Journal of Saudi Chemical Society (2017) 21, S120–S127
King Saud University
Journal of Saudi Chemical Society
www.ksu.edu.sa
www.sciencedirect.com
ORIGINAL ARTICLE
Removal of hazardous Rhodamine dye from water
by adsorption onto exhausted coffee ground
Kai Shen a, M.A. Gondal
b,*
a
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China
Laser Research Group, Physics Department and Center of Excellence in Nanotechnology, King Fahd University of Petroleum
and Minerals, Dhahran 31261, Saudi Arabia
b
Received 21 June 2013; revised 13 November 2013; accepted 17 November 2013
Available online 26 November 2013
KEYWORDS
Coffee grounds;
Batch adsorption;
Rhodamine dye;
Electrostatic;
Hydrophobic
Abstract Exhausted coffee ground powder (CGP) was proved to be an efficient adsorbent for the
removal of Rhodamine dyes (i.e. Rhodamine B and Rhodamine 6G) from aqueous solutions by
batch adsorption experiments. The morphology, chemical structure as well as the surface property
of the as-prepared CGP adsorbent were investigated by using SEM, FT-IR and contact angle meter
analytical techniques. The adsorption kinetics and isotherm behaviors of Rhodamine molecules
onto CGP were studied and compared using pseudo-1st, pseudo-2nd and Langmuir/Freundlich
models, respectively. The maximum adsorption capacities of Rh B and Rh 6G were calculated at
5.255 and 17.369 lmol g1 by Langmuir model fitting. The effects of temperature, ionic strength,
solution volume and the co-existing anions on the sorption behavior were also investigated.
Furthermore, the adsorption mechanism responsible for the efficient removal of dyes is discussed
in terms of adsorption process caused by electrostatic and intermolecular forces.
ª 2013 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under
the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
The textile industry is one of the major sources, which discharges large amounts of industrial waste water. The discharge
of such contaminated water into public streams is a great environmental challenge not only due to its treatment for reuse but
* Corresponding author. Tel.: +966 38602351; fax: +966 38602293.
E-mail address: magondal@kfupm.edu.sa (M.A. Gondal).
Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
also its toxicity to human beings and animals by contaminating
underground water reservoirs [1]. The Rhodamine dye is one of
fresh peach of synthetic dyes and it is widely used as a colorant
in the manufacturing of textiles and food stuffs. It has been
medically proven that drinking water contaminated with Rhodamine dyes could lead to subcutaneous tissue borne sarcoma
which is highly carcinogenic [2]. In addition, others kinds of
toxicity such as reproductive and neurotoxicity have been
widely and intensively investigated and proved as well by exposure to these dyes [2].
Various natural or wasted materials have been extensively
explored and investigated for the adsorption removal of different contaminants from aqueous solutions [3–6]. Coffee beans
are now produced and used in significant quantities worldwide.
http://dx.doi.org/10.1016/j.jscs.2013.11.005
1319-6103 ª 2013 King Saud University. Production and hosting by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Removal of hazardous rhodamine dye from water by adsorption onto exhausted coffee ground
According to the data reported by United States Department
of Agriculture (USDA), the annual global production capacity
of coffee beans in the year 2012/2013 was estimated as exceeding 150 million of 60 kg bags and in future more production
and waste of coffee ground are expected [7]. Therefore, the efficient utilization of the waste of coffee grounds has attracted
considerable attention as millions of posts of coffee are brewed
and millions of pounds of wet grounds are thrown every day
all around the world. The carbonized form of coffee grounds
has been attempted for soil remediation, adsorption removal
of hazardous molecules from aqueous or gas phases or waste
water desalination [8–10].
