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Geochimica et Cosmochimica Acta 74 (2010) 5718–5728

Modeling the adsorption of Cd (II), Cu (II), Ni (II), Pb (II)and Zn (II) onto montmorillonite

Xueyuan Gu a,⇑, Les J. Evans b, Sarah J. Barabash b

a State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing, 210093, PR Chinab School of Environmental Sciences, University of Guelph, Guelph, Ontario, Canada N1G 2W1

Received 15 December 2009; accepted in revised form 12 July 2010; available online 21 July 2010

Abstract

The adsorption of five toxic metallic cations, Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II), onto montmorillonite was investi-gated as a function of pH and ionic strength and a two-site surface complexation model was used to predict the adsorptiondata. The results showed that in the lower pH range, 3�6 for Cd, Cu, Ni and Zn, and 3�4.5 for Pb, the adsorption was greatlyaffected by ionic strength, while in the higher pH range, the adsorption was not. In the lower pH range, the metallic cationswere mainly bound through the formation of outer-sphere surface on the permanently charged basal surface sites (�X�),while in the higher pH range the adsorption occurred mainly on the variably charged edge sites (�SOH) through the forma-tion of inner-sphere surface complexes. Acid-base surface constants and metal binding constants for the two sites were opti-mized using FITEQL. The adsorption affinity of the five metallic cations to the permanently charged sites of montmorillonitewas Pb > Cu > Ni � Zn � Cd, while that to the variable charged sites was Pb� Cu > Zn > Cd > Ni.� 2010 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Adsorption and desorption reactions of toxic elementsonto clay minerals are critical geochemical processes thataffect their bioavailability and movement in both soilsand sediments. Montmorillonite, an expanding 2:1 phyllos-ilicate clay mineral, is a common component widely distrib-uted in warm and semi-arid temperate regions (Grim,1968). Because of its high cation exchange capacity(CEC), swelling properties and large surface area, montmo-rillonite is also routinely used as an effective barrier in nu-clear waste or hazardous chemical landfills to preventcontamination of groundwater and sub-soils (Murray,2000; Jo et al., 2006; Sampler et al., 2008).

Montmorillonite is a smectitic clay with a permanentnegative structural charge, �X�, generated largely in theoctahedral sheet through isomorphous substitution of

0016-7037/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2010.07.016

⇑ Corresponding author. Tel.: +86 25 8359 5682.E-mail addresses: xygu@nju.edu.cn (X. Gu), levans@uo-

guelph.ca (L.J. Evans), sbarabash@uoguelph.ca (S.J. Barabash).

Al3+ with Mg2+, and a smaller variable charge that canbe either positive, �S–OH2

+, or negative, �S–O�, gener-ated by proton adsorption/ desorption reactions at theedges of the mineral. The CEC of montmorillonites rangefrom 0.70 to 1.30 molc kg�1, with up to 80% of the ex-change capacity derived from the structural charge andthe rest from the negative variable charges on the edgesof the mineral (Weaver and Pollard, 1975). The permanentstructural charge derived from isomorphous substitution ornon-ideal octahedral occupancy can be calculated from achemical analysis of the clay.

The acid–base property of clay minerals has been stud-ied previously. Tournassat et al. (2004) used a MultisiteIon Complexation Model to predict the intrinsic protonbinding constants, Ka

int, for the edges of clay minerals, inwhich Ka

int values are calculated from the fractional chargeof a surface oxygen and from the bond valence of all itsligands (Hiemstra and van Riemsdijk, 1996). However themodel does not account for bond length relaxation at theedge surface (Bourg et al., 2007) which can cause predictedKa

int values to vary by as much as one log unit (Bickmoreet al., 2003). Bourg et al. (2007) concluded that no

Modeling adsorption of heavy metals onto montmorillonite 5719

combination of current models of edge surface structure,reactivity and electrostatics can quantitatively predict, with-out fitted parameters, the experimental titration data overthe entire range of pH (4.5–9) and ionic strength (0.001–0.5 M) covered by current available data.

Although theoretical calculations (Bickmore et al., 2003;Tournassat et al. (2004)) indicate that silanol, �Si–OH, alu-minol, �Al–OH, silica-alumina bridging sites, �SiAl–OH,and alumina–alumina bridging sites, �Al2–OH, all contrib-ute to proton binding at clay mineral edge surfaces, the�Al2–OH sites are probably the most important over thepH range 4.5–9 (Bourg et al., 2007).

Adsorption experiments utilizing the Langmuir and/orthe Freundlich isotherms are often used to compare theadsorption affinity of cations onto montmorillonite (Sposi-to et al., 1981, 1983; Fletcher and Sposito, 1989; Alvarez-Ayuso and Garcia-Sanchez, 2003; Bhattacharyya andGupta, 2007). However, in the past two decades, surfacecomplexation models (SCM’s) have been increasingly usedto describe the adsorption behavior of cations and anionsonto oxide minerals and phyllosilicate clay minerals(Dzombak and Morel, 1990; Davis and Kent, 1990; Gold-berg, 1992; Evans et al., 2010). These more mechanisticmodels provide the opportunity to study the adsorptionaffinity of ions based on their thermodynamic bindingconstants.

