will be compelexed with anions such as CI and hydroxides and hence
there will be less free Cd ions in the solution. Figure 29 shows a plot of Cd^^ activity as a
function of soil pH for soils 1,2,4,6, and 10, and (Fig. 30) shows a plot of Cd^"^ activity as
a function of soil pH for soils 11,12,13,14,15,16,17,18 and 19.
The relationships between pCd^" and soil pH were further plotted for all soils and
presented in Figure 31 for soils 1,2,4,6, and 10, and in Figiire 32 for soils 11,12,13,14,
15, 16, 17, 18 and 19. As shown in Figures 31 and 32, the Cd could not be precipitated as
CdCOH), according to the solubility product Ksp of 10"'^" in the added Cd concentrations
and the pH employed. Further, the Cd"* activity in all soils were plotted as a function of pH
and is given in Figure 33. Figure 33 showed that Cd*' activities in the soils investigated can
be predicated &om the following equation:
Log Cd^^ = -4.46x10-^ -0.92 pH
The summary of the regression models for estimating Cd"* activity in soils from
the individual soil are given in Table 10. The correlation of Cd^'^ activity and soil pH were
significant at p < 0.01. Depending on the soil properties the Cd^* activity may be predicted
from the regression output for the individual soil by Log Cd^* = A + B (pH), where A is
the regression constant or the intercept, and B is the slope of the line (Table 10).
Cadmium Mobility in Soils
Cadmium mobility in soils was investigated using soil thin layer chromatography
methods. Table 11, shows the cadmium mobility Rf values which were derived from the
chromatograph obtained for the different soils, which are considered to be a measure of
mobility in thin layer chromatography TLC.
103
The frontal Rf values was estimated using the following relationship:
Fm
Where F„ is frontal distance moved by the metal and Fj is the one moved by the developer
solution. The Rf values obtained ranged between 0.25 and 0.95. The results suggest that Cd
mobility in these soils was highly variable. Because the soil TLCtechnique has only recently
begtm to be used for estimating metal mobility such as Cd in soils, and also with the lack of
sufficient published data in this field, there is no mobility classification according to Rf
except the one proposed by Helling and Turner (1968) for pesticide mobility in soils.
11. Rf values for the mobility of cadmium in the soils studied.
Replications
Replications
Soil No.
Mean
S.D
•dl
•d2
•d3
IRf
2Rf
3Rf
1
2.60
2.50
2.30
0.26
0.25
0.23
0.25
0.01
2
2.40
2.30
3.10
0.24
0.23
0.31
0.26
0.04
4
3.10
3.00
2.50
0.31
0.30
0.25
0.29
0.03
6
2.60
2.30
2.90
0.26
0.23
0.29
0.26
0.02
10
5.30
4.90
4.90
0.53
0.49
0.49
0.50
0.02
11
4.30
4.50
3.80
0.43
0.45
0.38
0.42
0.03
12
4.70
4.30
4.90
0.47
0.43
0.49
0.46
0.02
13
2.60
2.60
2.60
0.26
0.26
0.26
0.26
0.00
14
2.60
3.10
3.60
0.26
0.31
0.36
0.31
0.04
15
4.40
3.50
4.10
0.44
0.35
0.41
0.40
0.04
16
9.50
9.90
9.00
0.95
0.99
0.90
0.95
0.04
17
2.00
2.90
3.90
0.20
0.29
0.39
0.29
0.08
18
3.50
2.60
3.50
0.35
0.26
0.35
0.32
0.04
19
4.20
3.80
3.90
0.42
0.38
0.39
0.40
0.02
Frontal distance in (cm) traveled by Cd^"
104
The cadmium mobility classifications in the soils studied according to Helling and
Turner (1968) are shown in Table 12. Based on this classification cadmium was slightly
mobile in 64%, moderately mobile in 29 % and very mobile in 7% of the soils investigated.
The distance moved by cadmium (Fig. 34a) was the highest in the soil with a high sand
percentage (soil 16) and the lowest for those of high silt and/or clay percentages (soil 1). In
general soils with high percentages of sand (soil 10,12, and IS) fall in the moderately
mobile class.
In order to determine the influence of the soil properties on Cd mobility, the simple
correlation coefficients between the Rf values and soil properties were obtained. The
correlation coefficients obtained are included in (Table 13).
"Class
Rf
Mobility
Soil No.
SoU (%)
1
<0.1
Immobile
0
0
2
0.10-.34
Slightly mobile
3
0.35-0.64 Moderately mobile
4
0.65-0.89
Mobile
0
0
5
0.90-1.0
Very mobile
16
7
1,2,4,6,11,13,14,17, 18
64
10,12, 15,19
29
'Classification according to Helling and Turner (1968).
There was a significant correlation at the 0.05 probability level between Rf values and sand
percentage. There were also a significant negative correlation (p < 0.05) between silt %,clay
%, exchangeable (Ca and K) soil contents and the Rf values. The other soil properties
including pH, CEC, and organic carbon content, where not significant at (p < 0.05). The
variabiliQr in the cadmium mobili^ in the soils is mostly accoimted for by sand (r = 0.63),
followed by silt %; (r=-0.59); the exchangeable Ca (r=-0.55); clay % (r=-0.55) and the
105
10
"TB"
9
8
. 7
E
•S
6
"2
1 5 -L
I 4
10
11
12
15
.a
® 3
^1
14
13
19
17
2
1
Soils
Figure 34a. Distance (cm) moved by cadmium overa 10 cm soil TLC.
18
106
exchangeable K (r = -0.49). The Rf values below 0.35 were generally measured for soils
with relatively a higher percentage of clay and silt than the rest of the soils studied. Both
clay and silt percentages showed a significant correlation with measured Rf values. The Soils
that have Rf > 0.35 or soils classified as moderately mobile with respect to cadmium
movements, shared either a higher sand % (soil 12, and 15), and /or lower pH values as the
case of soil 19. Only one soil, soil 16, classified as very mobile with regards to cadmiimi
movements has the highest amount of sand (94%) and the lowest clay content of (1%).
The Rf values of the soils were plotted as a function of sand % in soils and shown in
(Fig. 34b). The plots show that the higher the sand %, the higher will be the mobility of Cd
in soils.
Table 13. Simple correlation coefGcients (r) between Rf and soil
characteristics and the adsorption constants.
r
Sorption constants
Soil Property
r
sand
0.63*
b
-0.59*
K(l/g)
silt
-0.55*
clay
n
pH
-0.01
Kd
CEC
-0.11
-0.55*
Ca
-0.14
Mg
Na
-0.29
-0.49*
K
-0.04
Organic C
0.06
Feo-Fep
0.04
Feo
-0.09
Fep
0.07
Fed
0.10
Alo
0.11
Alp
Aid
0.15
* Significant at the 0.05 probability levels.
-0.19
0.02
-0.18
-0.15
107
35
30 —
25 —
|20 —
IM
ae
^
4
"S
as 15 —
10 —
Rf
0
0
20
40
60
80
100
Sand(%)
Figiire 34b. Red (Transport retardation factor for cadmium) and Rf (cm) versus sand (%
for fourteen soils.
108
The divalent cation. Ca*'. was negatively correlated with Rf (p < 0.05), which could
be a result of competition between Cd*' and exchangeable Ca-* for the sorbing sites. McBride
et al, (1981). found the cadmium retention capacity of soils to be dependent significantly on
the exchangeable Ca'" content of soils. Garcia-Miragaya and Page (1977), found the
competition of cadmium with exchangeable cations to decrease in the following order Al <
Ca < K < Na. The competition of Cd with these cations was also found in this study to
decrease in the order of Ca < K < Na < Mg which is consistence with the variation of the
correlation coefficients of Rf with the amounts of these elements in the soils.
The soil pH is considered by some researchers to be an important parameter affecting
the distribution and mobility of Cd in soils. The correlation between Rf values and soil pH was
insignificant and negative which indicates at least that increasing the soU pH will decease Rf
and cadiruum mobility in soils. Soil organic carbon content and CEC showed a negative and
insignificant correlation with Rf. which suggests that these factors can be important in
limiting cadmium mobility to a lesser degree than divalent cations specially Ca** or clay and
sQt contents. The correlation between Rf values and iron and aluminum was insignificant. The
contents of iron and aluminum oxides in the soils showed that they played an insignificant
role in cadmium mobility.
The correlation between Rf and the adsorption constants, b, n and kd were all
negative, while the correlation with k was positive but none were significant at p < 0.05. The
negative correlation between Rf and kd (r = -0.15), suggests that the higher the distribution
factor kd, the lower will be the mobility of Cd in soils. Sanchez-Camazano et al., (1993)
studied the mobility of Cd in various natural soils by soil thin-layer chromatography and
109
found that, kd and n were negatively correlated with Rf and insignificant.
The resuhs from the sorption equilibrium calculations in the sorption study also can
be use to calculate groundwater transport retardation factor for the sorbing cadmium. The
mobility of Cd also was evaluated by means of the cadmium retardation factor. Red.
The retardation factor Red, represents the ratios of the average linear velocity ofgroundwater
(u) to that of dissolved Cd (us).
n
vs
where pb the soil bulk density, n is the soil porosity, and the kd is the empirical distribution
coefficient for the sorbing Cd which is the solid phase concentration of Cd divided by the
dissolved Cd concentration. Also, the Red value can be calculated from, fw, which is the
form of a compound in the water solution in a volume containing both solid and water as
follows
fw =
1
where Ms is mass of soil used (0.5 g), and VM is the volume of the solution used (25 ml).
The Red is equal to 1/fw.
The kd factor is usually expressed in units of ml/g, derived as follows;
mg Cd
/g Soil
ml
mgCd/
" g
/ml Solution
The Red values of the soils studied at fixed total Cd load of 12.8 ppm and at their
natural pH varied fi'om 2.9 to 33.42. The corresponding Red values for the same soils at
110
four different levels of pH (S, 6, 7, and 8), also varied from 1.14 to 196.92 (Table 14).
The Red values for the soils studied were found to be increasing with increasing pH and also
to differ from soil to soil. In soil 1, for example the Red values increased from 12.80 at
pH 5 to 80 at pH 8. For soil 19, the Red values increased from 2.42 at pH S to 182.86
at pH 8. The Red values for all soils at their natural pH values were relatively low as
compared to the adjusted pH.
Table 14. Cadmium distribution coefiicients (Kd) and retardation factors (Red) values of the
soils studied.
'Kd (ml/g)
^Red
Soil No.
Adjusted pH
5.0
6.0
7.0
590.0 677.3 1634.2
1
286.8 760.1 1124.3
2
ll.O 102.4 2411.5
4
6
270.0 1034.7 4079.0
10
49.2 131,3 166.2
376.7 2277.3 1679.7
11
7.1
78.0 450.0
12
13
1016.7 4363.8 4521.4
424.1 4690.7 3950.0
14
1230.0 1778.6 3950.0
15
16
33.1 133.9 177.0
17
8.5 379.5 1311.7
18
157.8 1404.5 4216.7
19
70.8 2235.7 9796.2
^'Original soil pH.
Average of t^ee values.
