AN INVESTIGATION OF CHROMIUM AND NICKEL UPTAKE IN TOMATO PLANTS

AN INVESTIGATION OF CHROMIUM AND NICKEL UPTAKE IN TOMATO PLANTS
AN INVESTIGATION OF CHROMIUM AND NICKEL UPTAKE IN TOMATO PLANTS
IRRIGATED WITH TREATED WASTE WATER AT THE GLEN VALLEY FARM,
GABORONE, BOTSWANA
By
ADETOGUN ADEYEMO ADEKANMI
A dissertation submitted in partial fulfillment of the Requirements for the degree
MASTER OF SCIENCE
In the
FACULTY OF ENGINEERING, BUILT ENVIRONMENT AND INFORMATION
TECHNOLOGY
UNIVERSITY OF PRETORIA
December 2010
© University of Pretoria
DECLARATION
I, the undersigned, hereby declared that the work contained in this dissertation is my own
original work except where due reference is made and has not been and will not be submitted
for the award of any degree or diploma to any other institution of higher learning.
Author’s signature…………………………………………........
Date……………………………………………………………
ii
An Investigation of chromium and nickel uptake in Tomato plants when using treated
Waste water for Irrigation at the Glen Valley farm of Gaborone, Botswana
By
ADETOGUN ADEYEMO ADEKANMI (s2417622–3)
Supervisor:
Professor J.J. Schoeman
Department:
Chemical Engineering, Water Utilisation Division
Degree:
M.Sc. Applied Science
iii
DEDICATION
This work is dedicated to the glory of God who is the “Author and the finisher of my faith”.
Without God the journey would have remained but a fleeting illusion. I give God all the Glory,
thank you Jehovah.
iv
SUMMARY
The use of treated waste water for irrigation of vegetable crops is on the increase in Botswana
especially in the Glen Valley farms, a peri-urban settlement of Gaborone city. However, the
effects of this practice on heavy metals uptake by vegetable crops are uninvestigated.
Chromium and nickel have been reported to be accumulating in Gaborone crop soils and
cultivating vegetables in these soils with treated waste water could potentially lead to an
increased bio-availability of the heavy metals in the vegetable crops.
The main aim of this study was therefore to compare the uptake of chromium and nickel in
tomato plants, a vegetable grown in sludge amended Glen Valley soils, to those grown in sludge
absent Glen Valley soils using treated waste water at different pH values and tap water for
irrigation. The high water uptake and high water consumption rate of tomato plants made it
suitable for this study. Twenty five pots each containing 2.5 kg sludge amended Glen Valley
soils and 5 pots each containing 2.5 kg sludge absent soils were utilized. Fresh treated waste
water in a 50 L plastic container on a need by need basis was used. For the control experiments
5 pots each containing 2.5 kg standard commercial soils and fresh tap water were used. The
potted tomato plants were cultivated from early May to middle of October 2009. One leaf and
one fruit from each tomato plant was harvested and tested in this study.
The highest uptakes of chromium (0.819 mg/L) and nickel (0.327 mg/L) were experienced in
the leaves where the tomato plant were cultivated in standard commercial soil and irrigated with
tap water at pH 7.0. The least uptake of chromium (0.052 mg/L) and that of nickel (-0.030
mg/L) was found in the fruits, where the tomatoes were grown in sludge amended Glen Valley
soil and irrigated with normal Glen Valley treated waste water at pH 8.5. Increasing the pH of
the treated waste water from 5.0 to 6.0 caused increased bio-accumulation of chromium and
v
nickel in the leaves and the fruits of the tomato plants. Normal treated waste water (pH 8.5) and
treated waste water at pH 9.0, however, reduced the chromium and the nickel uptake by the
tomato plants. Treated waste water at pH 10.0 bio-accumulate more chromium and more nickel
in the leaves and fruits of tomato plants. The pH variation experiments suggested that the fruit
tissues accumulated more chromium and the leaf tissues accumulated more nickel. The mean
chromium uptake in the tomato plants exceeded the Food and Agriculture Organization
permissible limits but the Botswana Bureau of Standards effluent limit was not exceeded. The
mean nickel concentrations were below the threshold limits for both local and international
standards. Statistical analysis showed no significant difference between the mean chromium
and the mean nickel concentration in the leaves and the fruits of the tomatoes at the 5%
significant level. It can be concluded from this study that cultivating tomatoes with sludge
amended Glen Valley soil combined with normal treated waste water at pH 8.5 could reduce the
uptake of chromium and nickel uptake in tomato plants. However, an increase in the uptake of
chromium and nickel in the leaves and fruits of the tomato plants could be triggered at slightly
low pH (pH 5.0 and pH 6.0) and high pH (pH 10.0) of the treated waste water.
It is recommended that the current practices of using treated waste water combined with sludge
amended Glen Valley soil to cultivate tomatoes at the Glen Valley farm is good practice and
should be continued. Nonetheless, further studies need to be carried out at the farm to establish
possible phytotoxicity effects of these heavy metals on tomatoes when using treated waste
water combined with sludge amended and sludge absent soils.
Keywords: Treated waste water, Tap water, Tomato plants, Irrigation, Sludge absent soil,
Sludge amended soil, Chromium uptake, Nickel uptake, pH effect.
vi
ACKNOWLEDGEMENTS
I gratefully acknowledge the understanding, patience and support of my supervisor Professor
J.J. Schoeman during the entire journey of this project. Babatunde Akande, Sanusi Olarenwaju
and Kolawole Omomowo are friends who provided support and encouragement. Mr. E.
Ghodrati, a man whose kind gestures helped in pursuant of this course.
I am also indebted to the director of the Department of Waste Management and Pollution
Control Gaborone, Botswana, Mr. Jimi Opelo who used his good office to help facilitate the
analysis of the samples. Robert Wantle, a technical staff at the above mentioned department for
his help in processing the samples. Ms. Saniso Karingwa a laboratory officer at the Department
of Water Affairs for providing information on the equipments used for analyzing the samples.
Mr. George the manager at the research site (oldest farm) of Glen Valley Farms for granting me
access to the farm to collect soil and treated waste water samples.
Lastly, I like to thank my sister, Folasade, my brother Adeboye and mother Mary Adunni for
their love, support and encouragement in supporting this project.
vii
TABLE OF CONTENTS
DECLARATION……………………………………………………………………………....ii
DEDICATION………………………………………………………………………………...iv
SUMMARY………………………………………………………………………………........v
ACKNOWLEDGEMENTS ………………………………………………………………….vii
TABLE OF CONTENTS ……………………………………………………………………viii
LIST OF TABLES…………………………………………………………………………...xiii
LIST OF FIGURES…………………………………………………………………………..xvi
LIST OF ABBREVIATIONS ……………………………………………………………...xviii
CHAPTER 1: RESEARCH PROBLEM, AIM AND OBJECTIVES ....................................1
1.1
INTRODUCTION .........................................................................................................1
1.2
STATEMENT OF THE PROBLEM .............................................................................2
1.3
AIM AND OBJECTIVES OF THE STUDY ................................................................4
1.4
RESEARCH QUESTIONS ............................................................................................5
1.5
SIGNIFICANCE OF THE STUDY ..............................................................................6
1.6
OUTLINE OF STUDY ..................................................................................................6
CHAPTER 2: STUDY AREA …………………………………………………………………7
2.1
LOCATION ...................................................................................................................7
2.2
PHYSICAL ENVIRONMENT ......................................................................................8
2.3
HISTORY OF GLEN VALLEY ....................................................................................9
2.3.1
OVERVIEW OF THE SAMPLING AREA .................................................................10
CHAPTER 3: LITERATURE REVIEW..............................................................................12
3.1
BACKGROUND ON MUNICIPAL WASTE WATER .............................................12
viii
3.2
CONVENTIONAL WASTE WATER TREATMENT .............................................13
3.2.1
PRELIMINARY TREATMENT ..........................................................................................15
3.2.2
PRIMARY TREATMENT .................................................................................................15
3.2.3
SECONDARY TREATMENT ............................................................................................16
3.3
WASTE WATER STANDARDS AND AGRICULTURAL WATER QUALITY ..17
3.3.1
QUALITY OF IRRIGATION WATER USED FOR AGRICULTURAL PURPOSES .......................17
3.3.2
PARAMETERS USED IN THE EVALUATION OF AGRICULTURAL WATER QUALITY ............20
3.3.3
PARAMETERS OF HEALTH SIGNIFICANCE ……………………………………………..22
3.3.4
GUIDELINES
FOR
INTERPRETATION
OF
WATER
QUALITY
FOR
IRRIGATION
...................................................................................................................................................22
3.3.5
TRACE ELEMENTS AND HEAVY METALS ......................................................................24
3.4
HEAVY METAL REMOVAL METHODOLOGIES ...............................................25
3.5
EFFECTS OF IRRIGATED WASTE WATER USE ON PLANTS .........................25
3.5.1
THE
PH FACTOR ........................................................................................................25
3.5.1.1
Factors affecting pH ..................................................................................................26
3.5.1.2
Importance of pH ......................................................................................................27
3.5.2
EFFECTS
ON
PLANTS
DUE
TO
INCREASED
ACIDITY
OR
BASICITY
...................................................................................................................................................27
3.5.3
EFFECTS
ON PLANTS DUE TO INCREASED CONCENTRATIONS OF TRACE ELEMENTS
...............………………………………………………………………………………………28
3.6
BACKGROUND INFORMATION ON TEST CROP (TOMATO) .........................30
3.7
IRRIGATION FOR TOMATOES .............................................................................31
ix
3.8
TOMATO IRRIGATION REQUIREMENTS (IR) ...................................................31
3.9
NATURE OF HEAVY METALS OF CONCERN (CHROMIUM AND NICKEL) ....
………………………………………………………………………………………………...32
3.9.1
CHROMIUM ..................................................................................................................32
3.9.2
CHROMIUM IN WATER AND WASTE WATER ..................................................................33
3.9.3
CHROMIUM IN SOIL ......................................................................................................33
3.9.4
CHROMIUM CONCENTRATION IN PLANTS (CASE STUDIES) ............................................34
3.9.5
NICKEL......................................................................................................................36
3.9.6
NICKEL IN WATER AND WASTE WATER ........................................................................36
3.9.7
NICKEL IN SOIL ............................................................................................................37
3.9.8
NICKEL CONCENTRATION IN PLANTS (CASE STUDIES) ...................................38
3.10
WASTE WATER USE CASE STUDIES .................................................................40
3.10.1
WASTE WATER RE-USE FOR AGRICULTURAL IRRIGATION: CASE STUDY IN
LEÓN-GUANAJUATO, CENTRAL MEXICO. ..................................................................40
3.10.2
WASTE WATER USE CASE STUDIES IN CALIFORNIA ....................................................41
3.10.3
CURRENT AND FUTURE USE OF WASTE WATER IN TUNISIA .........................................42
3.10.4
WASTE
WATER
USE
CASE
STUDY
IN
BOTSWANA
...................................................................................................................................................44
3.11
SUMMARY OF FINDINGS AND PROBLEM TO BE ADDRESSED ..................47
CHAPTER 4: EXPERIMENTAL METHODS AND ANALYTICAL TECHNIQUES .......48
4.1
AN OVERVIEW OF THE PROJECT DESIGN .........................................................48
4.2
THE GREENHOUSE EXPERIMENTS .....................................................................49
4.3
THE CONTROL EXPERIMENTS .............................................................................51
x
4.4
BIO-ACCUMULATION
EXPERIMENTS ............................................................52
4.5
pH VARIATION EXPERIMENTS …………………………………………………53
4.6
SAMPLE COLLECTION AND PREPARATION .....................................................54
4.7
OPEN DIGESTION TECHNIQUE FOR THE TOMATO LEAVES AND FRUITS
...................................................................................................................................................55
4.8
ATOMIC ABSORPTION SPECTROMETRY ...........................................................55
CHAPTER 5: RESULTS AND DISCUSSION ...................................................................57
5.1
OVERVIEW ................................................................................................................57
5.2
CHROMIUM BIO-ACCUMULATION (CONTROL) ...............................................57
5.2
CHROMIUM UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER pH 5.0 ……………………………………………………………………...61
5.3
CHROMIUM UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER pH 6.0 ……………………………………………………………………...64
5.4
CHROMIUM UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER pH 9.0 ……………………………………………………………………………….66
5.5
CHROMIUM UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER pH 10.0 ……………………………………………………………………...69
5.6
SUMMARY OF CHROMIUM UPTAKE IN THE TOMATO PLANTS ....................73
5.7
NICKEL BIO-ACCUMULATION (CONTROL) .........................................................76
5.8
NICKEL UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE WATER
pH 5.0 …………………………………………………………………………...…….79
xi
5.9
NICKEL UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE WATER
pH 6.0 ………………………………………………………………………………....82
5.10
NICKEL UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE WATER
pH 9.0.……………….……………………………………………………………….85
5.11
NICKEL UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER pH 10.0 …………………………………………………………………….88
5.12
SUMMARY OF NICKEL UPTAKE IN THE TOMATO PLANTS ..........................91
CHAPTER 6: SUMMARY OF MAJOR FINDINGS ..........................................................94
6.1
SUMMARY OF FINDINGS .......................................................................................94
6.1.1
CHROMIUM ..................................................................................................................94
6.1.2
NICKEL ........................................................................................................................95
6.2
CONCLUSIONS ........................................................................................................96
6.3
RECOMMENDATIONS ………………………………………………………………...….97
REFERENCES .........................................................................................................................99
APPENDICES ........................................................................................................................112
xii
LIST OF TABLES
Table 3.1
Composition of municipal waste water .............................................................12
Table 3.2
Botswana Bureau of Standards for waste water effluent quality .......................19
Table 3.3
Heavy metal compositions in Gaborone industrial effluent ..............................20
Table 3.4
Some parameters used in the evaluation of agricultural water quality ..............21
Table 3.5
Guidelines for interpretation of water quality for irrigation ..............................23
Table 3.6
Compositions of secondary treated municipal waste water effluents and
irrigation water ..................................................................................................24
Table 3.7
Threshold levels of some trace elements for crop production ...........................29
Table 3.8
Chromium uptake in plants (Case studies) ........................................................35
Table 3.9
Nickel Uptake in plants (Case studies) ..............................................................39
Table 3.10
Types of crops irrigated with reclaimed water in California.............................41
Table 3.11
Average characteristics of treated waste water (TWW) and well waters (WW)
used for irrigation (in mg/L) in La Soukra compared to FAO recommended
maximum concentrations. .................................................................................44
Table 3.12
Mean ±SEM mineral concentration (mg/L) in the soils of the ryegrass and
Lucerne fields .................................................................................................46
Table 4.1
Some characteristics of Glen Valley treated waste water (GVTWW) and
ordinary tap water (OTW) used for experimental irrigation compared to FAO
recommended maximum concentrations ..........................................................51
Table 4.2
The Control experiments ...................................................................................52
Table 4.3
Bio-accumulation treatments .............................................................................52
Table 4.4
pH variation experiments ...................................................................................54
xiii
Table 5.1
Chromium uptake in the leaves and the fruits of the tomato plants: Tap water (at
pH 7.0) irrigation with standard commercial soil and sludge absent Glen Valley
soil. .....................................................................................................................57
Table 5.2
t-test for chromium to determine any significant differences in chromium
concentrations in the tomato leaves and fruits. ................................................60
Table 5.3
Chromium uptake in the leaves and the fruits of the tomato plants: Treated waste
water at pH 5.0 (sludge amended soils) compared with normal treated waste
water at pH 8.5 (sludge amended soils) and with tap water at pH 7.0 (sludge
absent soils). .......................................................................................................61
Table 5.4
Chromium uptake in the leaves and the fruits of the tomato plants: Treated waste
water at pH 6.0 (sludge amended soils) compared with normal treated waste
water at pH 8.5 (sludge amended soils) and with tap water at pH 7.0 (sludge
absent soils). .......................................................................................................64
Table 5.5
Chromium uptake in the leaves and the fruits of the tomato plants: Treated waste
water at pH 9.0 (sludge amended soils) compared with normal treated waste
water at pH 8.5 (sludge amended soils) and with tap water at pH 7.0 (sludge
absent soils). .......................................................................................................67
Table 5.6
Chromium uptake in the leaves and the fruits of the tomato plants: Treated waste
water at pH 10.0 (sludge amended soils) compared with normal treated waste
water at pH 8.5 (sludge amended soils) and with tap water at pH 7.0 (sludge
absent soils). .......................................................................................................70
Table 5.7
Nickel uptake in the leaves and the fruits of tomato plants: Tap water (at pH 7.0)
irrigation with standard commercial soil and sludge absent Glen Valley soil. ..76
xiv
Table 5.8
t-test for nickel to determine any significant differences in tomato leaves and
fruits. .................................................................................................................79
Table 5.9
Nickel uptake in the leaves and the fruits of the tomato plants: Treated waste
water at pH 5.0 (sludge amended soil) compared with normal treated waste water
at pH 8.5 (sludge amended soil) and with tap water at pH 7.0 (sludge absent
soils). ..................................................................................................................80
Table 5.10
Nickel uptake in the leaves and the fruits of the tomato plants: Treated waste
water at pH 6.0 (sludge amended soils) compared with normal treated waste
water at pH 8.5 (sludge amended soils) and with tap water at pH 7.0 (sludge
absent soils). .......................................................................................................83
Table 5.11
Nickel uptake in the leaves and the fruits of the tomato plants: Treated waste
water at pH 9.0 (sludge amended soils) compared with normal treated waste
water at pH 8.5 (sludge amended soils) and with tap water at pH 7.0 (sludge
absent soils). .......................................................................................................86
Table 5.12
Nickel uptake in the leaves and the fruits of the tomato plants: Treated waste
water at pH 10.0 (sludge amended soils) compared with normal treated waste
water at pH 8.5 (sludge amended soils) and with tap water at pH 7.0 (sludge
absent soils). .......................................................................................................89
xv
LIST OF FIGURES
Figure 2.1
Location map of study area: Glen Valley farms ...............................................11
Figure 3.1
Generalized flow diagram for municipal waste water treatment......................14
Figure 4.1
Frontal view of the greenhouse used for the tomato production ......................49
Figure 5.1
Average concentration of chromium in the tomato plants (control). Bars
represent SEM (n=5) .......................................................................................58
Figure 5.2
Average concentration of chromium in the tomato plants for the different
treatments (pH 5.0). Bars represent SEM (n=5)..............................................62
Figure 5.3
Average concentration of chromium in the tomato plants for the different
treatments (pH 6.0). Bars represent SEM (n=5)..............................................65
Figure 5.4
Average concentration of chromium in the tomato plants for the different
treatments (pH 9.0). Bars represent SEM (n=5)..............................................68
Figure 5.5
Average concentration of chromium in the tomato plants for the different
treatments (pH 10.0). Bars represent SEM (n=5)............................................71
Figure 5.6
Average concentration of chromium in the tomato plants for the different
treatments (summary). Bars represent SEM (n=5)..........................................74
Figure 5.7
Average concentration of nickel in the tomato plants for the different treatments
(control). Bars represent SEM (n=5) ................................................................77
Figure 5.8
Average concentration of nickel in the tomato plants for the different treatments
(pH 5.0). Bars represent SEM (n=5) .................................................................81
Figure 5.9
Average concentration of nickel in the tomato plants for the different treatments
(pH 6.0). Bars represent SEM (n=5) .................................................................84
xvi
Figure 5.10
Average concentration of nickel in the tomato plants for the different
treatments (pH 9.0). Bars represent SEM (n=5) ............................................87
Figure 5.11
Average concentration of nickel in the tomato plants for the different
treatments (pH 10.0). Bars represent SEM (n=5) ..........................................90
Figure 5.12
Average concentration of nickel in the tomato plants for the different
treatments (summary). Bars represent SEM (n=5) ........................................92
xvii
LIST OF ABBREVIATIONS
AF
Areni-Haplic- Lixisol/Ferralic-Arenosol Soils
BOS
Botswana Bureau of Standards
BMC
Botswana Meat Commission
DWA
Department of Water Affairs
DWMPC
Department of Waste Management and Pollution Control
FAAS
Flame Atomic Absorption Spectroscopy
FAO
Food and Agriculture Organization
GVTWW
Glen Valley treated waste water
n
No of samples
OTW
Ordinary tap water
SEM
Standard Error of the Mean
UNDP
United Nations Development Program
USEPA
United States Environmental Protection Agency
VL
Vertic-Cambisol/Vertic luvisol Soils
WHO
World Health Organization
xviii
CHAPTER 1
RESEARCH PROBLEM, AIM AND OBJECTIVES
1.1
INTRODUCTION
In Botswana, rainfall is generally low and highly variable and evaporation exceeds the
rainfall. Botswana thus experiences a hydro-climatological water scarcity, severely
restricting its agricultural potential (Arntzen, 2006). Application of sewage waste water on
agricultural land, otherwise known as irrigation, became an alternative water supply to crops
as well as an alternative waste disposal method. This practice brought other positive benefits
such as enhancing soil nutrients and the organic carbon content of the soils (Gupta, Narwal
and Antil., 1998), promoting good crop yields and replacing chemical fertilizers. However,
with increasing industrial effluent discharge, the heavy metal content and other pathogens in
waste water are posing a threat to human health (Cooper, 1991; Mendoza et al., 1996).