The adsorption of a range of toxic metals onto montmo-rillonite has been studied previously. These elements in-clude Cd(II) (Zachara and Smith, 1994; Barbier et al.,2000), Cu(II) (Stader and Schindler, 1993), Ni(II) (Brad-bury and Baeyens, 1997), Pb(II) (Barbier et al., 2000),Zn(II) (Ikhsan et al., 2005; Bradbury and Baeyens, 1997)and U(VI) (Pabalan and Turner, 1997; Hyun et al., 2001)(Table 1).

Using the Extended Constant Capacitance Complexa-tion Model (Ikhsan et al., 2005) successfully modeled theadsorption of Zn(II) onto montmorillonite with two typesof binding sites: one edge site, �SOH, and one permanentcharged site, �X�. Similarly, Marcussen et al. (2009) usedthe same two site model for adsorption of Ni(II) ontomontmorillonite but used the Diffuse Layer Model as theirsurface complexation model. By contrast, the adsorption ofCu (II) onto montmorillonite was modeled by Stader andSchindler (1993) using the Constant Capacitance Modelusing three adsorption sites with two edge sites, �SOH1

and �SOH2, and a permanent charged site, �X�. A similarmodel was used by Baeyens and Bradbury (1997) to exam-ine the adsorption of Ni and Zn but their model did notcontain an electrostatic term. The absence of this electro-static term has been criticized by a number of researchers(Morel, 1997; Morel and Kraepiel, 1997; Bourg et al.,2007).

Although many studies on metal adsorption onto mont-morillonite have been successfully modeled, there is no uni-versal consensus on adsorption sites, surface reactions andmodeling approaches. The associated proton and metalbinding constants are generally inconsistent because themethods and models used in the studies vary considerably.It is thus difficult to compare the adsorption affinity of var-ious metal cations with montmorillonite (Table 1). The

objectives of this study are to evaluate the adsorption affin-ity of five toxic metallic cations, Cd, Cu, Ni, Pb and Zn,onto montmorillonite by developing a surface complexa-tion model with the minimum number of input parameterswhile still successfully modeling the adsorption data atthree different ionic strengths. Ionic strength has a majorinfluence on the adsorption of metals to montmorillonite,especially in the lower pH range (Charlet et al., 1993; Za-chara et al., 1993; Zachara and Smith, 1994 and Baeyensand Bradbury, 1997). The model used in this study includetwo kinds of surface sites, a basal surface site �X� and anedge site �SOH, and Constant Capacity Model (CCM) wasused to model the surface electrostatic property. This modelsimplified the crystallographic properties of montmorillon-ite and may not sound completely chemically real. How-ever, it is useful to predict metal cations adsorptionbehavior by montmorillonite in the studied pH range andalso can give us some idea of the dominant chemical pro-cesses. In our previous studies, this model successfully de-scribed the adsorption properties of these metal ions ontoillite (Gu and Evans, 2007) and kaolinite (Gu and Evans,2008).

2. MATERIALS AND METHODS

2.1. Materials and chemicals

The montmorillonite sample used in this study was fromUpton, Wyoming, United States (distributed by Ward’sNatural Science Establishment Inc.). Before the experi-ments were started, the sample was dispersed in waterand the <2 lm fraction was collected by sedimentationtechniques. The clay was placed in dialysis tubing andsoaked for 5 h in 0.5 M NaNO3 at a pH of 3 to remove dis-solved impurities and to saturate the clay with Na+ ions.Excess salt was removed by washing ten times with ultra-pure water and the clay was then freeze-dried.

X-ray diffraction analysis (Co tube, Rigaku GeigerflexD-Max-B) showed that the sample was pure montmorillon-ite and that no quartz or other impurities were present. TheCEC’s of the Na-montmorillonite measured by Ba2+/NH4

+

exchange were 0.705 mol kg�1, measured at pH 4, and0.908 mol kg�1 measured at pH 8. Using the chemical datapresented in Weaver and Pollard (1975) for Upton mont-morillonite, it was calculated that the net structural chargeon the montmorillonite was 0.714 mol kg�1 and its struc-tural formula was Si4(Al1.54Fe3+

0.15Mg0.33)O10(OH)2,assuming an anionic charge of 22. Pusino et al. (1990) re-ported a CEC of Upton montmorillonite of 0.927 mol kg�1

and Pusino and Gessa (1990) a value of 0.902 mol kg�1.The edge specific surface was assumed to be 46 m2 g�1

(Green-Kelly, 1964). High purity (>99.99 %) metal nitratesalts were used to make up 0.0100 ± 0.0002 M bulk solu-tion in ultra-pure water. All other chemicals used were ana-lytical grade.