Adjusted pH
8.0
3950.0
2365.1
4216.7
6045.2
470.3
1591.0
478.9
3545.5
3505.6
2732.6
792.1
1473.8
4216.7
9092.9
5.0
pif
680.1 12.8
95.3
6.7
1.2
229.1
781.3 6.4
189.6 2.0
707 5
8.5
197.1 1.1
1057.0 21.3
1620.8 9.5
1310.5 25.6
546.5 1.7
373.8 1.2
1097.7 4.2
1013.9 2.4
6.0
14.5
16.2
3.0
21.7
3.6
46.5
2.6
88.3
94.8
36.6
3.7
8.6
29.1
45.7
7.0
8.0
33.7 80.0
23.5 48.3
49.2 85.3
82.6 121.9
4.3
10.4
34.6 32.8
10.0 10.6
91.4 71.9
80.0 71.1
80.0 55.7
4.5
16.8
27.2 30.5
85.3 85.3
196.9 182.9
pH"
14.6
2.9
5.6
16.6
4.8
15.2
4.9
22.1
33.4
27.2
11.9
8.5
23.0
21.3
In order to determine the influence of soil properties on the cadmium movement, the
simple correlation coefficient between thecadmium retardationfactor Red and soil properties
was conducted. Table 15 includes the single ccirelation coefficients between the Red values
Ill
and soil properties obtained. It can be seen from Table IS that there are highly significant
Table IS. Simple correlation coefiBcients (r) between cadmium retardation factor (Red) and
soil characteristics.
Soil Property
r
-0.50»»
sand
0.62***
silt
0.09
clay
-0.28
pH
0.19
CEC
0.17
Ca
0.30
Mg
-0.05
Na
0.29
K
Organic C
0.20
0 75#»#
Feo-Fep
0.79***
Feo
Fe p
0.35*
0.58^»
Fed
-0.13
A10
A1 p
-0.17
-0.13
Aid
*, **, ***significant at the 0.10,0.05,0.01 probability levels, respectively.
correlations between Red values and Feo, FeO-Fep, and silt (p < 0.01); and Fed at (p< 0.05);
Fep and the exchangeable Mg^"^ at (p <0.1). Also there is a significant, though negative
correlation between Red and sand % at ( p < 0.05 ). The Red values increase with
increasing amounts of the iron oxide fi-actions (Feo, Fep, and Fed); sUt% and exchangeable
Mg^* in the soils.
The quantities of extractable aluminum oxide fi'actions in the soils show a negative
but not significant correlation with measured Red. Soils with a high percentage of sand seem
to &vor cadmium mobility over retention or low Red values and high Rf values (Fig. 34b).
Both Red and Rf factors were significantly correlated (p < 0.05) with sand and silt
percentages. In addition, the Red in soils 1 and 15 are plotted (Fig. 35) as a function of pH
112
8 —
Wt. %Fe = 2.9(soill5)
1.6 —
cr
§>
1.4
Wt.%Fe =0.16 (soil I)
1.2 —
5.5
6.5
7.5
pH
Figure 35. Log Red (transport retardation factor for cadmium) versus pH for two
different amounts of hydrous ferric oxides(Feo-Fep) on a soil/water
system.
113
and weight percent of Feo-Fep. As shown in Fig. 35, Red increases rapidly with pH and is
somewhat consistently greater for soils with higher iron contents. The plots indicate that Cd
will move at a higher velocity at pH 5 or below for both soils, but at pH 8 Cd transport will
be retarded by factors ranging from 12.80 to 33.68 and from 25.6 to 80 for soil I and soil 15
respectively. The Red increased with increasing quantities of either poorly crystalline
inorganic or organically bound irons in soils.
Furthermore, in order to determine the influence ofthe soil properties on Red values,
a muhiple regression analysis was performed between Red and the soil properties at their
natural pH (Table 16). Two models were found to describe the variation on Red values, or
to better predict the Red on these soils. The first model, was a combinations of sand% and
Feo, the second model was the combination of silt % and the inorganic poorly crystalline,
Feo-Fep (Fig.36 and 37).
Table 16. A summary of best step-wise regression models (1, and 2) found to estimate
retardation factor of cadmium (Red) in soils studied.
Unstantdardized Standardized
t
Model
CoeiSBcients
Coefficients
SiK.
R
Std. Error
Beta
B
1
2
(Constant)
Feo
Sand
(Constant)
Feo-Fep
SUt
16.22
0.83
-0.20
-1.35
0.75
0.27
2.22
0.09
0.03
2.43
0.12
0.06
0.85
-0.54
0.72
0.54
7.31
9.20
-5.86
-0.55
6.22
4.63
0.00
0.00
0.00
0.59
0.00
0.00
0.953
0.923
Dependent Variable: Retardation factor (Red)
Both models are statistically significant at (p<0.01) with correlation coefficients R^
of 0.90 and 0.85 for the first and the second model respectively. The mobility of Cd
according to both factors, Red and Rf will increase with increasing sand and decreasing
sflt or clay percentage in the soils.
114
2 —
1.8 —
1.6 —
-r
0.8
0.6 —
V
0.4
0
0.5
1
1.5
2
2.5
3
Feo-Fep (%)
Figure 36. Log Red (Transport retardation factor for cadmium) versus Feo-Fep (%)
forforteen soils.
115
35
30 25
20 -
5-
0.5
1
1.5
2
2.5
3
3.5
Feo (%)
Figure 37. Log Red (Transport retardation factor for cadmium) versus Feo (%) in soils.
116
Modeling of Cadmium Sorption on Soik
The soq)tion isotherms such as the Langmuir isotherm model allows for the
determination of Cd sorption maxima, but they are not mechanistic and therefore can not be
used to accurately predict sorption at different pH values or for soil with different properties
from. Thermodynamically based surface complexation models can be used to predict the
sorption of metals e.g Cd on different humic materials (Bolton et al. 1996), phyllosillicat clays
(Schinder et al.l987), and hydrous ferric oxides (Dzombak and Morel 1986).
To support the results obtained by the sorption isotherms study, the computer
speciation program, HYDRAQL, (Papelis et al., 1988) was used to model the amount of
Cd adsorbed onto each of the humic materials and the hydrous ferric oxide surfaces as a
function of pH. Cadmium complexation with organic matter was modeled using a discrete
functional group model in which it was assumed that there were two unique diprotic
complexation sites, HjA and H^B, associated with the humic materials (Bolton et al., 1996)
The total concentrations of [HjAlx and [HjB]^ were estimated using Equation 14, while
the total concentration of adsorption sites associated with hydrous ferric oxide, [FeOOH]x
were estimated using Equation IS.
The concentration of the fractions of humic acid ([HjA]^ and [HjB]^) and hydrous
ferric oxides ([FeOOH]x weak,
and [FeOOHJ^ strong) which were used in the
complexation models are given in(Table 17). Thehumic materials fraction in thesoils studied
ranged for [HjAJtfrom 2.80x10*® Mto 5.15x10*' M and for[H2B]T from 1.26x10"' M to
2.31x10*' M. The hydrous ferric oxides content in thesofls ranged for [FeOOH]^ strong from
2.79x10-® M to 5.17x10*' M and for [FeCK)H]T weak from 1.12x10*^ M to 2.07x10*' M.
117
Table 17. Concentrations of the humic acid ([HjA]^ and [HjBJt ) and hydrous ferric
oxides (rFeOOHlj weak and fFeOOHlr strong) in the soils studied.
Soil
fHiAfr
rH31r fFeOOHlr weak fFeOOHlr strong
Mole/L
Mole/L
l.Typic Torrifluvent
1.82E-04
8.16E-05
2.79E-06
1.12E-04
2.Typic Natrargid
8.00E-05
3.59E-05
6.46E-06
2.59E-04
4.Typic Torrifluvent
5.20E-05
2.33E.05
4.93E-06
1.97E-04
6.Vertic Haplustoll
1.42E-04
6.37E-05
3.68E-06
1.47E.04
lO.Typic Calciorthid
2.80E.05
1.26E-05
7.77E-06
3.11E-04
1 l.Ustollic Camborthids
7.95E-04
3.56E-04
1.39E-05
5.57E-04
12.Typic Hydrudands
5.I5E-03
2.31E-03
1.27E-05
5.07E-04
13.Entic Hapludands
2.65E-03
1.19E-03
3.59E-05
I.44E-03
14.Typic Hydrudands
1.21E-03
5.42E-04
5.00E-05
2.00E-03
IS.Typic Ustivitrands
8.45E-04
3.79E-04
5.17E-05
2.07E-03
16.Typic Ustivitrands
1.55E-04
6.95E-05
2.08E-05
8.31E-04
n.Typic Hydrudands
1.60E-04
7.17E-05
1.98E-05
7.93E-04
IS.Typic Hydrudands
1.95E-03
8.76E-04
1.27E-05
5.07E-04
19.Typic Haplocryands
LI4E-03
5.13E-04
1.96E-05
7.86E.04
A comparison was made between the batchsorption data measured and the prediction
made using the non electrostatic and diffuse layer models (Table 7). As discussed in the
effect ofpH on cadmium sorption in Section 3.5, the amount of Cd sorbed increased with
increasing soil pH. Both the batch and complexation model predictions supported this. In
general, the complexation model showed a similar trend to the batch study on Cd sorption
on soils at the different pH levels used.
In addition, there was a good agreement between the experimental data and the
model results at pH 8, indicating that the model described Cd-himiic and hydrous ferric
118
oxides interactions reasonably well. The prediction however, varied from soil to soil. The
predicted Cd sorption onto soils using the surface complexation model were plotted together
with the experimental values (Fig. 38-51).
As it can be seen from these Figures, the complexation model prediction values for
the percentage of sorbed Cd are within 5% for all batch sorption experiments for soils 10,
11,12,13,14,15,16,17,18 and 19 at the pH value of 8. The prediction was based on the
amount of organic matter and hydrous ferric oxides, and therefore, the prediction of Cd
sorption on soils high in those materials showed relatively good agreements with the batch
study data as shown in (Fig 45, 50, and 51) for soils 13,18, and 19 respectively.
Moreover, when calculated according to the complexation model the Cd sorption
was overestimated in the range of 80 to 0.29% at all the pH levels for all soils with the
exception of soil 12 where theCd sorption was underestimated.The results of the modeling
indicate pH dependent sorption curves which are consistence with the experimental results
in that they both showed Cd sorption in soils is to be pH dependent.
Further, the results (Fig. 38-51), showed that much ofthe sorption ofCd takes place
on the organic and hydrous ferric oxides surfaces. Humic and hydrous oxide surfaces as
examples, accounted for as much as 13% for soil 1 and 80% for soil 17 of the sorbed Cd at
pH 5; and 67% and nearly 100% of the sorbed Cd at pH 8 for soil 1 and 17 respectively.
In addition, humic materials may be an important factor in Cd retention at pH values greater
than 3 and hydrous ferric oxide surfaces may become an important factor at pH values
greater than 7 as shown in (Fig. 52 and 53).
The distribution of Cd^" species in the presence of humic acid and hydrous ferric
119
Mumic Acid
Total (Humic&HFO)
measured
Figure 38. Measured and predicted Cd adsorption onto soil (1) using the
surface complexation model.
120
6
Humic Add
HFO
OA
Total (HumiciHFO) —— measured
E
"S
J3
O
M
"O
<
•o 2
U
0
5
5.5
6
6.5
PH
7
7.5
Figure 39. Measured and predicted Cd adsorption onto soil (2) using the
surface complexation model.
8
121
e^
0
—
5
Humic acid
HFO
Total (Humic & HFO)
measured
—
5.5
6
6.5
7
7.5
PH
Figure 40. Measured and predicted Cd adsorption onto soil (4) using the
surface complexation model.
