Adsorption of cd (II) in kaolinite

Hấp phụ của Cd hóa trị 2 trên khoáng kaolinite

COLLOIDS

SURFACES

ELSEVIER A: Physicochemical and Engineering Aspects 126 (1997) 137-147

Adsorption of cadmium(II) on kaolinite

Michael J Angove, Bruce B Johnson, John D Wells

La Trobe University, Bendigo, P.O Box 199, Bendigo, Bendige, Vic 3550, Australia

Received 2 August 1996; accepted 18 November 1996

Abstract

Three types of experiment were used to study the adsorption of Cd(II) onto two kaolinite samples at 25°C (1) Adsorption edges were characterised by a plateau around pH 5-6 separating an initial adsorption stage beginning about pH 4, and a second stage in the pH range about 7-9 The plateau was higher for the sample with greater face area

(2) Adsorption isotherms at constant pH could be fitted closely by a simple Langmuir model at pH 5.50, but a two-site Langmuir model was better for the data at pH 7.50 One of the model sites at pH 7.50 had a similar maximum adsorption as the single site at pH 5.50, but the equilibrium constant was greater At pH 5.50 one proton was released into the solution for the adsorption of about five cadmium ions, but at pH 7.50 the ratio was about 1:1

(3) Potentiometric titrations of kaolinite suspensions in the presence and absence of Cd(II) could be modeled very

closely by a surface complexation model assuming constant capacitance Parameters from this model were used in turn to predict the adsorption edges with remarkable precision

The results from all the experiments are consistent with the view that Cd(II) adsorbs to kaolinite by two distinct

processes: ion-exchange at the permanently-charged sites on the silanol faces, and complexation to aluminol and

perhaps silanol groups, which occur in particular at the crystal edges © 1997 Elsevier Science B.V

Keywords: Adsorption; Cadmium; Kaolinite; Langmuir; Surface complexation

1 Introduction

Adsorption and desorption of cadmium from

oxide, oxyhydroxide and clay particles largely

determine its availability within the environment

As cadmium is a highly toxic element which is

accumulated in plants its adsorption to soils and

soil minerals has been the subject of many studies

over recent years Factors influencing adsorption

of metal ions include pH [1], the nature of the

substrate [2-4], the nature and concentration of

the adsorbate [5,6], the presence of competing or

complexing ions or ligands [7,8], ageing of the

substrate and residence time of the metal ion at

the surface [9] and temperature [10-12]

Adsorption of heavy metals on phyllosilicates

0927-7757/97/$17.00 © 1997 Elsevier Science B.V All rights reserved

PIT 80927-7757(96)03990-8

has been shown to be significantly different from adsorption on oxides [2,13,14] In the case of oxides the fraction adsorbed usually increases smoothly from ~0 to ~ 100% over a pH range of

2 or 3 units, but the adsorption edges for layer

silicates such as smectite and kaolinite are often characterised by well-defined steps

Layer silicates differ from simple oxides and

oxyhydroxides in four obvious ways:

(1) 1:1 layer silicates, such as kaolinite, consist of

a tetrahedral silica sheet and an octahedral alumina sheet bonded together by the sharing

of oxygen atoms between the silicon and aluminium atoms in adjacent sheets The 1:1

layers are held together in the crystal by hydrogen bonding

(2) The crystals carry negatively charged sites at

all pH values as a result of isomorphous

substitution For kaolinite this is generally the

replacement of Si(IV) by AI(III) atoms in the

tetrahedral sheet

(3) For some layer silicates, particularly 2:1 miner-

als like smectites, the bonding between layers

is weak, allowing exchangeable cations to

reside in the interlayer region

(4) The edges of layer silicates contain both AIOH

and SiOH sites We shall call these jointly

SOH, and note that they can become SOH?