In this work, exhausted coffee ground powder (CGP) was
used directly as a zero-cost adsorbent for the application of
adsorption removal of series of Rhodamine dyes like Rhodamine B (Rh B) and Rhodamine 6G (Rh 6G) from aqueous
solutions. The crystal and chemical structures of the asprepared CGP were examined to understand the possible
adsorption mechanism for removal of dyes. In addition, the
adsorption kinetic, isotherm behaviors were compared and
the thermodynamic parameters were calculated as well. Furthermore, the adsorption mechanism responsible for the efficient adsorption removal behavior is discussed in terms of
adsorption caused by electrostatic and intermolecular forces.
2. Experimental
(k ¼ 0:15418 nm). The morphology of the as-prepared CGP
was studied by a scanning electron microscope (SEM, FEI
inspect F50). Fourier transform infrared spectra (FTIR) were
recorded at room temperature using a FTIR spectrophotometer (NEXUS 670). The zeta potentials of the as-prepared
adsorbent at different pHs were determined by a zeta potential
instrument (Malvern, Great Britain). The water contact angle
was measured by a contact angle meter (SL200B, Shanghai) at
ambient conditions.
2.3. Batch adsorption experiments
Batch adsorption experiments were conducted to examine the
adsorption kinetics, adsorption isotherm, the effect of temperature, solution pH and the ionic strength on the adsorption
process as well as desorption and regeneration. A certain
amount of CGP was mixed with Rhodamine dye aqueous solutions with a known initial concentration, and the mixture was
stirred in a stir machine at a constant stirring speed and temperature. The mixture was centrifuged at 4000 rpm in a centrifugation machine after batch adsorption experiments so that
the absorbance of Rh B and Rh 6G can be measured at 554
and 526 nm by means of UV–vis spectrophotometer (JASCO,
V-570), respectively. The concentrations of the solutions were
determined using linear regression equation.
2.3.1. Kinetics [11]
2.1. Materials
The coffee powders were purchased from local market in Saudi
Arabia. To obtain the final CGP product, each 5 g of coffee
powders was washed with 200 mL of boiled deionized water
3 times to get rid of any impurities. Salts of KCl and K2SO4
were purchased from BDH chemicals (Poole England) and
K2HPO4 was purchased from Sigma–Aldrich (USA). The Rh
B and Rh 6G dyes were purchased from Lambda Physik
(USA) and Merck (Germany), and their properties are listed
in Table 1. All the chemical reagents are of analytical grade
and used without further purification.
2.2. Characterization
The crystal structure of the as-prepared CGP adsorbent was
investigated by X-ray diffractometer (XRD, Bruker D8 ADVANCE) with 2h scope of 10–90 using Cu-Ka X-ray source
Table 1
Two kinetic models were used to fit the experimental data at
different temperatures. The pseudo-first-order rate expression
of Lagergren model is generally expressed as follows:
dq
¼ k1 ðqeq qÞ
dt
ð1Þ
where qeq and q are the amounts of adsorbed dye onto the
CGP at equilibrium and at time t, respectively. k1 is the rate
constant of first-order adsorption. The integrated form of
Eq. (1) is:
1
k1
1
¼
þ
q tqeq qeq
ð2Þ
The plots of 1/q against 1/t for the pseudo-first-order equation
give a linear relationship and k1 and qeq values can be determined from the slope and intercept of this equation, respectively.
The pseudo-second-order kinetic rate equation is expressed
as:
Selected properties of Rh B and Rh 6G.
Dyes
Rh B
Rh 6G
C28H31N2O3ÆCl
479.02
C27H29ClN2O3
464.98
Structure
Chemical formula
MW
S121
S122
K. Shen, M.A. Gondal
2
dq
¼ k2 qeq q
dt
ð3Þ
where k2 is the rate constant of second-order adsorption. After
integrating, the following equation is obtained:
t
1
t
¼
þ
q k2 q2eq qeq
ð4Þ
2.3.4. Influence of solution volume
The Rhodamine dye solution volume was adjusted continuously and increased from 10 to 200 mL with the constant
initial concentration of Rhodamine dye of 15 lmol L1, corresponding to the simulation of the waste water basin where the
solution volume varies with time and place [14].