Sewage waste water should be treated for safe agricultural production (Ghulam and Al-Saati,
1999). For this reason the Gaborone sewage treatment plant was commissioned in 1997 to
process about 40 000 m3/day municipal sewage effluents in and around the city of Gaborone
in Botswana. Despite the waste water treatment there is a need to assess its quality for crop
production application. Some of the essential variables in the waste water include pH,
salinity, major metals, anions, and heavy metals (trace metals). The pH though, has no direct
effect on plant growth; however, it affects the form and availability of metal nutrients to
plants (Quaghebeur et al., 2005; Kukier et al., 2004).
Heavy metals are normally present at trace amounts in water samples and they occur naturally
in the earth crust. The determination of heavy metal concentrations is important because they
are essential nutrients to plants, but can be toxic if they accumulate to high concentration
levels (Conolly and Guerinot, 2002).
Agricultural practice in the Glen Valley farms of Gaborone, Botswana make use of treated
waste water from the nearby Glen Valley waste water treatment plant. The uses of treated
waste water are restricted to specific crops not used for human consumption, such as
seedlings, and grass. However, current farm practices showed that these restrictions are not
being strictly adhered to in the Glen Valley agricultural farms of Gaborone city.
As a consequence of the foregoing, this research would shed light on the behavior and nature
of chromium and nickel uptake by the tomato plants and act as an early warning signs for the
increasing levels of chromium and nickel in the food chain as a result of waste water use and
sludge application to soil. Also the knowledge gained from this study can be useful by
farmers who want to commence the practices of waste water use and sludge application to
soils.
1.2
STATEMENT OF THE PROBLEM
Waste water quality is a concern as it contains a wide spectrum of contaminants that may be
assimilated in plants depending on the level of treatment. Therefore, the use of treated waste
water requires that its quality be assured in terms of its possible effects on soils, plants,
animals and humans (Scheltinga, 1987). The principle for evaluating the quality of treated
waste water deals with the total concentration and composition of soluble salts and trace
metals in water. Soluble salts and trace metals commonly found in waste water may have
undesirable effects on plants and may also be toxic to the plants. According to pilot tests
conducted with the Glen Valley treated waste water (Akande, 2007) it was found that the pH
of the treated waste water used for irrigation in Glen Valley was in the range of 9.52 – 10.25.
This high pH of the Glen Valley treated waste water could make a significant contribution to
2
the uptake of heavy metal contaminants in plants which could lead to the increase in
concentration levels of trace elements in vegetable crops and the food chain as a whole.
Sludge (from treated waste water) application to soils is also a viable practice in Glen Valley,
Gaborone, Botswana. Typical sludge consists of organic and inorganic materials including
plant nutrients, organic compounds, pathogens and heavy metals. Sludge composition varies
from one waste water treatment plant to the other depending on the treatment process
employed and the nature of waste water received at the plant.
Carlton-Smith (1987) investigated the use of treated waste water combined with the
application of sludge to soils and found that the combination poses a great concern when used
to cultivate agricultural crops for human consumption. Increases in metal concentrations in
the soil due to sludge application and waste water use produced significant increases in heavy
metal concentrations such as cadmium (Cd), nickel (Ni), chromium (Cr), copper (Cu) and
zinc (Zn) in the edible portion of most of the crops grown (wheat, potato, lettuce, red beet,
cabbage and ryegrass).
Zhai et al. (2003) reported the concentration levels of chromium and nickel heavy metals to
be at peak concentrations in agricultural crop soils of Gaborone, Botswana. A consequence of
this could lead to an increased bio-availability of chromium and nickel trace metals and hence
an increased assimilation of these contaminants into the food chain. The magnitude of the
bio-availability of the heavy metals and phytotoxicity in plants depends on the
interrelationship of a number of factors such as the rate and frequency of application of
treated waste water and sludge, soil characteristics and the plant species. However, additional
soil and plant factors further modify the uptake and the concentration of heavy metals in crops
(Food and Agriculture Organization, 1992).
3
Presently, the agricultural crops produced in Glen Valley irrigational farms include lettuce
(Appendix A), spinach, cabbage, green pepper, and tomatoes to mention a few. There is a
serious lack of in-depth of information regarding the uptake of heavy metals by vegetables
cultivated in the crop soils and irrigated with treated waste water in the study area.
Knowledge is also lacking concerning the problems that can be caused in the food chain by
heavy metals. This research will focus on tomato plants as the test plant due to its high water
uptake; its fruits alone contain 92.5 – 95.0 % water (Davies and Hobson, 1981) and because it
is a regular feature in most household diets in the country.
1.3
AIM AND OBJECTIVES OF THE STUDY
The aim of this study was to compare the uptake of chromium and nickel between tomato
plants grown in sludge-amended Glen Valley soil and those grown in sludge-absent Glen
Valley soil using treated waste water and tap water. The specific objectives were to:
 Determine chromium uptake by the leaves and fruits of the tomato plants
 Determine nickel uptake by the leaves and fruits of the tomato plants
 Determine the effect of the pH of the treated waste water on chromium uptake in the
leaves and fruits of tomato plants

Determine the effect of the pH of the treated waste water on nickel uptake in the
leaves and fruits of tomato plants
 Compare the chromium uptake by the leaves and fruits of the tomato plants grown in
sludge absent Glen Valley soil to those grown in standard commercial soil using tap
water in both cases.
4

Compare the chromium uptake by the leaves and fruits of the tomato plants grown
in sludge absent Glen Valley soil (using tap water) to those grown in sludge amended
Glen Valley soil (using normal treated waste water at pH 8.5 and treated waste water
at pH 5.0 ; pH 6.0 ; pH 9.0 and pH 10.0)
 Compare nickel uptake by the tomato leaves and fruits grown in sludge absent Glen
Valley soil to those grown in standard commercial soil using tap water in both cases.

Compare nickel uptake by leaves and fruits of tomato plants grown in sludge absent
Glen Valley soil (using tap water) to those grown in sludge amended Glen Valley soil
(using normal treated waste water at pH 8.5 and treated waste water at pH 5.0 ; pH 6.0
; pH 9.0 and pH 10.0)
1.4
RESEARCH QUESTIONS
1. Will there be a significant uptake of chromium and nickel contaminants in response to
treatments?
2. Will there be any observable differences in chromium and nickel concentrations in the
leaf and fruit tissues of tomato plants as a consequence of variations in the pH values
of the Glen Valley treated waste water?
3. At which pH value will tomato plants assimilate the highest concentrations of
chromium and nickel heavy metals?
4. Will there be a statistically significant difference in concentration of chromium and
nickel uptake in tomato leaves and fruits cultivated using sludge amended Glen
Valley soils with treated waste water as compared with using sludge absent Glen
Valley soils with tap water?
5
1.5
SIGNIFICANCE OF THE STUDY
 The results of this study could provide valuable insight into the behavior and
nature of chromium and nickel uptake by tomato plants to the food chain and
hence help farmers in developing techniques or methods that they could use for
reducing the assimilation of these heavy metals in tomato plants
 The results of this study could act as an early warning signs for the increasing
levels of chromium and nickel in the food chain as a result of treated waste water
use and sludge application to soil.
 The knowledge acquired from this study should provide valuable information for
farmers who want to start the practices of treated waste water use and sludge
application to soils.
1.6
OUTLINE OF STUDY
 The introduction to the study is given in chapter 1 followed by a background of the
study area in chapter 2. The relevant literature to the study is reviewed in chapter 3.
Thereafter, the experimental methods and analytical techniques are detailed in chapter
4. Results and discussions are presented in chapter 5. The summary of major findings
which include the conclusions and recommendations are presented in chapter 6
followed by the references and the appendices are inserted at the end.
6
CHAPTER 2
STUDY AREA
2.1
LOCATION
Botswana is located in Southern Africa and land locked by South Africa, Namibia, Zimbabwe
and Zambia. Gaborone the capital city of Botswana is situated in the southern part of the
country between latitudes 240 45 South and longitudes 250 55 East at about 1 000 meters
above sea level.
Glen Valley is a peri-urban area situated in North-Eastern Gaborone. The surrounding areas
are primarily residential, recreational or open space. The area is relatively flat and prone to
flooding due to the proximity of river channels. The Glen Valley farms which is the study
area (consist of 234 hectares) had been identified as ideal for agriculture. The soils are
suitable and treated waste water from a nearby treatment plant can be utilized (Government of
Botswana, 1998).
Gaborone has a semi-arid climate with a mean annual rainfall of between 250 mm and 450
mm (khupe, 1996). Almost all rain occurs during the months of October to April and its
incidence is highly variable in both time and space. The use of treated waste water is
becoming a viable option in the Glen Valley farms. As Botswana’s population grows, water
usage also grows thereby generating high volumes of effluent water discarded as waste water
throughout the country. Such high volumes of water in a country with persistent drought and
unreliable rainfall can be of great agronomic and economic importance. Irrigation with
treated waste water can increase the available water supply or release water for alternate uses
(Food and Agriculture Organization, 1992).
7
2.2
PHYSICAL ENVIRONMENT
The Glen Valley farms soils are predominantly sandy loam to sand occurring in an
alluvial-cum-colluvial landscape, with patches of vertisolic clayey materials alternating with
areas of more sandy and, even, gravelly deposits (Dikinya and Areola, 2010). The soils are
all texturally very similar irrespective of taxonomic classification and when mapped on a
scale 1:20 000, are classified as luvisols, lixisols, cambisols, calcisols, regosols and arenosols
(Food and Agriculture Organization, 1988).
The Glen Valley farms are situated within the eastern hardveld vegetation province. The
existing soil systems support crops that are cultivated under waste water irrigation and these
crops include tomatoes, spinach, okra, maize, cabbage, olive, Lucerne, butternuts, and green
pepper (Dikinya and Areola, 2010). Vegetable farming in the study area is exclusively
dependent on treated sewage water from the Glen Valley treated waste water plant. On the
ground survey showed that drip irrigation is the most common method used by farmers,
furrow irrigation and drag lines were observed in a handful of the farms. The study site
combines the use of drip irrigation and drag lines.
In terms of weather patterns, Botswana’s
annual climate ranges from months of dry
temperate weather during winter to days or weeks of sub-tropical humidity interspersed with
drier hot weather during summer. In summer (October to March) temperatures rise to above
340C (93.20F) in the extreme north and south-west. In winter (which lasts from April to
September) there is frequent frost at night and temperatures may fall below 20C (35.60F)
during the day, but skies are usually cloudless and sunny. Due to the clear skies and low
relative humidity, there is maximum insulation during the day and rapid energy loss at night.
This has resulted in a wide diurnal change in temperature with hotter days and relatively
8
colder nights. Evaporation rates are consequently very high ranging from 1.8 m to 2.0 m
annually for surface water (Khupe, 1996). Summer is heralded by a windy season, carrying
dust from the Kalahari, from about late August to early October (Parsons, 1999). Annual
rainfall, brought by winds from the Indian Ocean, averages 460 mm (18 in.), including a
range from 640 mm (25 in.) in the extreme north-east to less than 130 mm (5 in.) in the
extreme south-west. The rains are almost entirely limited to summer downpours between
December and April, which also mark the season for ploughing and planting. Cyclical
droughts, lasting up to five or six years in every two decades, can limit or eliminate harvests
and reduce livestock to starvation (Parsons, 1999).
2.3
HISTORY OF GLEN VALLEY
The intent of the Glen Valley horticultural plan was to create a well-designed irrigation
project which would cater to small scale commercial agricultural plots for horticultural
purposes with some other activities like flower gardening and perhaps poultry and small
livestock breeding. The idea was to allocate portions of land to agricultural investors who
were conversant with the irrigation systems and who would utilize the land to its fullest
potential, in order to produce and provide fresh agricultural produce for the city of Gaborone
and its surrounding areas (Mbiba, 1995). The plan was approved in September 1998 on
condition that an environmental impact assessment was clearly stipulated in the lease contract
and the planning authority stated that close environmental monitoring of the project at the
implementation stage was a prerequisite (Government of Botswana, 1998).
9
2.3.1
Overview of the sampling area
The farm area at the Glen Valley is about 10 km northeast of Gaborone city which is very
close in proximity to the Notwane River where about 234 hectares of cropland are being
cultivated with secondary treated waste water. The farms are located between the Botswana
Defence Force camp and the Gaborone sewage ponds between Latitudes 24°35’23.56’’S and
24°37’01.14’’S and between Longitudes 25°58’43.29’’E and 25°5816.74’’E. There are 47
different farms, varying in size from 1 to 10 hectares being managed by private farmers
raising a wide variety of arable crops (Dikinya and Areola, 2010). The size of the Glen Valley
farm plots varies from 1.5 to 4 hectares. All farm plots are laid sequentially and so are easily
serviced. The requirement from the Ministry of Agriculture is to reduce the buffer zone from
the Notwane River and its tributaries, in order to utilize the most fertile soils along the
riverbanks. As the farm area is a floodplain, investors were warned of the dangers of possible
loss of investments and properties. Those areas unsuitable for horticultural purposes (e.g.
with soil types susceptible to salinization) have been planned for other agricultural activities,
such as the raising of small livestock, poultry, etc. As a result, 63 plots have been designated
as good for horticultural purposes and 27 plots for other agricultural uses. No permanent
residences are allowed in the project area apart from farm sheds and small quarters to house
farm workers (Government of Botswana, 1998). A dozen or so farms have commenced
operations. This study was proposed to be conducted on the oldest active farm plot in the Glen
Valley area and soil samples were collected from this farm plot. Standard commercial soil
samples were also obtained and used for control experiments. Figure 2.1 shows a location
map of the Glen Valley project area from which soil samples were collected for this study.
This area is situated about 5 km north-east of the Glen Valley waste water treatment plant.
10
Figure 2.1
Location map of study area: Glen Valley farms
Source: Adapted from Botswana Department of Surveys and Lands (1987)
11
CHAPTER 3
LITERATURE REVIEW
3.1
BACKGROUND ON MUNICIPAL WASTE WATER
Municipal waste water consists of about 99.9% water, suspended organics and dissolved
inorganic solids. Some of the organic substances present in sewage are carbohydrates, fats,
soaps, proteins, and other natural synthetic organic chemicals from the process industries
(Food and Agriculture Organization, 1992). Table 3.1 shows the compositions of the major
constituents of strong, medium and weak municipal waste waters (Abdel-Ghaffar et. al,
1988). Municipal waste water includes domestic (or sanitary) waste water, industrial waste
water, infiltration and inflow into sewer lines and storm water runoff (Liu and Liptak, 1997).
Table 3.1. Composition of municipal waste water
Contaminant
1
Weak/(mg/L)
Medium/(mg/L)
Strong/(mg/L)
Solids, total (TS)
350
700
1200
Dissolved, total (TDS)
250
500
850
Suspended Solids (SS)
100
200
350
Nitrogen, total as (N)
20
40
85
Phosphorous, total (P)
6
10
20
Chlorides1
30
50
100
Alkalinity as (CaCO3)
50
100
200
Grease
50
100
150
BOD5
((CC(CaCo(CaCo3)
100
220
400
The amount of TDS and chloride should be increased by the concentrations of these
constituents in the carriage water.
Source: (Abdel-Ghaffar et. al, 1988)
12
A number of inorganic substances ranging from domestic to industrial up to and including
potentially toxic elements such as copper, zinc, arsenic, lead, chromium, mercury, cadmium,
etc., are also present in municipal waste water. Even if these potentially toxic elements are not
at concentrations that could endanger humans, they might as well be at phytotoxic levels thus
restricting their agricultural usage (Abdel-Ghaffar et al., 1988).