and SỐ” at low and high pH respectively

As a consequence of these properties there are

up to three different ways by which metal ions

may be sorbed to layer silicates: ion exchange at

the permanent negative charges, exchange in the

interlayer region and surface complex formation

at the SOH sites Since kaolinite has strong bond-

ing between the layers, exchange in the interlayer

region cannot occur, leaving ion exchange on the

siloxane faces and formation of surface complexes

with SOH groups

Schindler et al [2] proposed that adsorption of

Cu(II), Cd(H) and Pb(II) on kaolinite involves

two kinds of binding sites: weakly acidic XH sites

able to undergo ion exchange, and AIOH sites on

which specific adsorption occurs through the for-

mation of inner sphere complexes Because the

first stage of adsorption of Zn(II), Co(II), Cu(II)

and Cd(1I) on kaolinite began at the same pH for

all four metals, Spark et al {14] deduced that the

weakly acidic groups of Schindler et al were the

permanent negatively-charged sites of the silox-

ane faces

A contrary view has been offered by Schulthess

and Huang [13], who investigated adsorption of

NI(H), Zn(H), Cd(1I) and Pb(IT) on kaolinite,

montmorillonite and synthetic mordenite They

argued that all adsorption involved pH-dependent

ion exchange, with the weakly acid sites being

SiOH and the specific adsorption sites AIOH

The recent EXAFS study of Co(II) adsorption

on montmorillonite by Papelis and Hayes [15]

provides direct evidence of adsorption on two

quite different types of site At low pH and ionic

strength Co(II) adsorbs mainly at interlayer,

permanent-charge sites forming outer-sphere

complexes, while at higher pH Co(II) complexes with surface hydroxy groups

We report here a study of the adsorption of

Cd(II) on two kaolinite samples, in which we have used three different sets of experiments to obtain

data suitable for modeling the adsorption pro- cesses Adsorption edges were used to define the effect of pH on adsorption Two pH values were then chosen for measurement of constant-pH

adsorption isotherms, one characteristic of the first

adsorption stage and the other of the second The number of protons released during adsorption at each pH was determined by titration The constant

pH results were modeled with a simple Langmuir equation at pH 5.50 and a two-site Langmuir

equation at pH 7.50 The results are compared

with those for adsorption on alumina and silica substrates Finally, potentiometric titrations of

kaolinite suspensions were conducted in the

absence and presence of Cd(II), and the results modeled by use of a combined ion exchange — surface complexation model

2 Experimental

2.1 Preparation and characterization of substrates

One kaolinite sample, from Comalco Weipa, was supplied by the Advanced Mineral Products

Centre (University of Melbourne, Australia) after

treatment to remove residual iron oxide by electro- magnetic separation and extensive dialysis against

Milli-Q reagent grade water Another kaolinite

sample, from Ajax Chemicals, was used without further treatment The BET surface areas (mea- sured on a Micrometrics ASAP 2000) were

28.13+0.06 m? g~! (Comalco), and 14.73+0.02 m? g! (Ajax)

Scanning electron micrographs (Fig 1) showed that both samples had the flat plate-like structure

which characterizes kaolinite particles Comalco

kaolinite had an average face diameter of 290 nm

(range 180-650 nm) and edge thickness of 90 nm (range 50-140 nm) while Ajax kaolinite crystals

were significantly larger and thinner with an

average face diameter of 490nm_ (range

M.J Angove et al / Colloids Surfaces A: Physicochem Eng Aspects 126 (1997) 137-147 139

(b)

Fig 1 Scanning electron micrographs of (a) Ajax and (b) Comalco kaolinite showing crystals of similar shape but very different size distribution Secondary electron images at 20 kV using a Cambridge $150 SEM

180-1000 nm) and average edge thickness of 60 nm

(range 35—65 nm)

X-ray diffraction of the two kaolinite samples

generated d-spacings which corresponded closely

in both position and intensity with those in the

literature [16]

The alumina and silica samples were those used

by Spark et al [4]

2.2 Adsorption experiments

All solutions were prepared from AR grade chemicals dissolved in Milli-Q water Both the

adsorption experiments and the potentiometric titrations described below were performed in a

thermostated reaction vessel, at 25°C, under an

atmosphere of CO,-free nitrogen The pH

140 M.J Angove et al / Colloids Surfaces A: Physicochem Eng Aspects 126 (1997) 137-147

electrodes were contained completely within the

thermostated vessel and calibrated with NBS stan-

dard buffers at the reaction temperature In most

experiments the mass of kaolinite used was that

required to give a surface concentration of

94m?L1

Measurements of adsorption edges and adsorp-

tion isotherms at constant pH followed the meth-

ods described previously [11], with the following

minor modifications The supporting electrolyte

used in all experiments was 0.005M KNO3 Higher

electrolyte concentrations have been shown to

restrict the adsorption of Cd(II) at low pH [14]