2.3.5. Influence of ionic strength
2.3.2. Thermodynamics [12]
The Gibbs free energy change DGo, indicates the degree of
spontaneity of the adsorption process. For significant adsorption to occur, the free energy changes (DGo) of adsorption
must be negative. The Gibbs free energy change of adsorption
is defined as:
DGo ¼ RT lnK
To investigate the effect on ionic strength on the adsorption
capacity, the ionic strength varied from 0.001 to 0.1 M by adding different amounts of K2SO4, and the concentration of
Rhodamine dye was 15 lmol L1.
ð5Þ
The other thermodynamic parameters, change in the enthalpy
(DHo), and entropy (DSo), were determined by using following
equations:
K¼
CA
CS
DGo ¼ DHo TDSo
ð6Þ
ð7Þ
where K is the equilibrium constant; CA, the amount of dye
adsorbed on the adsorbent of the solution at equilibrium;
CS, the equilibrium concentration of the dye in the solution.
T is the solution temperature and R, the gas constant.
2.3.3. Isotherms [13]
In this study, two classical adsorption models, i.e. Langmuir
and Freundlich isotherms, were employed to describe Rh B
and Rh 6G adsorption equilibrium. The Langmuir isotherm
is valid for monolayer adsorption onto a surface with a finite
number of identical sites. The Langmuir model is based on
the assumption of adsorption homogeneity, such as equally
available adsorption sites, monolayer surface coverage, and
no interaction between adsorbed species. The Langmuir model
can be expressed as:
qe ¼
KL qm Ce
1 þ Ce
ð8Þ
where Ce is the equilibrium concentration of the adsorbates in
the solution, qm the maximum adsorption capacity and KL is
the adsorption equilibrium constant.
The Freundlich equation is the empirical relationship
whereby it is assumed that the adsorption energy of a protein
binding to a site on an adsorbent depends on whether or not
the adjacent sites are already occupied. One limitation of the
Freundlich model is that the amount of adsorbed solute increases indefinitely with the concentration of solute in the solution. This isotherm can be described as follows:
qe ¼ KF ðCe Þ1=n
ð9Þ
where qe is the adsorption capacity at equilibrium, Ce the equilibrium concentration of adsorbates in the solution, and KF
and n are the physical constants of Freundlich adsorption isotherm which indicate the adsorption capacity and adsorption
intensity, respectively.
Figure 1 Representative SEM images (a, b) and FTIR (c) of the
as-prepared CGP adsorbent. The inset figures a and c show the
particle size distributions (sample size = 168) and the image of a
water droplet on the surface of CGP surface demonstrating its
hydrophobic property, respectively.
Removal of hazardous rhodamine dye from water by adsorption onto exhausted coffee ground
2.3.6. Influence of co-existing anions
The effects of co-existing anions on the Rhodamine dye
adsorption were studied in 15 lmol L1 Rhodamine dye solution. Three kinds of salts of KCl, K2SO4 or K2HPO4 were
selected to study the effect of co-existing anions on the adsorption capacity.
2.3.7. Regeneration by photolysis and reuse
The regeneration of saturated adsorbent by photolysis was carried out under a Xenon lamp irradiation with full arc (460
Watts, Oriel instrument). FTIR measurement suggested that
the CGP adsorbent can be almost regenerated after irradiation
for 2 h as no FTIR signals of Rhodamine dye molecules can be
detected. The regenerated adsorbent was re-used in the next
cycle of adsorption.