2.2. Discontinuous titrations

Automatic potentiometric titration for montmorillonitewas first investigated using a Mandel PC-Titrate automated

Table 1Published model parameters for surface complexation models for montmorillonite.

logcKint

Stader andSchindler (1993)

Charlet et al.(1993)

Zachara andSmith (1994)

Bradbury andBaeyens (1997)

Barbieret al(2000)

Ikhsan et al.(2005) Marcussenet al.(2009)

Source of montmorillonite Wyoming, US SWy-1 Wyoming, US SWy-1 SWy-2 SWy-2 Gonzales,Texas

SWy-2

Surface acid/base reactions

�SOHo + H+� �SOH2

+ 8.16 4.80 5.78 (�0.95)b 4.5(6.0)d 4.38 5.0 3.9 5.1g

�SOHo� �SO� + H+ �8.71 (�5.77a) �6.96 �11.5 (�6.65)b �7.9(�10.5)d �5.26 �7.2 �7.2 �8.5g

�SOHo + Ca2+� �SOCa+ + H+ – – – – �6.9 �8.0 –

�X��H+ + Cat+� �X��Cat+ + H+ – – – – �8.67 �2.2 �2.2 3.6g

2�X��H+ + Ca2+� �X�2�Ca2+ + 2H+ �1.38 – – – – �0.3 �0.3 –

Surface cation binding reaction Cu Al Cd Ni Zn Cd Pb Zn Zn Ni�SOHo + Me2+

� �SOHMe 5.87 – – – – – – – –�SOHo + Me2+

� �SOMe+ + H+ �0.57 1.57 �2.70 (�10.01)b �0.1 (�3.1)d 1.6 (�2.7)d 2.44 1.56 – – �2.5�SOH + Me2+ + H2O � �SOMeOH+2H+

�6.76 �4.35 (�11.69)b – – – – �10.3 �11.0 –

2�X��Cat+ + Me2+� �X�2�Me2+ + 2Cat+ 0.30 �0.75 – 3.1 3.9 5.61 9.85 �0.1 0.2 1.8

Other parameters

Surface complexation model CCM CCM TLM 2SPNE modele DLM ECCM ECCM DDLMSite density �SOH (cmol kg�1) 5.98 (3.55a) 8.64 3.95(4.74)b 4.2(4.0)f 3.68 5.7 14.7 5.49g

Site density �X�H (cmol kg�1) 93 95.2 72 87 26 39.8 29.4 68Total metal concentration (mM) 0.1, 0.2, 0.5 0.001 0.05 0.4 0.1 0.1 0.00852Ss (m2 g�1) 91c 15 – 35 23.9 31.8 83.9 29.7Sd (g l�1) 10 2 0.8 1.1 1 3.14 1.19 0.01j (Farad m�2) 1 5.5 – – – 7, 3f 7, 3f –Background electrolyte Ca(ClO4)2 KCl NaClO4 NaClO4 NaNO3 KNO3 KNO3 NaNO3

a Indicates another similar site �TOH.b Values outside of parentheses are for site �AlOH and values in parentheses are for site �SiOH.c j�Ss = 91, optimized from FITEQLd Values outside of parentheses are for strong site and values in parentheses are weak sitee Two site protolysis model with no electrostatic termf j1 and j2g Adopted from Tertre et al. (2006).

5720X

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Modeling adsorption of heavy metals onto montmorillonite 5721

titration system. However it was difficult to obtain repro-ducible titration curves because they varied depending onthe titration rates and the equilibrium time. Under alkalinecondition, the titrations took many hours to achieve equi-librium. Therefore, a discontinuous titration method wasused in this study to investigate the proton-consuming bud-get, which is similar to that described by Baeyens and Brad-bury (1997).

0.015 g clay and 10 ml NaNO3 were added to a 50 mlNalgene HDPE centrifuge tube. Prior to use, the NaNO3

solution was purged with argon gas to exclude CO2. About0.2 ml of different concentrations of freshly prepared HNO3

or NaOH were added to the centrifuge tubes to adjust thepH to cover the pH range 3–9. The headspace of the centri-fuge tube was about 25 ml and was purged with argon toexclude CO2. Tubes were sealed, shaken for four daysand the pH of the suspension measured with an Orion com-bined PerpHect electrode 8272BN. Experiments were car-ried out in three concentrations of electrolyte: 0.1 M,0.01 M and 0.001 M NaNO3. One of the advantages of dis-continuous titrations is that the equilibrium time for eachpH point is the same and of sufficient time to allow for equi-librium to take place. To estimate the extent of dissolutionof the montmorillonite as a function of pH during the dis-continuous titrations, the supernatant solutions from theexperiment performed in 0.01 M NaNO3 were analyzedby ICP-AES for the major constituent elements Al, Siand Mg. All the experiments were performed in duplicateand at 25 �C.