8
122
6^
00
W
i= •A
Humic acid
HFO
Total (HumiciHFO)
measured
(/)
0
5
5.5
6
6.5
7
7.5
Figiire 41. Measured and predicted Cd adsorption onto soil (6) using the
surface complexation model.
8
123
I
'
Humicacid
HFO
Total (Humic & HFO)
measured
o>
I
<
•O 5
0
5
5.5
6
6.5
7
7.5
pH
Figure 42. Measured and predicted Cd adsorption onto soil (10) using the
surface complexation model.
8
124
T
6
o4
Humic acid
HFO
Total (Humic & HFO)
measured
s
0
5
5.5
6
6.5
DH
7
7.5
Figure 43. Measured and predicted Cd adsorption onto soil (11) using the
surface complexation model.
8
125
Humicacid
HFO
Total (Humic & HFO)
measured
00
OA
e
0
5
5.5
6
6.5
PH
7
7.5
Figure 44. Measured and predicted Cd adsorption onto soil (12) using the
surface complexation model.
8
126
6 —
Humic acid
Total (Humic & HFO)
Figure 45. Measured and predicted Cd adsorption onto soil (13) using the
surface complexation model.
127
5
so
o
s
£W
o
M
"O
<2
"O
U
Humic acid
HFO
Total (Humic & HFO)
measured
1
0
5
5.5
6
6.5
7
7.5
Figure 46. Measured and predicted Cd adsorption onto soil (14) using the
surface complexation model.
8
128
6
00
oA
•S
J3
w
O
1/1
•a
<
•o 2
U
Humic acid
HFO
Total (Humic & HFO)
Measured i
0
5
5.5
6
6.5
7
7.5
PH
Figure 47. Measured and predicted Cd adsorption onto soil (IS) using the
surface complexation model.
8
129
Humic acid
Total (Humic&HFO)
measured
Figure 48. Measured and predicted Cd adsorption onto soil (16) using the
surface complexation model.
130
Humic acid
HFO
Total (Humic & HFO)
measured
a>
l4
"O 5
5
5.5
6
6.5
7
7.5
Figure 49. Measured and predicted Cd adsorption onto soil (17) using the
surface complexation model.
8
Humic acid
HFO
— Total (Humic & HFO)
5
5.5
6
measured
6.5
PH
7
7.5
Figure SO. Measured and predicted Cd adsorption onto soil (18) using the
surface compiexation model
8
Humicadd
HFO
Total (Humic & HFO) — measured
5
5.5
6
6.5
PH
7
7.5
Figure 51. Measured and predicted Cd adsorption onto soil (19) using the
surface complexation model.
8
133
oxides (HFO) wascalculated using thecomputer speciation program, HYDRAQL, (Papelis
et al.,1988). The relative proportion of Cd'"^ species as a function of pH for Aridic soils
(average of soil 1,2,4,6, and 10) at the higher added Cd concentration (1.14x10"^ M), the
determined humic acid concentration, ([HjAJx = 9.70x10'® M); ([HiBJt = 4.30x10 ® M)
and HFO concentration [FeOOH]^ = 1.03x10'^ M are shown in (Fig. 52).
Also the relative proportion of Cd"* species as a function of pH for Andisols
(average of soil 12-19), the determined humic acid concentration, ([HiAJx = 1.66x10'^ M);
([HzBIt = 7.44x10'' M) and HFO concentration ([FeOOH],- = 3.76x10'^ M) is shown in
Figure 53. These calculations show that Cd complexation with H2A occurs at very low pH
values, resulting in the formation of CdHA" complexes (Fig. 52 and 53). As the pH increase,
the additional protons are released from the humic acid resulting in the formation of CdA°
complexes and the formation of CdHB" with HjB complexant sites. More over, atpH
values greater than 7 the formation of Fe-O-Cd"^ with HFO surfaces become the major Cd
complexant sites.
The fraction of humic acid responsible for most of the Cd complexation is
influenced by pH where higher pH values were found to promote a higher complexing
power for Cd ion. However, humic acid is a mixture of flmctional groups with different
fractions that may have different complexing properties and therefore humic acid can not
be regarded uniform in its complexing behavior . The study indicated that Cd forms
relatively strong complexes with both HFO and humic acids fictions
at pH values
characteristic of most soil environments (pH 5-8). Cadmium sorption in soil can therefore
be considerably influenced by the presence of these materials in the environment
134
Fe-O-Cd
0.8
Ardic soils
CdHA
0.2
CdHB
3
4
5
6
7
8
pH
Figure S2. Cadmium spedation for the nonelectrostatic and the diffuse layer models, where
fibAlT =9.70 X 10-5 M rH2BlT=4.30 x 10*'. and fFeOOHlr^ 1.03 x 10-»M.
9
135
Andisols
Fe-O-Cd"
Cd^0.8 —
IL
00.6
s
o
'S
8.
S
a. 0.4
CdA"
CdHA-
a
X.
0.2
CdHB*
6
PH
8
Figure 53. Cadmium speciation for the nonelectrostatic and the diffuse layer models, where
[H2AIT = 1.66 X 10-^ M. [H2B|T= 7.44 x LO"*, and [FeOOHlr = 3.76 x 10*^ M.
136
Solid Activity Coefficients of Cadmium Adsorbed
In this section, the adsorbed cadmium was examined further by the means of solid
solutions. Adsorption can be easily understood if it presented in the form of solid solutions
rather than in the conventional adsorption coefficient equations such as the Freundlich
Equation. Solid solution considers Ksp, mole fraction which is based on more specific
chemical properties than the method based on the solid mass (Bohn , 1992). The surface
complexation model (Dzombak and Morell, 1986), and other assumed solid activity
coefficients equal unity as for ideal mixing, yet the soil solution is a nom'deal mixing with
assumed solid homogeneity. Thus, better parameters for estimating the solid activity in the
solid solution may enhance existing complexation models
SAC of Cd Adsorbed (Unadjusted pH and varied Cd loads).
The aqueous Cd^" activities, ion activity products of Cd(0H)2, and solid activity
coefficients of adsorbed cadmium of the soils investigated at five levels of [Cd]^ (0.8-12.8
ppm) added and at their natural pH are given in Tables I8a, andlSb. The Cd"^ activities
in the soils were discussed in Sections 5.4. The ion activity products (lAP) of Cd(0H)2, in
the soils studied as calculated &om the Cd aqueous activities and equilibrium pH
measurements ranged from 2.3x10"^ M to 5.6x10"'® M. In general, the lAP of cadmium
hydroxide increased with increased total Cd added to the soil.
The results showed that the lAP of Cd(0H)2 varied among the soils. In soil 1, for
example the lAP of Cd(0H)2 increased from 9.4X10*" M at 0.8 ppm [Cd]^ added
to 6.5x10"'* M at 12.8 ppm [Cd]^ added. The increases in lAP of Cd(0H)2, however, over
the rangesof total cadmium added (0.8,1.8,3.2,6.4, and 12.8 ppm) were relatively small for
137
all soils. The averaged values of lAP of Cd(0H)2 over the whole range of [Cdj^ added varied
from a low of 2.1x10*^" M (soil 18) to a high of 5.6x10*" M (soil 10). The variations in the
lAP of Cd(0H)2 in soils can be explained by the corresponding pH, since the lAP of
cadmium hydroxide is pH dependent.
The lAP of Cd(0H)2 in the soils were substantially less than predicated from the
calculation of Ksp of the pure solid phase.The lAP of Cd(0H)2 increased with Cd added yet
it failed to approach the corresponding Ksp or even to reach a constant value. The over all
average of lAP was 10"'®^* M. The reason for the low values of the lAP of cadmium
hydroxide in the soils studied may be due to other factors such as 1) adsorption of cadmium
by soils; 2) a higher entropy state due to mixing and isomorphous substitution of ions
creating a lower solubility and chemical potential; 3) the dissolution reaction not attaining
equilibrium or non equilibrium conditions in the soil.
The saturation index or the lAP/Ksp ratios are also shown in Tables 18a and 18b. The
lAP/Ksp ratio is a valuable index to measure the change of the chemical potential of
Cd(0H)2 in the solid solution. At equilibrium the solubility product of pure Cd(0H)2, is
defined as being equal to lAP of Cd(0H)2. For nonequilibrium conditions the state of the
solid solution with regard to a solid phase can be defined as: lAP > Ksp as a supersaturation
condition with respect to the Cd(0H)2 favoring its precipitation, where an lAP < Ksp is
undersaturated condition promoting dissolution.
The lAP of Cd(0H)2 in the soils studied were all under saturated with respect to the
pure phase of cadmium hydroxide and therefore cadmium could not precipitate according to
the lAP measurements. The solid activity coefficients (SAC) is determine by the indices.
138
Ksp, lAP, and the concentration in the solid phase of the term X in Equation 9. (Csp is a
constant, and independent of soil pH and concentration of the element of interest. The lAP
however, depends on solution equilibrium pH (Table 19), ionic strength and other factors
such as temperature and varied widely from soil to soil. The mole fractions of adsorbed Cd
followed the same trend as the amounts of cadmium adsorbed by soils since its
calculation was based on the amount adsorbed and the 30 moles of oxides/Kg as calculated
by (Bohn and Bohn, 1987).
As can be seen from the results in Tables 18a, and 18b, the average mole fraction of
various [Cdly added varied slightly from 4.6x10'^ to 7.1x10'^ M . The average SAC of
Cd(0H)2 values on the other hand ranged froma low of 8.2x10'^ (soil 18) to a high of 470
(soil 10) withan over all mean value ofl3.4. Soil 10 has the lowest amount of Cd adsorbed
over theentire concentration rangeof added Cd, while soil 18 adsorbed much higher amount
The low value of SAC indicates that the capacity of Cd adsorption is large in soil 18 and
vice versa with soil 10. The Aridisols with relatively higher lAP and pH values than those
of Andisols showed higher SAC values.
The SAC values seemed to increase with increasing total Cd added (soil 10, and
soil 16), yet the increase is not constant. The large SAC values of Cd(0H)2 (soil 1,2,4,6,
and 10) imply that the substituting ion fits poorly into the mineral structure and that the
the mineral tends to emit the ion into another solid phase or into the aqueous solution
(Bohn, 1992). The single ion activity coefficientSACc^z^values areshown inTables 18a and
18b. The SACcd2^(Eq. 13) of adsorbed Cd can be more valuable than itscorresponding salts.
