Journal of Environmental Protection, 2011, 2, 1347-1352
doi:10.4236/jep.2011.210155 Published Online December 2011 (http://www.SciRP.org/journal/jep)
1347
Evaluation of the Adsorption of Hexavalent
Chromium on Kaolinite and Illite
Omar Ajouyed, Charlotte Hurel, Nicolas Marmier
Institute of Chemistry of Nice, Faculty of Science, University of Nice Sophia Antipolis, Nice, France.
Received September 21st, 2011; revised October 23rd, 2011; accepted November 28th, 2011.
ABSTRACT
The adsorption of hexavalent chromium on Kaolinite and Illite was studied in order to evaluate their potential for the
reduction of hexavalent chromium mobility and their possible application for the treatment of polluted sediment. The
influence of various parameters affecting the adsorption of hexavalent chromium, such as the pH of aqueous solution,
the ionic strength and the initial metal ion concentration were investigated. The optimal pH range corresponding to the
hexavalent chromium adsorption maximum on the Kaolinite and Illite is 2 - 4 and 2 - 2.6, respectively. The results
showed that hexavalent chromium sorption on Kaolinite and Illite was strongly influenced by the pH, the ionic strength
and the initial metal ion concentration. Langmuir and Freundlich adsorption isotherms are employed to understand the
nature of adsorption at room temperature. The characteristic parameters for each isotherm have been determined. This
showed that the Freundlich isotherm model well described the equilibrium data. The data suggest that the charge of the
clay mineral surface is one of the main factors controlling hexavalent chromium desorption at alkaline pHs.
Keywords: Hexavalent Chromium, Clay Mineral, Sediment, Adsorption, Stabilization
1. Introduction
Heavy-metal concentrations in aquatic ecosystems, espe-
cially chromium, have increased considerably as a result
of inputs from human production and consumption ac-
tivities. In the ecosystem, sediments are the main sink for
these elements, but when environmental conditions change
(pH, redox potential, etc.), sediments can act as a source
of metals. Sediments contaminated with heavy metals
have the potential to impart adverse effects to aquatic or-
ganisms and contribute to the degradation of ecosystem
function. Sediment accumulation in ports and other wa-
terways requires dredging to maintain navigation. If dre-
dged sediments are contaminated, it is therefore neces-
sary to propose appropriate treatment techniques that sa-
tisfy environmental as well as economic criteria.
Chromium is a common pollutant found in industrial
effluents; chromium salts are extensively used in several
industrial processes such as tanneries, electroplating, tex-
tile, dyeing, and metal finishing industries. Chromium is
found in various oxidation states ranging from −II to +VI.
Trivalent [Cr(III)] and hexavalent [Cr(VI)] chromium are
of major environmental significance depending on pH
and redox conditions [1,2]. As an hexavalent chromium
anions are highly mobile in sediment, soil and water en-
vironments. Trivalent chromium, on the other hand, is
known to be essential for protein, lipid and glucose me-
tabolism of mammals. Trivalent chromium is a cationic
species and is rather immobile due to its low solubility,
high adsorption and complexation. Due to the severe
toxicity of hexavalent chromium, the Agency for Toxic
Substances and Diseases Registry classifies it as the top
sixteenth hazardous substance [3]. Indicative limits for
total chromium concentrations in drinking water and re-
claimed wastewater for irrigation are 0.05 [4] and 0.1 - 1
mg/L [5], respectively.
Clay minerals are hydrous aluminosilicates broadly de-
fined as those minerals that make up the colloid fraction
(<2 µ) of soils, sediments, etc. and may be composed of
mixtures of fine grained clay minerals and clay-sized cry-
stals of other minerals such as quartz, carbonate and me-
tal oxides. Usually the term clay mineral is used for ma-
terials that become plastic when mixed with a small
amount of water. Clay minerals play an important role in
the environment by acting as a natural scavenger of pol-
lutants by taking up cations and anions which are toxic to
human and wildlife even at very low concentrations. Clay
minerals have been widely used in a range of applications
because of either high cation exchange capacity, swelling