2.3. Batch adsorption experiments

The retention of the metals to montmorillonite wasinvestigated by batch adsorption experiments using0.1 M, 0.01 M and 0.001 M NaNO3 as the indifferentbackground electrolyte. The experiments were carriedout in a similar manner to that of the discontinuous titra-tion experiments, except that a spike of 0.05 ml of metalsolutions was added to give a final metal concentrationof 4.88 � 10�5 M. After shaking four days, the tubes werecentrifuged and supernatant solutions filtered through a0.22 lm Nylon membranes. The pH was recorded imme-diately and metals in the solution were measured byFlame Atomic Absorption Spectrometry (Varian Spect-raAA 220). The amount of adsorbed metal was calculatedby subtracting the amount of added metal from theamount remaining in solution. Because the proton con-suming data was not needed to generate metal bindingconstants, there was no attempt to exclude CO2. Theinfluence of CO2 was included in the model by addingCO2(g) as an additional component. The partial pressureof CO2 was assumed to be atmospheric (0.00035 atm)and the aqueous carbonate species, H2CO3

o, HCO3�,

CO32�, MeHCO3

+, MeCO3o and, where necessary,

Me(CO3)22� were included in the model matrix. All the

aqueous equilibrium constants of all soluble complexes,including hydroxo and carbonate species, were takenfrom the NIST database version 8.0 (Martell and Smith,2004) and were adjusted to the different ionic strengthsusing the Davies equation.

3. A SURFACE COMPLEXATION MODEL FOR

BINDING OF PROTONS AND METALS TO

MONTMORILLONITE

Most of the surface charges on montmorillonite are gen-erated by isomorphous substitution or non-ideal octahedraloccupancy. These permanent negative charges are distrib-uted along the mineral basal surfaces and are balanced byabsorbing aqueous cations, such as Na+, K+, Ca2+ andMg2+. These cations can be exchanged with other cationsin solution and the exchange reactions are non-specific,stoichiometric and involve the formation of surface outer-sphere complexes. The edges of the silica tetrahedra andalumina or magnesia octahedra provide another type ofsorption site on montmorillonite. In kaolinite, the unitstructure is relatively thick and the 1:1 structures are heldtogether by hydrogen bonding and the cations in solutionscannot penetrate into the interlayer space. Hence the edgesites are the major contributors to the exchange capacityof kaolinite. By contrast in montmorillonite, the edge sitesaccount for a much smaller proportion of the exchangecapacity. The adsorption of metals to these sites involvesthe formation of surface inner-sphere complexes similarto the interaction of these metals with the surfaces of oxideminerals.

Previous studies (Schindler et al., 1987; Lackovic et al.,2003; Ikhsan et al., 2005; Gu and Evans, 2007, 2008) haveshown that a two-site model, including the permanentlynegative charged basal surface sites, �X�, and the variablecharged mineral edge sites, �SOH, could successfully pre-dict the adsorption of metals onto phyllosilicate clay miner-als. The surface reactions and equilibrium constants used inthe model are summarized as follows:

In the presence of a background electrolyte, such asNaNO3, and the added metallic cation the reactions forthe basal surface sites are:

� X� �Naþ þHþ� � X� �Hþ þNaþ ð1Þ

KXNa ¼½� X� �Hþ½NaþcHþ

½� X� �Naþ½HþcNa�

2 � X� �Naþ þMe2þ� � X� �Meþ þ 2Naþ ð2Þ

KXMe ¼½� X�2 �Me2þ½Naþ2c2

Hþ

½� X� �Naþ2½Me2þcMe2þ

For the edge sites, the corresponding reactions are:

� SOHo þHþ� � SOHþ2 ð3Þ

KðþÞ ¼½� SOHþ2

½� SOHo½HþcHþexpðwF =RT Þ

� SOHo þ � � SO� þHþ ð4Þ

Kð�Þ ¼½� SO�½HþcHþ

½� SOHo expðwF =RT Þ

� SOHo þMe2þ þ � � SOMeþ þHþ ð5Þ

KSOMe ¼½� SOMeþ½HþcHþ

½� SOHo½Me2þc2þMe

expðwF =RT Þ

where the square brackets represent solution concentra-tions, the term exp(±wF/RT) is derived from the Boltzman

3 4 5 6 7 8 9

-2*10-4

0

2*10-4

4*10-4

6*10-4

8*10-4

10-3

pH

[H+ ]a

dded

(M) I = 0.1 M

I = 0.01 MI = 0.001 M

Data Model

Fig. 1. Titration curves of Na-saturated montmorillonite,1.47 g L�1, in the presence of NaNO3 solution 25 �C. Dots areexperimental data and lines are fitted models calculated using theparameters given in Table 3.

5722 X. Gu et al. / Geochimica et Cosmochimica Acta 74 (2010) 5718–5728

equation and used to adjust for the electrostatic propertiesof the charged surfaces, w is the surface potential (volt), F isthe Faraday Constant, 96,485 C mol�1, R is the Gas Con-stant, 8.314 J mol�1 K�1, T is the absolute temperature(298.15 K), and c is the aqueous activity coefficient calcu-lated from the Davies equation. All the surface activitycoefficients were assumed to be unity.