The variation in the SACqq^ values of the soils studied were less than the SAC of cadmium
Table 18a. Aqueous Cd'" activities, ionactivity products lAP ofCdCOH);, soil mole fractions
X, saturation index (lAP/Ksp), solid activi^ coefficients, g' and SAC of adsorbed cadmiimi
at five levels of fCdlr added (ppm) and at their natural pH.
lAP/Ksp
g'
lAP
X
Soil [Cd]x
3cd2-f
SACcd(0H)2 SACqj2+
0.8
1.6
I
2
4
6
10
32
6.4
12.8
Mean
0.8
1.6
3.2
6.4
12.8
Mean
0.8
1.6
3.2
6.4
12.8
Mean
0.8
1.6
32
6.4
12.8
Mean
0.8
1.6
32
6.4
12.8
Mean
0.8
1.6
II
12
32
6.4
12.8
Mean
0.8
1.6
3.2
6.4
12.8
Mean
1.5E-07
1.8E-07
1.5E-07
1.5E-07
l.OE-06
3.3E-07
3.3E-07
3.4E-07
5.0E-07
7.6E-07
7.1E-06
1.8E-06
6.3E-07
1.6E-07
8.3E-07
1.5E-06
3.2E-06
1.3E-06
6.2E-08
U9E-07
2.2E.07
5.4E-07
l.lE-06
42E-07
2.2E-07
1.4E-06
1.6E-06
2.1E-06
1.4E-05
3.9E-06
2.0E-07
5.5E-07
7.9E.07
9JE-07
2.5E-06
l.OE-06
4.4E-07
lJE-06
62E-06
5.3E-06
8JE-06
4.4E-06
l.lE-05
2.3E-05
4.SE-05
8.9E^5
1.8E-04
6.9E-05
l.lE-05
L7E-05
3.4E-05
6.0E-05
I.lE-04
4.6E-05
l.lE-05
2.1E-05
4.0E-05
7.7E-05
1.4E.04
5.9E-05
l.lE-05
2.3E-05
4.6E-05
9.0E-05
1.8E-04
7.0E-05
9.9E-06
2.0E-05
3.7E-05
7.7E-05
I.4E-04
5.7E-05
1.2E-05
2.3E-05
4.5E.05
9.IE.05
I.8E-04
7.0E-05
l.lE-05
2.1E-05
2.7E-05
7.4E-05
1.3E-04
5JE-05
9.4E-19
2.9E-19
3.8E-19
3.8E-20
4.1E-19
4.1E.I9
8.3E-19
1.4E-18
8.3E-19
4.0E-19
1.5E-18
l.OE-18
4.0E-18
4.1E-20
I.lE-19
1.4E-19
9.1E.20
8.7E-19
1.6E-19
1.9E-19
3.5E-20
8.6E-20
4.4E-19
1.8E-19
5.6E.18
3.6E-17
1.6E-I7
3JE-17
3.6E-I7
2.5E-17
7.8E-23
22E-22
32E-22
3.7E-22
6.3E-22
32E-22
2.3E-25
1.3E-24
9.9E-24
5JE-24
4JE-24
4.2E-24
2.1E-04
6.5E-05
8.6E-OS
8.6E-06
9.2E-05
9.2E-05
1.9E-04
3.2E-04
1.9E-04
8.9E-05
3.5E-04
2.3E-04
8.9E-04
9.1E-06
2.4E-05
32E-05
2.0E-05
2.0E-04
3.5E-05
4.3E-05
7.9E-06
1.9E-05
9.8E-05
4.1E-05
1.3E-03
8.1E-03
3.6E-03
7.4E-03
8.0E-03
5.7E-03
I.7E-08
4.9E-08
7.1E-08
8.3E-08
L4E-07
72E-08
5.2E-11
3.0E-10
2.2E-09
UE-09
9.6E-10
9.4E-10
8.3E-14
1.3E-14
8.5E-15
4.3E-16
2.3 E-15
2.1E-14
7.6E-14
8.5E-14
2.4E-14
6.6E-15
1.5E-14
4.1E-14
3.5E-13
1.9E-15
2.7E.15
1.8E-15
6.4E-16
72E-I4
1.4E.14
8.5E-15
7.7E.16
9.5E-I6
2.4E-15
5JE.15
5.6E.13
1.8E-12
4.3E-13
4JE-13
2.5E-13
7.0E-13
6.7E-18
9.6E-18
7.0E.18
4.1E-18
3.6E-18
62E-18
22E-20
6JE-20
3.7E-19
7.2E-20
32E.20
t.lE-l9
18.6
2.9
1.9
O.l
0.5
4.8
16.9
19.1
5.4
1.5
3.2
9.2
792
0.4
0.6
0.4
0.1
162
3.1
1.9
02
02
0.5
12
125.9
407.9
95.4
96.8
56.5
156.5
1.5E-03
2.1E-03
1.6E-03
9.1E-04
8.0E-04
1.4E-03
4.8E-06
1.4E-05
82E-05
1.6E-05
7.1E-06
2JE-05
2.7
1.4
12
0.5
0.8
1.3
2.6
2.7
1.8
1.1
1.5
1.9
4.3
0.8
0.8
0.7
0.5
2.5
1.5
12
0.6
0.6
0.8
0.9
5.0
7.4
4.6
4.6
3.8
5.4
0.1
O.l
0.1
O.I
0.1
0.1
0.02
0.02
0.04
0.03
0.02
0.03
140
Table 18b. Aqueous
activities, ion activity products lAP of Cd(0H)2, soil mole
fractions X, saturation index (lAP/Ksp), solid activiQr coefficients, g' and SAC of
adsorbed cadmium at five levels of fCdlr added (ppm) and at their natural pH.
g' SACcj(0H)2 SACca+
X
lAP lAP/Ksp
Sou [Cdlx
acd2^
0.8
1.6
13
32
14
6.4
12.8
Mean
0.8
1.6
3.2
6.4
12.8
Mean
0.8
1.6
15
16
17
18
19
32
6.4
12.8
Mean
0.8
1.6
3.2
6.4
12.8
Mean
0.8
1.6
3.2
6.4
12.8
Mean
0.8
1.6
3.2
6.4
12.8
Mean
0.8
1.6
32
6.4
12.8
Mean
5.3E-08
4.7E.08
6.IE-08
1.2E-07
1.5E-05
3.0E-06
5.3E-08
8.7E-08
1.3E.07
1.4E-07
2.6E-07
1.3E-07
8.9E-08
6.2E-08
1.7E-07
7.6E-07
8.5E-07
3.9E-07
3.6E-08
5.3E-07
I.lE-06
3.1E-06
6.IE-06
2.2E-06
1.3E-06
1.7E-06
22E-06
5.7E-06
1.3E-05
4.8E-06
1.6E-06
1.6E-06
t.9E-06
3.7E-06
I.lE-05
4.0E-06
l.tE-06
12E-06
1.4E-06
2.0E-06
l.lE-^)5
3JE-06
l.lE-05
2.3E-05
4.7E-05
9.4E-05
1.8E-04
7.1E-05
12E-05
2JE-05
4.6E-0S
9.3E-05
1.8E-04
7.1E.05
UE-05
2.3E-05
4.6E-05
9.2E-05
1.8E-04
7.1E-05
1.2E-05
2.3E-05
4.5E-05
9.0E-05
1.7E-04
6.8E-05
1.2E-05
2.3E-05
4.5E-05
8.8E-05
1.6E<04
6.5E-05
1.2E-05
2.3E-05
4.6E-05
9.2E-05
1.8E-04
7.1E-05
1.2E-05
2.3E-05
4.6E-05
9JE-05
1.8E-04
7.1E-05
3.4E-22
3.0E-22
3.9E-22
7.3E-22
9.1E-20
1.9E-20
5.9E-22
9.6E-22
1.4E-21
1.5E-21
2.8E-21
1.5E-21
5.1E-21
4.1E-21
l.OE-20
4.3E-20
4.8E-20
2.2E-20
\2E-20
1.8E-19
3.8E-19
I.OE-18
2.0E-18
7.2E-19
8.3E-2I
2.1E-20
2.0E-20
3.9E.20
l.OE-19
3.8E-20
8.3E-2S
8.4E-25
l.OE-24
1.9E-24
5.9E-24
2.IE-24
1.8E-22
2.0E-22
2JE-22
3JE-22
1.8E-21
5.5E.22
7.5E-08
6.7E-08
8.7E-08
1.6E-07
2.0E-05
42E-06
1.3E-07
2.2E-07
32E-07
3.3E-07
6.3E-07
3.3E-07
I.lE-06
9.2E-07
2.3E-06
9.7E-06
I.IE-OS
4.9E-06
2.6E-06
4.0E-05
8.5E-05
2.3E-04
4.6E-04
1.6E-04
1.9E-06
4.6E-06
4.5E-06
8.8E-06
2.3E-05
8.6E-06
1.8E-10
1.9E-10
2.3E.10
4.3E-10
1.3E-09
4.7E-10
4.1E-08
4.4E-08
5JE-08
7.4E-08
4.0E-07
12E-07
3.1E-17
1.3E-17
8.3E-18
7.8E-18
5.0E-16
1.3E-16
5.1E-17
4.1E-17
3.1E-17
1.6E-17
1.6E-17
3.1E-17
4.4E-16
1.8E-16
2.2E-16
4.7E-16
2.6E-16
3.1E-16
l.OE-15
7.8E-IS
8.4E-15
l.IE-14
1.2E-14
8.9E-15
7.1E-I6
8.9E-16
4.5E-16
4.5E.16
6.7E-16
8.9E-16
7.1E-20
3.6E-20
22E-20
2.1E-20
3.2E-20
3.6E-20
1.6E-17
8.4E-I8
5.1E-18
3.6E-t8
9.9E-18
8.9E-18
7.0E-03
3.0E-03
1.9E-03
1.7E-03
0.1
0.03
I.1E.02
9.3E-03
6.9E-03
3.6E-03
3.5E-03
7.0E-03
0.1
0.04
0.05
0.1
0.1
0.07
0.2
1.8
1.9
2.5
2.6
2.0
0.2
0.2
O.l
0.1
0.2
02
1.6E-05
8.1E-06
4.9E-06
4.7E-06
7.2E-06
8.2E-06
3.5E-03
1.9E-03
l.lE-03
8.0E-04
2.2E-03
2.00e-03
0.2
0.1
0.1
0.1
0.5
0.3
02
02
02
02
0.2
02
0.5
0.3
0.4
0.5
0.4
0.4
0.6
12
12
1.4
1.4
1.2
0.5
0.6
0.5
0.5
0.5
0.6
0.03
0.02
0.02
0.02
0.02
0.02
02
0.1
0.1
0.1
0.1
0.1
141
hydroxides. The SAQdj^ values of the soils studied ranged from 0.017 to 7.8 with an over
all averaged value of 1.43. In the Aridic soils (1,2,4,6, and 10) the SACqq^- values were
between 1.56 to 7.8 with an average of 3.40, whileintheAndisols, it varied from 0.02 to
1.2 with an average of 0.31.
Table 19. pH measurements at equilibrium.
1.6
0.0
0.8
6.4
3.2
[Cdlr added
ppm
Soil
'Solution pH at Equilibrium
12.8
l.Typic Torrifluvent
8.4
8.1
8.2
7.7
7.8
7.9
2.Typic Natrargid
8.2
8.3
8.1
7.9
7.7
7.4
4.Typic Torrifluvent
8.4
7.7
7.6
7.5
7.2
7.3
6.Vertic Haplustoll
8.2
8.0
7.6
7.6
7.8
7.6
lO.Typic Calciorthid
8.7
8.7
8.5
8.6
8.2
8.5
11.Ustollic Camborthids
6.3
6.3
6.3
6.3
6.2
6.2
12.Typic Hydrudands
4.9
5.0
5.1
5
4.9
4.8
13.Entic Hapludands
6.9
6.9
6.9
6.9
6.9
6.6
14.Typic Hydrudands
7.0
7.0
6.9
7.1
6.9
7.5
IS.Typic Ustivitrands
7.2
6.9
6.6
6.6
6.9
6.6
16.Typic Ustivitrands
7.4
7.0
7.0
6.8
6.7
6.7
17.Typic Hydrudands
6.9
6.5
6.6
6.4
6.2
6.1
IS.Typic Hydrudands
5.8
5.7
5.9
5.9
5.6
5.5
19.Typic Haplocryands
6.8
6.2
6.0
5.5
5.8
5.6
'Average of three values
SAC of Cd Adsorbed (adjusted pH and fixed Cd load).