Evaluation of the Adsorption of Hexavalent Chromium on Kaolinite and Illite 1348
capacity, high specific surface area, and consequential
strong adsorption capacity [6]. Kaolinite and Illite are
two of the most important mineral components in crys-
talline and sedimentary rock formations as far as adsorp-
tion is concerned. For this reason we have decided to
study these clay minerals and to investigate, qualify and
quantify their role in the immobilization of metals in a
polluted sediment.
The aim of this work is to examine the hexavalent
chromium ions adsorption behavior from aqueous solu-
tion on commercial adsorbents (Kaolinite and Illite) by a
batch method as a function of the pH, ionic strength and
initial metal ion concentration of the solution. Adsorption
isotherms have been analysed in terms of Langmuir and
Freundlich equations. This study falls under the context
of the management of dredged sediments, and focus on
the mineralogical fraction which could accumulate ani-
onic species of metals in a polluted sediment.
2. Materials
The Kaolinite and Illite (Illite du Puy) used in this study
come from Sigma-Aldrich and the region of Le Puy-en-
Velay, France, repectively. These adsorbent materials
were chosen on the basis of their availability, potential
efficiency and market-price.
A stock solution containing hexavalent chromium ions
was prepared from the analytical grade K2Cr2O7 (Sigma-
Aldrich, purity equal to 99.5%) in ultrapure water. NaNO3
was used as the supporting electrolyte to maintain the
ionic strength constant during the adsorption experiments.
Solutions of (0.01, 0.1 and 1 M) NaOH and HNO3 were
used for pH adjustment. The pH of the solutions was
measured using a Consort pH meter C 561, calibrated
using buffer solutions at pH 4.00 and 7.00 at room tem-
perature. Chromium concentrations in the supernatant
were determined by Inductively Coupled Plasma-Optical
Emission Spectrometry (ICP-OES). The content (%) of
hexavalent chromium adsorbed by solid was determined
from the difference between initial Ci and final Cf con-
centrations of hexavalent chromium ion in aqueous solu-
tion, before and after contact. The following equation
was used for calculations:

%Cr VI Adsorbed100
if
i
CC
C






3. Batch Adsorption Experiments
All adsorption experiments were conducted in 50 mL
polypropylene tubes at room temperature by using batch
technique. The effect of solution pH (range 2 - 11), ionic
strength (0.01, 0.05 and 0.1 M) and initial Cr(VI) con-
centrations (0.1 and 0.5 mg/L) on the adsorption were
studied. A constant mass (0.2 g) of Kaolinite or Illite was
equilibrated with chromium solution in the presence of
sodium nitrate. A small amount of acid or base was
added to the dispersions to fix pH. The dispersions were
continuously stirred during 24 h, centrifuged at 4000 rpm
for 15 min, filtered through 0.45 µm pore size acetate
filters, acidified, and analysed for metal ion concentra-
tions.
The adsorption isotherms were determined by a batch
technique in a background electrolyte of 0.01 M NaNO3.
In the experiments, 0.2 g of Kaolinite or Illite were mixed
with 50 mL solutions of various hexavalent chromium
concentrations between 0.1 mg/L and 16 mg/L. The pH
of the system was maintained at 8, characteristic of the
pH value for sediment equilibrated with water. After that,
the samples were shaken, the dispersions were centri-
fuged, filtered and acidified for later analyses of anion
concentration.
4. Results and Discussion
Hexavalent chromium exists in solution as chromic acid
(H2CrO4), dichromate (), bichromate (4
2
27
Cr OHCrO

) or
chromate (2
4
CrO

) depending on the pH and the hexava-
lent chromium concentration [7]. The pH and concentra-
tion ranges were used in this study meant that bichromate
(4 HCrO

) in an acidic medium and chromate (2
4
CrO

) in
an neutral and basic medium would be the predominant
form of hexavalent chromium which participate in ad-
sorption [8]. For practical reasons, the hexavalent chro-
mium writing will be used for both chromates and bich-
romates.
4.1. Effet of Solution pH
The pH of the solution is one of the prime factors that
drastically influence the adsorption behavior of a system.
In the present studies, the effect of pH has been observed
by varying the pH of the dispersion in the range 2 - 11.
The results are depicted in Figure 1, which clearly reveal
that the optimum adsorption is at pH  4 for Kaolinite
and pH  2.6 for Illite while the adsorption of hexavalent
chromium decreases as solution pH increased.
It is generally accepted that clay minerals possess two
distinctly different surfaces, that of the basal faces, which
possesses charge arising from isomorphous substitution
within both tetrahedral and octahedral sheets (e.g., Al3+
for Si4+ in tetrahedral coordination, or Mg2+ for Al3+ in
octahedral coordination), and that of the broken bond
surface at the edge. These surfaces are expected to the
adsorption process for cations and anions by cation ex-
change and a surface complexation [9,10]. In our case,
surface complexation at amphoteric edge sites (SOH) is
expected to be the most relevant adsorption process for
chromium.
In aqueous systems, the surface groups of the clay mi-