The potentiometric titration data for the montmorillon-ite at the three ionic strengths was used to estimate theintrinsic proton binding constants, K(+), K(�), the bindingconstants for the background electrolyte, KXNa, and thespecific capacitances of the various adsorbing surfaces usingthe least square fitting program FITEQL version 3.1 (Herb-elin and Westall, 1994). The site densities of montmorillon-ite are critical parameters needed by SCM’s. For thepermanently charged site �X�, it is normally obtained bymeasuring the CEC (Charlet et al., 1993; Stader and Schin-dler, 1993; Zachara and Smith, 1994; Bradbury and Baey-ens, 1997). However, accurate estimates of the edge sitedensities of montmorillonite still remain uncertain (Bourget al., 2007) and this has lead to a large range of reportedvalues in the literature (Table 1). Edge site densities are ob-tained either from potentiometric titrations (Bradbury andBaeyens, 1997), or optimized by FITEQL or other similarprogram (Charlet et al., 1993; Stader and Schindler, 1993;Zachara and Smith, 1994; Tertre et al., 2006; Barbieret al., 2000; Ikhsan et al., 2005). In this study, the total sitedensity of both permanent and edge sites was assumedequal to the CEC value measured at pH 8 (0.908 mol kg�1).The permanently charged site density was assumed to beequal to the CEC value measured at pH 4 (0.705 mol kg�1),and the variable charge density was obtained by subtractingthe CEC measured at pH 4 from the CEC measured at pH 8(0.203 mol kg�1). The applicability of the Diffuse LayerModel (DLM), the Constant Capacitance Model (CCM),the Extended Constant Capacitance Model (ECCM) andthe Triple Layer Model (TLM) was investigated usingFITEQL (Herbelin and Westall, 1994).

4. RESULTS AND DISCUSSION

4.1. The acid/base chemistry of montmorillonite

Results from the discontinuous titrations showed thationic strength had little or no effect on the titration curvesof the montmorillonite at mildly acidic to alkaline pH val-ues (Fig. 1). This is in contrast to the behavior observedwith oxide minerals which normally possess an intersectionpoint at different ionic strengths (Dzombak and Morel,1990; Hayes et al., 1991; Lumsdon and Evans, 1994). Sim-ilar phenomena for montmorillonite have been reported byBaeyens and Bradbury (1997), Avena and dePauli (1998),Duc et al. (2006), Tertre et al. (2006) and Rozalen et al.(2009). However, in a study by Wanner et al. (1994), ionicstrength was shown to have a larger impact on the titrationcurves. However, this might be because they used relativelyshort equilibrium time (10 mins) after the addition of ti-trants. The results indicate that the hydroxyl groups onthe montmorillonite edges, similar to the same groups onoxides, are not the only surface functional groups reacting

with protons. Other surface sites, such as the permanentcharge sites on the basal surfaces, are also involved in theproton consuming budget.

To investigate whether silanol groups, Si-OH, in addi-tion to the aluminol groups, Al-OH, were important inthe proton consuming budget, the total number of edgesites was apportioned in a 2:1 ratio to these two sites. How-ever analysis of the data using FITEQL failed to convergeto reasonable results when an additional site was consid-ered. It may be because too many logK (totally 5) wereoptimized at one time. Therefore as in our previous twostudies with illite and kaolinite (Gu and Evans, 2007,2008), a two-site model, including the permanently chargedsurface site �X� and variable charged edge site �SOH waschosen in this study. Four surface complexation modelswere investigated – the DLM, the CCM, the ECCM andthe TLM. Because the site densities of the two surface siteswere fixed as the results from CEC measurement, DLMfailed to converge under such condition. ECCM andTLM have more adsorbing planes than CCM and hence re-quire more fitting parameters. Considering on the edge site,only inner-sphere complexation involved for metal and pro-ton adsorption. CCM, one of the most widely used SCM’s,was chosen to describe the electrostatic term on the edgesite. It assumes that all the surface complexes are inner-sphere complexes and there is a linear relationship betweensurface charge potential, w, (J C�1) and surface charge den-sity, r (C m�2).

r ¼ jw ¼ T sF =SsSd

where j is the capacitance, Farad m�2, Ts is the total sur-face charge, mol L�1, Ss is the specific surface, m2 kg�1,and Sd is the suspension density (kg L�1).

Some studies have included the aqueous speciation of Alas part of the proton consuming budget (Tertre et al.,2006). In this study no corrections were made for the disso-lution of Al ions from the montmorillonite as their concen-tration was low enough not to affect proton consumption(Fig. 2). Although there was some dissolution of Mg ions,they will not contribute to the proton consuming budgetuntil relatively high pH values (pb-1,1,1, MgOH+ = 11.42).

3 4 5 6 7 80

50

100

150

200

pHCon

cent

ratio

nof

elem

enti

nso

lutio

n(

M)

AlSiMg

SD =1.46 g L-1

Fig. 2. Dissolution of Na-montmorillonite as a function of pH inthe discontinuous titration experiments.

Modeling adsorption of heavy metals onto montmorillonite 5723

The point of zero net proton charge (PZNPC) of mont-morillonite was observed to occur within a pH range from6.4 to 6.8 at the three ionic strengths used in this study andwas consistent with those reported by Ikhshad et al. (2005).A wide range of PZNPC’s has been reported in the litera-ture – about 5.5 in Duc et al. (2006); 7.0–8.0 in Baeyensand Bradbury (1997) and Tertre et al. (2006); 7.7 in Staderand Schindler (1993) and over 9 in Barbier et al. (2000).