Samples removed from the reaction system for

Cd(II) analysis by flame atomic absorption spec-

trometry (Varian SpectrAA10) were filtered

through 0.22 pm polycarbonate filters in all experi-

ments In the constant-pH experiments the number

of protons released per metal ion adsorbed was

estimated from the volume of KOH dispensed by

the Metrohm 614 Impulsomat to maintain the

selected pH

2.3 Potentiometric titrations

The titration procedure was based on the

method of Yates [17] The required mass of Ajax

kaolinite was added to the reaction vessel together

with 300 mL of 0.005M KNO; solution The sus-

pension was stirred using a PTFE-coated magnetic

stirrer under an atmosphere of CO,-free N, for

18-24 h The initial pH was noted, and the suspen-

sion titrated to pH 3.5 with 0.100 m HNO; From

pH 3.5 the system was titrated in steps up to pH 9

with 0.100M KOH and then with acid down to

the initial pH After each addition of acid or base

the pH was monitored until the drift was less than

0.01 pH units per minute, a criterion that was

typically achieved within 20-30 min

A similar set of titrations was performed on

Ajax kaolinite suspensions in the presence of

5.0 x 1075M Cd(II)

2.4 Modeling the adsorption results

The one-site Langmuir adsorption isotherm was

chosen to model adsorption data collected at

pH 5.50:

Naki C N=

1+K,C where N is the amount of Cd(II) adsorbed per

unit area of substrate and C is the equilibrium

solution concentration of Cd(JI) The equation contains two adjustable parameters: N,,,, the max- imum adsorption density per unit area of substrate, and K,, the equilibrium constant for the overall adsorption process Multi-site adsorption models

were considered, but these either fitted the data

poorly or yielded unrealistic parameter values The derivation of the one-site Langmuir equation assumes that all adsorption sites are of equal energy and that only monolayer adsorption occurs The data collected at pH 7.50 were modeled by use of a two-site Langmuir equation

Nm Ki C + Nm K2C

~~ 14K,C 1+ÑC

The two-site model assumes that there are two independent populations of sites responsible for adsorption, and contains four adjustable parame-

ters: N,, and K for each of the site types While it

has been found to fit adsorption data over a wide

range of concentrations there is generally no reason

to suppose that there are just two site types present

at the surface [18] However, in this case two quite different populations of surface sites are expected,

so its use has a practical justification Values for

adjustable parameters were estimated by use of an

iterative procedure based on a nonlinear least-

squares algorithm as outlined by Rodda et al [19]

The potentiometric titration data for Ajax kaolinite suspensions were analysed by the use of

a combined ion exchange-surface complexation model, assuming constant capacitance, similar to that used by Schindler et al [2] The surface reactions assumed were:

XK*H=XH+K*

where X~ represents a negatively charged

exchange site, and,

SO” +H* =SOH SOH + H* =SOH}

M.J Angove et al / Colloids Surfaces A: Physicochem Eng Aspects 126 (1997) 137-147 141

Values for the equilibrium constants were eval-

uated by use of FITEQL version 3.1 [20] Initial

estimates of {X ~} and {SOH} were derived from

the values of the Langmuir parameters N,,, and

Ny at pH 7.50

The parameters estimated from the kaolinite

titrations (equilibrium constants and site densities)

were introduced as fixed values when modeling the

Cd(II)- kaolinite titration data The likely surface

reactions for Cd(II) were found by use of

FITEQL Outputs from this modeling were equi-

librium constants for the proposed adsorption

reactions on the exchange (X_) sites and the

variable-charge (SOH) sites

3 Results

3.1, Adsorption edges

Fig 2 shows adsorption edges for both of the

kaolinite samples: identical surface areas were used

Fig 2 Adsorption of Cd(II) from 5 x 107°M solution at 25°C

on kaolinite samples from Ajax (@) and Comalco (©)