3. Results and discussion
3.1. Characterization of CGP adsorbent
The amorphous characteristic was proved by XRD measurement (not shown) as there is no apparent diffraction peak in
the 2h range of 1–90. Fig. 1(a and b) depicts the SEM image
of the as-prepared CGP adsorbent. The average particle size
is calculated at 11.4 lm under the sample size of 168. Most of
the particle sizes are ranged 1–20 lm. The enlarged image of
the adsorbent is shown in Fig. 1(b), which suggests the hierarchical morphology of the as-prepared CGP adsorbent. As clear
from Fig 1(b), the particles of adsorbent are of micrometer size
S123
and also consisted of numerous nanoparticles of hundreds of
nanometers size. FTIR spectrum of the as-prepared CGP
adsorbent was measured to study the surface chemical structure. As depicted in Fig. 1(c), the hydrophobic functional group
of the C–H group was centered at wavenumbers 2921 and
2841 cm1, and the C–O group at 1741 cm1. The hydrophobic property of the CGP surface was also studied. The image
of a water droplet on the surface of the CPG layer was captured
and shown in the inset of Fig. 1(c). The contact angle of the
droplet was measured at 113, further suggesting the hydrophobic nature of the CPG surface.
3.2. Adsorption kinetics
As depicted in Fig. 2, a sharp increase can be observed by raising the adsorption temperature from 278 to 292 K. After
adsorption for 2 h, the adsorption–desorption equilibrium
can be reached and the adsorption capacity of Rh B was calculated at 3.02 and 1.661 lmol g1 at 292 and 278 K for Rh B,
respectively. However the saturated adsorption capacity of
Rh 6 G was found to be 4.16 and 5.45 times that of Rh B,
which is calculated at 12.7 and 7.00 lmol g1 at 292 and
278 K, respectively. The fitted constants by the first-order
and second-order kinetic models for Rh B and Rh 6G adsorption are listed in Table 2. The correlation coefficients for the
first-order kinetic and the second-order kinetic models were
0.99, close to 1.0 for Rh 6G at 292 and 278 K, Rh B at
292 K. Therefore, the adsorption of Rh B and Rh 6G by the
adsorbent CGP can be approximated favorably both by the
pseudo-first-order model and the pseudo-second-order model.
3.3. Adsorption thermodynamics
It can be noticed clearly from Fig. 2 that the adsorption capacity of Rh B and Rh 6G on CGP are increased by increasing the
adsorption temperature, demonstrating the endothermic nature of the adsorption process, which is further demonstrated
by the calculated positive value of DHo. As shown in Table 3,
the positive value of DSo indicates the reversible characteristic
of the adsorption of Rh B and Rh 6G onto CGP. DGo values
were positive indicating that the adsorption process led to an
increase in Gibbs free energy. Positive DGo values of Rh B
indicate the nonspontaneity of the adsorption process and
the negative DGo value of Rh 6G indicates the adsorption process was spontaneous.
Figure 2 The variations of adsorption capacity of Rh B and Rh
6G as a function of contact time by the as-prepared CGP
adsorbent at 278 and 292 K (experiment conditions: adsorbent
dosage = 50 mg, solution volume = 50 mL; contact time = 3 h;
temperature = 292 K).
Table 2
Dye
3.4. Adsorption isotherms
Fig. 3 depicts the adsorption amount of RhB and Rh 6G on the
CGP adsorbent at different equilibrium concentrations, and the
Constants for the pseudo first-order and pseudo second-order kinetics for Rh B and Rh 6G adsorption on CGP adsorbent.
Temperature (K)
First-order kinetics
k1 (min1)
qeq (lmol g1)
Second-order kinetics
r2
k2 (g lmol1 min1)
qeq (lmol g1)
r2
Rh B
292
278
37.944
14.599
4.018
1.661
0.999
0.981
6.56
41.22
4.018
1.661
0.999
0.981
Rh 6G
292
278
37.537
28.808
16.702
9.057
0.999
0.990
1.59
3.83
16.703
9.058
0.999
0.990
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Table 3
K. Shen, M.A. Gondal
Thermodynamic parameters for the adsorption of Rh B and Rh 6G on CGP adsorbent.