Table 2Summary of all the parameters for the metal adsorption mo

Surface acidity constants: I = 0.001 M I

log K(+) 5.83 6log K(�) �6.60 �log K(XNa) 0.18 0I = 0Surface binding constants: Cd Clog K(SOMe) �2.93 �log K(X2Me) 2.37 2

Other parameters

Site density [�SOH]T (mol kg�1) 0.203 SSite density [�X� ] (mol kg�1) 0.705 SK (Farad m�2) 3.2b E

a Averaged from values in the three ionic strengths whichequation.

b Optimized from FITEQL.

Fig. 3. The predicted surface species distribution of the Na-saturated

The best fit parameters for the acid/base surface proper-ties of the studied montmorillonite sample are summarizedin Table 2. The predicted surface species distribution ofmontmorillonite at various ionic strengths was shown inFig. 3. Results indicated that ionic strength had a major im-pact on the distribution of the surface species �X��H+ and�X��Na+, but not on the edge site species�SOH2

+,�SOHo

and�SO�. These parameters were then fixed and used in theadsorption edge experiments to estimate the values for theintrinsic metal binding constants KX2Me and KSOMe.

4.2. Adsorption of metallic cations onto montmorillonite

The species/component matrix used to calculate the me-tal binding constants using the CCM are shown in Table 3.For all the surface and aqueous reactions and log K valuesused in the model, please refer to the **8 Appendix A table.

The adsorption of Cd (II), Cu (II), Ni (II), Pb (II) andZn (II) onto montmorillonite in solutions of 0.1 M,0.01 M and 0.001 M NaNO3 were investigated by batchadsorption experiments. Two phases of adsorption can beseen from the experimental data (Fig. 4). In the lower pHrange, the adsorption was greatly influenced by the ionicstrength. At higher ionic strengths, the amount of adsorp-tion was decreased, while at lower ionic strengths the extentof adsorption increased. In the pH range 5–7, the adsorp-tion data at the three strengths showed similar trends,

del of Na-saturated montmorillonite.

= 0.01 M I = 0.10 M I = 0 a

.12 6.22 6.046.67 �6.83 �6.63

.18 0.18 0.18

u Ni Pb Zn2.25 �3.16 0.49 �2.80

.49 2.40 2.56 2.39

pecific surface, Ss (m2 g�1) 46uspension density, Sd (g L�1) 1.5lectrolyte 10�1, 10�2, 10�3 M NaNO3

were adjusted to zero ionic strength using the Davis

montmorillonite (1.47 g L�1) at three ionic strengths of NaNO3.

Table 3Species/component matrix used in FITEQL for the batch adsorption data to calculate metal binding constants.

Species Components

�SOHo e(�wF/RT) �X��Na+ Me2+ Na+ CO2(g) H+

1 Me2+ 0 0 0 1 0 0 02 MeOH+ 0 0 0 1 0 0 �13 Me(OH)2

o 0 0 0 1 0 0 �24 Me(OH)3

� 0 0 0 1 0 0 �35 MeHCO3

+ 0 0 0 1 0 1 �16 MeCO3 0 0 0 1 0 1 �27 Me(CO3)2

2� 0 0 0 1 0 2 �48 �SOH2

+ 1 1 0 0 0 0 19 �SOHo 1 0 0 0 0 0 0

10 �SO� 1 �1 0 0 0 0 �111 �X��H+ 0 0 1 0 �1 0 112 �X��Na+ 0 0 1 0 0 0 013 �SOMe+ 1 1 0 0 0 0 �114 �X�2�Me2+ 0 0 2 1 �2 0 015 Na+ 0 0 0 0 1 0 016 H2CO3 0 0 0 0 0 1 017 HCO3

� 0 0 0 0 0 1 �118 CO3

2� 0 0 0 0 0 1 �219 OH� 0 0 0 0 0 0 �120 H+ 0 0 0 0 0 0 1

0.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

epr

opor

tion

ofm

etal

adso

rbed

bym

ontm

orillo

nite

.

Cd Cu Ni

3 4 5 6 7 8 90.0

0.2

0.4

0.6

0.8

1.0

pH

Pb

3 4 5 6 7 8 9pH

I = 0.1 MI = 0.01 MI = 0.001 M

Zn

Fig. 4. The relative proportions of adsorbed metal cations onto Na-montmorillonite at different ionic strengths. Symbols are experimentaldata and lines are fitted using the parameters in Table 3.

5724 X. Gu et al. / Geochimica et Cosmochimica Acta 74 (2010) 5718–5728

increasing sharply with increasing pH at all three ionicstrengths. A previous study (Hayes and Leckie, 1987) fordivalent cations adsorbed onto oxides showed that the ef-fects of ionic strength on metal adsorption could be usedto distinguish between the formation of inner-sphere andouter-sphere surface complexes. The adsorption behaviorin the lower pH range was greatly affected by ionic strength,which was consistent with an adsorption mechanism involv-ing the formation of outer-sphere surface complexes, whilethe adsorption in the higher pH range was more indicativeof the formation of inner-sphere surface complexes. Con-sidering the structure of montmorillonite, the former typeof adsorption is a cation exchange reaction occuring at

the permanently charged basal surfaces, while the later isan inner-sphere surface complexation at the variablecharged edge sites �SOH. Similar adsorption trends werealso observed in our previous studies for kaolinite and illite(Gu and Evans, 2007, 2008). However, ionic strength hadmuch less effect on metal adsorption onto kaolinite and il-lite in the lower pH range because montmorillonite has amuch larger permanent structural charge than either kao-linite or illite.