Background electrolyte 5 x 10~3M KNO,;; BET surface area

94 m? L~! for each substrate The points were obtained directly

from adsorption experiments The lines were calculated inde-

pendently, by use of parameters from surface complexation

modeling of the potentiometric titration data shown in Fig 5

in the experiments represented by these data In contrast to results for Cd(II) sorption by oxides and oxyhydroxides, which show a sharp, but regu- lar, increase in the fraction adsorbed with pH [4,6], adsorption on kaolinite reached a plateau between pH 5 and 7 before increasing again at higher pH Adsorption before and after the plateau

can be considered as separate adsorption stages

The adsorption edges coincide at pH values above 7.50 for both kaolinite samples and for alumina and silica These results are not presented here but are essentially the same as those found

by Spark et al [14] Although the first stage of adsorption began at about the same pH for both kaolinite samples, heights of the plateau regions were significantly different: about 45% adsorption for Ajax kaolinite and 20% for the Comalco

sample

3.2 Adsorption isotherms Adsorption isotherms were measured at

pH 5.50, which represents adsorption at the end

of the first adsorption stage, and at pH 7.50, which

is after the onset of the second stage for both kaolinite samples Adsorption isotherms at both

pH values are shown for Ajax kaolinite in Fig 3 and Comalco kaolinite in Fig 4 Fig 4 (for the Comalco sample) includes data from experiments

in which the surface area of kaolinite was

94m?L~!, as well as some at the surface area

70 m? L~ 1 The results are indistinguishable The lines represent the best fit obtained by use of the simple Langmuir equation for pH 5.50, or the two- site Langmuir equation for pH 7.50 Values of the parameters are given in Table 1 The isotherms for

pH 5.50 show clearly that in the first adsorption stage the Ajax sample adsorbed two to three times

as much Cd(II) as the Comalco sample, over the

whole range of concentrations studied The extra adsorption at pH 7.50, represented by the differ- ence between the isotherms at pH 7.50 and 5.50,

was similar for the two kaolinite samples, suggest-

ing that they contain a similar number of sites for the second adsorption stage

We also measured the adsorption of Cd(II) at

fixed pH on alumina and silica samples, at the same BET surface area as for the kaolinite

142 M.J Angove et al / Colloids Surfaces A: Physicochem Eng Aspects 126 (1997) 137-147

10° C/ (mol m”) Fig 3 Cd(II) adsorption isotherms measured at 25°C on the

Ajax kaolinite sample at pH 7.50 (@) and pH 5.50 (©) The

line at pH 5.50 was calculated from a single-site Langmuir equa-

tion, while that at pH 7.50 was calculated from a two-site

Langmuir equation: the parameters are given in Table 2

10° C { (mol m*) Fig 4 Cd(IE) adsorption isotherms measured at 25°C on the

Comalco kaolinite sample at pH 7.50 (@) and pH 5.50 (0)

The line at pH 5.50 was calculated from a single-site Langmuir

equation, while that at pH 7.50 was calculated from a two-site

Langmuir equation: the parameters are given in Table 2

experiments At pH 5.50 the amount adsorbed was too small to provide meaningful results While

significant adsorption occurred on both silica and

alumina at pH 7.50, the amount adsorbed was much less than that for either kaolinite sample, reaching about 1.8 x 107’ mol m~? for each sub-

strate when the Cd(II) concentration was 8 x

10 ”M

Table 1 shows that N,,,;, while essentially the

same at pH5.50 and 7.50 for each kaolinite sample, was much smaller for Comalco kaolinite

than for the Ajax sample Conversely there is

reasonable agreement between the values of K, for the two samples at either pH, but for both samples

K, was about three times as great at pH 7.50 as it

was at pH 5.50 The values of N,,, and K, were the same, within error, for the two kaolinite

samples

3.3 Proton stoichiometry

Table 2 shows the proton stoichiometry, y, (the number of protons released per Cd(II) ion adsorbed) measured during the constant pH experiments The two kaolinite samples gave sim- ilar values, but the results at pH 5.50 and 7.50

were quite different At pH 5.50 the value of y (0.2) indicates the release, on average, of one proton for every five metal ions adsorbed, which

is substantially lower than has been found for the adsorption of metals by hydrous oxides [4,6, 19]

While the result at pH 7.50 (~ 1.0) was also rather

low, it represents the average for adsorption in both the first and second stages The low value for

x in the first stage suggests that the proton stoichi- ometry in the second stage alone would be nearer 2