3.2
CONVENTIONAL WASTE WATER TREATMENT
Conventional waste water treatment follows a combination of physical, chemical, and
biological processes and operations to remove solids, organic matter and, sometimes,
nutrients from waste water. Different degrees of treatment, in order of increasing treatment
level, are preliminary, primary, secondary, and tertiary and/or advanced waste water
treatment. In some countries, disinfection to remove pathogens sometimes follows the last
treatment step (Food and Agriculture Organization, 1992). A generalized waste water
treatment flow diagram is shown in Figure 3.1
13
Preliminary
Primary
Advanced
Secondary
Effluent
Effluent
Low Rate Processes
Stabilization ponds
Disinfection
Aerated lagoons
Disinfection
Disinfection
Nitrogen Removal
Nitrification
-
Screening
High Rate Processes
Comminution
Activated sludge
Selective ion exchange
Trickling filters
Break point chlorination
Rotating bio-contactors
Gas stripping
Grit Removal
Sedimentation
Denitrification
Overland flow
Phosphorus Removal
Secondary
Chemical Precipitation
Sedimentation
Suspended Solids Removal
Chemical Coagulation
Filtration
Sludge Processing
Biological
Non biological
Thickening
Thickening
Digestion
Conditioning
Dewatering
Dewatering
Filter
Filter
Centrifuge
Centrifuge
Drying beds
Incineration
Organics and Metals Removal
Carbon adsorption
Dissolved Solids Removal
Reverse osmosis
Electro dialysis
Distillation
Disposal
Figure 3.1
Generalized flow diagram for municipal waste water treatment
14
3.2.1
Preliminary treatment
The preliminary treatment aims to remove coarse solids and other large materials often found
in raw waste water. Removal of these materials is necessary to enhance the operation and
maintenance of subsequent treatment units. Preliminary treatment operations typically
include coarse screening, grit removal and, in some cases, combination of large objects
(Aganga et al., 2005).
In grit chambers, the velocity of the water through the chamber is maintained sufficiently
high, or air is used, so as to prevent the settling of most organic solids. Grit removal is not
included as a preliminary treatment step in most small waste water treatment plants.
Comminutors are sometimes adopted to supplement coarse screening and serve to reduce the
size of large particles so that they will be removed in the form of sludge in subsequent
treatment processes. Flow measurement devices, often standing-wave plumes, are always
included at the preliminary treatment stage (Food and Agriculture Organization, 1992).
3.2.2
Primary treatment
The objective of primary treatment is the removal of settleable organic and inorganic solids
by sedimentation, and the removal of materials that will float (scum) by skimming.
Approximately 25% to 50% of the incoming biochemical oxygen demand (BOD5), 50% to
70% of the total suspended solids (SS), and 65% of the oil and grease are removed during
primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated
with solids are also removed during primary sedimentation but colloidal and dissolved
constituents are not affected. The effluent from primary sedimentation units is referred to as
primary effluent (Aganga et. al., 2005).
15
The minimum level of pre-application treatment required for waste water irrigation in most
industrialized countries is the primary treatment (Food and Agriculture Organization, 1992).
This may be sufficient enough if the waste water is used for irrigating crops that are not
consumed by humans or to irrigate orchards, vineyards and some processed food crops.
However, as a precautionary measure against nuisance conditions in storage or
flow-equalizing reservoirs, some form of secondary treatment is a normal requirement in
these countries, even in the case of non-food crop irrigation. It may be possible to use at least
a portion of primary effluent for irrigation if off-line storage is provided (Aganga et al., 2005).
3.2.3
Secondary treatment
In the secondary treatment, there is further treatment of the effluent from primary treatment to
remove the residual organics and suspended solids. In most cases, secondary treatment
follows primary treatment and involves the removal of biodegradable dissolved and colloidal
organic matter using aerobic biological treatment processes. Aerobic biological treatment is
performed in the presence of oxygen by aerobic micro-organisms (principally bacteria) that
metabolize the organic matter in the waste water, thereby producing more micro-organisms
and inorganic end-products (principally CO2, NH3, and H2O). Several aerobic biological
processes are used for secondary treatment differing primarily in the manner in which oxygen
is supplied to the micro-organisms and in the rate at which organisms metabolize the organic
matter (Asano et al., 1985). High-rate biological processes are characterized by relatively
small reactor volumes and high concentrations of micro-organisms compared with low rate
processes. Consequently, the growth rate of new organisms is much greater in high-rate
systems because of the well-controlled environment. The micro-organisms must be separated
from the treated waste water by sedimentation to produce clarified secondary effluent.
16
The biological solids removed during secondary sedimentation, called secondary or
biological sludge, are normally combined with primary sludge for sludge processing.
Common high-rate processes include the activated sludge processes, trickling filters or
bio-filters, oxidation ditches, and rotating biological contactors (RBC). A combination of two
of these processes in series (e.g., bio-filter followed by activated sludge) is sometimes used to
treat municipal waste water containing a high concentration of organic material from
industrial sources (Bhatia, 2005).The Gaborone city council sewage department waste water
treatment plant is a conventional treatment plant employing processes or procedures that go
only as far as secondary treatment for its waste water intake.
Among the natural biological treatment systems available, stabilization ponds and land
treatment have been used widely around the world and a considerable record of experience
and design practice has been documented (Food and Agriculture Organization, 1992).
3.3
WASTE WATER STANDARDS AND AGRICULTURAL WATER
QUALITY
3.3.1
Quality of irrigation water used for agricultural purposes
The quality of irrigation water is of key significance in arid zones where extremes of
temperature and lower relative humidity result in high rates of evaporation, with consequent
deposition of salt which tends to accumulate in the soil profile. The physical and mechanical
properties of the soil, such as dispersion of particles, stability of aggregates, soil structure and
permeability, are very sensitive to the type of exchangeable ions present in irrigation water
(Food and Agriculture Organization, 1992).
17
One other area of agricultural irrigation of concern is the effect of total dissolved solids (TDS)
in the irrigation water on the development and growth of plants. Dissolved salts tend to
increase the osmotic potential of soil water and an increase in osmotic pressure of the soil
solution increases the amount of energy, which plants must expend to take up water from the
soil. As a consequence, respiration is increased and the growth and yield of most plants
decline progressively as the osmotic pressure increases. Although most plants respond to
salinity as a function of the total osmotic potential of soil water, some plants are susceptible to
specific ion toxicity (Akande, 2007).
Many of the ions which are harmless or even beneficial at relatively low concentrations may
become toxic to plants at higher concentrations, either through direct interference with
metabolic processes or through indirect effects on nutrients, which might be rendered
inaccessible (Morishita, 1988). As a result of the foregoing, waste water quality standards
(Table 3.2) are put in place to guide the discharge and possible reuse in agricultural areas
where they pose no threats to plants and the food chain. It is also necessary to check if
industrial effluent discharges are the sources of some selected heavy metals in the sludge
(Table 3.3) and the final effluent water discharged at the Gaborone city council waste water
treatment plant in Glen Valley.
18
Table 3.2.
Botswana Bureau of Standards for waste water effluent quality
Unit
Upper limit
Class 3
and range
potable water
TCU
50
50
O0C
35
Comment
Determinant
Colour
Acceptable
Not
Temperature
comparable
pH value at 25oC
Chemical
6.0-9.0
5-10
Acceptable
Unit
requirements-micro
determinants
mg/L
0.25
Not
Chromium VI as Cr
comparable
Chromium as Cr (total)
mg/L
0.5
50
Acceptable
Cobalt as Co
mg/L
1.00
1000
Acceptable
Copper as Cu
mg/L
1.00
1000
Acceptable
Nickel as Ni
mg/L
0.30
20
Acceptable
Note: Acceptable means waste water standards are acceptable as class 3 drinking water
Not comparable means standards are not comparable
Source: (Botswana Bureau of Standards, 2004)
19
Table 3.3.
Heavy metal compositions in Gaborone industrial effluent
Heavy Metal Compositions in Gaborone industrial effluent
Type of Industry
Ni (µgL-1)
Fe(µgL-1)
Zn(µgL-1)
Cd(µgL-1)
Pb(mgL-1)
Brewery
72.7
0.0
0.0
0.0
0.0
Paints
92.5
0.0
0.0
0.0
0.0
Pharmaceutical
85.9
0.0
0.0
0.0
0.0
Soaps
66.7
0.0
0.0
0.0
0.0
Phytography
87.5
669.5
0.0
0.0
0.0
>233.8
>1443.6
>427.9
0.0
>125.5
5.6
5.2
0.0
0.0
0.0
20.0 mgL-1
20.0 mgL-1
20.0 mgL-1
5.0 mgL-1
5.0 mgL-1
Typical
Gaborone Sludge
Typical
Gaborone
Gaborone
Secondary
Effluent
council
city
sewer
discharge
Sludge value: µgkg-1.
0.0: means no detectable analyte
guideline
Source: (Nkegbe and Koorapetse, 2005)
3.3.2
Parameters used in the evaluation of agricultural water quality
Priority agricultural water quality parameters include a number of specific properties of water
that are relevant in relation to the yield and quality of crops, maintenance of soil productivity
and protection of the environment. These parameters mostly consist of certain physical and
20
chemical characteristics of the water. Table 3.4 presents a list of some of the important
physical and chemical characteristics that are used in the evaluation of agricultural water
quality (Kandiah, 1990).
Table 3.4.
Some parameters used in the evaluation of agricultural water quality
Parameters
Symbol
Unit
Ecw
dS/m1
T
°C
Physical
Electrical conductivity
Temperature
NTU/JTU2
Colour/Turbidity
Chemical
Acidity/Basicity
Trace metals
mg/L3
Heavy metals
mg/L
Nitrate-Nitrogen
NO3-N
mg/L
Phosphate Phosphorus
PO4-P
mg/L
K
mg/L
Potassium
1
pH
dS/m = deciSiemen/metre in SI Units (equivalent to 1 mmho/cm)
2
NTU/JTU = Nephelometric Turbidity Units/Jackson Turbidity Units
3
mg/L = milligrams per litre = parts per million (ppm); also, mg/L ~ 640 x EC in dS/m
Source: (Kandiah, 1990)
21
3.3.3
Parameters of health significance
The regulation of water quality for irrigation is of international importance because trade in
agricultural products across regions is growing and products grown with contaminated water
may cause health effects at both the local and transboundary levels (Beuchat, 1998). Issues of
integration of the various measures available to attain effective health protection were
discussed and reported in a technical report by the World Health Organization, (1989). The
effluent quality guidelines for health protection (Food and Agriculture Organization, 1992)
based its standard on the view that the actual risk associated with irrigation using treated
waste water is far much lower than previously reported.
3.3.4
Guidelines for interpretation of water quality for irrigation
Water quality criteria for irrigation are by nature imprecise. The end result of quality
evaluation depends on plant, soil and climatic variables all of which can be interdependent.
However, a guideline which serves to identify potential crop production problems inherent in
the use of conventional water sources was proposed by Ayers and Westcot (Food and
Agriculture Organization, 1985). Table 3.5 shows classification of irrigation water into three
groups based on salinity, toxicity and miscellaneous effect.
22
Table 3.5.
Guidelines for interpretation of water quality for irrigation
Potential irrigation problems
Degree of restriction on use
Units
Salinity (affects crop water availability)
Ecw1
dS/m
None
Slight to moderate
Severe
< 0.7
0.7 - 3.0
> 3.0
< 450
450 – 2000
> 2000
or
TDS
mg/L
Infiltration (affects infiltration rate of water into the soil. Evaluate using ECw and SAR
together
SAR2 = 0 - 3
and ECw
=
> 0.7
0.7 - 0.2
< 0.2
=3-6
=
> 1.2
1.2 - 0.3
< 0.3
= 6 - 12
=
> 1.9
1.9 - 0.5
< 0.5
= 12 - 20
=
> 2.9
2.9 - 1.3
< 1.3
= 20 - 40
=
> 5.0
5.0 - 2.9
< 2.9
3–9
>9
Specific ion toxicity (affects sensitive crops)
Surface irrigation
SAR
<3
Sprinkler
me/L
<3
>3
Surface irrigation
me/L
<4
4 – 10
Sprinkler
m3/L
<3
>3
Chloride
irrigation (Cl)
irrigation
pH
1
> 10
Miscellaneous effects
Normal range 6.5-8.5
ECw = electrical conductivity, a measure of the water salinity, reported in deciSiemens per
meter at 25°C (dS/m). TDS = total dissolved solids, reported milligrams per litre (mg/L)
2
SAR = sodium adsorption ratio. At a given SAR, infiltration rate increases as water salinity
increases.
Source: (Food and Agriculture Organization, 1985)
23
3.3.5
Trace elements and heavy metals
Several elements exist in relatively low concentrations, usually less than a few mg/L, in
conventional irrigation waters and they are called trace elements. These elements are not
normally included in routine analysis of regular irrigation water, but attention should be paid
to them when using sewage effluents, particularly if contamination with industrial waste
water discharges is suspected. These include Aluminium (Al), Beryllium (Be), Cobalt (Co),
Fluoride (F), Iron (Fe), Lithium (Li), Manganese (Mn), Molybdenum (Mo), Selenium (Se),
Tin (Sn), Titanium (Ti), Tungsten (W) and Vanadium (V) (Food and Agriculture
Organization, 1992). Heavy metals belong to a special group of trace elements which have
been shown to create definite health hazards when taken up by plants. Under this group are
included, Nickel (Ni), Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Lead
(Pb), Mercury (Hg) and Zinc (Zn). These are called heavy metals because in their metallic
form, their densities are greater than 4 g/cm3. Table 3.6 refers to chromium and nickel
concentration levels in secondary treated municipal waste water effluents and irrigation
water.
Table 3.6.
Compositions of secondary treated municipal waste water effluents and
irrigation water
Parameter
a
Range
Secondary Effluenta
Irrigation Water Quality
Typical
Criteria b
7.0
6.5–8.4
pH
6.8–7.7
Nickel (ug/L)
5–500
10.0
200
Chromium (ug/L)
<1–100
1.0
100
Adapted from Asano et al., (1985) and Treweek (1985)
b
From Westcot and Ayers (1985) and National Academy of Sciences (1972)
24
3.4
HEAVY METAL REMOVAL METHODOLOGIES
In the past few decades several methods have been devised for the treatment and removal of
heavy metals from waste water and the degree of success varies. Most commonly used
procedures for removing metal ions from aqueous streams include reverse osmosis, solvent
extraction, lime coagulation, chemical precipitation and ion exchange (Rich and Cherry,
1987). A particular study (Patoczka et. al., 1998) of trace heavy metal removal with ferric
chloride from waste water showed that chromium and nickel concentrations of 0.10 mg/L and
0.08 mg/L, respectively, being targeted, could not be achieved by lime precipitation or ion
exchange. Upon polishing of the lime precipitation supernatant with ferric chloride at 30
mg/L dose, it removed both chromium and nickel to the 0.01 mg/L range in unfiltered
samples.
3.5
3.5.1
EFFECTS OF IRRIGATED WASTE WATER USE ON PLANTS
The pH factor
pH is a measure of how acidic or basic water is. It can be defined as the negative logarithm of
the activity of H+ ions: pH = -log [H+]
where [H+] is the concentration of H+ ions in moles per liter (a mole is a unit of measurement,
equal to 6.022 x 1023 atoms). The range goes from 0-14. The pH measures accurately the
relative amount of free hydrogen and hydroxyl ions in the water. Pure water for example is
said to be pH neutral, with a pH value close to 7.0 at 25 0C. The pH of waste water needs to
remain between 6.0 and 9.0 to protect organisms. Solutions with a pH less than 7 (at 250C) are
said to be acidic and solutions with a pH greater than 7.0 (at 250C) are said to be basic or
25
alkaline. When an acid is dissolved in water, the pH will be less than 7.0 (if at 25 0C ) and
when a base , or alkali is dissolved in water the pH will be greater than 7.0 (if at 250C ). A
solution of a strong acid, such as hydrochloric acid, at concentration 1 mol dm-3, has a pH 0.0.
A solution of a strong alkali, such as sodium hydroxide, at concentration 1 mol dm-3 has a pH
14.0. The pH can be affected by chemicals in water and waste water; hence pH is an important
indicator of water quality that is changing with chemical addition. It is reported in
"logarithmic units," like the Richter scale, which measures earthquakes. Each number
represents a 10-fold change in the acidity or basicity of the water. Water with a pH 5.0 is ten
times more acidic than water having a pH 6.0. The pH of water and waste water can be
measured with a pH meter.
3.5.1.1
1.
Factors affecting pH
The concentration of carbon dioxide in the water: Carbon dioxide (CO2) enters a water
body from a variety of sources, including the atmosphere, runoff from land, release from
bacteria in the water, and respiration by aquatic organisms. This dissolved CO2 forms a weak
acid. Natural, unpolluted rainwater can be as acidic as pH 5.6, because it absorbs CO2 as it
falls through the air. Because plants take in CO2 during the day and release it during the night,
pH levels in water can change during the day and at night.
2.
Geology and Soils of the watershed: Acidic and alkaline compounds can be released
into water from different types of rock and soil. When calcite (CaCO3) is present, carbonates
(HCO3, CO3-2) can be released, increasing the alkalinity of the water, which raises the pH.
When sulfide minerals, such as pyrite, or "fool’s gold," (FeS2) are present, water and oxygen
interact with the minerals to form sulfuric acid (H2SO4). This can significantly drop the pH of
26
the water. Drainage water from forests and marshes is often slightly acidic, due to the
presence of organic acids produced by decaying vegetation.
3.
Drainage from Mine Sites: Mining for gold, silver, and other metals often involves the
removal of sulphide minerals buried in the ground. When water flows over or through
sulphide waste rock or tailings exposed at a mine site, this water can become acidic from the
formation of sulphuric acid. In the absence of buffering material, such as calcareous rocks,
streams that receive drainage from mine sites can have low pH levels.
4.
Air Pollution: Air pollution from car exhaust and power plant emissions increases the
concentrations of nitrogen oxides (NO2, NO3) and sulfur dioxide (SO2) in the air. These
pollutants can travel far from their place of origin, and react in the atmosphere to form nitric
acid (HNO3) and sulfuric acid (H2SO4). These acids can affect the pH of streams by
combining with moisture in the air and falling to the earth as acid rain or snow.
3.5.1.2
Importance of pH
The pH of water determines the solubility (amount that can be dissolved in the water) and
biological availability (amount that can be utilized by aquatic life) of chemical constituents
such as nutrients (phosphorus, nitrogen, and carbon) and heavy metals (nickel, copper,
cadmium, chromium etc.). In the case of heavy metals, the degree to which they are soluble
determines their toxicity and possible uptake by plants. Metals tend to be more toxic at lower
pH because they are more soluble and more bio-available.
3.5.2
Effects on plants due to increased acidity or basicity
The pH hardly poses any problems on its own. The normal pH range for irrigated waste water
is from 6.5 to 8.4; pH values that fall outside this range are indicators that the water is
27
abnormal in quality. A low pH can result in a possible toxicity of iron, manganese, zinc and
copper in certain plants. It could also cause the deficiency of calcium and / or magnesium and
leads to ammonium sensitivity in plants. High basicity on the other hand could most probably
cause deficiency of iron, manganese, zinc, copper and boron. Result of tests carried out
during this study on the pH level of irrigated treated waste water used for tomato production
in the farm site of Glen Valley farms of Gaborone, Botswana gave an average pH 8.5 and the
average pH of ordinary tap water used was 7.0.