A two-site metal adsorption model, which included thepermanent charged surface site, �X�, and the variablecharged edge site, �S–O�, was used to predict the relativeproportions of each of these sites in the retention of the

Modeling adsorption of heavy metals onto montmorillonite 5725

divalent metals onto Na-montmorillonite. The data fromthe batch adsorption experiments were used to generatethe metal binding constants using FITEQL to analyze theadsorption data. Two adsorbed surface species, �X2�Me+,and �SO–Me+, were found to fit the data well (Fig. 5)and the best-fit intrinsic metal binding constants are shown

0.0

0.2

0.4

0.6

0.8

1.0Cd

I = 0.1M

0.0

0.2

0.4

0.6

0.8

1.0Cu

0.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

epr

opor

tion

ofm

etal

ions

adso

rbed

onto

mon

tmor

illoni

te

Ni

0.0

0.2

0.4

0.6

0.8

1.0Pb

3 4 5 6 7 8 90.0

0.2

0.4

0.6

0.8

1.0Zn

I = 0.01

3 4 5

Fig. 5. Modeled surface speciation diagrams of Ni(II), Cd(II), Cu(II),1.47 g L�1, at various ionic strengths and 25 �C. Symbols are experimentaare the species �SOMe+. Solid lines are the total metal adsorbed ontparameters given in Table 3.

in Table 2. Inclusion of an additional surface metal-hydro-xo complex, �SO–MeOHo did not improve the fit.

The intrinsic constants for the edge sites, log KintSOMe,

were optimized from the experimental data at 0.1 MNaNO3 because at this high ionic strength, adsorption ontothe permanently charged sites was at a minimum. The bind-

M

6 7 8 9pH

I = 0.001 M

SOMe+

X2- .Me2+

Total adsorbed

3 4 5 6 7 8 9

Pb(II) and Zn(II) adsorption onto Na-saturated montmorillonite,l data, dotted lines are the species �X–

2�Me2+ and the dashed lineso montmorillonite. Modeled species were calculated by the using

5726 X. Gu et al. / Geochimica et Cosmochimica Acta 74 (2010) 5718–5728

ing constants for the basal sites, log KintX2Me were optimized

from the data at the intermediate ionic strength, 0.01 M.Because only one set of parameters was used to fit the threeionic strengths, the model underestimated metal adsorptionin the lower pH range at the highest ionic strength, 0.1 M,particularly for Cu and Zn (Fig. 5).

Analysis of the results from the batch adsorptionexperiments for the five metals onto montmorillonite sug-gested that there were two different mechanism ofadsorption. In the lower pH range, the metallic cationswere adsorbed onto the permanently charge sites, �X�,through the formation of outer-sphere surface complexes,�X2�Me2+ complexes. As the pH values increased, themetal were adsorbed onto the negatively charged edgesites, �S–O�, to form inner-sphere surface complexes,�S–OMe+. Previous studies conducted by other research-ers using X-ray absorbance spectroscopy (XAS) provideother more direct evidence to confirm our suggestedmechanisms. These studies have shown that at low pHvalues and ionic strengths, metals, such as Cu, Pb, Cdand Co, form outer-sphere surface complexes with thepermanently charge sites of montmorillonite and withincreasing pH and ionic strength, the metal ions form in-ner-sphere surface complexes with the edge sites (Papelisand Hayes, 1996; Strawn and Sparks, 1999; Zacharaet al., 1993; Morton et al., 2001). Strawn and Sparks(1999) used X-ray absorbance fine structure spectroscopy(XAFS) to investigate the mechanisms of adsorption ofPb2+ ions onto montmorillonite. They found that at alow ionic strength, 0.006 M, in the pH range 4.5–6.4,only outer-sphere surface complexes were formed, whileat a pH of 6.8, both inner- and outer-sphere surface com-plexes were formed. In the highest ionic strength of 0.1 Mused in their studies, and at the same pH value, inner-sphere surface complexes are formed. These results areconsistent with the predicted surface species for Pbadsorption onto montmorillonite in this study (Fig. 5).

XAS evidence has also shown that Ni can form inner-sphere mononuclear surface complexes at the edges ofmontmorillonite at pH 7.2 in the presence of 0.3 M Na-ClO4 (Dahn et al., 2003). Those results are consistentwith the surface species proposed in this study. Althoughsome other XAS studies have suggested that surface poly-mer or/and surface precipitation were also importantmechanisms for metal adsorption onto montmorillonite,the metal concentrations used in those studies were high-er than those used in this study (Papelis and Hayes, 1996;Elzinga and Sparks, 1999; Morton et al., 2001). At lowermetal coverage at the surface, the formation of mono-dentate surface complexes was predominated (Hayesand Katz, 1996).