Dye
Temperature (K)
K
Rh B
292
278
0.369
0.125
Rh 6G
292
278
3.923
1.327
DGo (kJ/mol)
DHo (kJ/mol)
DSo (J/(mol K))
2.420
4.806
52.185
170.429
3.318
0.654
52.246
190.286
of Rh B and Rh 6G was theoretically calculated at 5.255 and
17.369 lmol g1 by Langmuir model fitting, respectively.
For different adsorbents, the saturated adsorption capacity
for adsorbates can be compared by calculating the amount of
adsorbates adsorbed on the adsorbents through adsorption
isotherms. Some of the natural (waste) adsorbent materials
and their adsorption capacities are given in Table 5. As shown
in Table 5, the low cost natural (waste) adsorbent, such as kaolinite, wasted biological sludge had been used to remove RhB
or Rh 6G, and the corresponding adsorption capacity can be
reached to as high as 46 and 16.3 mg g1, respectively. Some
natural adsorbents, such as natural zeolite and Na+-Montmorillonite are not inexpensive, but nevertheless their adsorption
capacities for Rhodamine molecules are low and the reuse of
such exhausted coffee grounds is appealing in terms of resource utilization.
Figure 3 Adsorption isotherms of Rh B and Rh 6G on CGP
adsorbent at 292 K (experiment conditions: adsorbent dosage = 50 mg, solution volume = 50 mL; contact time = 3 h;
temperature: 292 K).
3.5. Influence of solution volume
corresponding fitted results are listed in Table 4. It can be seen
that the adsorption isotherms of Rh B and Rh 6G on the CGP
adsorbent can be fitted better by the Langmuir model than the
Freundlich model, indicating the homogeneous (monolayer)
adsorption characteristic. The monolayer adsorption capacity
Table 4
Langmuir and Freundlich isotherm constants for adsorption of Rh B and Rh 6G on CGP adsorbent.
Dye
Langmuir isotherm model
Rh B
Rh 6G
Table 5
Fig. 4 depicts the effect of solution volume on the adsorption
capacity of Rh B and Rh 6G over the as-prepared CGP adsorbent. It can be found that the adsorption capacity of Rh 6G
and Rh B decreases linearly from 15.2 to 9.3 lmol g1, and
5.3 to 1.6 lmol g1 with rising solution volume from 10 to
Freundlich isotherm model
qm (lmol g1)
KL (L lmol1)
r2
KF (lmol11/n L1/n g1)
n
r2
5.255
17.369
4.364
1.053
0.958
0.882
1.190
8.161
2.062
2.952
0.884
0.749
Comparison of adsorption capacity of a few natural (waste) adsorbents for Rh B and Rh 6G removal.
Adsorbents
Adsorption capacity (mg g1)
Refs.
Kaolinite
Bentonite
Luffa cylindrical
Na+-Montmorillonite
Australian natural zeolite
Coal ash
Cellulosic waste orange peel
Anaerobic sludge
Cellulose-based wastes
Biological sludge
Trichoderma harzianum mycelial waste
Na+-Montmorillonite
Coffee ground powder
46.1 (Rh B)
7.7 (Rh B)
9.9 (Rh B)
38.27 (Rh B)
2.12 (Rh B)
2.86 (Rh B)
3.23 (Rh B)
19.52 (Rh B)
20.6 (Rh B)
16.3 (Rh 6G)
3.40 (Rh 6G)
0.4 (Rh 6G)
5.26 (Rh B)
17.37 (Rh 6G)
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
This study
Removal of hazardous rhodamine dye from water by adsorption onto exhausted coffee ground
S125
ionic Rhodamine molecules to the adsorbent surface. It is also
interesting to find that there is almost no apparent effect of ionic strength on the adsorption capacity of Rh B, indicating the
less important role of electrostatic force in the adsorption process of Rh B.
