Advances in Environmental Research 6 Ž2002. 533᎐540
Selective adsorption of chromiumž VI/ in industrial
wastewater using low-cost abundantly available
adsorbents
M. Dakiky U , M. Khamis, A. Manassra, M. Mer’eb
Faculty of Science and Technology, Al-Quds Uni¨ ersity, P.O. Box 20002 East Jerusalem
Abstract
The removal of poisonous CrŽVI. from industrial wastewater by different low-cost abundant adsorbents was
investigated. Wool, olive cake, sawdust, pine needles, almond shells, cactus leaves and charcoal were used at different
adsorbentrmetal ion ratios. The influence of pH, contact time, metal concentration, adsorbent nature and
concentration on the selectivity and sensitivity of the removal process was investigated. The adsorption process was
found to follow a first-order rate mechanism and the rate constant was evaluated at 30⬚C. In the case of wool, the
rate constant was the highest Ž39.6= 10y3 miny1 . and the cactus leaves gave the lowest value Ž6.8= 10y3 miny1 ..
Langmuir and Freundlich isotherms were applicable to the adsorption process and their constants were evaluated.
The thermodynamic equilibrium constant and the Gibbs free energy were calculated for each system. The ⌬G o for
the absorption by wool Žy2.26 kJ moly1 . and that for the cactus leaves Ž2.8 kJ moly1 . supported the findings that
wool was the best among the selected adsorbents for the selective removal of CrŽVI. at pH 2 and an adsorbent
concentration of 16 g ly1 at 30⬚C, for which the removal was 81% out of 100 ppm CrŽVI. after 2 h of stirring. A
comparison between a simulated sample containing 100 ppm CrŽVI. and a true wastewater sample containing 100
ppm CrŽVI., 19 ppm Al, 30 ppm Mg, 49 ppm Ca, and 10 ppm B, showed that the adsorption process is satisfactory
and selective for CrŽVI.. 䊚 2002 Elsevier Science Ltd. All rights reserved.
Keywords: ChromiumŽVI.; Adsorption; Adsorption isotherm; pH effect; Adsorbent; Thermodynamics
1. Introduction
Chromium occurs most frequently as CrŽVI. or
CrŽIII. in aqueous solutions. The two oxidation states
have different chemical, biological and environmental
properties ŽWorld Health Organization, 1988.. CrŽIII.
is relatively insoluble, and an essential micronutrient
ŽSaner, 1980., while CrŽVI. is a primary contaminant
U
Corresponding author. Fax: q972-2-2796960.
E-mail address: dakiky@planet.edu ŽM. Dakiky..
because of its toxicity to humans, animals, plants and
microorganisms ŽUS Department of Health and Human Services, 1991; Cieslak-Golonka, 1995.. Chromium
has widespread industrial applications; hence, large
quantities of chromium are discharged into the environment. The major industries that contribute to water
pollution by chromium are mining, leather tanning,
textile dyeing, electroplating, aluminum conversion
coating operations, plants producing industrial inorganic chemicals and pigments, and wood preservatives
ŽUdy, 1956.. The level of chromium in discharged
wastewater should be reduced, or recycled if possible.
1093-0191r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved.
PII: S 1 0 9 3 - 0 1 9 1 Ž 0 1 . 0 0 0 7 9 - X
534
M. Dakiky et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 533᎐540
Several methods are utilized to remove chromium from
industrial wastewater. These include: reduction followed by chemical precipitation ŽZhou et al., 1993.; ion
exchange ŽTiravanti et al., 1997.; reduction ŽSeaman et
al., 1999.; adsorption ŽDahbi et al., 1999.; electrochemical precipitation ŽKongsricharoern and Polprasert, 1996.; solvent extraction ŽPagilla and Canter,
1999.; membrane separation ŽChakravarti et al., 1995.;
cementation ŽLin et al., 1992.; evaporation; reverse
osmosis; foam separation; freeze separation; and biosorption ŽAksu and Kutsal, 1990; Aksu et al., 1996..