By comparing the intrinsic equilibrium constants, logKint, the order of the adsorption affinity for the five metalsto the variable charged edge sites was Pb� Cu > Zn >Cd > Ni, which is consistent with the order found for hy-drous ferric oxide (Dzombak and Morel, 1990), and similarto the order for kaolinite and illite found in our previousstudies (Gu and Evans, 2007, 2008). The adsorption affinityorder for the permanently charged surface sites of montmo-rillonite is Pb > Cu > Ni � Zn � Cd, which is the same for

kaolinite and illite. As the adsorption mechanisms of the fivemetallic cations onto the phyllosilicate clay minerals kaolin-ite, illite and montmorillonite are similar, it is reasonable toexpect a similar adsorption affinity.

5. CONCLUSIONS

The acid/base chemistry of a Na-saturated Wyomingmontmorillonite using 0.1 M, 0.01 M and 0.001 M NaNO3

as the indifferent background electrolyte was investigatedby discontinuous titrations at 25 �C. A two-site adsorptionmodel using the CCM was used to fit the experimental data– a permanently charged site, �X�, for the basal surfacesand one variably charged negative site �SO�, for the min-eral edges.

The results of the batch adsorption experiments in whichCu(II), Cd(II), Ni(II), Pb(II) and Zn(II) were added tomontmorillonite showed that two mechanisms of adsorp-tion were involved in the studied pH range 3–9. Outer-sphere surface complexes are formed at the permanentlycharged basal sites at low pH range and the formation ofthese surface complexes was greatly affected by ionicstrengths. At higher pH range, inner-sphere surface com-plexes are formed at the variably charged edge sites. Theformation of these surface complexes was much less affectedby changes in ionic strength.

The outer-sphere surface complex could be modeledusing a bidentate surface species, �X2

��Me2+. The inner-sphere complex formed of the edges of the mineral couldbe modeled using the surface species, �SOMe+. The mont-morillonite surface acidic constants and metal binding con-stants were optimized by FITEQL. The total number ofthese sites was estimated from the CEC of montmorillonitemeasured at pH 4 and 8. This two-site surface complexationmodeling fully considered the effect of ionic strengths,which is an important factor to the adsorption of metal cat-ions onto the basal surface sites, especially at lower pHrange (3–6). The adsorption log K values were obtainedthrough experiments in different ionic strengths and it couldpredict all the adsorption data under different ionicstrengths, indicating it has a wider applicability than thosederived from only one ionic strength. However one shouldbe noted that the two-site model and two adsorption specieswere simplified from real chemical processes and indicatedthe major processes during the adsorption. Application ofthis modeling approach to other adsorption mechanismsor condition, for example outer-sphere complexation orwider pH range, need to be carefully checked.

The affinity order of the five metal cations onto the per-manently charged sites was Pb > Cu > Ni � Zn � Cd, whilethat for the variably charged edge sites was Pb� Cu > Zn> Cd > Ni.

ACKNOWLEDGMENTS

We thank to Peter Smith and Glen Wilson of the AnalyticalLaboratory, Land Resource Science, University of Guelph, fortheir help in ICP-AES and X-ray diffraction. We also thank Na-tional Natural Science Foundation of China (20807002) for partof financial support.

Modeling adsorption of heavy metals onto montmorillonite 5727

APPENDIX A. ALL THE REACTIONS AND LOG KVALUES USED IN THE MODEL (I = 0).

Reactions Surface reactions log K

H Cd Cu Ni Pb Zn

1 �SOH + H+ = �SOH2+ 6.04

2 �SOH = �SO� + H+ �6.633 �X��Na+ + H+ = �X��H+ + Na+ 0.184 �SOH + Me2+ = �SOMe+ + H+ �2.93 �2.25 �3.16 0.49 �2.805 2�X��Na+ + Me2+ = �X�2�Me2+ + 2Na+ 2.37 2.49 2.40 2.56 2.39

Aquatic reactions

6 Me2+ + H2O = MeOH+ + H+ �10.13 �7.53 �9.93 �7.63 �9.037 Me2+ + 2H2O = Me(OH)2 + 2H+ �20.32 �17.32 �19.02 �17.12 �16.928 Me2++3H2O = Me(OH)3

� + 3H+ �32.50 �27.79 �29.99 �28.09 �28.399 Me2+ + CO2 + H2O = MeCO3 + 2H+ �13.82 �11.41 �13.61 �11.70 �13.42

10 Me2+ + CO2 + H2O = MeHCO3+ + H+ �7.49 �6.05 �5.76 �6.48 �6.35

11 Me2+ + 2CO2 + 2H2O = Me(CO3)22� + 4H+ �26.03 �26.3

12 CO2(g) + H2O = H2CO3 �1.4713 CO2(g) + H2O = HCO3

� + H+ �7.7314 CO2(g) + H2O = CO3

2� + 2H+ �17.88

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Associate editor: Dimitri A. Sverjensky