3.5.3
Effects on plants due to increased concentrations of trace elements
In conventional irrigation waters, some elements are normally present although in relatively
low concentrations, say a few mg/L and these are termed trace elements. They are often
excluded in routine analysis of regular irrigation water; nonetheless, care must be taken when
applying sewage effluents and sewage sludge. Heavy metals, however, are species of trace
elements, and are known to create definite health hazards when taken up by plants. Under this
species are included, arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb),
mercury (Hg), nickel (Ni) and zinc (Zn). The densities of these heavy metals are greater than
4 g/ cm3 in their metallic form (Kandiah, 1990). Table 3.7 presents phytotoxic threshold levels
of some selected trace elements.
28
Table 3.7.
Threshold levels of some trace elements for crop production
Symbol
/(Element)
Cd/(cadmium)
Recommended
maximum concentration
Remarks
(mg/L)
0.01
Toxic to beans, beets and turnips at
concentrations as low as 0.1 mg/L in nutrient
solutions. Conservative limits recommended
due to its potential for accumulation in plants
Co/(cobalt)
0.05
and soils
to concentrations
thatinmay
be
Toxic
to tomato
plants at 0.1 mg/L
nutrient
harmful toTends
humans.
solution.
to be inactivated by neutral
and alkaline soils.
Cr/(chromium)
0.10
Not generally recognized as an essential
growth
element.
Conservative
limits
recommended due to lack of knowledge on its
toxicity to plants.
Ni/(nickel)
0.20
Toxic to a number of plants at 0.5 mg/L to 1.0
mg/L; reduced toxicity at neutral or alkaline
pH.
Zn/(zinc)
2.0
Toxic to many plants at widely varying
concentrations; reduced toxicity at pH > 6.0
and in fine textured or organic soils.
Source: (Food and Agriculture Organization, 1985)
29
3.6
BACKGROUND INFORMATION ON TEST CROP (T OMATO)
The Tomato (Lycopersicon esculentum) is an herbaceous, usually sprawling plant in the
nightshade family that is typically cultivated for the purpose of harvesting its fruit for human
consumption. The fruit of most varieties ripens to a distinctive red color.
Tomato plants typically reach 1–3 metres (3–10 feet) in height, and have a weak, woody stem
that often vines over other plants. The leaves are 10–25 centimeters (4–10 inch) long, odd
pinnate, with 5–9 leaflets on petioles (Acquaah, 2002) and each leaflet measures up to
8 centimeters (3 inch) long, with a serrated margin; both the stem and leaves are densely
glandular-hairy. The flowers are 1–2 centimeters (0.4–0.8 inch) in width, yellow, with five
pointed lobes on the corolla; they are borne in a cyme of 3–12 together. Tomatoes are
perennial and are often grown outdoors in temperate climates as an annual crop.
There are two types of tomatoes commonly grown; determinate and indeterminate tomatoes.
Most commercial varieties are determinate. These “bushy” types have a defined period of
flowering and fruit development. Most heirloom garden varieties and greenhouse tomatoes
like the ones used in this research are indeterminate which means they produce flowers and
fruit throughout the life of the plant (Kelley and Boyhan, 2006). Specifically, tomato variety
used in the project was the “25 tomato super beef steak seeds”. This variety grows firm and
strong, offer better resistance to blight and fungus though it takes longer time to produce
fruits; nonetheless of good quality (see appendix B)
Most cultivated tomatoes require around 75 days from transplanting to first harvest and can
be harvested for several weeks before production declines. Ideal temperatures for tomato
growth are 700F-850F or 210C-290C during the day and 650F-700F or 180C-210C at night.
30
Significantly higher or lower temperatures can have negative effects on fruit set and quality.
The optimum pH range for tomato production is 6.2 to 6.8 (Kelley and Boyhan, 2006).
Jones (2008) provided instructions on how a grower can produce transplants for greenhouse
tomato production. He stated that it is best to have a seedling greenhouse, or growth chamber,
where the growing conditions (temperature, humidity, light etc.) can be precisely controlled.
The common procedure is to transplant seedlings from a seedling bed or cube into a larger
cube or pot before transplanting. The transplants should be set into the soil 1 inch (2.54
centimeters) deeper than previously grown, or up to the cotyledon leaves.
3.7
IRRIGATION FOR TOMATOES
For a tomato plant, irrigation is used to replace the amount of water lost by transpiration and
evaporation. This amount is also called crop evapotranspiration (ETc) (Simonne, 2003).
Tomato water requirement (ETc) depends on the stage of growth and evaporative demand.
ETc can be estimated by adjusting the reference evapotranspiration (ETo) with a correction
factor called the crop factor (Kc; equation (1)):
Crop water requirement = crop coefficient X reference evapotranspiration
ETc = Kc X ETo…………………….. Eq. (1)
3.8
TOMATO IRRIGATION REQUIREMENTS (IR)
Irrigation systems are generally rated with respect to application efficiency (Ea) which is the
fraction of the water that has been applied by the irrigation system and that is available to the
plant for use (Simonne, 2003). In general, Ea is 60-80% for overhead irrigation, 20-70% for
seepage irrigation, and 90-95% for drip irrigation.
31
A tomato irrigation requirement (IR) is determined by dividing the desired amount of water to
provide to the plant (ETc) by Ea as a decimal fraction (Eq. 2).
Irrigation requirement (IR) = Crop water requirement divided by application efficiency
IR = ETc/Ea…………………………… Eq. (2)
3.9
3.9.1
NATURE OF HEAVY METALS OF CONCERN (CHROMIUM AND NICKEL)
Chromium
Chromium is a non-essential element to plants and its compounds are highly toxic to plants.
There are many valence states of chromium which are unstable and short lived in biological
systems however, the most stable forms of Chromium are the trivalent Chromium (III) and
the hexavalent Chromium (VI) species. Its complex electronic chemistry has been a serious
challenge in unraveling its uptake and bio-accumulation pattern in plants. It is probably due
to this reason that chromium has received little attention from plant scientists as compared
to other toxic trace metals like mercury, cadmium, aluminum and lead. In soil, chromium
concentrations range between 1 and 3 000 mg/kg, in sea water 5 to 800 µg/L, and in rivers
and lakes 26 µg/L to 5.2 mg/L (Kotaś and Stasicka, 2000). The relation between chromium
(III) and chromium (VI) strongly depends on the pH and oxidative properties of the location;
however, the chromium (III) is the dominating species (Kotaś and Stasicka, 2000). In some
areas the ground water can contain up to 39 µg/L of total chromium of which 30 µg/L is
present as chromium (VI) (Gonzalez, Ndung'u and Flegal, 2005).
32
3.9.2
Chromium in water and waste water
Many chromium compounds are relatively water insoluble. Chromium (III) oxides are only
slightly water soluble; therefore, concentrations in natural waters are limited. Cr3+ ions are
rarely present at pH values over 5.0, because hydrated chromium oxide is hardly water
soluble. Hexavalent chromium in industrial waste waters mainly originates from tanning and
painting. Waste water usually contains about 5 ppm of chromium. Kotlhao, Ngila and
Emongor (2006) gave an assessment of 111.00 ± 6.56 µg/L in the month of August 2004 and
98.35 ± 4.81 µg/L in the month of March 2005, respectively, for the level of Cr3+ detected in
secondary treated sewage water for crop irrigation in Gaborone, Botswana. The maximum
allowable value of hexavalent chromium for irrigation water is 0.1 mg/L (Food and
Agriculture Organization, 1985). Botswana Bureau of Standards (2004) also set a maximum
limit of 0.50 mg/L for chromium effluent quality.
3. 9.3
Chromium in soil
Chromium solubility in soil water is lower than that of other potentially toxic metals. This
explains the relatively low plant uptake. The solubility of chromium (III) in soil is dependent
on pH (Palmer and Wittbrodt, 1991) and decreases dramatically at pH > 4.5. Chromium (VI)
compounds are toxic at low concentrations for both plants and animals. The mechanism of
toxicity is also pH dependent. Chromium (VI) is more mobile in soils than chromium (III)
compounds, but is usually reduced to chromium (III) compounds within a short period of
time. Soluble chromates are converted to insoluble chromium (III) salts and consequently,
availability for plants decreases. This mechanism protects the food chain from high amounts
of chromium. Chromate mobility in soils depends on both soil pH and soil sorption capacity,
33
and on temperature. Adsorption of chromium (VI) is considerably less at neutral to alkaline
pH than at more acidic pH values (Bartlett and Kimble, 1976; Bartlett and James, 1983). The
intensity of adsorption will depend on the type and quantity of soil components, as well as the
pH and the presence of competing ligands such as phosphate. The availability of soil
chromium to the plant depends on the oxidation state of chromium, pH, and the presence of
colloidal binding sites and chromium-organic complexes that would influence its total
solubility (Hossner et al., 1998).
3.9.4
Chromium concentration in plants (case studies)
The first interaction that chromium has with a plant is during the uptake process (Shanker et
al., 2005). In a study carried out by Mangabeira et al. (2005) of the uptake, transport and
localization of chromium in tomato plants using Secondary Ion Mass Spectrometry (SIMS)
and Electron Probe Microanalysis (EPMA) they detected chromium in decreasing order of
concentration in the roots, stems and leaves. They reported no detection of chromium in the
fruits of tomato plants. Golovatyj et al., (1999) showed that chromium distribution in crops
had a stable character which did not depend on the soil properties and concentration of this
element in the water; the maximum quantity of element contaminant was always contained in
the roots and a minimum in the vegetative and reproductive organs. The use of metabolic
inhibitors diminished chromium (VI) uptake whereas it did not affect chromium (III) uptake,
indicating that chromium (VI) uptake depends on metabolic energy and chromium (III) does
not (Skeffington et al., 1976). In contrast, an active uptake of both chromium species, slightly
higher for chromium (III) than for chromium (VI), was found in the same crop
(Ramachandran et al., 1980). In 7 out of 10 crops analyzed, more chromium accumulated
34
when plants were grown with chromium (VI) than with chromium (III) (Zayed et al., 1998).
Table 3.8 gives an account of chromium uptake and accumulation by some crops.
Table 3.8.
Chromium
Chromium uptake in plants (Case studies)
concentration
Uptake and accumulation
in medium
Crop/plant
Reference
Spinach
Singh
pattern
0, 5, 30, 45, 60, 75, 90, 2.8 Cr(III) and 3.14 Cr(VI) µg/g
105, 120 and 135 mg/Kg
(2001)
Cr(III) and Cr(VI)
6, 12, 24 mg/L Cr
Cr more in roots than shoots in A:Dactylis
Shanker
A and more in shoots than roots glomerate
(2003)
in B
0.5, 1.5, 25 µg/mL
B:Medicago
sativa
Cr Progressive increase with more Rice
51
radio-labeled
Mishra
et
Cr in roots than shoots
al., (1997)
Roots took up more than shoots Tomato
Moral et al.
and not detected in fruits
(1996)
Progressive increase with more Sunflower,
Kocik and
(51Cr is a radioactive
isotope of chromium)
0, 50, 100 mg/L Cr(III)
0-200 mg/Kg
Cr in roots than shoots
0.25 and 1.0 mg/L
maize
and Ilavsky
Viciafaba
75-100% steady state removal; Lemna minor
(1994)
Wahaab et
1-2 mg/Kg dry weight at the rate
al. (1995)
of 250-667 mg/day m2
Source: (Shanker et al., 2005)
(1994)
35
3.9.5
Nickel
Nickel is the twenty-fourth most abundant chemical element in the earth's crust, occurring at
an average concentration about 75 µg/g. Nickel has an atomic number of 28 and an atomic
weight of 58.71. Although it has oxidation states of -1, 0, +1, +2, +3, and +4, the most
common valence state in the environment is Ni2+ (Cotton and Wilkinson, 1988; Nieboer et al.,
1988). Nickel occurs in nature as a trace constituent in a wide variety of minerals, particularly
those containing large amounts of iron and magnesium, such as olivine and pyroxenes (Avias,
1972). In minerals in which it is an essential component, it occurs most frequently in
combination with sulphur, arsenic, or antimony. Examples include millerite (NiS), red nickel
ore (mainly niccolite (NiAs), pentlandite (Ni, Fe)9S8, and deposits consisting primarily of
NiSb, NiAs2, NiAsS, or NiSbS. In Botswana, the most important commercial deposits of
nickel contain up to 8% Ni as 70% pyrrhotite, 20% pentlandite and 7% pyrite. This exists in
the Phoenix mine (Palmer and Johnson, 2005) and is operated by the Tati nickel mining
company near Francistown, Botswana.
3.9.6
Nickel in water and waste water
Nickel is a naturally occurring element that is present in the environment principally in the
divalent state (Ni2+).
Nickel in dissolved and the particulate form enters the aquatic
environment in effluents and leachates, as well as through atmospheric deposition after
release from anthropogenic sources (Canadian Environmental Protection Act, 1994). The
most water-soluble nickel compounds are nickel chloride hexahydrate (2 500 g/L), nickel
sulphate hexahydrate (660 g/L), nickel sulphate heptahydrate (760 g/L), and nickel nitrate
hexahydrate (2 400 g/L) (Lide, 1993). Less soluble nickel compounds include hexa-ammine
36
nickel nitrate (45 g/L), nickel (II) hydroxide (0.13 g/L), and nickel carbonate (0.09 g/L)
(Lide, 1993). Nickel subsulphide and nickel oxide are considered to be "insoluble" in water,
but both are soluble in acids (Cotton and Wilkinson, 1980; International Program on
Chemical Safety, 1991). Nickel is a relatively mobile heavy metal. In natural waters, nickel is
transported in both particulate and dissolved forms. The pH, oxidation-reduction potential,
ionic strength, type, and concentration of organic and inorganic ligands (in particular, humic
and fulvic acids), and the presence of solid surfaces for adsorption (in particular, hydrous iron
and manganese oxides) can all affect the transport, fate, and biological availability of nickel
in fresh water and seawater ( Semkin, 1975; Callahan et al., 1979). Nkegbe and Koorapetse
(2005) reported the level of nickel concentrated in Gaborone industrial effluent at 5.6 µg/L
and the amount concentrated in the Glen Valley sludge varied from 27.5 – 33.1 mg/Kg
(Nkegbe, 2005). The maximum allowable value of nickel for irrigation water is 0.2 mg/L
(Food and Agriculture Organization, 1985). Botswana Bureau of Standards (2004) has a
permissible limit of 0.30 mg/L for nickel effluent quality.
3.9.7
Nickel in soil
The bio-availability of nickel in soils varies, depending in particular upon the forms of nickel
present and the soil pH. Nickel that is bound in the lattice of naturally occurring silicate
minerals (e.g., olivine or pyroxenes) is relatively unavailable for uptake by plants compared
to water soluble forms, such as nickel sulphate, which may be deposited on surface soils from
the atmosphere (Canadian Environmental Protection Act, 1994). Generally, bio-availability
increases with decreasing soil pH. In acidic soils, nickel-bearing sulphide and, to a lesser
extent, silicate minerals (and possibly nickel oxide) can dissolve over time, and relatively
little nickel is removed from soil pore waters by adsorption processes. Nickel complexed by
37
organic ligands dissolved in soil pore waters is expected to be less bio-available than free
nickel ions (Canadian Environmental Protection Act, 1994).
3.9.8
Nickel concentration in plants (case studies)
Nickel is very easily extracted from soils by plants and the nickel contents of plants are simple
functions of the nickel content of soils (Kabata-Pendias and Mukherjee, 2007). The
concentration of nickel in plant tissues provides an indication of the concentrations of
bio-available forms of nickel in the soils in which they are growing. Although the transport
and storage of nickel by plants seems to be metabolically controlled, it is mobile in plants and
is likely to accumulate in both the leaves and seeds (Kabata-Pendias and Pendias, 2001). Both
plant and soil factors affect nickel uptake by plants, although the most important factor is the
influence of soil pH; uptake is reduced significantly by increasing soil alkalinity
(Kabata-Pendias and Pendias, 2001). Table 3.9 summarizes the uptake of nickel by some
plants.
38
Table 3.9.
Nickel Uptake in plants (Case studies)
A study of uptake of trace They
observed
that
nickel (Moral et al., 1994)
metals by tomato plants accumulated in the fruit more than
from a nutrient solution.
observed with other metals including
chromium and cadmium.
The
uptake of nickel Observation was that nickel was (Woodward et al., 2003)
and cobalt by tomato concentrated in the roots but was
plants in a series of pot transferred to all parts of the plant
experiments
The study of uptake of Nickel
concentration
generally (Brake et al., 2004)
nickel by tomato and declined as the plant matured,
squash plants from soil suggesting that early uptake was
amended with coal fly diluted by growth. Daily watering
ash.
might have leached available nickel
from the rhizosphere soil.
Study of uptake of nickel Of the nickel taken up by the tomato (Poulik, 1999)
by tomatoes in a series of plants about 75% was translocated
pot experiments with soil to the shoots and only 25% to the
amended
with
nickel fruits
chloride solution
Source: (Environment Agency Science Report, 2009)
39
3.10
3.10.1
WASTE WATER USE CASE STUDIES
Waste water re-use for agricultural irrigation: Case study in
León-Guanajuato, central Mexico.
The city of León-Guanajuato with a population 1.2 million is one of the fastest growing cities
in Mexico, North America, and is highly dependent on groundwater for public supply (United
Nations Environment Program, 2003). Groundwater is abstracted mainly from aquifers
downstream of the city, including areas where waste water is used for agricultural irrigation.
Studies (Foster, 1992; Chilton et al, 1998) showed that high rates of recharge from excess
waste water irrigation on alfalfa and maize south-west of the city (coupled with no
agricultural abstraction) have helped maintained groundwater levels within 10 metres depth,
despite intensive abstraction from deeper horizons for municipal water supply. In adjacent
areas water levels are falling at 2 to 5 m/a.
Further observation (United Nations Environment Program, 2003) showed that though the
waste water contained large concentrations of chromium salts, the chromium content of the
groundwater remained low. Test carried out on the soil samples showed that both chromium
and other heavy metals were accumulating in the soil, with very little passing below a depth
of 0.3 metre. These accumulations of heavy metals on the surface soil could potentially
impact on their uptake by plants. Experimental results from solution culture and greenhouse
potted plants have shown that plant uptake usually increases with increased trace-element
concentration in the growing medium (Chang and Page, 1977).