Adsorption is an effective and versatile method for
removing chromium, particularly when combined with
appropriate regeneration steps. This solves the problems of sludge disposal and renders the system more
economically viable, especially if low-cost adsorbents
are used ŽBailey et al., 1999.. Several recent publications utilized different inexpensive and locally abundantly available adsorbents Že.g. activated carbon
ŽLeyva-Ramos et al., 1995., agricultural by-products
ŽSamantaroy et al., 1997., waste materials ŽNamasivayam and Yamuna, 1995., and charge minerals ŽSingh
et al., 1992.. However, the literature is still insufficient
to cover this problem, and more work and investigations are needed to deal with other locally available
and cheap adsorbents to eliminate CrŽVI. from industrial wastewater samples with different compositions
and characteristics. The simultaneous quantitative determination of the micronutrient CrŽIII. and the carcinogenic CrŽVI. is of great importance in environmental
analysis and biological studies. Different speciation
techniques are applied prior to detection. Chromatography ŽPantsar-Kallio and Manninen, 1996., solvent
extraction ŽNygren and Wahlberg, 1998., co-precipitation ŽZou et al., 1996., capillary ion electrophoresis ŽJia
et al., 1996. and electro-deposition ŽBermejo-Barrera
et al., 1998. are the methods most utilized.
In this paper, wool, olive cake, sawdust, pine needles, almond shells, cactus leaves and charcoal, all
abundant, low-cost locally available adsorbents, were
studied to determine their efficiency in removing
chromium from simulated contaminated samples. The
effects of pH, contact time, adsorbent concentration
and metal ionradsorbent ratio were investigated at
30⬚C.
2. Experimental
2.1. Materials
All primary chemicals used were of analytical reagent
grade. K 2 Cr2 O 7 , CrCl 3 ⭈ 6H 2 O, NaOH, and H 2 SO4
were purchased from Merck. The seven adsorbents
used in the study: wool, olive cake, sawdust, pine nee-
dles, almond shells, cactus leaves and coal, were taken
from local Palestinian natural resources. Wool, freshly
cut from sheep, was washed with water and detergent,
dried to constant weight at room temperature and then
sized to 1-cm-long fibers prior to use. The other adsorbents were cleaned, dried to constant weight and ground
to pass through a 50-mesh screen Žparticle size, 200
␮m.. No further characterization or analysis was performed on the adsorbents.
2.2. Instruments
A Varian Vista charged-coupled device axial simultaneous inductively coupled plasma-atomic emission
spectrometer ŽVesta CCD ICP-AES. was used for
chromium determination. The pH of the solutions was
measured with a 3310 Jenway pH meter using a combined glass electrode calibrated with buffers of pH 2, 4,
and 7. The solutions were shaken with a Untritronic-OR
ŽSelecta P. thermostated electronic shaker. Centrifugation was performed on a Sigma 2-3 centrifuge.
2.3. Method
Stock solutions Ž5000 ppm. of CrŽVI. and CrŽIII.
were prepared by dissolving 7.0719 g of AR grade
K 2 Cr2 O 7 and 12.8111 g of AR grade CrCl 3 ⭈ 6H 2 O,
respectively, in 500 ml of deionised, double-distilled
water. Standard solutions of the required CrŽVI. and
CrŽIII. concentrations were prepared by appropriate
dilution. Batch adsorption studies were carried out for
different adsorbents using 100-ml conical flasks containing 25 ml of the test solutions at the desired initial
chromium concentration and pH. The required amount
of the adsorbent material was then added and the flask
contents were shaken for the required contact time at
30⬚C in an electric thermostated shaker. The contents
of the flask were filtered through filter paper, centrifuged and the supernatant was analyzed for final pH,
and final chromium concentration using ICP. The percentage removal of chromium was calculated as follows:
% removal of Cr s Ž Cint y Cfin . = 100rCint
where Cint and Cfin are the initial and final chromium
concentrations, respectively.
Throughout the study, the contact time was varied
from 30 to 300 min, the pH from 1.0 to 10, the initial
chromium concentration from 20 to 1000 ppm and the
amount of adsorbent from 2 to 24 g ly1. A comparison
between true wastewater samples from an aluminum
powder coating factory and laboratory-simulated synthetic samples was performed.
The kinetic parameters for the adsorption process
M. Dakiky et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 533᎐540
535
were studied on the batch adsorption of 100 ppm of
CrŽVI. at 30⬚C at pH int 2. The contact time was varied
between 0.5 and 2.0 h and the percent removal of
CrŽVI. was monitored. The data were fitted to the
Lagergren equation ŽNamasivayam and Yamuna, 1995.:
3. Results and discussion
log Ž qe y q . s log qe y K ad tr2.303
The selected adsorbents Žwool, pine needles, sawdust, olive cake, almond shells, cactus leaves and coal.
were used at concentrations ranging from 2 to 24 g ly1
in a batch adsorption technique at 30⬚C. At 8 g ly1 of
adsorbent, the removal of CrŽVI. was found to be
between 68.7% Žfor wool. and 19.8% Žfor almond
shells.. Increasing the adsorbent concentration resulted
in an increase in the percentage removal of CrŽVI.