3.4 Potentiometric titrations

Results from potentiometric titrations for Ajax

kaolinite with and without 5 x 10-°M Cd(II) are

shown in Fig 5 The lines, which represent the best-fit obtained from the surface complexation model assuming constant capacitance, fit the data

very closely Values of the model parameters for Ajax kaolinite are shown in Table 3 A measure of

the goodness of fit of the model for titration data

is the weighted sum of squares divided by the

MJ Angove et al | Colloids Surfaces A: Physicochem Eng Aspects 126 (1997) 137-147 143

Table 1

Langmuir model parameters for adsorption of Cd(II) on kaolinite at pH 5.50 and 7.50

Proton stoichiometry for adsorption of Cd(II)

Parameters from surface complexation modeling on Ajax

kaolinite

Comalco 5.50 0.2 SOH = SO~ +H* —7.15

7.50 1.0 XH+K* = XK+H* — 2.88

Silica 7.50 0.3 Cd?* 4+2XK = X,Cd+2K* 3.01

Concentration of surface sites: {SOH}=(3.2+0.1)x

er 7 an excellent fit For kaolinite alone V(y) =12.6,

Ni - while for the Cd(II) — kaolinite system V(y) =

16.6 The value of the specific capacitance, x, was

Š „ for adsorption of Cd(II) on kaolinite

3 7 The titration data in the presence of Cd(II) was

Cd** +2SOH=Cd —(SO), +2H*

Other reaction stoichiometries and adsorbing

6 ; L—— 1 : — species were investigated, but none fitted the exper-

pH

Fig 5 Results for potentiometric titration of Ajax kaolinite

with (®) and without (©) 5x 107°M Cd(II) The lines were

calculated using a constant capacitance surface complexation

model: the parameters are given in Table 3

degrees of freedom, or V(y) The magnitude of

V(y) is given in the FITEQL output: a value of

V(y) between 1 and 20 is considered to indicate

imental data over the whole pH range as well

as these

Knowledge of the equilibrium constants for the proposed adsorption reactions, together with the

site densities, allows the calculation of the concen-

trations of the adsorbed species as a function of

pH Thus it is possible to construct an adsorption

edge from the parameters of the surface complex- ation model The line for adsorption on Ajax

kaolinite in Fig.2 shows the adsorption edge

144 M.J Angove et al | Colloids Surfaces A: Physicochem Eng Aspects 126 (1997) 137-147

predicted by the surface complexation model

compared with the experimental results shown as

points This line has been calculated from parame-

ters derived from surface complexation modeling

of potentiometric titration results, without refer-

ence to the adsorption edge data The fitted line

for Comalco kaolinite was constucted using the

equilibrium constants from the Ajax sample, and

the concentrations of surface reactive sites, {xX ~}

and {SOH}, from the Langmuir constants N,,,

and Nz of the Comalco sample

4 Discussion

4.1 Adsorption edges

Adsorption of Cd(IT) on kaolinite below pH 6.5

(Fig 2) was quite different from that on either

alumina or silica, which may be considered to be

analogues of kaolinite However, above pH 6.5 the

adsorption edge on kaolinite corresponded closely

with those for silica and alumina Similar results

have been reported by Spark et al [14] The fact

that an adsorption plateau appears in the adsorp-

tion edge for kaolinite suggests that there exists

on kaolinite an entirely different set of sites which

are responsible for the initial adsorption phase

Those sites are most probably the permanent nega-

tive charges which are due to isomorphous substi-

tution on the siloxane face

The difference in the percent adsorption in the

plateau region reflects the different face areas for

the two kaolinite samples shown in the Scanning

Electron Micrographs (Fig 1) Because of the

different face to edge ratios the relative area avail-

able for adsorption is significantly smaller for the

Comalco sample than for the Ajax sample

4.2 Adsorption at pH 5.50

The Langmuir one-site equation provides an

excellent fit to the data for both kaolinite samples

at pH 5.50 (Figs 3 and 4), suggesting that adsorp-

tion in the first stage occurs on identical, non-

interacting sites This is consistent with adsorption

onto the permanent negative charges of the silox-

ane face, which are relatively scarce and far apart

There was little or no improvement in the fit when the two-site Langmuir equation was tested, and

the fitted value of K, was generally negative, which

is physically impossible

In agreement with the adsorption edge results,

the value of N,,, for Ajax kaolinite is almost twice that for the Comalco sample These maximum site densities, calculated from the value of N,, to be about 1 meq per 100 g of dry clay, fall within the

range of permanent-charge densities reported for kaolinite crystals [21] Thus there are sufficient sites on the permanent charge kaolinite face to account for all the adsorption that occurs at

pH 5.50 At the site densities found for the two

samples, each adsorbed Cd(II) species occupies approximately 1 nm? of the siloxane face, about

twice the area (0.44nm7’) required to adsorb a hydrated Cd(II) ion [11]