The SAC values of adsorbed Cd were evaluated at a constant [Cd]x (12.8 ppm) and
at four pH levels (5, 6, 7, and 8). The aqueous Cd^^ activities, ion activiQr products of
Cd(0H)2, and solid activiQr coefficientsof adsorbed cadmium in the soils investigated at four
142
levels of pH and a constant [CdJ^ of 12.8 ppm are shown in Tables 20a, and 20b. The lAP
of Cd(0H)2 increased with increasing pH values. For example, the lAP of Cd(0H)2
increased from 8.1x10'^* M at pH S to 1.4x10'^° M at pH 8 in soil 1, and from 6.5x10'^
M at pH 5 to 2.7x10*" M at pH 8 in soil 16.
The lAP of cadmium hydroxides as a function of soil pH are shown in Figure 54
for soils 1,2,4,6 and 10, and in Figure SS for soils 11-19. The Figures showed that the lAP
of cadmium hydroxide is a strongly pH dependent and for all soils it increased about 10 fold
for each one unit increment in soil pH. The range of lAP values of Cd(0H)2 over the range
of the pH values used in this study were similar despite the fact that these soils have
considerably different chemical and physical properties. It indicated that the soil pH is the
master variable controlling the cadmium hydroxide solubility.
The corresponding SAC values of the soils followed ahnost the same trend as lAP
values, because the increments in the mole fractions were almost negligible in the pH ranges
and the [Cdj-r used. The values of the SACoi(0H]2
different pH values and constant Cd load
are given in Table 20a, and 20b. The SACcd(0H)2 values ranged from 3.9x10"^ to 4.6, and
with a mean of 0.10 and standard deviation of 0.56. The SAQdcoH)! values were very small
in all soils as compared to the results obtained at the soils natural pH values. It increased,
however, with increasing pH. For example, the SAC of cadmium hydroxides in soil 1,
were 4.5xl0*^ 2.0x10'^, 0.18, and 4.6atpH values of 5, 6, 7, and 8 respectively. Each
one unit increase in soil pH (soil 1) caused about a 100 fold increase in the corresponding
SACcd(0H)2-
solid activiQr of adsorbed Cd or the single activity coefficient also increased
with increased pR
143
1E-171E-18^
1E-19^
^ 1E-20w
^ 1E-21
1E-22
soil 10
1E-24
5.5
6
6.5
pH
Figure S4. lAP of Cd(0H)2 as a function of soil pH.
7.5
144
1E-18 —
soil II
1E-19^
soil 12
£ 1E-20^
soil 13
soil 14
soil 15
g' 1E-22
soil 16
1E-231
soil 17
1E-24
5.5
6
6.5
pH
Figure 55. LAP of Cd(0H)2as a function of soil pH.
7.5
8
soil 18
119
145
The averaged values of SACcd2+ (Table 20a, and 20b) wereO.2,0.2,0.14,0.13,0.6,
0.12,0.20,0.20,0.22,0.20,0.40,0.13, 0.11, and 0.10 forsoilsl, 2, 4,6,10, 11,12,13,
14, 15, 16, 17,18, and 19 respectively. The SACcd2> values ranged over the pH values of
(5-8) from 0.016 to 0.2, with an over all average of 0.20 and standard deviation of 0.23. The
measured SAC of Cd(0H)2 at the adjusted soil pH were considerably lower than that
measured at the soil, natural pH. Even if the level of added Cd and the approximate natural
soil pH in the range of pH used is considered, the SAC values are much lower than the
unadjusted pH indicating that these soils adsorbed much higher Cd when their pH was
altered. The SAC of Cd(0H)2 at the soil natural pH (no complexing of oxidizing-reducing
agents added), were considered to be more important and therefore were further evaluated
by means of simple correlation with the major soil characteristics.
The simple correlation coefficient between these constants and soil properties are
presented in Table 21. As can be seen from the results of the correlations, the SAC values
of Cd(0H)2 were significantly, though negatively correlated with silt %, at (p < 0.1), and
also strongly correlated with soil pH, and hydroxide activities.The pH and (OH ) are factors
in calculating the lAP of Cd(0H)2 which were also used to calculate the SAC and therefore
the correlations wereexpected. Soils with highsilt percentages are expected to have a higher
surface area than those of sand texture and hence higher sorption capacity or lower SAC
values. The single solid activity^ SACc^q* (Eq.l3) values, on the other hand were highly
significant with most of the important soil properties. If the lAP, the soil mole fraction (30
moles oxides /kg soil) is constant, the SAC would predicate a decreasing sorption with
increasing SAC and vice versa.
Table 20a. Aqueous Cd'" activities, acd2+. ion activity products lAP, soil mole fractions X,
saturation mdex (lAP/Ksp), solid activity coefBcients, g' and SAC of adsorbed cadmium
at the highest [Cdlr added.
LAP/Ksp
g'
lAP
Soil pH
X
SACcd(om2 SACQJ2+
acd2+
Cd(0EI)2
No.
I
2
4
6
10
11
12
5 8.1E-06
6 4.4E-06
7 8.9E-07
8 1.4E-08
Mean 3.4E-06
5 1.4E-05
6 3.1E-06
7 2.9E-06
8 1.8E-08
Mean 5.0E-06
5 9.1E-05
6 3.3E-05
7 2.7E-07
8 6.2E-09
Mean3.IE-05
5 1.3E-05
6 4.8E-06
7 1.3E-07
8 1.6E-08
Mean 4.6E-06
5 3.8E-05
6 1.5E-05
7 1.4E-05
8 3.6E-06
Mean 1.8E-05
5 I.2E-05
6 4-6E-07
7 8.0E-08
8 2.3E-08
Mean 3.1E-06
5 9.1E-05
6 8.5E-06
7 5.3E-08
8 6.2E-08
Mean 2.5E-05
1.7E-04
1.8E-04
1.8E-04
1.9E-04
1.8E-04
1.6E-04
1.8E-04
1.8E-04
1.9E-04
1.8E-04
3.4E-05
1.3E-04
1.9E-04
1.9E-04
1.3E-04
I.6E-04
I.8E-04
I.9E-04
1.9E-04
1.8E-04
9.4E-05
1.4E-04
1.5E-04
1.7E-04
1.4E-04
1.7E-04
1.9E-04
1.8E-04
1.8E-04
1.8E-04
2.4E-05
1.2E-04
1.7E-04
L7E-04
1.2E-04
8.1E-24
4.4E-22
8.9E-21
1.4E-20
5.9E-21
1.4E-23
3.1E-22
2.9E-20
1.8E-20
1.2E-20
9.1E-23
3.3E-21
2.7E-21
6.2E-21
3.1E-21
1.3E.23
4.8E-22
1.3E-21
1.6E-20
4.6E-21
3.8E-23
1.5E-21
1.4E-19
3.6E-18
9.3E-19
1.2E-23
4.6E-23
8.0E-22
2.3E-20
6.0E-21
9.1E-23
8.5E-22
5.3E-22
6.2E-20
1.6E-20
1.8E-09
l.OE-07
2.0E-06
3.2E-06
1.3E-06
3.1E-09
7.0E-08
6.6E-06
4.0E-06
2.7E-06
2.0E-08
7.5E-07
6.0E-07
1.4E-06
6.9E-07
3.0E-09
I.IE-07
2.9E-07
3.7E-06
l.OE-06
8.6E-09
3.4E-07
3.1E-05
8.0E-04
2.1E-04
2.7E-09
LOE-08
1.8E-07
5.2E-06
1.3E-06
2.0E-08
1.9E-07
1.2E-07
I.4E-05
3.6E-06
4.6E-20 l.OE-05
2.5E-18 5.7E-04
5.IE-17 l.lE-02
8.1E-17 . 1.8E-02
3.4E-17 7.5E.03
7.9E.20 1.8E-05
1.8E-18 4.0E-04
1.7E-16 3.8E-02
l.OE-16 2.3E-02
6.8E-17 1.5E-02
5.2E-I9 1.2E-04
1.9E-17 4.3E-03
1.5E-17 3.4E-03
3.6E.17 8.0E-03
1.8E.17 3.9E-03
7.7E-20 1.7E-05
2.7E-18 6.1E-04
7.4E-18 1.7E-03
9.4E-17 2.1E-02
2.6E-17 5.8E-03
2.2E-19 4.9E-05
8.7E-18 2.0E-03
7.8E-16 1.8E-01
2.0E-14 4.6E+00
5.3E-15 1.2E+00
6.8E-20 1.5E-05
2.6E-19 5.9E-05
4.6E-18 l.OE-03
1.3E-16 3.0E-02
3.4E-17 7.7E-03
5.2E-19 1.2E-04
4.9E-18 LlE-03
3.1E-18 6.8E-04
3.6E-16 8.0E-02
9.1E-17 2.0E-02
0.02
0.08
0.22
0.26
0.20
0.03
0.07
0.33
0.28
0.20
0.05
0.16
0.15
0.20
0.14
0.03
0.09
0.12
0.28
0.13
0.04
0.13
0.56
1.66
0.60
0.02
0.04
0.10
0.31
0.12
0.05
0.10
0.09
0.43
0.20
Table 20b. Aqueous
actmties, acd2+. ion activity products lAP, soil mole fractions X,
saturation index (lAP/Ksp), solid activity coeflBcients, g'and SAC of adsorbed cadmium
at the highest fCdlr
• • 1 added.
g'
LAP/Ksp
lAP
SACCJ(0H)2 SACcj2-iX
Soil pH
aciG^
Cd(OH)2
No.