40
3.10.2
Waste water use case studies in California
Beneficial use of waste water has been in practice since the 1890s in California, USA. By the
turn of the century, say around 1987, more than 0.899 Mm3/d of municipal waste water (7-8%
of the production) was used for various farm applications. Historically, agricultural use has
dominated, and continues to play significant roles; however, the past decade has seen
reclaimed waste water utilized for landscape irrigation in urban areas and for ground water
recharge. Most of the reclaimed water (78%) is used in the central valley and south coastal
regions of California. In agricultural use of treated effluent, at least twenty different food
crops are being irrigated as well as at least eleven other crops and nursery products as
indicated in Table 3.10.
Table 3.10.
Types of crops irrigated with reclaimed water in California
Food Crops
Non-food Crops
Apples, corn, asparagus, grapes, peaches,
Alfalfa ,Christmas trees, Clover, Corn,
Avocados, lettuce, barley, beans, plums,
Cotton,
Peppers, broccoli, pistachios, cabbage,
hay, sod trees, vegetable seeds
eucalyptus
trees,
flower
seeds
Cauliflower, squash, celery, sugar beets
Citrus and wheat
Source: (California State Water Resources Control Board, 1990)
41
In several surveys reported in the review of the California municipal waste water reclamation
in 1987, all the waste water treatment plants producing effluents for beneficial uses were
found to provide at least secondary treatment.
3.10.3
Current and future use of waste water in Tunisia
Waste water use for agricultural purposes has been practiced in Tunisia for several decades
and currently it is an integral part of the national water resources strategy. In the year 1988,
the volume of treated waste water available was 78 million m3 and in the year 2000 it would
probably exceed 125 million m3 (Bahri, 1988). Use of treated effluents is seasonal in Tunisia
(spring and summer time) and the effluent is often mixed with groundwater before being
applied to irrigate citrus and olive trees, forage crops, cotton, golf courses and hotel lawns.
In the period 1981 to 1987, the Ministries of Agriculture and Public Health, with assistance
from the United Nations Development Program carried out studies designed to assess the
effects of using treated waste water and dried, digested sewage sludge on crop productivity
and on the hygienic quality of crops and soil. Treated waste waters and dried, digested sludge
from the La Cherguia (Tunis) and Nabeul activated sludge plants were used in the studies and
irrigation with groundwater was used as a control. At La Soukra, tests were conducted on
sorghum (Sorghum vulgare) and pepper (Capsicum annuum) using flood irrigation and
furrow irrigation, respectively. Clementine and orange trees were irrigated at Oued Souhil
Nabeul (Bahri, 1988).
The average quality characteristics of the treated waste water from La Soukra are shown in
Table 3.11. The effluent contains moderate to high salinity but presents no alkalization risk
and trace element concentrations are below toxicity thresholds.
42
Evaluation of the fertilizing value of the effluent in relation to crop uptake suggests that the
mean summer irrigation volume of 6 000 m3/ha would provide an excess of nitrogen (N) and
potassium (K2O) but a deficit of phosphorus (P2O5). Application of treated effluent would
balance the fertilizing elements but could provide an excess for crop requirements. Excess
nitrogen would be of concern from the point of view of crop growth and in relation to
groundwater pollution (Bahri, 1988). The application of treated waste waters and sewage
sludge at the La Soukra and Oued Souhil experimental stations, where the soils are alluvial
and sandy-clayey to sandy, has not adversely affected the physical or bacterial quality of the
soils. However, the chemical quality of the soils varied considerably, with an increase in
electrical conductivity and a transformation of the geochemical characteristics of the soil
solution from bicarbonate-calcium to chloride-sulphate- sodium. Trace elements
concentrated in the surface layer of soil, particularly zinc, lead (Pb) and copper (Cu), but did
not increase to phytotoxic levels in the short term of the study period (Bahri, 1988).
43
Table 3.11.
Average characteristics of treated waste water (TWW) and well waters
(WW) used for irrigation (in mg/L) in La Soukra compared to FAO
recommended maximum concentrations.
Parameter
TWW
WW
FAO
pH
7.6
7.6
6.5-8.5
EC
2.97
2.61
3.0
TDS (g/L)
1.82
1.71
2.0
Cr
0.02
NA
0.1
Ni
0.06
0.05
5
Fe
0.33
0.11
5
Pb
0.19
0.16
2
Co
0.05
0.04
0.05
Mn
0.05
0.01
0.2
Cd
NA
NA
0.01
NA: Not Available, EC: Electrical Conductivity (in dS/m at 25°C)
TDS: Total Dissolved Solids
Source: (Bahri, 1988)
3.10.4
Waste water use case study in Botswana
In Botswana Aganga et al. (2005) conducted a study to determine the effect of sewage water
on soils and forages irrigated with treated sewage water at the Botswana College of
44
Agriculture’s farm. The study was conducted for a period of 120 days using established
forage pastures of ryegrass (Lolium multiforum) and Lucerne (Medicago sativa). Heavy
metals determined were Fe, Mn, Cu, Zn, Ni, Pb and Cd. Generally the treated sewage water
contained relatively low levels of heavy metals. Zn, Ni and Mn concentrations were below the
detectable levels in the sewage water while soils and plants had low levels of heavy metals.
Comparatively the soil and plants heavy metal levels were much higher than those in the
water and the difference was significant (p<0.05). There was a low correlation between trace
element contents in the water and soil. In addition there was some significant difference
(p<0.05) in the heavy metal concentration in the sewage water between the months during
which the analyses were carried out. However, the sewage water, soils and forage mineral
concentrations were within the internationally allowable heavy metal concentration with
respect to irrigation, soil loadings and animal feeds. The results showed that the water
contained Fe, Cu, Zn, Pb and Cd. The concentration of Mn and Ni were below levels
detectable by the Atomic Absorption Spectrophotometer (AAS) procedure. There was no
variation in heavy metal concentration with months except for Fe which increased with
respect to seasons. The decline was significantly different at p<0.05 and similarly the increase
in Fe was significantly different at p<0.05. The mineral concentrations were within the
typical and allowable concentrations required for irrigation water compared to the levels in
Table 3.12. Both forages contained some heavy metals including non-essential trace elements
such as Pb and Cd. There were some significant differences (p<0.05) in concentrations of
most constituents determined for Lucerne and rye grass except for Cu (Table 3.12.)
45
Table 3.12.
Mean ±SEM mineral concentration (mg/L) in the soils of the ryegrass
and Lucerne fields
Months
Forage
Mn
Fe
Cu
Zn
Ni
Pb
Cd
September
Rye
11.344
4.471
0.120
0.367
0.293
0.180
0.030
Lucerne 12.019
5.127
0.216
0.233
0.280
0.260
0.035
11.344
4.471
0.120
0.367
0.293
0.180
0.030
Lucerne 12.019
5.127
0.216
0.233
0.280
0.260
0.035
12.442
5.896
0.109
0.188
0.203
0.244
0.040
Lucerne 10.849
6.149
1.163
0.206
0.206
0.244
0.040
13.037
2.929
0.112
0.291
0.325
0.330
0.035
Lucerne 9.7465
3.498
0.180
0.333
0.241
0.366
0.040
0.2053
0.0506
0.0014
0.0207
0.0164
0.0134
0.0014
1.139
0.7288
0.5024
0.0239
0.0222
0.0522
0.0020
Field
field
October
Rye
Field
field
November
Rye
Field
field
December
Rye
Field
field
SEM
Rye
Field
Lucerne
field
Source: (Aganga et al., 2005)
46
3.11
SUMMARY OF FINDINGS AND PROBLEM TO BE ADDRESSED
There is a serious lack of in-depth of information regarding the uptake of heavy metals by
vegetables cultivated in the crop soils irrigated with treated waste water in the Glen Valley
farms of Gaborone, Botswana. Nevertheless the use of treated waste water combined with
application of sludge to soils could pose a great concern when used to cultivate agricultural
crops for human consumption. Reports had shown that agrochemical activities in Gaborone
crop soils was causing chromium and nickel accumulation on the surface soils and this could
potentially impact on their uptake by plants. One chief factor that could influence the
transport, fate and biological availability of these heavy metals in plants is the pH of the
treated waste water irrigation. At the Glen Valley farms, the average pH value of the treated
waste water used for tomato production is pH 8.5. This pH is beyond the optimum pH range
of 6.2 to 6.8 which is suitable for tomato production.
Henceforth, the present study would compare the uptake of chromium and the uptake of
nickel between tomato plants (leaves and fruits) grown in sludge-amended Glen Valley soils
to those grown in sludge-absent Glen Valley soils using treated waste water at pH 8.5 and tap
water at pH 7.0. This study will also compare the chromium uptake by the leaves and fruits
of the tomato plants grown in sludge absent Glen Valley soils using tap water at pH 7.0 to
those grown in sludge amended Glen Valley soil using treated waste water at pH 8.5 and
treated waste water at pH 5.0 , pH 6.0, pH 9.0 and pH 10.0. The results of this pH variation
experiments could tell if there is any significant contribution to the uptake of chromium and
nickel in the tomato leaves and fruits which invariably could lead to the increase in
concentration levels of these heavy metals in tomato plants, the food chain as well as impact
on human health.
47
CHAPTER 4
EXPERIMENTAL METHODS AND ANALYTICAL TECHNIQUES
4.1
AN OVERVIEW OF THE PROJECT DESIGN
The project design consisted of three sets of experiments conducted in the greenhouse after on
the ground survey had been carried out at the actual research site in the oldest Glen Valley
farm of Gaborone, Botswana. The treated waste water used for the experiments was
transported from the Glen Valley site in 50 liter plastic containers. Physical and chemical
checks of agricultural significance were carried out on the treated waste water. Standard
commercial soil was also collected and used in the control experiment. Chemical parameters
analyzed were pH and heavy metals determined by Atomic Absorption Spectrometry. The
soil type in the Glen Valley project site was a mixture of Vertic-Cambisol/Vertic-Luvisol
soils and Areni-Haplic-Lixisol Ferralic-Arenosol soils. The soil used was uniform for two
sets of the experiments which came from the Glen Valley farm site. The third soil type came
from a standard commercial soil and contained a mixture of Virgin Vertisol and Cleared
Vertisol soils. Parameters such as soil pH, soil electrical conductivity, and Cation Exchange
Capacity (CEC) were not the primary focus of this research and hence were not determined.
However, the soil analytical data for the Glen Valley farm are presented (Appendix C) for
reference purposes. Further routine laboratory analysis of the soil samples had been reported
elsewhere (Dikinya and Areola, 2010). Tissue analysis was conducted to determine
concentrations of chromium and nickel within the fruits and leaves of the plants; because the
fruit is the edible portion of the tomato and also in order to compare between the fruits and the
leaves. This was primarily achieved through nitric and sulphuric acid digestions and the
48
quantitative determination of heavy metals concentration was done using a Varian Spectr
AA-10/20 Atomic Absorption Spectrometer (AAS).
4.2
THE GREENHOUSE EXPERIMENTS
The greenhouse preparation started in late April of 2009. The greenhouse was constructed
with the help of a commercial farmer and expert advice of the Botswana College of
Agriculture (BAC). The greenhouse was made out of treated timbers as the base and the
fascia; the gusset was made of treated plywood and the covering was ultra violet resistant net
shedding as shown in Figure 4.1.
Figure 4.1
Frontal view of the greenhouse used for the tomato production
49
The next step was to collect sludge amended soil and sludge absent soil of the same type and
put them into 30 flower pots. The soil was collected within a 2 m x 2 m grid in a central
location on the farm to ensure uniformity and the soil that was dug was at the root level zone.
Standard commercial soil was purchased for the control experiments and 5 pots were utilized.
At the greenhouse each pot was weighed to ensure that it contained exactly 2.5 kg of soil and
the soil was kept wet until seedlings were planted. Tomato cultivation was originally started
on Monday 16 March 2009 but after about 7 weeks, a heavy rainstorm destroyed everything.
Subsequently, fresh seeds were planted on Sunday 10 May 2009. A total of 35 pots with each
pot containing about 2.5 kg of soil were again collected from the Glen Valley farm site. All
pots were watered with 100 mL/day of ordinary tap water from 10 May to 24 July (11 weeks)
-germination and early growth phase (Tomatoes grow best in slightly acidic soil with an
optimum pH level between 6.5 and 7.0 hence the need to use tap water for the initial
germination and early development stage).
Treatment commenced on 25 July 2009 with all the 35 germinating potted tomato plants
receiving closer monitoring. Treatment with tap water at pH 7.0, treated waste water at pH 8.5
and adjusted treated waste water (at pH 5.0, 6.0, 9.0 and 10.0) was carried out on a need by
need basis of each plant. However, to ensure uniformity and consistency in pattern each plant
received an equal amount of water (250 mL) by the end of each week; the rate and time of
day(s) were determined by the individual plant response to treatment. Some of the plants were
watered early in the day to cut down on evaporation losses and also to give the plants plenty of
time to dry out. A drip irrigation technique which delivered water right at the soil surface and
not on the leaves was used; this was to make sure that water was made available all the time.
Irrigation at midday was avoided because that was when evaporation losses were the highest.
The average characteristics of representative samples of the Glen Valley treated waste water
50
and ordinary tap water used for experimental irrigation treatments (in mg/L) compared to the
Food and Agricultural Organization (FAO) recommended maximum concentrations are
shown in Table 4.1.
Table 4.1.
Some characteristics of Glen Valley treated waste water (GVTWW) and
ordinary tap water (OTW) used for experimental irrigation compared to
FAO recommended maximum concentrations
PARAMETER GVTWWa
OTWb
FAOc(mg/L)
pH
7.200
6.5 to 8. 5
7.900
NA
0.400
0.020
0.0 - 10
0.014
0.013
0.0 - 2.0
9.700
pH with LAN 10.400
fertilizer
Nitrate
nitrogen
Phosphate
Source: a, b (Akande, 2007) c (Food and Agriculture Organization, 1992)
The project contained three experimental designs which were the control, bio-accumulation
and pH variation experiments.
4.3
THE CONTROL EXPERIMENTS
The control experiments had 5 tomato pots each filled with 2.5 kg standard commercial soil
and treated with ordinary tap water (the pots were labeled CON1, CON2, CON3, CON123a
and CON123b for the purpose of identification and analysis). A summary of the control
experiments is shown in Table 4.2.
51
Table 4.2.
The Control experiments
TREATMENT TYPE
CODE
T1: Standard commercial soil irrigated
CON1 (Replicates:CON2, CON3, CON123a
with tap water (pH 7.0)
and CON123b)
4.4
BIO -ACCUMULATION
EXPERIMENTS
The bio-accumulation experiments had 10 tomato pots each filled with 2.5 kg soil. The pots
were labeled B1, B1a, B1b, B1c, B1d, B2, B3, B4, B5 and B6. B1 contained Sludge Absent
Glen Valley soil irrigated with ordinary tap water at pH 7.0. B1a, B1b, B1c, and B1d were
replicates of B1. B2 contained Sludge Amended Glen Valley soil irrigated with treated waste
water collected from the farm site. B3, B4 and B5 and B6 were replicates of the B2 set up. A
summary of the bio-accumulation treatments is shown in Table 4.3.
Table 4.3.
Bio-accumulation treatments
TREATMENT TYPE
CODE
T2: Sludge absent (ordinary) Glen Valley soil
B1 (Replicates: B1a, B1b, B1c, B1d)
irrigated with tap water (pH 7.0)
T3: Sludge amended Glen Valley soil irrigated
B2 (Replicates: B3, B4, B5 and B6)
with treated waste water (pH 8.5)
52
4.5
pH VARIATION EXPERIMENTS
The experimental set-up consisted of 20 tomato pots which were sub divided into 4 set-ups.
Set up 1 contained 5 tomato pots each filled with 2.5 kg sludge amended Glen Valley soil and
irrigated with treated waste water at pH 5.0 (the pots were labeled pHa, pHb, pHc, pHabc1,
and pHabc2 respectively). Set-up 2 contained 5 tomato pots each filled with 2.5 kg Glen
Valley sludge amended soil and irrigated with treated waste water at pH 6.0 (The pots were
labeled pHd, pHe, pHf, pHdef1, and pHdef2). Set-up 3 contained 5 tomato pots each filled
with 2.5 kg sludge amended Glen Valley soil and irrigated with treated waste water at pH 9.0
(The pots were labeled pHg, pHh, pHi, pHghi1, and pHghi2). Set-up 4 also contained 5 tomato
pots each filled with 2.5 kg sludge amended Glen Valley soil and irrigated with treated waste
water at pH 10.0 (the pots were labeled pHj, pHk, pHl, pHjkl1, and pHjkl2). Dilute 0.1 M
hydrochloric acid (HCL) was used as acid pH adjuster and dilute 0.1 M sodium hydroxide
(NaOH) was used to adjust the basicity level. The acid and base were used to increase or
reduce the level of acidity of the treated waste water to pH 5.0 or pH 6.0 or increase the level
of basicity to pH 9.0 or pH 10.0. The potted tomato plants were irrigated regularly with the
treated waste water at different pH values (Appendix B). A summary of the pH variation
experiments is shown in Table 4.4.
53
Table 4.4.
pH variation experiments
TREATMENT TYPE
CODE
T4: Sludge amended Glen Valley soil pHa (Replicates: pHb, pHc, pHabc1, and pHabc2)
irrigated with treated waste water at
pH 5.0
T5: Sludge amended Glen Valley soil pHd (Replicates: pHe, pHf, pHdef1, and pHdef2)
irrigated with treated waste water at
pH 6.0
T6: Sludge amended Glen Valley soil pHg (Replicates: pHh, pHi, pHghi1, and pHghi2)
irrigated with treated waste water at
pH 9.0
T7: Sludge amended Glen Valley soil pHj (Replicates: pHjkl2, pHl, pHjkl1, and pHjkl2)
irrigated with treated waste water at
pH 10.0
4.6
SAMPLE COLLECTION AND PREPARATION
Tomato leaf and fruit samples were hand harvested at the full red stage (vine-ripe) by mid
October 2009. The largest leaf and the biggest fruit from each tomato plant were selected for
testing (financial considerations also played a role in making this decision). Harvested leaves
and fruits were transported immediately to the Department of Waste Management and
Pollution Control, Gaborone for laboratory preparation and subsequent analysis by the
laboratory of the Department of Water Affairs also in Gaborone, Botswana.