The low proton stoichiometry (y=0.2) at

pH 5.50 is consistent with ion exchange at surface sites which are initially occupied mainly by cations

from the background electrolyte

4.3 Adsorption at pH 7.50

Adsorption was much stronger at pH 7.50 than

at pH 5.50 A multi-site model was required to describe adsorption at this pH, which implies that sites with different binding energies must be consid- ered The likely change between pH 5.50 and

pH 7.50 is the availability of variable-charge

hydroxyl sites at the higher pH

The values of the two-site Langmuir parameters are of particular interest If adsorption on the first site type involved only ion exchange on the silox-

ane face, the parameters might be expected to be

similar to those found at pH 5.50 The values of

Nm Show excellent concordance at the two pH

values for both of the kaolinite substrates, suggest- ing strongly that they represent adsorption on the

same set of surface sites Values for K, are, how- ever, quite different, increasing by a factor of

about three as the pH increases from 5.50 to 7.50 This result is not surprising, as the Langmuir

equation makes no allowance for the changing chemistry of surface reactions Even though the

proton stoichiometry of the ion exchange process

is small (0.2), protons are released The 100-fold

M.J Angove et al / Colloids Surfaces A: Physicochem Eng Aspects 126 (1997) 137-147 145

decrease in concentration of H* from pH 5.50 to

pH 7.50 is therefore expected to result in a signifi-

cant change in the value of K,

Values for N,, and K, are less certain, but

indicate that more adsorption sites are available

in the second stage than the first The smaller

value of N,,, is not surprising since adsorption in

the first stage most likely involves interaction with

permanent-charge sites, which are sparsely distrib-

uted in kaolinite [21] The uncertainty in the value

of N„¿; does not allow conclusions to be drawn

about the relative numbers of variable-charge sites

available on the two kaolinite samples Evidence

of the relative numbers of sites occupied at a given

equilibrium Cd(II) concentration is provided,

however, from the difference in the amounts

adsorbed at pH 7.50 and 5.50 on the two kaolinite

samples Figs 3 and 4 indicate identical amounts

adsorbed within experimental error, corresponding

to 2.5x 1077 mol m~? when C is 2x 10~5M, and

3.5 x 1077 mol m~? when C is 4x 10°°M for both

samples

The higher proton stoichiometry for adsorption

at pH 7.50 suggests that adsorption in the second

stage involves the formation of inner sphere com-

plexes with surface hydroxyl groups, resulting in

the release of protons Complexation reactions

similar to those below have been suggested [6] for

Cd(II) adsorption on oxide surfaces:

Cả? +SOH=SOCd* +H*

Cd?* +2SOH=(SO),Cd+2H*

Benjamin and Leckie [6] found that adsorption

of Cd(JI) on an amorphous iron oxyhydroxide

surface was accompanied by the release of 1.8

protons per Cd(II) ion adsorbed They suggested

that a bidentate adsorption reaction, similar to the

second reaction, may be responsible for the proton

release Given the low proton stoichiometry in the

first adsorption stage, the number of protons

released per ion adsorbed on kaolinite in the

second stage must be close to two, as found by

Benjamin and Leckie

The proton stoichiometry for adsorption on

alumina and silica at pH 7.50 also provides useful

information y was about 1.3 for alumina but only

0.3 for silica under the same conditions The higher

value on alumina suggests that aluminol groups are responsible for most of the adsorption occur- ring on the variable-charge sites of kaolinite at this pH Schindler et al [2] reached the same conclusion on the basis of surface species stability constants