13
14
15
16
17
18
19
5
6
7
8
Mean
5
6
7
8
Mean
5
6
7
8
Mean
5
6
7
8
Mean
5
6
7
8
Mean
5
6
7
8
Mean
5
6
7
8
Mean
3.0E-06
l.OE-06
2.8E-07
5.3E-08
l.lE-06
l.lE-05
l.OE-06
4.4E-07
2.0E-07
3.2E-06
4.4E-06
2.7E-06
1.2E-06
4.4E-08
2.1E-06
6.5E-05
2.0E-05
1.4E-05
2.7E-07
2.5E-05
7.9E-05
1.3E-06
6.9E-07
8.9E-09
2.0E-05
2.7E-05
2.IE-06
4.1E-08
1.2E-08
7.3E-06
4.4E-05
2.1E-06
3.2E-08
6.2E-09
l.lE-05
I.8E-04
1.9E.04
1.9E-04
1.9E-04
1.9E-04
1.7E-04
1.9E-04
1.9E-04
1.9E-04
1.8E-04
1.8E-04
1.8E-04
1.9E-04
1.9E-04
1.9E-04
7.6E-05
1.4E-04
1.5E-04
1.8E-04
1.4E-04
2.8E-05
1.7E-04
1.8E-04
1.8E.04
1.4E-04
1.4E-04
1.8E-04
1.9E-04
1.9E-04
1.8E-04
1.1E-(W
L9E-04
I.9E-04
1.9E-04
I.7E-04
3.0E-24
l.OE-22
2.8E-21
5.3E-20
1.4E-20
l.lE-23
l.OE-22
4.4E-2I
2.0E-19
5.2E-20
4.4E-24
2.7E-22
1.2E-20
4.4E-20
1.4E-20
6.5E-23
2.0E-21
1.4E-19
2.7E-19
l.OE-19
7.9E-23
1.3E-22
6.9E-21
8.9E-21
4.0E-21
2.7E-23
2.1E-22
4.1E-22
1.2E-20
3.IE-2I
4.4E-23
2.1E-22
3.2E-22
6.2E-21
L7E-21
6.8E-10
2.3E-08
6.4E-07
1.2E-05
3.2E-06
2.5E-09
2.3E-08
l.OE-06
4.6E-05
1.2E-05
9.8E-10
6.1E-08
2.7E-06
l.OE-05
3.2E-06
1.5E-08
4.5E-07
3.2E-05
6.0E-05
2.3E-05
1.8E-08
2.8E-08
1.6E-06
2.0E-06
9.0E-07
6.1E-09
4.8E-08
9.2E-08
2.6E-06
6.8E-07
9.8E-09
4.8E-08
7.2E-08
1.4E-06
3.8E.07
1.7E-20
5.9E-19
1.6E.17
3.1E-16
8.0E.17
6.4E-20
5.8E-19
2.5E-17
1.2E-15
3.0E-16
2.5E-20
1.6E-18
7.0E-17
2.5E-16
8.1E-17
3.7E-19
I.IE.17
8.2E-16
1.5E-15
5.9E-16
4.5E-19
7.2E.19
4.0E-17
5.1E.17
2.3E-17
1.6E.19
1.2E-18
2.3E-18
6.6E-17
1.7E-17
2.5E-19
1.2E.18
1.8E-18
3.6E-17
9.7E-18
3.9E-06
1.3E-04
3.6E-03
6.8E-02
1.8E-02
1.4E-05
1.3E-04
5.7E-03
2.6E-01
6.7E-02
5.6E-06
3.5E-04
1.6E-02
5.7E-02
1.8E-02
8.4E-05
2.5E-03
1.8E-01
3.4E-01
1.3E-01
l.OE-04
1.6E-04
8.9E-03
I.lE-02
5.1E-03
3.5E-05
2.7E-04
5.2E-04
1.5E-02
3.9E-03
5.6E-05
2.7E-04
4.1E-04
8.0E-03
2.2E-03
0.02
0.05
0.15
0.41
0.20
0.02
0.05
0.18
0.64
0.22
0.02
0.07
0.25
0.38
0.20
0.04
0.14
0.57
0.70
0.40
0.05
0.05
0.21
0.22
0.13
0.03
0.06
0.08
0.25
O.ll
0.04
0.06
0.07
0.20
0.10
148
The LAP of the solid of interest would increase with increasing SAC values and
also there could be less adsorption. The S
values can be considered to behave similar
to the retardation factor. Red (Section S.S), with the difference that increasing the Red
values would indicate an increasing adsorption and increasing the SAC^^^ values would
mean a decreasing adsorption with the assumption stated earlier.
Table 21. Simple correlation coefficients (r) between solid activity coefficients
of cadmium and soil characteristics.
Soil Characteristics
SACca2+
SACcd(0H)2
0.34^
-0.42^
-0.09
*
O
0.23
-0.39^
%
0.17
(1:1)
0.69^^^
(M)
0.89^^^
0.98^^^
-0.27
-0.39^
-0.18
0.08
-0.28
-0.14
cmol/kg
-0.09
0.12
Na
-0.24
-0.08
K
-0.26
-0.46^^
Organic C
g/kg
.0.2S
-0.46^^
Peo-Pep
-0.28
-0.S3^^
Peo
-0.23
-0.42^
Pep
-0.29
-O.SS^^
Ped
%
-0.19
-0.35^
Alo
-0.17
-0.29
Alp
-0.18
-0.35^
Aid
•,
•••significant at the O.IO, 0.05,0.01 probability levels, respectively.
sand
silt
clay
PH
OHCEC
Ca
Mg
By comparing the results of simple correlations between Red values (Table 15) and
SACqq^ values (Table 21) with soil properties they behaved similar in that both correlated
significantly with the same soil properties namely silt %, and iron oxides fraction (Feo-Fep,
Feo, Pep and Ped). Moreover, increasing the Red values would indicate an increasing
adsorption and increasing the SACcdz^^ values would mean a decreasing adsorption.
149
The SACqq^ values were significantly correlated yet negative with silt %, CEC,
Fep, Aid, and Alo at
0.1), and with O.C and iron fractions (Feo-Fep, Feo, and Fed) at
p < O.OS. The pH and hydroxide activi^ were also strongly correlated with the SAC^jj^
valiies at p < 0.01, yet these parameters are used in calculating the SAC as mentioned
previously. These parameters can have a indirect effect where increasing the pH or hydroxide
activity^ would result in a higher lAP of Cd(0H)2 and a higher corresponding SACqj(oh)2 or
SACqq^ with assuming a fixed
mole fraction and Ksp.
The results of the correlations between SACc^q^ and soil characteristics suggest that
the soil adsorptioncapacity forCd will increase with increasing iron fraction (Feo-Fep, Feo,
and Fed), CEC, and organic carbon in soils. In fact the organic carbon and Feo-Fep were
found according to the complexation model as discussed in section S.6 to be the most
important soil properties accounting for most of the adsorbed cadmium.
150
CHAPTER6
SUMMARY AND CONCLUSIONS
Summary
The term sorption may include several mecham'sms such as ion exchange, surface
induced precipitation reactions, formation of chemical bonds on the solid surface, and
absorption into mineral lattices. Experimentally, these mechanisms can not be separated for
a complex sorbent matrix such as soil, and therefore, their overall contribution is described
as sorption. In this study, the sorption isotherms were obtained by batch experiment at a
solid:solution ratio of 1:50 or 0.5 g soil and 25 ml of solution. The solutions consisted
of 0.025 M NaNO] background electrolyte containing 0.8,1.6, 3.2,6.4, and 12.8 ppm of
Cd^". These were added to 0.5 g soil samples (air dried and passed through 2 mm nylon
mesh) in 50 mL polyethylene centrifuge tubes and shaken at room temperature (25 ± 2°C)
for24hr.
A selective ion electrode procedure was also employed for the determination of the
concentration of five ionic Cd, (Cd^*) in the soil solution using a cadmium electrode (Orion
Model 944800) with single-junction reference electrode (Orion Model 900100). In addition,
the cadmiimi mobility was studied by the means of soil thin layer chromatography(soiI TLC).
The fourteen soils investigated in this research exhibited a range of pH values &om
strongly acidic (pH of 4.9) to strongly alkaline(pH of 8.8), a wide range of cation exchange
capacities from very low of 4.1 cmoi/kg to very high of 99.0 cmoHcg, and a wide range of
organic carbon contents finm 0.6 g/kg to 103.0 g/kg. The soils also have wide rangesof iron
and aluminum oxides.
151
The maxiinuin soiption capaciQr (b) of the soils ranged fit)m 0.59 to 3.84 (mg/g).
The sorption parameter, k, which is related to the energy of sorption in the Langmuir
isotherm ranged from 0.23 to 3.241/g. The distribution coefBcient of adsorbed Cd , kd,
ranged from 0.10 to 1.62 (1/g) and n values ranged from 0.49 to 0.87. The maxima soil
sorption capaci^ was significantly correlated with the inorganic poorly crystalline iron
firaction, Feo-Fep, and the poorly crystalline (oxalate extractable), Feo, at the probability
level of (p < 0.05); and with silt %, and the organically bound (pyrophosphate extractable),
Fep, soil contents at
0.1). The sand % also was significantly correlated though negative
with b at (p < 0.1).
The parameter k values were significantly correlated with the total Fe-oxides,
hydroxides and oxyhydroxides (ditionite-citrate-bicarbonate extractable), Fed, and with
Feo-Fep at (p < 0.05) and (p < 0.1) respectively. Moreover, the n values are significantly
correlated with silt and clay soilcontents at (p <0.01) and with exchangeable Ca^^ Na\ and
K"" at the (p < 0.1). They are also negatively correlated with sand % and Alo soil content at
(p < 0.1). The kd values on the other hand, were significantly correlated with silt %, FeoFep; Feo, at (p < 0.01); with Fed at (p < 0.05); and with Fep at ^ < 0.1). The sand % was
also significantly correlated with kd at (p < 0.05), though negative.
The Cd^* activities in the soils studied varied firom 10''** to 10"^" M with an over
all average of 10"^ °^ M. It increased with increasing total Cd added. The Cd^"^ activities at
constant [Cd]^ added at 12.8 ppm concentration, and at di£ferent levels of pH (5, 6, 7,
and 8) on the other hand for the same soils ranged firom 10**^ to lO'*"* M with an over all
average of 10*^ M, and were inversely related to the soil pH .The relationship between the
152
logarithm of cadmium activiQr and soil pH was highly significant (p > 0.01), withcorrelation
coefficients, (R^), ranging firom 0.79 to 0.99.
The cadmium mobility Rf values ranged between 0.25 and 0.95. The results
suggested that Cd mobility in these soils is highly variable. The cadmium mobility
classification in the soils studied according to Helling and Turner (1967) were found to be
slightly mobile in 64 %, moderately mobile in 29 % and very mobile in 7 % of the soils
investigated. The variability in the cadmium mobility in the soils is mostly accounted for by
sand% (r = 0.63), followed by silt %; (r=-0.59); the exchangeable Ca (r=-0,55); clay %
(r = -0.55) and the exchangeable K (r = -0.49).
The retardation factor, Red, values of the soils studied at fixed total Cd load of
12.8 ppm and at their natural pH varied from 2.9 to 33.42, while the corresponding Red
values for the same soils at four adjusted levels of pH (5,6,7, and 8) varied from 1.14 to
196.92. The results indicate highly significant correlations between Red values and Feo,
Feo-Fep, and silt (p< 0.01); and Fed at (p< 0.05); Fep and the exchangeable Mg^" at (p
<0.1). Also there was a significant, though negative correlation between Red and sand %
at ( p < 0.05). The Red values were increased with increasing amounts of the iron oxide
fractions (Feo, Fep, and Fed); silt % and exchangeable Mg^^ in the soils. The Red values also
increased with increasing quantities of either crystalline inorganic or organically bound irons
in soils.
The complexation model prediction values for the percentage of sorbed Cd are
within 5% for all batch sorption experiments for most of the soils at the pH value of 8. The
prediction was based on the amount of organic matter and hydrous ferric oxides, and
153
therefore, the prediction of Cd sorption on soils high in those materials showed relatively
good agreements withthe batch study dataas for soils 13,18,andl9respectively.Thehumic
materials may be an important factor in Cd retention at pH values greater than 3 and hydrous
ferric oxide surfaces may become an important factor at pH values greater than 7. The
predicted relative proportion of Cd^"* species at a different pH values indicated that the Cd
complexation with H2A occurs at very low pH values, resulting in the formation of CdHA"
complexes. As the pH increases additional protons are released from the humic acid
resulting in the formation CdA° complexes and the formation of CdHB"* with HjB
complexant sites. Moreover, at pH values greater than 7 the formation of Fe-O-Cd" with
HFO surfaces become the major Cd complexant sites.
The solid activity coefBcients (SAC) is determine by the indices, Ksp, lAP, and
the concentration in the solid phase of the term X in Equation 9. Ksp is a constant, and
independent of soil pH and concentration of the element of interest. The lAP, however;
depends on solution equilibrium pH, ionic strength and other factors such as temperature
and varied widely from soil to soil.