54
4.7
OPEN DIGESTION TECHNIQUE FOR THE TOMATO LEAVES AND
FRUITS
The tomato plants (leaves and fruits) were harvested on 14 October 2009. The leaves and
fruits were washed thoroughly with ordinary tap water, rinsed with deionized water and oven
dried at 600 C. Each sample was weighed (about 2 g each of leaves and of fruits) and then 10
mL of concentrated HNO3 was added to the samples and the samples were covered with a
ribbed watch glass. The samples were then brought to the boil on a hot plate and evaporated to
15 – 10 mL. Thereafter 5 mL of concentrated HNO3 and 10 mL of concentrated H2SO4 were
added and the flask was cooled between additions. The flask and its content were then
transferred to a hot plate to allow its contents to evaporate until dense white fumes of SO3 just
appear. Heat was then applied to remove all the HNO3 before treatment. The next step was to
cool and to dilute the flask contents to about 50 mL with water and subsequently heated to
almost boiling to dissolve the soluble salts. Finally, the samples were filtered and ready for
analysis (Jackson, 1967). A blank was also run under similar conditions.
4.8
ATOMIC ABSORPTION SPECTROMETRY
The concentrations of chromium were determined with the Varian SpectrAA 10/20 system
(SpectrAA-10/20, 1985) and that of nickel with the Shimadzu AA6300 Atomic Absorption
Spectrometer (Shimadzu AA6300, 2003). In atomic absorption spectrometry, a light beam is
directed through a flame, into a monochromator and then onto a detector that measures the
amount of light absorbed by the atomized element in the flame. For some metals, atomic
absorption exhibits superior sensitivity over other techniques such as the flame emission
technique. Since each metal has its own characteristic absorption wavelength, a source lamp
55
composed of that element was used; this makes the method relatively free from spectral or
radiation interferences. The amount of energy at the characteristic wavelength absorbed in the
flame is proportional to the concentration of the element in the sample over a limited
concentration range (Wilis, 1962).
56
CHAPTER 5
RESULTS AND DISCUSSION
5.1
OVERVIEW
The chromium and nickel concentrations in the leaves and fruits of the tomato plants after
treatment with Glen Valley soils using tap water and treated waste water are presented and
discussed in this chapter.
5.2
CHROMIUM BIO-ACCUMULATION (CONTROL)
The chromium concentration in the leaves and the fruits of the tomato plants after treatment
with standard commercial soil and sludge absent Glen Valley soil and irrigated with tap water
are shown in Table 5.1 and Figure 5.1
Table 5.1.
Chromium uptake in the leaves and the fruits of the tomato plants: Tap
water (at pH 7.0) irrigation with standard commercial soil and sludge
absent Glen Valley soil.
Chromium concentration (mg/L)
Leaves
Treatment Type
T1:Standard commercial soil
Mean
Standard Error
Fruits
Mean
of the Mean
Standard
Error
0.819
0.242
0.599
0.153
of the Mean
0.740
0.028
0.511
0.009
with tap water at pH 7.0
T2: Sludge absent Glen Valley
soil with tap water at pH 7.0
57
1.400
1.200
Cr concentration (mg/L)
Leaves
Fruits
1.000
0.800
0.600
0.400
0.200
0.000
T1: Standard commercial soil
(tap water pH 7.0)
Figure 5.1
T2: Sludge Absent Glen Valley soil
(tap water pH 7.0)
Average concentration of chromium in the tomato plants (control). Bars
represent SEM (n=5)
Table 5.1 and Figure 5.1 show that more chromium bio-accumulate in the leaves than in the
fruits with both treatments. The highest concentration of chromium was found in the leaves
(0.819 mg/L) where the tomatoes were grown in standard commercial soil irrigated with tap
water and the lowest concentration (0.511 mg/L) in the fruits where the tomatoes were grown
in sludge absent Glen Valley soil irrigated with tap water. These results agree with findings of
Mangabeira et al. (2005). The mean concentrations of chromium bio-accumulation in both
treatments exceeded the 0.10 mg/L permitted limit suggested by the Food and Agriculture
Organization (1985) for crop production. The 0.50 mg/L effluent quality limits set by the
Botswana Bureau of Standards (2004) was also exceeded in both cases. Other studies
58
(Kirkham, 1986 and Omran et al., 1988) have reported high concentrations of heavy metals
bio-accumulation in tomato plants when using tap water as a source of irrigation water. This
could be ascribed to high concentrations of heavy metals in the soil irrigated with the tap
water source. The sludge absent Glen Valley soil bio-accumulates somewhat less chromium
in the leaves and the fruits as compared with the standard commercial soil when using tap
water for irrigation. In conclusion, the standard commercial soil or possibly the tap water
contains chromium levels above the Food and Agriculture Organization safe limits.
To determine if there were any significant differences in the concentration levels of
chromium uptake in the tomato leaves and fruits cultivated using sludge amended Glen
Valley soils with treated waste water compared with using sludge absent Glen Valley soils
treated with tap water, a statistical tool known as the Graph Pad software which uses the
student t-test was employed (this was necessary to analyze, graph and organize the data sets).
The student t-test determines if the mean values of two data columns are equal. The
hypotheses used are the null and alternate hypothesis and they are stated as follows:
The null hypothesis (HO): There are no statistically significant differences in concentration
levels of chromium uptake in the tomato leaves and fruits cultivated using sludge amended
Glen Valley soils with treated waste water compared with using sludge absent Glen Valley
soils treated with tap water.
The alternate hypothesis (HA): There are significant differences in concentration levels of
chromium uptake in the tomato leaves and fruits cultivated using sludge amended Glen
Valley soils with treated waste water compared with using sludge absent Glen Valley soils
treated with tap water.
To determine whether the null hypothesis is accepted or rejected statistically, the probability
value (P-value) is used. The P-value ranges from 0 to 1, and it has been statistically accepted
59
that if the P-value is > 0.5 then the null hypothesis is accepted, but if the P-value is < 0.5 then
the null hypothesis is rejected and the alternate hypothesis is accepted (Appendix D). By
conventional criteria (Appendix D), the difference observed in the concentrations of
chromium in the leaves and fruits of the tomato plants in this study are considered to be not
statistically significant as shown in Table 5.2, therefore the null hypothesis is accepted.
Table 5.2.
t-test for chromium to determine any significant differences in chromium
concentrations in the tomato leaves and fruits.
Heavy
Treated waste water Tap water irrigation t-test
Metal
irrigation
sludge
Glen
P-values
Remarks
with with sludge absent values
amended Glen
Valley
soil
Valley soil (Mean values)
(Mean values)
Chromium
0.2970
0.2930
0.0253
0.9803
No
significant
difference
60
5.2
CHROMIUM UPTAKE IN THE TOMATO PLANTS AT TREATED
WASTE WATER PH 5.0
Chromium concentrations in the tomato leaves and fruits using sludge amended Glen Valley
soils treated with waste water (normal waste water at pH 8.5 and waste water adjusted to pH
5.0) compared with chromium concentrations in tomato leaves and fruits treated with sludge
absent Glen Valley soils irrigated with tap water are presented in Table 5.3 and Figure 5.2
Table 5.3.
Chromium uptake in the leaves and the fruits of the tomato plants: Treated
waste water at pH 5.0 (sludge amended soils) compared with normal
treated waste water at pH 8.5 (sludge amended soils) and with tap water at
pH 7.0 (sludge absent soils).
Chromium concentration (mg/L)
Leaves
Mean
Treatment Type
T2: Sludge absent Glen Valley
Standard Error
Fruits
Mean
Standard Error
0.740
of the Mean
0.028
0.511
of the Mean
0.009
0.052
0.007
0.063
0.009
0.231
0.090
0.165
0.067
soil with tap water at pH 7.0
T3:
Sludge
amended
Glen
Valley soil with normal treated
waste water at pH 8.5
T4:
Sludge
amended
Glen
Valley soil with treated waste
water at pH 5.0
61
0.90
Leaves
Fruits
0.80
Cr concentration (mg/L)
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
T2:Sludge Absent Glen Valley soil
(tap water pH 7.0)
Figure 5.2
T3: Sludge Amended Glen Valley soil T4: Sludge Amended Glen Valley soil
(treated waste water pH 8.5)
(treated waste water pH 5.0)
Average concentration of chromium in the tomato plants for the different
treatments (pH 5.0). Bars represent SEM (n=5)
Gaborone crop soils which include the Glen Valley soils are enriched in chromium (59 mg/kg
– 240 mg/kg) (Zhai et al. 2003). Growing tomato plants in sludge amended Glen Valley soils
with treated waste water has been shown to reduce chromium translocation to the leaves and
fruits of the tomatoes. This pattern is observed (Figure 5.2) where the uptake of chromium
was significantly different for the different treatments. The highest uptake of chromium was
observed in the tomato plants treated with sludge absent Glen Valley soil and tap water (0.740
62
mg/L for leaves and 0.511 mg/L for fruits). This could be due to chromium having the affinity
to be sorbed to a slightly greater extent at pH 7.0 compared to other pH values. Chromium
uptake was reduced in tomato plants cultivated in sludge amended Glen Valley soil using
treated waste water at pH 5.0 (0.231 mg/L for leaves and 0.165 mg/L for fruits). This could be
ascribed to chromium species (particularly trivalent chromium) which tend to form
hydroxides that precipitate at low pH 5.0 (Appendix E1). This hydroxide of chromium
formed is mostly retained in the roots and minimally translocated to the leaves and fruits of
the tomato plants. Shewry and Peterson (1974) observed a similar trend when studying the
uptake and translocation of Cr042- from nutrient solutions by barley seedlings. They suggested
that most of the chromium were retained in the roots and very little translocation of chromium
took place from the roots to the tops. The lowest chromium uptake in the tomato plants was
obtained when using normal treated waste water at pH 8.5 with sludge amended Glen Valley
soil (0.052 mg/L for leaves and 0.063 mg/L for fruits). This is an indication of the minimum
solubility of chromium at this treated waste water pH 8.5 (Appendix E1) and consequently
less chromium uptake. On the average the tomato leaves tend to accumulate more chromium
than the fruits because the rate of transpiration is higher in the leaves compared with the
fruits. Moreover, fruits are mostly phloem loaded and heavy metals are generally poorly
mobile in the phloem. The work of Zheljazkov and Neilsen (1996) on concentrations of heavy
metals in vegetables buttressed this point. They found that the concentrations of heavy metals
in vegetables per unit dry matter generally follow the order: leaves >fresh fruits >seeds.
63
5.3
CHROMIUM UPTAKE IN THE TOMATO PLANTS AT TREATED
WASTE WATER PH 6.0
The chromium concentration in the leaves and fruits of tomatoes after treatment with normal
treated waste water and treated waste water at pH 6.0 using sludge amended Glen Valley soils
compared with treatment using tap water on sludge absent Glen Valley soils are shown in
Table 5.4 and Figure 5.3
Table 5.4.
Chromium uptake in the leaves and the fruits of the tomato plants: Treated
waste water at pH 6.0 (sludge amended soils) compared with normal
treated waste water at pH 8.5 (sludge amended soils) and with tap water at
pH 7.0 (sludge absent soils).
Chromium concentration (mg/L)
Leaves
Mean
Treatment Type
T2: Sludge absent Glen Valley
Standard Error
Fruits
Mean
Standard Error
0.740
of the Mean
0.028
0.511
of the Mean
0.009
0.052
0.007
0.063
0.009
0.406
0.009
0.427
0.036
soil with tap water at pH 7.0
T3:
Sludge
amended
Glen
Valley soil with normal treated
waste water at pH 8.5
T5:
Sludge
amended
Glen
Valley soil with treated waste
water at pH 6.0
64
0.90
Leaves
Fruits
0.80
Cr concentration (mg/L)
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
T2:Sludge Absent Glen Valley soil
(tap water pH 7.0)
Figure 5.3
T3: Sludge Amended Glen Valley soil T5: Sludge Amended Glen Valley soil
(treated waste water pH 8.5)
(treated waste water pH 6.0)
Average concentration of chromium in the tomato plants for the different
treatments (pH 6.0). Bars represent SEM (n=5)
Treatment with sludge amended Glen Valley soil (Figure 5.3; T5) using treated waste water at
pH 6.0 increased the chromium uptake in the leaves and fruits of the tomato plants compared
with treatment using treated waste water at pH 5.0 (Figure 5.2; T4). The chromium uptake in
the leaves increased from 0.231 mg/L (Figure 5.2; T4) to 0.406 mg/L (Figure 5.3; T5).
Similarly, the chromium uptake in the fruits increased from 0.165 mg/L (Figure 5.2; T4) to
0.427 mg/L (Figure 5.3; T5). These observation showed that at pH 6.0 chromium
accumulation in both leaf and fruit is roughly double that at pH 5.0. However, the chromium
uptake in the leaves and fruits of the tomato plants with tap water (pH 7.0) treatments in
65
sludge absent Glen Valley soils (Figure 5.3; T2) still remain higher compared with normal
treated waste water at pH 8.5 in sludge amended Glen Valley soils (T3) and treated waste
water at pH 6.0 in sludge amended Glen Valley soil (T5). The trend of increasing chromium
uptake in the tomato leaves and fruits from pH 5.0 to 6.0 and further up to pH 7.0 show
evidence for the strong dependence of chromium uptake on the pH of the water or waste water
used for irrigation. Thus far, these results agree with the findings of Cary et al., (1975) where
they studied the effect of pH on the uptake of chromium from solutions of pH 5.0, 6.0, 7.0 and
8.0 using wheat plants 25 to 30 cm tall. They reported that chromium (VI) which is highly
soluble and more mobile than chromium (III) was sorbed to a slightly greater extent at a pH
6.0 to 7.0.
5.4
CHROMIUM UPTAKE IN THE TOMATO PLANTS AT TREATED
WASTE WATER PH 9.0
The chromium concentrations in the leaves and fruits of tomatoes after treatment with normal
treated waste water and treated waste water at pH 9.0 using sludge amended Glen Valley soils
compared with treatment using tap water in sludge absent Glen Valley soils are shown in
Table 5.5 and Figure 5.4.
66
Table 5.5.
Chromium uptake in the leaves and the fruits of the tomato plants: Treated
waste water at pH 9.0 (sludge amended soils) compared with normal
treated waste water at pH 8.5 (sludge amended soils) and with tap water at
pH 7.0 (sludge absent soils)
Chromium concentration (mg/L)
Leaves
Treatment Type
T2: Sludge absent Glen
Mean
Standard Error
Fruits
Mean
Standard Error
0.740
of the Mean
0.028
0.511
of the Mean
0.009
0.052
0.007
0.063
0.009
0.079
0.011
0.054
0.008
Valley soil with tap
water at pH 7.0
T3: Sludge amended
Glen Valley soil with
normal treated waste
water at pH 8.5
T6:
Sludge amended
Glen Valley soil with
treated waste water at
pH 9.0
67
0.90
Leaves
Fruits
0.80
Cr concentration (mg/L)
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
T2:Sludge Absent Glen Valley soil
(tap water pH 7.0)
Figure 5.4
T3: Sludge Amended Glen Valley soil T6:Sludge Amended Glen Valley soil
(treated waste water pH 8.5)
(treated waste water pH 9.0)
Average concentration of chromium in the tomato plants for the different
treatments (pH 9.0). Bars represent SEM (n=5)
Cultivating tomatoes with sludge amended Glen Valley soil (Figure 5.4; T6) using treated
waste water at pH 9.0 decreased the chromium uptake in the leaves and fruits of the tomato
plants compared with treatments at pH 6.0 (Figure 5.3; T5). Chromium was reduced from
0.406 mg/L (Figure 5.3; T5) to 0.079 mg/L (Figure 5.4; T6) and from 0.427 mg/L (Figure 5.3;
T5) to 0.054 mg/L (Figure 5.4; T6) in the leaves and fruits, respectively. The result of T6
68
treatments (sludge amended Glen Valley soil) where treated waste water at pH 9.0 was used
was close to T3 treatments (sludge amended Glen Valley soil) where normal treated waste
water at pH 8.5 was the source of irrigation. This close trend that was observed could be
ascribed to the optimum precipitation of chromium occurring around a pH slightly greater
than 8.5 (Appendix E1). It thus makes chromium less available for plant uptake. Another
possible cause of the low chromium uptake when using treated waste water at pH 9.0 is the
lower solubility of chromium hydroxide at this pH. Therefore, less chromium is available for
uptake by the tomato plants.
5.5
CHROMIUM UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER PH 10.0
The chromium concentration in the leaves and fruits of the tomatoes after treatment with
normal treated waste water and treated waste water at pH 10.0 using sludge amended Glen
Valley soils compared with treatment using tap water on sludge absent Glen Valley soils are
shown in Tables 5.6 and Figure 5.5
69
Table 5.6.
Chromium uptake in the leaves and the fruits of the tomato plants: Treated
waste water at pH 10.0 (sludge amended soils) compared with normal
treated waste water at pH 8.5 (sludge amended soils) and with tap water at
pH 7.0 (sludge absent soils).
Chromium concentration (mg/L)
Leaves
Fruits
Standard Error
Mean
Treatment Type
Standard Error
Mean
of the Mean
T2: Sludge absent Glen
of the Mean
0.740
0.028
0.511
0.009
0.052
0.007
0.063
0.009
0.271
0.047
0.538
0.151
Valley soil with tap
water at pH 7.0
T3:Sludge
amended
Glen Valley soil with
normal
treated
waste
water at pH 8.5
T7:Sludge
amended
Glen Valley soil with
treated waste water at
pH 10.0
70
0.90
Leaves
Fruits
0.80
Cr concentration (mg/L)
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
T2:Sludge Absent Glen Valley soil
(tap water pH 7.0)
Figure 5.5
T3: Sludge Amended Glen Valley soil
(treated waste water pH 8.5)
T7: Sludge Amended Glen Valley soil
(treated waste water pH 10.0)
Average concentration of chromium in the tomato plants for the different
treatments (pH 10.0). Bars represent SEM (n=5)
Using sludge amended Glen Valley soil (Figure 5.5; T7) and treated waste water at pH 10.0
increased the chromium uptake in the leaves and fruits of the tomato plants compared to
treatment with sludge amended Glen Valley soil and treated waste water at pH 9.0 (Figure
5.4; T6). This effect could be linked to the amphoteric nature of chromium hydroxide which
formed from the precipitation reaction when adding sodium hydroxide (to adjust the pH to
9.0) to the normal treated waste water at pH 8.5. This amphoteric nature of chromium
hydroxide makes it increasingly soluble in the treated waste water at both low and high pH
71
values. At pH 10.0 the chromium hydroxide formed goes back into solution and makes
chromium available for tomato plant uptake. Another reverse trend that was observed when
using treated waste water at pH 10.0 was that more chromium, 0.538 mg/L, bio-accumulate in
the fruits of the tomatoes planted (Figure 5.5; T7) compared to a lesser amount of chromium,
0.054 mg/L, in the fruits of the tomatoes planted with treated waste water at pH 9.0 (Figure
5.4; T6). This reverse trend could be traceable to the tomato roots surface where increasingly
negative charges could have built up at high pH (pH 10.0) of the treated waste water that
could have attracted the positively charged chromium ions more strongly into the fruits.