ŽFig. 1.. At 16 g ly1 of wool, the removal of CrŽVI.
from solution was found to be 81.3%. The variation in
the sorption capacity between the various adsorbents
could be related to the type and concentration of
surface groups responsible for interaction with the
metal ions. The selected adsorbents are from two different classes of fibers. Wool is a protein-based animal
fiber, with many amino and carboxylic groups that may
play a major role in metal binding. The other adsorbents are cellulose-based plant fibers, with many hydroxy groups that may bind the CrŽVI. ion. The presence of a particular functional group or binding site
does not necessarily guarantee its accessibility as a
sorption site, due to the possible coexistence of steric,
conformational, or other types of barriers. The advan-
Ž1.
where q is the amount of CrŽVI. Žmg gy1 adsorbent.
removed at time t, qe is the amount of CrŽVI. removed
at equilibrium and K ad is the rate constant of adsorption Žminy1 ..
The adsorption isotherms for the CrŽVI. removal
were studied using initial concentrations of CrŽVI.
between 20 and 1000 ppm at adsorbent concentration
of 80 g ly1 at 30⬚C. The data obtained were then fitted
to the Langmuir adsorption isotherm ŽNamasivayam
and Yamuna, 1995.:
Cerqe s 1rQbq CerQ
Ž2.
where Ce is the equilibrium concentration of adsorbate
Žmg ly1 ., qe is the amount adsorbed at equilibrium Žmg
gy1 adsorbent., and Q Žmg gy1 . and b Žl mgy1 . are the
Langmuir constants related to the adsorption capacity
and energy, respectively. The adsorption data were also
fitted to the Freundlich isotherm ŽNamasivayam and
Yamuna, 1995.:
log qe s log K f q 1rnlogCe
3.1. Effect of adsorbent type and concentration
Ž3.
where qe is the amount of adsorbate adsorbed per unit
weight Žmg gy1 adsorbent., Ce is the equilibrium concentration Žmg ly1 . of adsorbate and K f is the Freundlich constant.
The thermodynamic equilibrium constant Ž K co . for
each system was obtained at 30⬚C by calculating the
⬘
apparent equilibrium constant Kc at different initial
concentrations of CrŽVI. and extrapolating to zero:
K c⬘ s CarCe
Ž4.
where Ca is concentration of CrŽVI. on the adsorbent
at equilibrium in mg ly1 and Ce is the concentration of
CrŽVI. in solution in mg ly1.
The Gibbs free energy Ž ⌬G o . for the adsorption
process was obtained at 30⬚C using the formula:
⌬G o s yRT ln K co
Ž5.
where R is the ideal gas constant Ž8.314 J moly1 Ky1 .
and T is temperature in K.
Fig. 1. Effect of adsorbent concentration on CrŽVI. removal
by selected adsorbents: pH 2; wCrŽVI.x s 100 ppm; contact
time, 2 h; and temperature, 30⬚C.
536
M. Dakiky et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 533᎐540
tage of wool for the removal of CrŽVI. ions from
solution over the other adsorbents may arise from both
the high concentration of the sorption sites and the
loose nature of the fiber, allowing easier penetration of
the metal ion to the sorption sites.
3.2. Effect of pH
An increase in the pH of all solutions from the
initial value, pH int , took place in all samples after
stirring of the batch mixture for 2 h with a sample
solution containing 100 ppm CrŽVI.. For all adsorbents,
the percentage removal of CrŽVI. from solution was
affected dramatically by the pH int at which the batch
adsorption was performed. The percentage removal
reached a maximum value at a pH int of approximately
2.0 ŽFig. 2.. It is well known that the dominant form of
CrŽVI. at this pH is HCrO4y ŽNamasivayam and Yamuna, 1995.. Increasing the pH will shift the concentration of HCrO4y to other forms, CrO42y and Cr2 O 72y.