4.4 Potentiometric titrations

The constant capacitance surface complexation model used here fitted the potentiometric titration

data for Ajax kaolinite very closely For kaolinite

alone the surface reactions were ion exchange on X~ sites, together with protonation and deproto- nation of variable-charge SOH sites In the presence of 5x10 ÝM Cd(II), two bidentate

adsorption reactions were added to the model, one

on the exchange sites and the other on the variable-

charge sites The concentrations of X~ and SOH sites (Table 3) were slightly more than double the values of Nj; and N,, found from Langmuir modeling of the adsorption isotherm of Cd(II) on

Ajax kaolinite at pH 7.50 (Table 2) This repre-

sents excellent agreement since the surface com- plexation model requires two adsorption sites per adsorbed Cd(II) species

The values obtained for the surface complex- ation equilibrium constants in Table 3 are similar

in magnitude to those found by Schindler et al

[2] Schindler and coworkers point out that the

inclusion of only two types of adsorption site does not allow for the heterogeneity of the clay, and is therefore an oversimplification of the system While this is undoubtedly true, surface complex- ation modeling does provide a useful means of

identifying those chemical processes that are most

likely to make a significant contribution to adsorption

The equilibrium model proposed by Schindler

et al [2] included two reactions in addition to those used in this study They were:

Cd,, +SOH2Cd—SO* +H*

SOH +Na*=Na—SO+H*

Inclusion of these reactions in our model saw

their equilibrium constants converge towards zero, suggesting that these reactions do not contribute

significantly to the adsorption process It should

be noted that Schindler et al used a background

electrolyte containing Na* rather than K* as used

in this work

Using a constant capacitance surface complex-

ation model for adsorption of Cd(II) on goethite,

Gunneriusson [22] found that inclusion of

the surface complex =FeOHCd?* dramatically

improved the fit of the model The inclusion of an

analogous complex for the Cd(II)-kaolinite

system, however, did not improve the statistical fit

at all, and furthermore its equilibrium constant

tended to zero

Another possibility for adsorption is surface

complexation of hydroxy complexes such as

CdOH* Such a reaction might involve the forma-

tion of the surface species SOCdOH Adding this

reaction to the system did not improve the fit of

the model, and once more the estimated equilib-

rium constant was extremely small, suggesting that

this reaction plays a negligible role in adsorption

Our confidence in the proposed reaction scheme

is strengthened by the excellent agreement between

the adsorption edge predicted for Ajax kaolinite

from the surface complexation parameters in

Table 3 and the experimental results (Fig 2) Even

more remarkable is the coincidence between the

model line and the experimental points for

Comalco kaolinite Here the equilibrium constants

found for Ajax kaolinite were used together with

estimates of surface site concentrations obtained

from two-site Langmuir modeling of the pH 7.50

isotherm for Comalco kaolinite

5Š Conclusions

The sorption of Cd(II) by kaolinite occurs on

two different populations of surface sites Surface

reactions involve bidentate ion exchange onto per-

manent negatively-charged sites on the silanol

faces, and formation of bidentate inner sphere

complexes on pH-dependent surface hydroxyl

groups Ion exchange is characterized by uptake

of Cd(II) at lower pH (24) than is usually found

on oxides or oxyhydroxides, and a low proton

stoichiometry (y=0.2) The surface complexation

process has the characteristics normally associated

with adsorption on oxide or oxyhydroxide

surfaces Adsorption occurred at pH values (>7)

similar to those found on alumina, silica and

goethite, and the proton stoichiometry mirrored those found on oxide substrates Comparison of

the proton stoichiometry for the surface complex- ation reaction with those found on alumina and silica suggests that AIOH groups adsorb more Cd(11) than SiOH groups

The results of the three different sets of experi- ments provide a remarkably consistent picture of adsorption Surface complexation and Langmuir modeling gave very similar estimates of the

concentration of X~ and SOH sites, while surface

complexation parameters obtained from potentio- metric titration experiments predicted the experi- mental adsorption edge remarkably well

Acknowledgment

Financial support was provided by the Australian Research Council Small Grants Scheme M.J.A is the recipient of an Australian Postgraduate Award with stipend Stephen Johnson (AMPC) is gratefully acknowledged for providing the Comalco kaolinite sample and for determining the surface areas of all substrates We also thank the Microscopy and X-ray Analysis Facility at La Trobe University, Bendigo, for the scanning electron micrographs

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