Evaluation of the Adsorption of Hexavalent Chromium on Kaolinite and Illite1349
Figure 1. Eff ect of pH on the adsorption of he xaval ent chro-
mium onto kaolinite and illite.
neral can be protolyzed in two different ways, as oxide
mineral surfaces [11-13]. First, in acid media, the surface
may be protonated according to ≡SOH + H+  2 SOH

;
Second, in alkaline solutions the surface hydroxyl groups
can dissociate ≡SOH  SO– + H+. Therefore, the con-
centrations of surface species (≡SOH uncharged surface
groups, ≡2 positive charged surface groups, ≡SO–
negatively charged groups) change at different pH values.
With increasing pH, the number of negatively charged
≡SO– groups increases and this leads to the decrease of
hexavalent chromium adsorption due to the electrostatic
repulsion.
SOH
4.2. Effet of Initial Concentration
The content (%) metal removal efficiency as a function
of pH of the Kaolinite and Illite dispersions for various
concentrations of hexavalent chromium is presented in
Figure 2. The results indicate that the content removal of
hexavalent chromium decreases as the initial concentra-
tion of hexavalent chromium increased. It was found that
the adsorption of hexavalent chromium onto Kaolinite
(Figure 2(a)) and Illite (Figure 2(b)) was strongly de-
pendent on initial metal ion concentration and pH values.
This discrepancy is explained by the saturation of the
surface sites of Kaolinite and Illite by chromium.
At low initial hexavalent chromium concentrations, the
available adsorption sites were easily occupied by hexa-
valent chromium resulting in higher removal efficiencies.
However, as the initial concentration of hexavalent chro-
mium increased, most of the available adsorption sites
became occupied, leading to a decrease in the removal
efficiency.
4.3. Effet of Ionic Strength
Adsorption of hexavalent chromium on Kaolinite and
Illite as a function of NaNO3 (0.01, 0.05 and 0.1 M) con-
centration and pH of the solution (2 - 11) is shown in
(a)
(b)
Figure 2. Adsorption of hexavalent chromium onto (a) kao-
linite and (b) illite as a function of pH at different initial
concentration.
Figure 3. The results showed that hexavalent chromium
adsorption is strongly affected by ionic strength, espe-
cially at higher pH values.
It is somewhat surprising that increased ionic strength
would result in increased anion adsorption. The possible
explanation for the observed behavior is related to the
surface charge characteristics of the clay mineral surface,
specifically, the silanol groups of overall formula (>SiOH)
[10,14] and the permanent negative charge. It is known
that substitutions in the inorganic crystalline network,
resulting in a negatively charged surface are independent
of the physico-chemical conditions in the surrounding
medium and that silanol groups have a low PZC (Point of
Zero Charge), resulting also in a negatively charged sur-
face. These negative charges would tend to reduce anion
fractional uptake. At the higher ionic strength, more coun-
terions (positively charged ions) would be attracted to the
surface, thereby screening the negative surface charge
more effectively and allowing more negatively charged