3.7. Influence of co-existing anions
Figure 4 Effect of solution volume on the Rh B and Rh 6G by
CGP adsorbent (experiment conditions: adsorbent dosage = 50 mg, solution volume = 50 mL; contact time = 3 h;
temperature: 292 K).
100 mL, respectively. The adsorption capacity decrease might
be due to the overlapping of adsorption sites as a result of
overcrowding of adsorbent particles [27]. This procedure
may correspond to simulation of the adsorption removal that
takes place in a lake or river where the rate of solute and
solvent are relatively constant and the solution volume varies
with time and place.
3.6. Influence of ionic strength
It is well known that the ionic strength could affect the electrostatic interactions between adsorbents and adsorbates to a
certain extent, meanwhile the electrolytes can affect the
adsorption behavior by competing with adsorbate ions on
the surface of adsorbent as well. The effect of ionic strength
on the uptake of Rhodamine molecules by the as-prepared
CGP is shown in Fig. 5. A negative correlation between
adsorption capacity of Rh 6G molecules and ionic strength
can be found. The reason could be possibly the fact that the
increased concentration of K2SO4 could neutralize the surface
negative charge, and an increase in the electrostatic repulsion
between the negatively charged CGP adsorbent surface and
the cationic Rh 6G molecules, inhibiting the adsorption of cat-
Figure 5 Effect of ionic strength on the Rh B and Rh 6G by
CGP adsorbent (experiment conditions: adsorbent dosage = 50 mg, solution volume = 50 mL; contact time = 3 h;
temperature: 292 K).
The anions of chloride, sulfate and hydrogen phosphate which
are considered to exist commonly in actual groundwater, were
adopted to investigate the influence of anions on the adsorption capacity of Rhodamine dyes onto the as-prepared CGP
adsorbent. As depicted in Fig. 6, it can be found that anions
of hydrogen phosphate lead to the greatest decrease of the
adsorption capacity of Rh B, but no apparent negative effect
of chloride anions on the adsorption capacity was found for
both Rh B and Rh 6G. The decrease of adsorption capacity
could possibly be caused by the competition adsorption of anions with target model compounds (Rhodamine dye molecules) on the surface of adsorbent. Besides, an electrostatic
field might form around the surface of adsorbent as well,
which also can promote the electrostatic repulsion.
3.8. Regeneration by photolysis and reuse
The saturated CGP adsorbent can be regenerated easily upon
UV light exposure. In our study, a simulated sunlight irradiated from Xenon lamp was used as light source for adsorbent
regeneration. FTIR measurements (not shown here) clearly
show that almost no Rh 6G and Rh B signals can be detected
after 2 h of exposure, suggesting that the saturated CGP adsorbent was regenerated almost completely. The adsorption
capacities of Rh B and Rh 6G in the successive five cycles
are shown in Fig. 7. After the first cycle of reuse, the adsorption capacity of Rh B and Rh 6G decreases from 3.4 and
12.5 lmol g1 to 2.1 and 9.3 lmol g1, respectively. After five
cycles of reuse, their capacities decrease to 0.9 and
5.5 lmol g1, respectively. The decrease in adsorption capacity
Figure 6 Effect of co-existing anions on the adsorption of Rh B
(left) and Rh 6G (right) on the CGP adsorbent (experiment
conditions:
adsorbent
dosage = 50 mg,
solution
volume = 50 mL; contact time = 3 h; temperature: 292 K).
S126
Figure 7 Changes of adsorption capacity of Rhodamine molecules onto adsorbent in five successive adsorption–desorption
cycles.
after regeneration could be attributed to the unsuccessful
regeneration sites on the as-prepared CGP adsorbent.