There is also more chromium in the fruits compared with the leaves when the pH of the
treated waste water was raised to 10.0. This agrees with research findings of Khairiah et al.
(2002) where they reported higher chromium concentration in the fruits and roots than in the
leafy vegetables in their study of the bioavailability of chromium in vegetables of selected
agricultural areas in Malaysia. However, other researchers (Grubinger et al., 1994; Soane and
Saunder, 1959) pointed to the fact that chromium uptake and distribution in plants are often
dictated by the type of cultivar.
The role of pH in water or treated waste water irrigation is significant albeit controversial in
the uptake of chromium in tomato leaves and fruits. The contribution of the sludge amended
soil to chromium uptake is also not well defined and further research needs to be carried out
on the combined use of waste water and sludge amended soil. One thing that emerged from
the pH variation experiments is that the cultivation of tomatoes with the Glen Valley treated
72
waste water at pH 8.5 on sludge amended Glen Valley soil has been shown to reduce the level
of chromium uptake compared to using tap water on sludge absent Glen Valley soil. This
should be good news for farmers in the Glen Valley farms. However caution must be
exercised to avoid prolonged use of treated waste water that may trigger the buildup of
chromium and cause possible harm to the food chain and impact adversely on human health.
5.6
SUMMARY OF CHROMIUM UPTAKE IN THE TOMATO PLANTS
A summary of the chromium concentration in the leaves and fruits of the tomatoes after
treatment with normal treated waste water and treated waste water at different pH levels (in
sludge amended Glen Valley soils) and tap water (in sludge absent Glen Valley soil and
standard commercial soil) are shown in Figure 5.6
73
1.20
Leaves
Fruits
Cr concentration (mg/L)
1.00
0.80
0.60
0.40
0.20
0.00
T1:Standard
T2:Sludge
T3: Sludge
T4: Sludge
T5: Sludge
T6:Sludge
T7: Sludge
commercial soil Absent Glen Amended Glen Amended Glen Amended Glen Amended Glen Amended Glen
(tap water pH Valley soil
Valley soil
Valley soil
Valley soil
Valley soil
Valley soil
7.0)
(tap water pH (treated waste (treated waste (treated waste (treated waste (treated waste
7.0)
water pH 8.5) water pH 5.0) water pH 6.0) water pH 9.0) water pH 10.0)
Figure 5.6
Average concentration of chromium in the tomato plants for the
different treatments (summary). Bars represent SEM (n=5)
Tap water irrigation of the tomato plants in standard commercial soil induced the highest
accumulation of chromium, 0.819 mg/L (Figure 5.6; T1), in the tomato leaves. Chromium
accumulation, 0.740 mg/L (Figure 5.6; T2), was also high in the tomato leaves when using tap
water to irrigate tomatoes planted in sludge absent Glen Valley soil. This could be ascribed to
gpre-existing high chromium levels in standard commercial soil and sludge absent Glen
Valley soil which is being translocated easily to the tomato leaves. Another factor could be
the rate of transpiration that is higher in the leaves than the fruits and again the fruits are
mostly phloem loaded where heavy metals are generally poorly mobile. The lowest
accumulation of chromium, 0.052 mg/L (Figure 5.6; T3), was recorded in the tomato leaves
74
when using treated waste water irrigation with sludge amended Glen Valley soils. This could
be ascribed to a strong affinity of chromium for organic matter which may be present in the
normal treated waste water and also sludge amended Glen Valley soils, making it easily
complexed and reducing its availability for plant uptake.
Tap water irrigation (pH 7.0) in tomato plants cultivated in standard commercial soil and in
sludge absent Glen Valley soil showed a higher accumulation of chromium in their leaves and
their fruits compared to treated waste water irrigation (and treated waste water at different pH
values; T4, T5, T6, T7) in the tomato plants cultivated in sludge amended Glen Valley soils. It
is important to note that tomato plants cultivated in sludge amended Glen Valley soil using
treated waste water at pH 10.0 increased the chromium uptake in the leaves and fruits of the
tomato plants compared to treatment with sludge amended Glen Valley soil and treated waste
water at pH 9.0. All these show that chromium solubility and subsequent bioavailability in
tomato plant are pH dependent. Also, the theoretical solubility of chromium hydroxide
(Appendix E1) showed how chromium is directly controlled by pH. The affinity and binding
capacity of chromium in solution for organic matter contained in soil as well as its tendency to
form complexes become reduced as pH increases from pH 5.0 to pH 6.0 increasing
availability for plant uptake (Guertin et al., 2004).
Mean chromium concentration in the leaves was higher than in the fruits but statistical
analysis shows no significant difference between them at the 5% significant level. The mean
chromium concentration in the leaves and fruits of tomato plants in this study exceed the 0.1
mg/L recommended maximum level of chromium for crop production (Food and Agriculture
Organization, 1985). However, the maximum limits of 0.50 mg/L for chromium effluent
quality set by the Botswana Bureau of Standards (2004) was only exceeded when tomato
75
plants were irrigated with tap water at pH 7.0 and treated waste water at pH 10.0 (in case of
the tomato fruits).
5.7
NICKEL BIO-ACCUMULATION (CONTROL)
The nickel concentration in the leaves and fruits of the tomato plants after treatment with
standard commercial soil and sludge absent Glen Valley soil with tap water are shown in
Table 5.7. and Figure 5.7.
Table 5.7.
Nickel uptake in the leaves and the fruits of tomato plants: Tap water (at
pH 7.0) irrigation with standard commercial soil and sludge absent Glen
Valley soil.
Nickel concentration (mg/L)
Leaves
Treatment Type
T1:Standard commercial soil
Mean
Standard Error
Fruits
Mean
Standard Error
0.327
of the Mean
0.204
0.224
of the Mean
0.174
0.217
0.000
-0.003
0.000
with tap water at pH 7.0
T2: Sludge Absent Glen Valley
soil with tap water at pH 7.0
76
0.70
Leaves
Fruits
Ni concentration (mg/L)
0.60
0.50
0.40
0.30
0.20
0.10
0.00
-0.10
T1: Standard commercial soil
(tap water pH 7.0)
Figure 5.7
T2: Sludge Absent Glen Valley soil
(tap water pH 7.0)
Average concentration of nickel in the tomato plants for the different
treatments (control). Bars represent SEM (n=5)
Crop soils of Gaborone (including the Glen Valley farm soils) are high in nickel, ranging
from 40 mg/kg to 161 mg/kg (Zhai et al. 2003). Using standard commercial soil and tap water
for irrigation, T1 of Figure 5.7, and sludge absent Glen Valley soil with tap water, T2 of
Figure 5.7, to cultivate tomato plants produced significant differences in the nickel uptake in
the leaves and the fruits of the tomato plants. Nickel uptake was 0.327 mg/L in the leaves
when standard commercial soil with tap water was used and 0.217 mg/L in the leaves when
sludge absent Glen Valley was used with tap water. There was also a high nickel (0.224
mg/L) uptake in the fruits when standard commercial soil with tap water was used. However,
nickel desorption was experienced in the case of the sludge absent Glen Valley soil treated
77
with tap water (-0.003 mg/L) for the fruits. The unavailability of nickel in the tomato fruits
when irrigating sludge absent Glen Valley soil could be due to the low transpiration rates of
the fruits as compared with the leaves which are more tolerant to nickel at pH 7.0. Again, the
fruits as storage organs are largely phloem-loaded and heavy metals are generally poorly
mobile in the phloem (Krijger et al., 1999). Al-Lahham et al., (2003) observed a similar trend
in their study where they recorded no accumulation of nickel in tomato fruits when irrigating
with potable water.
The tomato leaves in the case of the standard commercial soil and sludge amended Glen
Valley soil (treated with tap water at pH 7.0) and the fruits in the case of the standard
commercial soil (treated with tap water at pH 7.0) accumulate nickel beyond the limit of 0.20
mg/L as set by the Food and Agricultural Organization (1985) for crop production. It also
exceeds the 0.30 mg/L permissible limits set by the Botswana Bureau of Standards (2004).
The nickel concentration in the tomato fruits (-0.003 mg/L) when using tap water at pH 7.0 to
irrigate tomatoes planted in sludge absent Glen Valley soil is not significant. This indicates
that tap water at pH 7.0 combined with sludge absent Glen Valley soil could reduce the
uptake of nickel in the fruits but not in the leaves of the tomato plants. However, this
indication is not conclusive and further studies need therefore be carried out to properly
understand the uptake mechanism of nickel when using tap water to irrigate tomato plants in
sludge absent soils.
To determine if there were any significant differences in the concentration levels of nickel
uptake in the tomato leaves and fruits cultivated using sludge amended Glen Valley soils with
treated waste water compared with using sludge absent Glen Valley soils with tap water, the
same statistical tool described in section 5.1 for chromium was also used for nickel and the
78
same convention followed (Appendix D). By conventional criteria, the differences observed
in the leaves and fruits were considered to be not statistically significant (Table 5.8).
Table 5.8.
t-test for nickel to determine any significant differences in tomato leaves
and fruits
Heavy
Treated waste water Tap water irrigation t-test
Metal
irrigation with sludge with sludge absent values
amended Glen Valley Glen
soil (Mean values)
Nickel
5.8
0.0918
Valley
P-values
Remarks
soil
(Mean values)
0.0545
0.5180
0.6157
No
significant
difference
NICKEL UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER PH 5.0
The nickel concentration in the leaves and the fruits of the tomatoes after treatment with
normal treated waste water and treated waste water at pH 5.0 using sludge amended Glen
Valley soils compared with treatment using tap water on sludge absent Glen Valley soils are
shown in Table 5.9. and Figure 5.8.
79
Table 5.9.
Nickel uptake in the leaves and the fruits of the tomato plants: Treated
waste water at pH 5.0 (sludge amended soil) compared with normal
treated waste water at pH 8.5 (sludge amended soil) and with tap water at
pH 7.0 (sludge absent soils).
Nickel concentration (mg/L)
Leaves
Mean
Treatment Type
T2: Sludge absent Glen
Fruits
Standard Error
Mean
Standard Error
0.217
Of the Mean
0.000
-0.003
Of the Mean
0.000
-0.025
0.078
-0.030
0.030
0.085
0.022
0.020
0.047
Valley soil with tap
water at pH 7.0
T3:Sludge
amended
Glen Valley soil with
normal
treated
waste
water at pH 8.5
T4:Sludge
amended
Glen Valley soil with
treated waste water at
pH 5.0
80
0.25
Leaves
Fruits
0.20
Ni concentration (mg/L)
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.15
T2: Sludge Absent Glen Valley soil T3: Sludge Amended Glen Valley T4: Sludge Amended Glen Valley
(tap water pH 7.0)
soil (treated waste water pH 8.5) soil (treated waste water pH 5.0)
Figure 5.8
Average concentration of nickel in the tomato plants for the different
treatments (pH 5.0). Bars represent SEM (n=5)
Figure 5.8 compares tap water irrigation of tomato plants (in sludge absent Glen Valley soil)
with treated waste water irrigation (in sludge amended Glen Valley soil). It can be deduced
that irrigating tomato plants with treated waste water at pH 5.0 in sludge amended Glen
Valley soils has been shown to reduce its translocation to the leaves, 0.085 mg/L (Figure 5.8;
T4). The level of nickel was quite high, however, in the tomato leaves, 0.217 mg/L, when
using tap water in sludge absent Glen Valley soil (Figure 5.8; T2). There was nickel
desorption in the tomato fruits, -0.003 mg/L when using tap water to irrigate tomatoes planted
in sludge absent Glen Valley soil. No nickel was taken up in the tomato leaves and fruits when
81
irrigating sludge amended Glen Valley soil with normal treated waste water and a possible
reason could be the higher basicity (pH 8.5) of the normal treated waste water which caused
reduced solubility and mobility of the nickel in plants at the higher pH of the irrigation water
(Kabata-Pendias
and Mukherjee, 2007). The concentration of nickel in the tomato plants
(leaves and fruits) was below the recommended threshold of 0.30 mg/L set by the Botswana
Bureau of Standards and also lower than the 0.2 mg/L limits set by the Food and Agricultural
Organization for crop production when irrigating the sludge amended Glen Valley soil with
treated waste water at pH 5.0. This could be correlated with the free nickel ion activity in the
soil solution because the plant uptake of nickel is dependent on the soil pH as well as other
factors such as the organic matter, iron and manganese oxide content of the soil (Ge et al.,
2000; Kabata-Pendias and Mukherjee, 2007). Another possibility is that the tomato plants
might have less tolerance for nickel at pH 8.5 and this observation agreed with work by the
Environment Agency (2009) that plants generally differ in their tolerance and ability to
uptake nickel. These differences could be ascribed to the plants ability to respond differently
to nickel ion activity in the soil solution. Nickel uptake becomes more readily available in its
simple ionic form (Ni2+) than as inorganic and organic complexes (Kabata-Pendias and
Mukherjee, 2007)
5.9
NICKEL UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER PH 6.0
The nickel concentration in the leaves and the fruits of tomatoes after treatment with normal
treated waste water and treated waste water at pH 6.0 using sludge amended Glen Valley soils
82
compared with treatment using tap water on sludge absent Glen Valley soils are shown in
Table 5.10. and Figure 5.9.
Table 5.10.
Nickel uptake in the leaves and the fruits of the tomato plants: Treated
waste water at pH 6.0 (sludge amended soils) compared with normal
treated waste water at pH 8.5 (sludge amended soils) and with tap water
at pH 7.0 (sludge absent soils).
Nickel concentration (mg/L)
Fruits
Leaves
Treatment Type
T2:
Sludge
Absent
Glen
Mean
Standard Error
Of the Mean
Mean
Standard Error
Of the Mean
0.217
0.000
-0.003
0.000
-0.025
0.078
-0.030
0.030
0.262
0.138
0.147
0.097
Valley soil with tap water at
pH 7.0
T3: Sludge Amended Glen
Valley
soil
with
normal
treated waste water at pH 8.5
T5: Sludge amended Glen
Valley soil with treated waste
water at pH 6.0
83
0.50
Leaves
Fruits
Ni concentration (mg/L)
0.40
0.30
0.20
0.10
0.00
-0.10
-0.20
T2: Sludge Absent Glen Valley T3: Sludge Amended Glen Valley T5:Sludge Amended Glen Valley
soil (tap water pH 7.0)
soil (treated waste water pH 8.5) soil (treated waste water pH 6.0)
Figure 5.9
Average concentration of nickel in the tomato plants for the different
treatments (pH 6.0). Bars represent SEM (n=5)
As shown in Figure 5.9., there was an increased uptake of nickel when treated waste water at
pH 6.0 was used to irrigate sludge amended Glen Valley soil compared to irrigation of treated
waste water at pH 5.0 (Figure 5.8; T4). This might be ascribed to nickel being rather weakly
sorbed to clay and iron minerals in the soil at the higher pH than at the lower pH and thus
becoming more mobile and available for plant uptake (Agency for Toxic Substance and
Disease Registry, 2005; McGrath, 1995). However, one contrary report stated that it was
possible that the type of plant species affects the pH behavior of nickel uptake. Cataldo et al.,
(1978) reported in their study of nickel in plants that nickel uptake in soybean seedlings lack a
84
pH effect; they reported nickel uptake from 20 µM/L solutions by 15-day old plant to be
independent of pH from 4.5 to 7.0
5.10
NICKEL UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER PH 9.0
The nickel concentration in the leaves and the fruits of tomatoes after treatment with normal
treated waste water and treated waste water at pH 9.0 using sludge amended Glen Valley soils
compared with treatment using tap water on sludge absent Glen Valley soils are shown in
Table 5.11 and Figure 5.10.
85
Table 5.11.
Nickel uptake in the leaves and the fruits of the tomato plants: Treated
waste water at pH 9.0 (sludge amended soils) compared with normal
treated waste water at pH 8.5 (sludge amended soils) and with tap water
at pH 7.0 (sludge absent soils)
Nickel concentration (mg/L)
Fruits
Leaves
Treatment Type
T2: Sludge absent Glen
Mean
Standard Error
Mean
of the Mean
Standard Error
of the Mean
0.217
0.000
-0.003
0.000
-0.025
0.078
-0.030
0.030
-0.050
0.020
-0.007
0.053
Valley soil with tap water
at pH 7.0
T3:Sludge amended Glen
Valley soil with treated
waste water at pH 8.5
T6:Sludge amended Glen
Valley soil with treated
waste water at pH 9.0
86
0.25
Leaves
Fruits
0.20
Ni concentration (mg/L)
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.15
-0.20
T2: Sludge Absent Glen Valley
soil (tap water pH 7.0)
Figure 5.10
T3: Sludge Amended Glen Valley T6: Sludge Amended Glen Valley
soil (treated waste water pH 8.5) soil (treated waste water pH 9.0)
Average concentration of nickel in the tomato plants for the different
treatments (pH 9.0). Bars represent SEM (n=5)
Treatment with sludge amended Glen Valley soil (Figure 5.10; T6) using treated waste water
at pH 9.0 showed a negative correlation of nickel uptake in the leaves and fruits of the tomato
plants compared with treatments at pH 6.0 (Figure 5.8; T5). Nickel was reduced from 0.262
mg/L (Figure 5.8; T5) to -0.050 mg/L (Figure 5.10; T6) and from 0.147 mg/L (Figure 5.8; T5)
to -0.007 mg/L (Figure 5.10; T6) for the leaves and fruits, respectively. The observed trend in
the T6 treatments with nickel desorption in both the leaves and the fruits of tomato plants
cultivated in sludge amended Glen Valley using treated waste water at pH 9.0 is similar to the
T3 treatments (sludge amended Glen Valley soil with normal treated waste water at pH 8.5)
87
where there was also nickel desorption in both the leaves and the fruits of the tomato plants.
The similar trend observed could be ascribed to the reduced availability of the free nickel ion
in the soil water at high pH (pH 9.0) (Vijayakumaranj et al., 2009). From these observations it
can be concluded that the measured and controlled Glen Valley treated waste water at pH 8.5
can significantly lower if not eliminate the uptake of nickel in the tomato plants, especially
the edible fruit portion of it.
5.11
NICKEL UPTAKE IN THE TOMATO PLANTS AT TREATED WASTE
WATER PH 10.0
The nickel concentration in the leaves and the fruits of tomato plants after treatment with
normal treated waste water and treated waste water at pH 10.0 using sludge amended Glen
Valley soils compared with treatment using tap water on sludge absent Glen Valley soils are
shown in Table 5.12 and Figure 5.11.