It can be concluded that the active form of CrŽVI. that
can be adsorbed by all adsorbents chosen in this study
is HCrO4y. The increase in pH with contact time from
pH int to pH fin can be explained by hydrolysis of the
adsorbent in water, which will create positively charged
sites. Upon adsorption of HCrO4y, a net production of
hydroxide ions will occur, as shown in Eq. Ž6.:
y.
y
qŽ
⬅OHq
2 q HCrO4 l ⬅OH 2 HCrO4
Ž6.
This change in pH is very small at low pH, since the
Table 1
Relationship between pH and adsorption selectivity
Adsorbent
Removal Ž%.
pH 2
Wool
Olive cake
Sawdust
Pine needles
Almond
Coal
Cactus
pH 5
CrŽVI.
CrŽIII.
CrŽVI.
CrŽIII.
69.3
47.1
53.5
42.9
23.5
23.6
19.8
0.0
0.0
0.0
0.0
0.0
0.0
0.0
5.8
8.4
13.8
13.0
2.3
2.4
8.2
58.3
74.8
96.8
79.4
60.0
99.4
55.2
wCrx s 100 ppm; adsorbent dosage, 8 g ly1 ; contact time, 2
h; and temperature, 30⬚C.
solutions are well buffered by the acids used in this pH
range. This mechanism is in agreement with the findings of previous studies on other adsorbents ŽNamasivayam and Yamuna, 1995.. It is worth mentioning
that, on conducting similar experiments on CrŽIII. under the same conditions, no removal of CrŽIII. by any
of the adsorbents was observed, due to repulsion of the
positive CrŽIII. ions by the positively charged active
centers on the adsorbents at this pH ŽTable 1.. Hence,
this process is very selective for the removal of the
toxic form of chromium from any matrix under these
conditions ŽpH 2.. The adsorption of CrŽIII., on the
other hand, occurs at higher pH values and reaches a
maximum at pH 5. At this pH, the number of negatively charged groups on the adsorbent matrix increases
and enhances the removal of the CrŽIII. species by
coulombic attraction, as shown in Eq. Ž7.:
q
q
2 Ž ⬅Oy . q Cr Ž OH. 2 l Ž ⬅Oy. 2 Cr Ž OH. 2
Ž7.
CrŽVI. adsorption is highly reduced at pH 5 due to the
same mechanism Žsee Table 1..
3.3. Effect of time
Fig. 2. Effect of pH on the removal of CrŽVI. by selected
adsorbents: wCrŽVI.x s 100 ppm; adsorbent dosage, 8 g ly1 ;
contact time, 2 h; and temperature, 30⬚C.
Fig. 3 shows the effect of contact time on the batch
adsorption of 100 ppm CrŽVI. at 30⬚C and pH int 2. It is
obvious that the increase in contact time from 0.5 to
2.0 h increased the percentage removal of CrŽVI.. A
further increase in contact time had a negligible effect
on the % removal. The nature of the adsorbent and its
compactness affected the time needed to reach equilibrium. For wool, this time was 1.5 h. For the other
adsorbents, a contact time of 2 h was needed for
equilibrium to be established, and hence all experiments on the physical properties of adsorption were
conducted after 2 h of contact time. From the time
versus % removal curves, the kinetics of CrŽVI. adsorption on the different adsorbents was found to follow
M. Dakiky et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 533᎐540
Fig. 3. Effect of contact time on the removal of CrŽVI. by
selected adsorbents: pH 2; wCrŽVI.x s 100 ppm; adsorbent
dosage, 8 g ly1 ; and temperature, 30⬚C.
the first-order rate law derived by Lagergren ŽEq. Ž1...
Fig. 4 shows the Lagergren plot for all adsorbents. The
linearity of these plots indicates that a first-order
mechanism is indeed followed in this process. The rate
constants for each system were calculated from the
linear least-squares method and are given in Table 2.
3.4. The adsorption isotherms
The relation between the initial concentration of
537
Fig. 4. Lagergren plot for the adsorption of CrŽVI. by selected
adsorbents: pH 2; wCrŽVI.x s 100 ppm; adsorbent dosage, 8 g
ly1 ; and temperature, 30⬚C.
CrŽVI. and its percentage removal from solution was
studied for all adsorbents included in the study. The
initial CrŽVI. concentrations tested were 20, 100, 200,
300, 400, 500 and 1000 mg ly1 at an adsorbent concentration of 8 g ly1. The adsorption process was found to
follow the Langmuir adsorption isotherm ŽEq. Ž2...