4. Possible mechanism
To understand the possible adsorption mechanism, the effect
of pH condition on the adsorption capacity of Rh B and Rh
6G was investigated and results are depicted in Fig. 8(a). It
can be found that the adsorption capacity is highly dependent
on the pH of the solution which affects the degree of ionization
of adsorbate. The maximum adsorption capacities of Rh 6G
and Rh B were achieved at 25.1 and 7.18 lmol g1 under
conditions of pH = 2. However, only adsorption capacities
K. Shen, M.A. Gondal
Figure 9 The possible adsorption process trend on CGP
adsorbent surface.
of 9.1 and 1.7 lmol g1 were obtained for the adsorption of
Rh 6G and Rh B in aqueous solutions of pH = 10, respectively, indicating that the static electric absorption mechanism
occurred during the adsorption process. The zeta potentials of
the CGP adsorbent surface under different pH conditions were
measured. As shown in Fig. 8(b), it can be noticed that the zero
point of zeta-potential (isoelectrical point, IEP) of the
as-prepared CGP adsorbent was examined at 3.5, indicating
the positive surface charge of samples at pH < 3.5. The initial
pH value for Rh B and Rh 6G aqueous solutions with a concentration of 15 lmol L1 is around 6.0, therefore the negative
charge of the CGP adsorbent surface can be expected. The
electrostatic interaction might have occurred between the negatively charged surface of CGP and the positive charge of two
kinds of Rhodamine molecules. It is worth noticing that CGP
adsorbent exhibits a much higher adsorption capacity of Rh
6G than Rh B and threefold improvement of capacity can be
found by the batch adsorption mentioned above. The reason
should be possibly due to the repulsion forces that existed between the dissociated carboxylic (–COOH) group (pKa = 3.1
[28]) and the negatively charged surface of CGP adsorbent,
which might inhibit the adsorption process through electrostatic forces. However, the adsorption can still be found under
basic conditions, even though the adsorption capacity is much
lower, indicating that the inter-molecular interaction might
possibly be involved.
On the other hand one can expect, the hydrophobic interaction between the surface of CPG (which has been proved
above) and the ester group (–COOCH3) in Rh 6G molecules
might took place in the adsorption process as well, which facilitates the efficient uptake on the surface of CGP adsorbent.
Therefore, on the basis of the electrostatic and inter-molecular
interactions, it can be preliminarily concluded that the much
more higher adsorption capacity of Rh 6G should be caused
by the repulsion forces from the dissociated carboxylic (–
COOH) group in Rh B molecules, and the hydrophobic interaction forces from the ester group (–COOCH3) in Rh 6G molecules (as illustrated in Fig. 9).
5. Conclusion
Figure 8 Changes of adsorption capacity of Rhodamine molecules (a) and zeta potential (b) on the surface of the as-prepared
CGP adsorbent as a function of pH conditions.
In this work, exhausted coffee ground powder has been proved
to be an efficient adsorbent for the removal of Rhodamine
dyes of Rh B and Rh 6G from aqueous solutions. The pseu-
Removal of hazardous rhodamine dye from water by adsorption onto exhausted coffee ground
do-2nd order adsorption kinetic model can describe best the
adsorption process of both Rh B and Rh 6G on CGP adsorbent. Temperature-dependent adsorption experiments suggest
that the adsorption process is of exothermic and endothermic
nature. The maximum adsorption capacities toward Rh B and
Rh 6G were estimated at 5.255 and 17.369 lmol g1 according
to the Langmuir model. Anions of hydrogen phosphate were
found to significantly interfere with Rh B and Rh 6G adsorption. Both the increase of ionic strength and solution volume
could not be related to the improved uptake capacity. The saturated CGP can be regenerated easily by the photolysis method. The higher adsorption capacity of Rh 6G onto CGP
adsorbent may be caused by the repulsion forces from the dissociated carboxylic (–COOH) group in Rh B molecules and
through the hydrophobic interaction forces from the ester
group (–COOCH3) in Rh 6G molecules.
Acknowledgments
The support under project # R15-CW-11 (MIT11109 &
MIT11110) and RG1011-1 by KFUPM is highly appreciated.
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