The ion activity products (lAP) of Cd(0H)2, in the soils studied as calculated from
the Cd aqueous activities and equilibrium pH measurements ranged from 2.3x10'^ M
to 5.6xlO'''M. The lAP ofCd(OH)2 in the soils studied were all undersaturated with respect
to the pure phase of cadmiimi hydroxide and therefore cadmium could not precipitate
according to the lAP measurements. The lAP of Cd(0H)2in the soils were substantially less
than predicated fromthecalculationofKspofthepuresolidphase.lt
increased with added
Cd yet it failed to approach the corresponding Ksp or even to reach constant values.
154
The average SAC of Cd(0H)2 values ranged from a low of 8.2x10'^ (soil 18) to
a high of470 (soil 10) with an over all mean value of 13.4. The lAP of cadmium hydroxide
is strongly pH dependent and for all soils it increased about 10 fold for each one unit
increment in soil pH. The values of SAC cd(0H)2 values were very small in all soils as
compared to the results obtained at the soils natural pH values. It increased, however, with
increasing pH. For example, the SAC of CdCOH); in soil 1, were 4.SxlO'^ 2.0xl0'\ 0.18,
and 4.6atpH values of S, 6, 7, and 8 respectively. Each one unit increase in the soil pH
caused about a 100 fold increase in the corresponding SAC cdcoHu*
SACcd2^ values were
sigm'ficantly correlated yet negative with silt %, CEC, Fep, Aid, and Alo at (p< 0.1), and
with O.C and iron fractions (Feo-Fep, Feo, and Fed ) at p < O.OS. The results of the
correlations between SACcd2> and soil characteristics suggest that soil sorption capacity for
Cd will increase with increasing iron (Feo-Fep, Feo, and Fed), CEC, and organic carbon in
soils.
Conclusions
In general the following conclusions can be derived from the results of this research:
1.
In all soils used in the experiments, the relationship between the liquid and solid
phase at equilibrium, the best fit for cadmium sorption is with the Freundlich
sorption isotherm.
2.
The maxima soil sorption capacities were higher for soils with high amounts of
inorganic poorly crystalline iron (Feo-Fep), the poorly crystalline (oxalate
extractable), Feo,silt%,andtheorganicallybound(pyrophosphateextractable),Fep.
3.
The sorption constant, k, values weresignificantlycorrelatedwithFedand Feo-Fep.
4.
The n values were significantly correlated with silt and clay soil contents at (p <
0.01) and with exchangeable
Na\and K" at the (p< 0.1). They were also
negatively correlated with sand % and Alo soil contents at (p < 0.1).
5.
The distribution coefficient, kd, values were significantly correlated with silt %,
Feo-Fep; Feoat(p< 0.01); with Fed at (p<O.OS);andwithFepat (p< 0.1). Also
it decreased with increased sand percentage.
6.
The average of Cd^" activities in the soils was 10*^ °^ M.
7.
The Rf values obtained ranged Gvm 0.25 to 0.95 which suggested that Cd mobility
in these soils is highly variable.
8.
The cadmium mobility was found to be slightly mobile in 64%, moderately mobile
in 29% and very mobile in 7% of the soils investigated.
9.
The variability in the cadmium mobility in the soils is mostly accounted for by the
sand % (r=0.63), followed by the silt %;(r=-0.59); the exchangeable Ca
(r=-0.55); the clay % (r=-0.55) and the exchangeable K (r=-0.49).
10.
The retardation factor. Red, values of the soils studied at fixed total Cd load of 12.8
ppm and at their natural pH were between 2.9 to 33.42.
11.
The Red values increased with increasing amounts of the iron oxide fiactions (Feo,
Fep, and Fed); silt % and exchangeable Mg^^ in the soils.
12.
Both Red and Rf factors showed Cd mobility to be higher in those soils with a high
percentage of sand.
13.
TheCdsorption predictionmade using the non electrostatic and difiuse layermodels
were based on the amount of organic matter and hydrous ferric oxides, and as a
156
result, the predicted Cd sorption values were in relatively good agreement with the
batch study data for soil with high contents of organic matter and iron hydroxides.
14.
The humic materials are an important factor in Cd retention at pH values greater than
three and hydrous ferric oxide surfaces are an important factor at pH greater than
seven.
15.
The ion activity products of Cd(0H)2 ranged from 2.3x10*" M to 5.6x10*" M,
while the solid activity coefficients of Cd(0H)2 values varied form 8.2x10*^ to 470.
16.
The correlations between SACcd2> and soil characteristics suggest that soil sorption
capacity for Cd would increase with increasing iron fraction (Feo-Fep, Feo, and Fed),
CEC, and organic carbon in soils.
157
REFERENCES
Alhaji, S. Jeng, and Bai Ram Singh.1993.Partioning and distributk)n ofcadmium and zinc in
selected cultivated soils in Norway. SoQ Sci.156:240-250.
AUoway, B.J., AJL Tills, and H. Morgan. 1985. The speciation and availability of cadmium
and lead in polluted soils. IN D. D. Hemphill (ed.). Trace Substances in
Environmental Health, pp.187-201. University of Missouri, Columbia, MO.
Anderson, P.R., and TJl .Chirstensen.l988.Distribution coefBcients of Cd, Co, Ni,and Zn
in soils. J. Soil Sci.39:15-26.
Blakemore, S.C., P.S. Searle, B.K. Daly. 1987. Methods for chemical analysis of soils. N.Z.
Bur, Sci. Rp.80. N.Z. Soil Bureau, Sower Hutt, N.Z.
Barrow, N. J.I986a.Testing a mechanistic model, n. The effect of time and temperature on
the reaction of zinc with a sofl. J. of Soil Sci.37:277-286.
Barrow, N. J.1984. Modeling the effects of pH on phosphate adsorption by soOs. J. of Soil
Sci.35:283-297.
Barrow, N.J. and A. S. Ellis.1986.Testing a mechanistic model. V. The point of zero salt
effect for phosphate retention, Zmc retention and for acid/alkali titration of soil. J. of
SoaSci.37:303-310.
Basta, N.T.,and M. A. Tabatabai.1992. Effect of cropping systems on a desorption of metals
by soU. n. Effect of pH. Soil Sci.153:195-204.
Basta, N.T., D. J. Pantone and M. A. Tabatabai.1993. Path analysis of heavy metal
adsorption by soil. Agron. J. 85:1054-1057.
Benjamin, M.M., and J. O. Leckie. 1981. Muk^le-site adsorption ofCd, Cu, Zn, and Pb on
amorphous iron hydroxide. J. Coll. Interf Sci. 79:209-221.
Berggren, D.1992. Speciation and mobilization of aluminum and cadmiimi in Podozols and
Cambisols of S. SWEDEN. Water, Air ,and Soil Pollute,62:125-126.
Boekhold, A. E.,and S. E. A. M. Van der Zee.l99lXong-term effects of soil heterogeneity
on cadmium behavior in soil. J. Contam. Hydrol.7:371-390.
Boekhold, A. E.,and S. E. A. T. M. Van der Zee.l992.A scaled sorption model validated at
the column scale to predict cadmium content in a spatialty variable field soil .SoO
Sci.Soc.AnLJ.54:105-120.
158
Boekhold, A. E., and S. E. A. T. M. Vander Zee.l993.InfhieQce ofelectrofyte composition
and pH on cadmium sorption by an acid sandy soil. J. Soil Sci.44:85-96.
Boekhold, A. E., and S. E. A. T. M. Van der Zee.l992.Significance of soil chemical
heterogeneity for spatial behavior of Cd in field soils. Soil Sci.Soc.Am.J.56:747-754.
Bohn, H. L. 1992. Chemical activity and aqueous solutions solubility of soQ solid solutions.
Soa Sci. 154:357-365.
Bohn, H. L. 1983. Ion activity products m sofl solutions. Chemical mobility and reactivity m
soil systems (D.W. Nelson, ed). AHL SOC. Agron. 11:1-10.
Bohn,H.L., and R.K.BohiL1986. Solid activity coefiBcients ofsoil components. Geoderma
38:3-18.
Bohn, H. L., and R. K. Bohn.1987. Solid activities of trace elements in soils. Soil Sci.
143:398-403.
Bolan, N.S., J. K. Syers and R.W. Tillman. 1986. Ionic strength effects on surface charge
and adsorption of phosphate and sulphate by soO. J. of SoQ Sci. 37:379-388.
Bolton, K. A., and L. J. Evans. 1996. Cadmium adsorption capacity of selected Ontario
Soils. Can. J. SoO Sci.76:183-189.
Bolton, K. A.; S. Sjdberg, and L. J. Evans. 1996. Proton binding and Cd complexation
constants for a soil humic acid using a quasi-particle model. Soil Sci. Soc. Am.
1.60:1064-1072.
Brummer, G.W., J. Gerth, and FC. G. Tiller.1988.Reaction kinetics of adsorption and
desorption of nickel, zinc and cadmium by geothite. I. Adsorption and difilision of
metals. J. Soil Sci.39:37-52.
Buchter, B., B. David, M. C. Amacher, C. Hinz, I.K. Skandar and H.M. Selim .1989
correlation of Freundlich Kd and n retention parameters withsoils and elements. Soil
Sci.143:370-379.
Carey AE. 1979. Soil cadmium monitoring data. Memorandum, July 23. Environmental
Protection Agency, Washington, DC.
Cavallaro, N., and M. B. McBride.1978. Copper and cadmium adsorption characterstks of
selected acid and calcareous soils. Soil ScLSoc.Am.J.42:550-556.
Cavallaro, N., and M. B. McBride.1980.Activities of Cu2+ and Cd2-)- m soil solutions as
affected by pH. Sofl ScL 44:729-732.
159
Christensen,Ti{.1984.cadimuin soil sorption at lowconcentrations: Effect oftime, cadmium
loading, pH and calcium. Water, Air and Soil Pollut.21:1573-1576.
Chnstensen,TiI.1987.Cadmium soil sorption at low concentrations. I. Effect oftime,
cadmium soil load, pH, and calcium. Water, Air, and Soil Pollut.43;293-303.
Cbristensen,T.H.l989a.Cadmium soil sorption at low concentrations. VII. E£fect of stable
solid waste leachate complexes. Water, Air, and Soil PoUut. 44:43-56.
Cowan, C.E., J. M. Zachara, S.C. Smith, and C. T Resch. 1992. Individual sorbent
contributions to cadmium sorption on Ultisols of mixed mineralogy. Soil Sci. Soc.
Am. J.56:1084-1094.
Curtin, D., R. Naidu, and J. K. Syers. 1991. Chemical and mmeralogical characteristics of
strongly weathered Fijian sofl. Some fertility implications. Geoderam. 48:263-372.
Dzombak, D. A., and P.M. Morel. 1990. Sur&ce complexation modelmg: Hydrous oxides.
WUey-Interscience, Toronto, pp.393.
Dzombak, D. A., and F.M. Morel. 1986. Sorption ofcadmiumon hydrous ferric oxideat high
sorbate/sorbent ratios: equilibrium kineticsand modeling. J. Coll. Interf.Sci. 112:588598.
Edmeandes, D.C., D.M. Wheeler and O.E. Clinton. 1985. The chemical composition and
ionic strength ofsoil solutions from New Zealand topsoils. Aust. J. of Sofl Research.
23:151-165.
El-Swaify,SA.1976.Changes in the physical properties of sofl clays due to precipitate
aluminum and iron hydroxides. H. CoUoidal interactk)ns in the absence ofdrying. Sofl
Sci.Soc.Am.J.40:516-520.