Linear least-squares fitted plots were obtained for the
Table 2
Adsorption constants for the removal of CrŽVI. by the selected adsorbents at pH 2 and 30⬚C
Adsorbent
Wool
Olive cake
Sawdust
Pine needles
Almond
Coal
Cactus
Adsorption
kinetics
Lagergren
Rate
constant
Kad
Žminy1 .
3.96= 10y2
8.99= 10y3
9.00= 10y3
1.18= 10y4
8.80= 10y3
7.44= 10y3
6.80= 10y3
Adsorption isotherms
Langmuir
Q
Žmg gy1 .
b
Žmg ly1 .
41.15
33.44
15.823
21.50
10.616
6.78
7.082
7.15 = 10y3
4.70= 10y3
9.17= 10y3
5.44= 10y3
5.46= 10y3
11.50= 10y3
6.13= 10y3
Adsorption thermodynamics
Freundlich
Kf
N
Equilibrium
constant
KC
Gibbs
free
energy
⌬Go
ŽkJ moly1 .
2.23
0.489
0.877
0.27
0.141
0.207
0.094
2.459
1.450
2.295
0.948
0.3388
0.3712
0.3296
y2.26
y0.94
y2.02
0.134
2.73
2.50
2.80
2.295
1.575
2.29
1.44
1.46
1.68
1.419
538
M. Dakiky et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 533᎐540
low concentration region ŽFig. 5., where the adsorption
isotherms obeyed the Langmuir equation; both Q and
b were evaluated and are given in Table 2. However,
the interpretation of the plots as straight lines in the
low concentration region does not exclude the possible
interpretation as two straight lines when the concentration limit is extended to higher values. The adsorption
process was also found to obey the Freundlich isotherm
ŽEq. Ž3.., where the plots of log qe vs. logCe resulted in
a linear correlation ŽFig. 6.. The K f values were calculated from the linear least-squares fitting and are given
in Table 2. The results obtained showed that wool has
the largest capacity and affinity for the selective removal of hexavalent chromium ions from solutions
under the conditions studied.
Wool was found to fit better to the Langmuir
isotherm, which is based on the formation of layers on
the active sites, while the other adsorbents fit better to
the Freundlich isotherm, which is an empirical expression for the adsorption process ŽAtkin, 1998.. This is
attributed to the difference in the micro and macro
structures of the adsorbents. The adsorption of CrŽVI.
is controlled by three diffusion steps Žfrom bulk solution to the film surrounding the adsorbent, from the
film to the adsorbent surface, and from the surface to
Fig. 6. Freundlich plot for the adsorption of CrŽVI. by selected adsorbents: pH 2.0; adsorbent dosage, 8.0 g ly1 ; contact
time, 2.0 h; and temperature, 30⬚C.
the internal sites. and the binding of the metal ions to
the active sites. Wool, as a polypeptide loose fiber, has
a high absorption capacity in acidic medium. The three
diffusion steps occur very rapidly, allowing a high concentration of CrŽVI. metal ions to bind electrostatically
to the active sites in a monolayer mechanism at low
concentration ŽFig. 5., in which the concentration of
active sites is relatively high. At higher concentrations
of CrŽVI., saturation of the binding sites will occur and
the formation of multilayers will take place. However,
the other adsorbents are cellulose fibers that shrink in
acidic solution, increasing their compactness. This
makes the diffusion steps relatively slow, and diffusion
becomes the rate-determining step in the binding of
CrŽVI. to the active sites of these adsorbents. The
adsorption in this case is random, due to the existence
of a distribution of energetically different active sites.
An empirical equation is more suitable to fit such
behavior. For the wool fiber, the mode of binding is
electrostatic interaction between the metal ion
.
ŽHCrO4y. and the active sites Ž ᎐NHq
3 , whereas for the
cellulose adsorbents, it is hydrogen bonding.
3.5. Thermodynamics of adsorption
Fig. 5. Langmuir plot for the adsorption of CrŽVI. by selected
adsorbents: pH 2; adsorbent dosage, 8 g ly1 ; contact time, 2 h;
and temperature, 30⬚C.
The process of chromiumŽVI. adsorption can be
summarized by the following reversible process, which
represents a heterogeneous equilibrium:
Cr Ž VI . in solution l Cr Ž VI . y adsorbent
M. Dakiky et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 533᎐540
539
Table 3
CrŽVI. removal by wool from simulated samples w100 ppm
CrŽVI.x, and aluminum powder coating wastewater samples
w100 ppm CrŽVI., 19 ppm Al, 30 ppm Mg, 49 ppm Ca, and 10
ppm Bx at different wool concentrations
Wool
Žgrl.
