Evaluation of the Adsorption of Hexavalent Chromium on Kaolinite and Illite

doi:10.4236/jep.2011.210155

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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.

E-mail: [email protected]

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













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

Cr OHCrO



) or

chromate (2

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

HCrO



) in an acidic medium and chromate (2

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:

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:



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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