88
Table 5.12.
Nickel uptake in the leaves and the fruits of the tomato plants: Treated
waste water at pH 10.0 (sludge amended soils) compared with normal
treated waste water at pH 8.5 (sludge amended soils) and with tap water
at pH 7.0 (sludge absent soils).
Nickel concentration (mg/L)
Leaves
Standard Error
Fruits
Mean
Standard Error
Mean
Treatment Type
T2: Sludge absent Glen
Of the Mean
Of the Mean
0.217
0.000
-0.003
0.000
-0.025
0.078
-0.030
0.030
0.062
0.008
0.200
0.237
Valley soil with tap water at
pH 7.0
T3: Sludge amended Glen
Valley soil with normal
treated waste water at pH
8.5
T7: Sludge amended Glen
Valley soil with treated
waste water at pH 10.0
89
0.50
Leaves
Fruits
Ni concentration (mg/L)
0.40
0.30
0.20
0.10
0.00
-0.10
-0.20
T2: Sludge Absent Glen Valley soil T3: Sludge Amended Glen Valley T7: Sludge Amended Glen Valley
(tap water pH 7.0)
soil (treated waste water pH 8.5) soil (treated waste water pH 10.0)
Figure 5.11
Average concentration of nickel in the tomato plants for the different
treatments (pH 10.0). Bars represent SEM (n=5)
Treatment with sludge amended Glen Valley soil (Figure 5.11; T7) and treated waste water at
pH 10.0 increased the nickel uptake in the leaves and fruits of the tomato plants compared to
treatment with sludge amended Glen Valley soil and treated waste water at pH of 9.0 (Figure
5.10; T6) where there was nickel desorption. This effect could be linked to the amphoteric
nature of the hydroxide of nickel that formed from the precipitation reaction when adding
sodium hydroxide to the normal treated waste water at pH 8.5. This amphoteric nature of
nickel hydroxide makes it increasingly soluble in the treated waste water for irrigation at high
pH (pH 10.0) so that the nickel goes back into solution and makes it available for tomato plant
uptake. One opposite pattern observed when cultivating tomato plants in sludge amended
90
Glen Valley soil using treated waste water at pH 10.0 compared with cultivating tomato
plants in sludge amended Glen Valley soil using treated waste water at pH 9.0 was that more
nickel bio-accumulate in the fruits of the tomato at pH 10.0 than at pH 9.0 (Figure 5.11; T7).
This could be ascribed to the presence of root surface hydroxyl (OH-) radicals at pH value
higher than 9.0 (Argun and Dursun, 2007) and thus creating a competition between nickel
ions, decreasing the aggregation of nickel and thereby causing an increase in the adsorption of
nickel into tomato shoots and partioning it into the fruits and the leaves.
5.12
SUMMARY OF NICKEL UPTAKE IN THE TOMATO PLANTS
A summary of the nickel concentration in the leaves and fruits of the tomatoes after treatment
with normal treated waste water and treated waste water at different pH levels (sludge
amended Glen Valley soils) and tap water (in sludge absent Glen Valley soil and standard
commercial soil) are shown in Figure 5.12.
91
0.60
Leaves
0.50
Fruits
Ni concentration (mg/L)
0.40
0.30
0.20
0.10
0.00
-0.10
-0.20
T1:
T2: Sludge
Standard Absent Glen
commercial Valley soil
soil (tap
(tap water
water pH
pH 7.0)
7.0)
Figure 5.12
T3: Sludge
Amended
Glen Valley
soil (treated
waste water
pH 8.5)
T4: Sludge T5:Sludge
Amended
Amended
Glen Valley Glen Valley
soil (treated soil (treated
waste water waste water
pH 5.0)
pH 6.0)
T6: Sludge
Amended
Glen Valley
soil (treated
waste water
pH 9.0)
T7: Sludge
Amended
Glen Valley
soil (treated
waste water
pH 10.0)
Average concentration of nickel in the tomato plants for the different
treatments (summary). Bars represent SEM (n=5)
Nickel accumulation in the tomato plants tends to fluctuate. The highest accumulation
occurred in the tomato leaves, 0.327 mg/L (Figure 5.12; T1), when using tap water irrigation
for tomatoes planted in standard commercial soil. There was also a very high concentration of
nickel in the leaves (0.262 mg/L) of the tomato plants irrigated with treated waste water at
pH 6.0 (in sludge amended Glen Valley soil) and the leaves (0.217 mg/L) (Figure 5.12; T2),
of the tomato plants irrigated with tap water at pH 7.0 (in sludge absent Glen Valley soil).
High concentration of nickel was also experienced in the fruits (0.224 mg/L) of the tomato
plants irrigated with tap water (in standard commercial soil) and in the fruits (0.200 mg/L) of
the tomato plants irrigated with treated waste water at pH 10.0 (in sludge amended Glen
Valley soil). Again, nickel concentration was high in the fruits (0.147 mg/L) of the tomato
92
plants irrigated with treated waste water at pH 6.0 (in sludge amended Glen valley soil). In
contrast, nickel desorption took place in the tomato plants cultivated in the sludge amended
Glen Valley soils using treated waste water at pH 8.5 and pH 9.0. This could be ascribed to a
combination of factors such as reduced availability of the free nickel ion in the soil water at
high pH and raising of the pH of the normal treated waste water solution to 9.0 with sodium
hydroxide which precipitated nickel hydroxide from the solution. The foregoing trends are
comparable to the theoretical solubility of nickel hydroxide (Appendix E2) where the
solubility of the metal is directly controlled by pH. Statistical analysis shows no significant
difference between the mean nickel concentration in the leaves and that of fruits of the
tomatoes at the 5% significant level. Most of the mean nickel concentrations in the leaves and
fruits of the tomato plants in this study were lower than the 0.2 mg/L recommended maximum
level for crop production (Food and Agriculture Organization, 1985). The recommended
permissible limit of 0.30 mg/L set by the Botswana Bureau of Standards (2004) was also not
exceeded. However there were a few exemptions where these limits were exceeded. One is
the mean nickel concentration in the tomato plant leaves cultivated in standard commercial
soil using tap water irrigation and another is the mean nickel concentration in the tomatoes
planted in sludge amended Glen Valley soil with treated waste water (pH 6.0) for irrigation.
93
CHAPTER 6
SUMMARY OF MAJOR FINDINGS
6.1
SUMMARY OF FINDINGS
The objective of this study is to compare the uptake of chromium and nickel in tomato plants
irrigated with treated waste water (using sludge amended Glen Valley soils) to that irrigated
with tap water (using sludge absent Glen Valley soils). The summary, conclusions and
recommendations that can be made are as follows;
6.1.1
CHROMIUM
Chromium was detected in significant concentrations in the leaves (0.052 mg/L –
0.819 mg/L) and the fruits (0.054mg/L – 0.599mg/L) of the tomatoes for most
treatments.
Irrigation of the tomato plants with normal Glen Valley treated waste water at pH 8.5
in sludge absent Glen Valley soils has been shown to reduce the uptake of chromium
in the leaves and the fruits of the tomato plants compared to irrigation of the tomato
plants with tap water in sludge absent Glen Valley soils. A similar reduction pattern
was observed in the leaves and the fruits of the tomato plants irrigated with treated
waste water at pH 9.0 in sludge amended soils
Accumulations of chromium in the tomato leaves and the fruits tend to increase as the
pH of the treated waste water increased from slightly acidic (pH 5.0 to pH 6.0) and
close to a neutral pH of 7.0. Chromium uptake increased more than two fold in the
tomato leaves and fruits when the pH of the irrigating waste water was raised from 5.0
to 6.0.
94
Tomato leaves and fruits cultivated in standard commercial soil with tap water take up
slightly more chromium than tomato plants cultivated in sludge absent Glen Valley
soil with tap water.
Uptakes of chromium in the tomato leaves and fruits grown in sludge absent Glen
Valley soils using tap water are greater than the uptake of chromium in the tomato
leaves and fruits grown in sludge amended Glen Valley soil using treated waste water
at pH 5.0; 6.0; 8.5 and 9.0. However, there was an exception when the tomato plants
were irrigated with treated waste water at pH 10.0 (in sludge amended Glen Valley
soil); the fruits of the tomato bio-accumulate more chromium than the other
treatments in this case.
 The Food and Agriculture Organization permissible limits and the Botswana Bureau
of Standards effluent quality limits for chromium were exceeded as a result of treated
waste water irrigation in sludge amended Glen Valley soil for tomato production.
6.1.2
NICKEL
Nickel was detected in the tomato leaves (-0.050 mg/L – 0.327 mg/L) and fruits
(-0.030 – 0.224 mg/L) but the concentrations were not significant for most treatments.
Desorption of the nickel occurred in the fruits of the tomato plants cultivated with
sludge absent Glen Valley soil using tap water for irrigation. Nickel was also desorbed
in the tomato leaves and fruits cultivated with sludge amended Glen Valley soils using
normal treated waste water at pH 8.5 and treated waste water at pH 9.0 for irrigation
Irrigation of the tomato plants with normal Glen Valley treated waste water at pH 8.5
in sludge absent Glen Valley soils has been shown to eliminate the uptake of nickel in
95
the leaves and the fruits of the tomato plants compared to irrigation of the tomato
plants with tap water in sludge absent Glen Valley soils. A similar reduction pattern
was observed in the leaves and the fruits of the tomato plants irrigated with treated
waste water at pH 9.0 in sludge amended soils
Accumulations of nickel in the tomato leaves and fruits tend to increase as the pH of
the treated waste water increased from slightly acidic to neutral pH values. Nickel
uptake increased more than two fold in the tomato leaves and fruits when the pH of the
irrigating waste water was raised from pH 5.0 to 6.0.
Uptake of nickel in the leaves of the tomato plants grown in sludge amended Glen
Valley soil with treated waste water at a pH 6.0 was slightly higher than uptake of
nickel in the leaves of the tomato plants grown in sludge absent Glen Valley soil using
tap water.
 The Food and Agriculture Organization permissible limits and the Botswana Bureau
of Standards effluent quality limits for nickel were not exceeded as a result of treated
waste water irrigation in sludge amended Glen Valley soil for tomato production.
6.2
CONCLUSIONS
In terms of chromium uptake, use of treated waste water at pH 8.5 was the most suitable for
the production of tomatoes. At this of pH 8.5 chromium concentration in the leaves and fruits
of the tomato plant was found to bio-accumulate the least amount of chromium compared to
other treatments. For nickel uptake, desorption occurred in the tomato leaves and fruits when
using treated waste water at pH 8.5 and this may be good because nickel toxicity may be
avoided at this pH of 8.5 but it may also be bad because repeated application of treated waste
96
water at pH of 8.5 could trigger leaching of nickel in the soil and consequently to the ground
water table. Irrigation of tomato plants by Glen Valley treated waste water at pH 5.0; 6.0 and
10.0 has increased mean chromium concentration in the tomato leaves and fruits to quantities
above the maximum permitted concentrations, hence usage at these pH levels should not be
allowed. However, the mean nickel concentration is reduced at these pH levels.
In the final analysis, irrigation of tomato plants with normal Glen Valley treated waste water
at pH 8.5 appears to be safe and should be continued, but with caution. As highlighted in this
study, the mean chromium uptake in the tomato plant samples exceeded the permissible safe
limit for agriculture production. Consequently, the edible portions of the tomato plants grown
with treated waste water and sludge amended Glen Valley soils should be subject to testing
and analysis before passing them on to consumers.
6.3
RECOMMENDATIONS
The Glen Valley farmers should consider reducing the pH of the treated waste water
by acid addition to pH 8.5 or pH 9.0 because this should help to reduce or minimize
the uptake of chromium by tomato plants and possibly eliminate the nickel uptake.
Further studies into the uptake and the phytotoxicity effects of other heavy metals in
tomato plants when irrigating with treated waste water should be carried out. It should
pose questions on the bio-assimilation of heavy metals in tomato plants.

Analyze the content of chromium, nickel and other heavy metals in the tomato leaves
and fruits and, compare them to the levels in the soil around the root zone.
Utilization of treated waste water in the long term irrigation plans at the Glen Valley
should be based on an environmentally sound, accurate, and a well-managed
97
approach. Also, programs need to be monitored periodically for the quality of the
properties of the water, treated waste water and plants, in order to minimize the risk of
negative effects to the food chain and human health.
Government should partner farmers to initiate, fund and encourage more research
work on chromium, nickel and other heavy metals of concerns as a result of treated
waste water use. Also, due to few empirical data available from Botswana for this
present study it is imperative for the government of Botswana to actively take a
leading role in the campaign for academicians, researchers, financial backers and the
private sector to contribute in improving the situation.










98
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APPENDICES
Appendix A
Production of vegetables (Lettuce) using treated waste water in Glen valley
Agricultural production of vegetable (green pepper) using treated waste water in Glen Valley
112
Appendix B
Potted tomato plants irrigation with treated waste water at different pH values
Ripen tomato plants just before harvesting
113
Appendix C
Soil analytical data
Soil pH was reported to be basically neutral in the Glen Valley sludge amended soils and
sludge absent soils as shown in table 16.
Table C1: Active and inactive soil pH of Glen Valley sludge amended and sludge absent soils
Soil sample
Active pH
Inactive pH kcl
Glen Valley sludge
7.90 at temp 23.200C
6.05 at temp 23.200C
7.34 at temp 23.200C
5.19 at temp 23.200C
absent soils
Glen Valley sludge
amended soils
Source: (Akande, 2007)
P1
P2
P3
PLATES: P1, P2 and P3 (some potted tomato plants containing withered plants after the
second day of treatment with nutrients, N: P: K)
P4
P5
P6
P7
PLATES: P4, P5, P6 and P7 (some potted tomato plants containing sludge absent and sludge
amended Glen Valley soil and Standard commercial soil after second day of treatment)
114
Appendix D
t-test calculator
The t-test used compares one variable (chromium/nickel) between two groups; using treated
waste water with sludge amended soil on one hand and tap water with sludge absent soil on
the other hand. Group one represents leaves of tomatoes and group two represents fruits of
tomatoes. The data sets are entered according to Table D1 and the GraphPad software
calculates the t-tests. Data set for chromium is presented below (nickel values were obtained
in a similar version).
Table D1: Chromium concentration in the leaves and fruits of tomato plants
Treatment
Group one
Group two
(treated waste water with sludge
(tap water with sludge absent soil)
amended soil)
Mean Values
Mean values
T2
0.217
-0.003
T3
-0.025
-0.030
T4
0.085
0.020
T5
0.262
0.147
T6
-0.050
-0.007
T7
0.062
0.200
115
Unpaired t-test results-Chromium
P value and statistical significance:
The two-tailed P (probability of the result) value, assuming the null hypothesis (Ho) equals
0.9803. By conventional criteria; this difference is considered to be not statistically
significant.
The conventional criteria assume the following:

The null hypothesis (Ho) has priority and is not rejected unless there is strong
evidence against it.

If one of the two hypotheses is 'simpler' it is given priority so that a more 'complicated'
theory is not adopted unless there is sufficient evidence against the simpler one.

In general, it is 'simpler' to propose that there is no difference between two sets of
results than to say that there is a difference.

The outcome of a hypothesis testing is "reject H0" or "do not reject H0". If we
conclude "do not reject H0", this does not necessarily mean that the null hypothesis is
true, only that there is insufficient evidence against H0 in favor of HA. Rejecting the
null hypothesis suggests that the alternative hypothesis may be true.
116
Confidence interval:
The mean of Group One minus Group Two equals 0.00350
95% confidence interval of this difference: From -0.30433 to 0.31133
Intermediate values used in calculations:
t-test value = 0.0253
degree of freedom = 10
standard error of difference = 0.138
Unpaired t-test results for nickel: P value and statistical significance
The two-tailed P (probability of the result) value, assuming the null hypothesis (Ho) equals
0.6157. By conventional criteria, this difference is considered to be not statistically
significant.
Confidence interval:
The mean of Group One minus Group Two equals 0.03283
95% confidence interval of this difference: From -0.10839 to 0.17406
Intermediate values used in calculations:
t-test value = 0.5180
degree of freedom = 10
standard error of difference = 0.063
The Relationship between error bars and statistical significance
From the data in this research a figure as the one shown in Figure 6 is drawn and one may be
tempted to draw conclusions about the statistical significance of differences between group
117
means by looking at whether the error bars overlap. Let's look at two contrasting examples of
chromium uptake between the leaves and fruits when cultivating tomato using sludge absent
Glen Valley soil with tap water on one hand and using sludge amended Glen Valley soil with
treated waste water on the other hand. Figure 6: Chromium concentration in tomato plants in
different treatment (Error bars and statistical significance compared). Bars represent SEM
(n=5)
What can be concluded when standard error bars do not overlap?
When standard error (SE) bars do not overlap, one cannot be sure that the difference between
two means is statistically significant. Even though the error bars do not overlap in experiment
T2 (sludge absent Glen Valley soil with tap water), the difference is not statistically
significant (P=0.9803 by unpaired t test).
What can be concluded when standard error bars do overlap?
When Standard Error bars overlap, as in experiment T3 (sludge amended Glen Valley soil
with treated waste water) one can be sure the difference between the two means is not
statistically significant (P>0.05).
In conclusion, if two Standard Error bars overlap one can conclude that the difference is not
statistically significant, but that the converse is not true (Motulsky, 2002).
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Appendix E
Both of the Figures in appendix E1 and E2 illustrate how the solubility of chromium and
nickel is directly controlled by pH. The y-axis displays the concentration of dissolved
metal in the waste water, in milligrams/liter (mg/L). There is a wide variation in scale. The
upper part of the scale shows a dissolved concentration of 100 mg/L. The lowest number
on the scale is 0.001 mg/L. These solubility graphs display regions where the metals are
soluble or insoluble.
The region above the dark lines (the shaded areas)
for
each
metal signifies that the metals should precipitate as metal hydroxides. This is
referred to as the precipitation region.The region below or outside of the dark lines illustrates
where the metals are dissolved in solution, no precipitation occurs, and no metal removal
takes place.
Appendix E1: Theoretical Solubility of Chromium Hydroxide (Ayres et al., 1994)
Theoretical Solubility of
Chromium Hydroxide vs. pH
Concentration
Dissolved Metal
100
10
1
0.1
Chromium
0.01
0.001
2
3
4
5
6
7
pH
8
9
10
11
12
119
Appendix E2: Theoretical Solubility of Nickel Hydroxide (Ayres et al., 1994)
Theoretical Solubility of
Nickel Hydroxide vs. pH
Concentration
Dissolved Metal
100
10
Nickel
1
0.1
0.01
0.001
2
3
4
5
6
7
pH
8
9
10
11
12
120
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