Evaluation of the Adsorption of Hexavalent Chromium on Kaolinite and Illite 1350
(a)
(b)
Figure 3. Adsorption of hexavalent chromium onto (a) kao-
linite and (b) illite as a function of pH at different ionic
strengths.
ions (anions) to adsorb.
As shown in Figure 3, one might argue that adsorption
is increasing with increasing ionic strength at the higher
pH region. This can be explained based on the above hy-
pothesis. It is reasonable to assume that the effect would
be more pronounced at higher pH, where the negative
charge of the mineral surface is higher. It is very pro-
nounced for Illite (Figure 3(b)) and not very pronounced
for Kaolinite (Figure 3(a)). This is not surprising if one
considers the proportion of SiO2 present in the chemical
compositions of the Illite (Table 1) and the most impor-
tant properties of Illite is that they carry a large surface
charge. This is a net negative charge and much of it is a
permanent charge originating within the lattice structure
by isomorphic substitution, which accounts for the cation
exchange capacity (CEC) of Illite. Its value is very high
when compared with Kaolinite [10].
Table 1. Chemical composition (in wt%) of clay minerals.
Elements Kaolinite Illite
SiO2 45.6 53.9
Al2O3 39.7 24.3
Fe2O3 0.3 8.3
K2O 0.9 6.5
MgO 0.1 4.0
Na2O 0.07 1.2
CaO 0.01 0.4
4.4. The Evaluation of Langmuir and Freundlich
Isotherm
In order to optimize the design of an adsorption system,
analysis of the adsorption equilibrium data is important.
Two isotherm equations namely the Langmuir and Fre-
undlich isotherm models were tested for the adsorption
phenomenon of hexavalent chromium onto Kaolinite and
Illite.
The Langmuir model is valid for monolayer adsorption
on a surface with a finite number of similar active sites
[15,16]. The well-known expression of the Langmuir
model is given by the following Equation:
1
ee
e
CC
qQbQ


Ce(mg/L) is the concentration of adsorbate left in solu-
tion at equilibrium, b is the Langmuir bonding energy
coefficient, Q(mg/g) is the adsorption maximum, and qe
(mg/g) is the amount of adsorbate adsorbed per unit mass
of adsorbent.
The empirical Freundlich equation based on adsorp-
tion onto a heterogeneous surface [15,16] is given below
by the following Equation:

1n
efe
qK C
where Kf(mg/g) and n are the empirical Freundlich con-
stants characteristic of the system.
To gain further understanding of the behaviour and
mechanisms involved in the Kaolinite and Illite hexava-
lent chromium interactions, the linearized Langmuir and
Freundlich plots are studied as indicated in Figure 4 and
Figure 5, repectively.
The values of parameters calculated from the slope
and intercept of Figure 4 and Figure 5 are tabulated in
Table 2. These data indicate that the correlation coeffi-
cients (R2) for the Freundlich equation are slightly better
than those for the Langmuir equation, closer to 1. It sug-
gests that there is not only single molecular layer adsorp-
tion, but also asymmetric adsorption on the adsorbent
surface. Vázquez [17] suggested that the disagreement
with the Langmuir model might be due to the heteroge-

Evaluation of the Adsorption of Hexavalent Chromium on Kaolinite and Illite1351
Figure 4. Langmuir isotherm plots for adsorption of hexava-
lent chromium onto kaolinite and illite.
Fig ure 5. F reundlich is other m plots f or adsorpti on of hex a-
valent chromium onto kaolinite and illite.
Table 2. Constant parameters and correlation coefficients
calculated for Langmuir and Freundlich adsor ption mod els.
Langmuir Freundlich
Adsorbent Q (mg/g) b (L/mg) R2 K
f (mg/g) n R2
Kaolinite 0.571 0.098 0.903 0.047 1.2620.995
Illite 0.276 0.152 0.812 0.033 1.4050.979
neity of the adsorbent surface with resulting variation in
adsorption energy. As has been observed by other au-
thors [18,19].
The magnitude of n gives an indication of the favoura-
bility of adsorption. Values of n larger than 1 show the
favorable nature of adsorption. The value of n (Table 2)
suggests that hexavalent chromium is favorably adsorbed
by the Kaolinite and Illite surface.
5. Conclusions
Kaolinite and Illite are found to able to adsorb hexavalent
chromium from aqueous solution under various experi-
mental conditions. The adsorption was found to be strong-
ly dependent on pH, initial hexavalent chromium con-
centration and ionic strength. The uptake of hexavalent
chromium by Kaolinite and Illite was maximal at acidic
medium, and remains significant for neutral pH values.
Furthermore, the results obtained in this study showed
good fits to the Freundlich adsorption isotherm, which
could suggest a multi sites adsorption process. Even if
Kaolinite and Illite are less efficient than iron oxides for
the adsorption of hexavalent chromium, these adsorbents
could be responsible of one part of the uptake of hexava-
lent chromium in sediments.
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