Uptake Žmgrg.
4
8
12
16
13.1
8.5
6.4
5.0
Simulated
Removal Ž%.
True
10.0
7.0
5.5
4.4
Simulated
True
52.1
68.7
77.2
81.3
40.0
56.2
65.8
70.6
Contact time, 2 h; and temperature, 30⬚C.
3.6. True sample ¨ ersus simulated one
Fig. 7. Apparent equilibrium constants versus equilibrium
concentration for the adsorption of CrŽVI. by selected adsorbents: pH 2; adsorbent dosage, 8 g ly1 ; and temperature,
30⬚C.
In this case, the activity should be used instead of
concentration in order to obtain a unitless equilibrium
constant ŽAtkin, 1998.. To achieve the standard state
where the concentration can be used instead of activity,
extrapolation of the apparent equilibrium constant to
the limit of infinite dilution is made Fig. 7. At this
condition, the activity coefficient is equal to unity and
the concentration is then equal to activity. The thermodynamic equilibrium constant obtained is used to calculate all other thermodynamic parameters ŽTable 2..
The Gibbs free energy for the adsorption process was
obtained at 30⬚C using Eq. Ž5. ŽTable 2.. The Gibbs
free energy indicates the degree of spontaneity of the
adsorption process, where more negative values reflect
a more energetically favorable adsorption process. The
negative ⌬G o values obtained in this study for some
adsorbents confirm the feasibility of these adsorbents
and spontaneity of the adsorption. The ⌬G o value for
wool Žy2.26 kJ moly1 . shows that it has the largest
capacity and affinity for the selective removal of CrŽVI.
compared to the other adsorbents used in this study. A
similar ⌬G o value Žy2.53 kJ moly1 . was reported for
the removal of CrŽVI. by biogas residual slurry under
similar conditions ŽNamasivayam and Yamuna, 1995..
Comparing a true sample taken from an aluminum
coating factory w100 ppm CrŽVI.x and a laboratory
simulated sample w100 ppm CrŽVI.x showed that the
percent removal of CrŽVI. from the simulated sample
was higher than that for the industrial effluent ŽTable
3.. The percentage removal of CrŽVI. from the
aluminum powder coating wastewater as a function of
initial CrŽVI. concentration in the effluent sample is
also shown in Table 4. The difference between the
CrŽVI. uptake from the simulated sample and the
effluent sample is attributed to the presence of other
ions and impurities in the effluent, in which a slight
hindrance to the CrŽVI. adsorption may occur compared to the pure CrŽVI. simulated sample.
4. Conclusion
Selective removal of the poisonous hexavalent form
of chromium from solutions was possible using several
abundantly available low-cost adsorbents. Natural wool
from sheep was the most effective, for which the re-
Table 4
A comparison between simulated samples and true aluminum
powder coating wastewater samples at different initial CrŽVI.
concentrations
wCrŽVI.x
Žppm.
Uptake Žmgrg.
100
300
500
1000
8.5
19.8
26.9
34.5
Simulated
True
10.0
18.1
26.5
30.5
Conditions: pH 2; adsorbent dosage, 8 grl; contact time, 2
h; and temperature 30⬚C.
540
M. Dakiky et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 533᎐540
moval reached 70% of CrŽVI. at 30⬚C. The optimum
pH for removal was found to be 2, at which CrŽVI.
exists mostly as the most easily adsorbed form, HCrO4y.
Increases in the concentration of adsorbent, initial
CrŽVI. concentration and contact time were found to
increase the % removal of CrŽVI.. The kinetics of the
CrŽVI. adsorption on the different adsorbents was
found to follow a first-order rate mechanism. The Gibbs
free energy obtained for wool as an adsorbent showed
that it has the largest capacity and affinity for the
selective removal of the metal. More studies are needed
to optimize the system from the regeneration point of
view and to investigate the economic aspects.
Acknowledgements
The authors are grateful to the National Aluminum
and Profiles Company in Nablus, the Al-Tawfeeq Tanning Factory in Hebron, and the Al-Najafa Electroplating Factory for their help and assistance during this
study. Our gratitude is extended to the Center for
Chemical and Biological Research at Al-Quds University for carrying out the ICP measurements. The generous support from the Palestinian Ministry for Environmental Affairs is highly appreciated.
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