Elliot, H.A.1983. Adsorption behavior ofcadmium in response to sofl solution as affected by
pH. Sofl Sci.Soc.Am.J.44:723-732.
Elliott, H.A.,and C. M. Denney.1982.Sofl adsorption of cadmium &om solutions containing
organic ligands. J. Envvon. Qual.4:658-663.
Elliott, FI.A.,M. R. Liberatv and C. P. Huang.I986.Competitive adsorption of heavy metals
by sofls. J. Environ. Qual.l5:2I4-2I9.
Escrig, I., and I. Morell. 1998. Efl^t of cakium on the sofl adsorption of cadmium and zmc
in some Spanish sandy sofls. Water, Air, and sofl poll.l05:507-520.
160
EPA (Envvonmental Protection Agency). 1981. Health Assessment Document for Cadmium.
Environmental Criteria and Assessment OfBce, Research Triangle Park, NC. EPA600/8-81-023.
EPA (Environmental Protection Agency). 198Sa. Cadmium Contamination of the
Environment: An Assessment ofNationwide Risk. Washington, DC. EPA-440/4-85023.
Evans. L. J. and W.G. Wilson. 1985. Extractable Fe, Al, Si and C in B horizons of Podozoiic
and Brunisolic soils from Ontario. Can. J. of Soil Sci. 65:489-496.
Farrah, H. and W.F. Pickering. 1977. The sorption of lead and cadmium species by clay
minerals. Aust. J. Chem. 30:1417-1422.
Fey, M. V., and J. LeRoux. 1977. Properties and quantitative estimation of poorly crystal!^
components in sesquioxidic soil clays. Clays Clay Miner. 25:285-294.
Forbes, E., A.M. Posner, and JJP. Quirk. 1976. The specific adsorption of divalent Cd, Co,
Cu, Pb, and Zn on geothite, J of Sofl Sci. 27:154-166.
Fuller,W.H.1977.Movement of selected metals. Asbestos and cyanide m soQ: Application to
waste disposal problem. EPA-600/2-77-020. Solid and Hazardous waste Research
Cincinnati, OH.
Garcia-Miragaya, J.,andA.L.Page.l977. Influence, of exchangeable cation on the sorption
oftrace amounts of cadmium by montmorillonite. SoD Sci. Soc. Am. J. 41:718-721.
Garcia-Miragaya, J.,and A. L. Page.l978.Sorption of trace quantities of cadmiimi by soils
with different chemical and mineralogical composition. Water, Air, and soil
poU.9:289-299.
Haghiri, F. 1974. Plant uptake of cadmium as influenced by cationexchange capacity, organic
matter, zinc, and soil temperature. J. Environ. Qual. 3:180-183.
Hanafi M.M and J. Sjiaola. 1998. Cadmium and zinc in acid tropical soOs: I. Soil physochemical properties effect on their adsorptbn. Commun. Soil Sci. Plant Anal. 29:1114.
Hatch, D. J., Jones. L.H.,and Burau,R.G.1988.The effect of pH on the uptake of cadmiimi
by four plant species grown in flowing solution culture. Plant and soil 108:121-126.
He, Q3.and B. R Singh. 1993. Effect of organic matter on the distribution, extractabli^and
uptake of cadmium in soils. J. of Soil ScL 44:641-650.
161
Helling, C. S.,and B.C. Turner.1968.Pesticide mobility: Determination by soil thin-layer
chromatography. Science (Washington, DC) 162:562-563.
Hendricks, D. M. 1985. Arizona Soils. College of Agric. Univ. Of. Arizona, Tucson, Az.
Hendrickson, L. L.,and R. B. Corey.1981.Effect of equilibrium metal concentrations on
apparent selectivity coefBcients of soil complexes. Soil ScL131:163-171.
Hershafl, A.1972.Solid waste treatment technology. Envfron. Sci. Technol.6:412-413.
John MJC1972.Cadmium adsorption maxima of soils as measured by the Langmur isotheroL
Can. J. Sofl SCL52:343-350.
John, M. K., Chuah, H. H. and Van Laerhoven, C.1972a.Cadmium contamination of soil and
its uptake by Oats. Environ. Sci. Technol.6:555-557.
Johnston, A.E.,and K.C. Jones.1992.The cadmium issue-long term changes in the cadmium
content ofsoils and the crops grown on them. In:International Fertilizer Development
Center(ed).proceeding international workshop on fertilizers and the environment.
pp.255-269.27 niarchl992. Florida, USA.
Kirkham, M. B. 1977. Uptake by barley of water table or sur&ce applied cadmium. Soil
Sci.Soc.Am.J.41:1123-1123
Kilmer, V. J., and Alexander, L. T. 1949. Methods of making mechanical analysis of soils.
Soil Sci. 68:15-24.
Korte, N. E., J. Skopp ,G. E. Niebla, and W, H. FuUer.l975.A baselme study on trace metal
elution from diverse soQ types. Water, air and soil pollut.5:149-156.
Kuo, S., and A.S. Baker.1980.Sorption of copper, zinc, and cadmium by some acid soils. Soil
Sci.Soc.Am.J.44:969-974.
Lagerwerfi^ J V,1972. Lead, mercury and cadmium as environmental contaminants. Soil Sci.
Soc. Am. Proc.36:734-737.
Levi-Menzies, R.,G. F. Soldatini, and R. Rifi^di.1976. Cadmium adsorption by soils. J. of
Sofl Sci. 27:10-15.
Levy, R., and C. W. Francis. 1976. Adsorption and desorption of cadmium by synthetic and
natural organo-clay complexes. Geoderma. 15:361-370.
Lindsay,W.L.1979.Chemical equilibrium in soils. John Wiley, New York.
162
Lowe .1973. Ammo acid distribution in forest humus layers m British Columbia. Soil Sci.
Am. Proc. 37:569-603.
Lund, L. J., A. L. Page, and C.O. Nelson.1976.Movement of heavy metals below sewage
disposal ponds. J. Environ. Qual.5:330-334.
Maclean, A.J.1976.Cadmium in different plant species and its availability in soils as influenced
by organic matter and additions of lime, P, Cd, and Zn. Can. J. Soil Sci.56:129-133.
McBn'de, M. B., L. D. Tyler, and D.A. Hovde.1981. Cadmium adsorption by soils and
uptake by plants as affected by soil chemical properties. Soil Sci. Soc. Am. J.45:739744.
McDufSe, B.,E1-Barbary, I., Hollod, G. J.,and Tiberio,R.D.l976a. In D. Hemphill (ed.).
Trace substances in environmental health Univ. of Columbia, P.8S.
Meranger J C, Subramian K. S, Chalifoux C. 1981. Survey for cadmium, cobah, chromium,
copper, nickel, lead, zinc, calcium, and magnesium in Canadian drmking water
supplies. J Assoc Off Anal Chem 64:44.
Naidu, R.,N.S. Bolan, R. S. Kookana, K.G. Tiller.1994Jonk; strength and pH effects on the
sur&ce charge and sorption of cadmium by soil. J. of SoQ Sci. 45:419-29.
Orion research. Inc., 1990. Model 94-48. Cadmium electrode instruction manual.
Page, G. W. 1981. Comparison of groundwater and sur&ce water for patterns and levels of
contamination by toxic substances. Envffon Sci Technol 15:1475-1481.
Papelis, D., Hayes, K. F. And Leckie, J. O. 1988. HYDRAQL: A program for the
computation ofchemical equihbrium composition of aqueous batchsystem s including
sur&ce complexation modeling of ion adsorption at the oxide/solution inter&ce.
Technical report 306. Department of Civil Engendering . Stanford University,
Stanford CA.
Sanchez-Martin, M. J.,andM. Sanchez-Camazano.1993.Adsorptionand mobilityofcadmium
in natural, uncultivated soils. J. Environ. Qual.22:737-742.
Schindler, P. W.,and W. Stumm.1989.The sur&ce chemistry of oxides, hydroxides, and
oxides minerals. In: Aquatic surt^e chemistry (ed.W.Stumm).
Schindler, P. W.,P. Liechti and J. C. WestaU.1987. Adsorption of copper, cadmium and lead
from aqueous solution to the kaolmite/water interne. Netherlands. L of Agric.
Sci.35:219-230.
163
Scokat, P.O.JC Mecus-verdume and R, De. Borger.1983. Mobility of heavy metals in
polluted soils near zinc smelters. Water, Aff, and Soil PoUut.20:451-463.
Singh, M.1971.Retention of added copper by soils as af^ted by organic matter, CaC03,and
exchangeable ions. Geodenna.5;219-227.
Sposito, G.1986. Sorption of trace metals by humic materials in soil and natural waters. Crit.
Rev. Environ. Control 16:193-229.
Sposito, G.1984. The surface chemistry of soils. Oxford Unv. Press, New York.
Street. J. J. W.L. Lmdsay, and B. R. Sabey 1977.The solubility and plant uptake of cadmium
in soils amended with cadmium and sewage sludge. J. Envffon. Qual. 6:72-77.
Syers, J. K., M.G. Brownian, G.W. Similler, and R. B. grey. 1973. Phosphate sorption by
soils evaluated by the Langmuir adsorption equation. Soil Sci. Soc of Am. Proc.
37:358-363.
Temminghof^ E. J. M., S. E. A. T.M. Van der Zee and F. A. M. Dehaan.I995.Speciation
and calcium competition effects on cadmium sorption by sandy sofl at various pHs,
European J. of Soil Sci.46:649-655.
Tiller, K.G.1989.Heavy metals in soils and their environmental significance. Adv. In Soil
Sci.9:l 13-114.
Unruh, B. L., J .C. Silvertooth, and D. M Hendricks. 1994. Potassium fertility status of
several Sonora desert soils. Soil Sci. 158:435-441.
Weber, W. J. and H.S. Posselt. 1974. Equilibrium models and precipitation reactions for
cadmium(II). In: Aqueous envffonmental chem^ry of metals, (ed. Rubin A.J.) Ann
Arbor Sci, Pub. Inc.
Wayne, P. R. 1998. Precipitation/Dissolution reaction in soils. In: Soil physical chemistry
(ed. Sparks D. L).
White, M.C., and R. L. Chaney. 1980. Zinc, cadmium and manganese uptake by soybean
from two zmc and cadmiiun amended costal plain soils. Soil Sci.Soc.Ani.J.44:308313.
WiOcens, B.J.,and J. P.G. Loch.l992.Retention processes of cadmium and Zinc relevant m
acid sandy soil affected by difiiise pollution. Water-Rock mteraction, Kharaka &
Maest (e^). Balkema,Rotterdam.ISBN9054100753. pp.453-456.
164
Yuan, G., and L. M. Lavkulkh. 1997. Sorption befaavrar of copper, zmc, and cadmium in
response to snmilated changes m soil propertKS. Commun. Soil ScL Plant AnaL
28:571-587.
Yost, K J. 1983. Source-specific exposure mechanisms for envvonmental cadmium. In:
Wilson D, Voipe RA, eds. Cadmium 83: Edited Proceedings— Fourth International
Cadmium Con^nce—Munich. New York, NY: Cadmium Council.
Zachara, J. M., S.C. Smith, C.T. Resch and CX. Cowan. 1992a. Cadmium sorption to soil
separates contaming layer silicates and iron and alummum oxides. Soil Sci. Soc. Am.
J. 56:1074-1084.
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