Colloids and Surfaces A: Physicochem. Eng. Aspects 332 (2009) 71–77

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Colloids and Surfaces A: Physicochemical andEngineering Aspects

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Interaction of selenite with MX-80 bentonite: Effect of minor phases,pH, selenite loading, solution composition and compaction

G. Montavona,∗, Z. Guoa, J. Lützenkirchenb, E. Alhajji a, M.A.M. Kedziorekc,A.C.M. Bourgc, B. Grambowa

a Subatech Laboratory, CNRS/In2P3/Ecole des Mines de Nantes/Université de Nantes, 4 rue Alfred Kastler, BP 20722, 44307 Nantes, Franceb Institut für Nukleare Entsorgung, FZK, Postfach 3640, D-76021 Karlsruhe, Germanyc LHGE Environmental Hydro Geochemistry, Dept. Earth Sciences, BP 1155, Université de Pau et des Pays de l’Adour, 64013 Pau, France

a r t i c l e i n f o

Article history:Received 26 March 2008Received in revised form 20 August 2008Accepted 1 September 2008Available online 12 September 2008

Keywords:SeleniteMontmorilloniteMX-80 bentoniteSorptionCompacted state

a b s t r a c t

We propose a simple model describing the retention of selenite, Se(IV), by the MX-80 bentonite in asynthetic groundwater (SGW) in dispersed and compacted states. The model was calibrated on a puremontmorillonite from data obtained for 4 < pH < 10 and total selenite concentrations between 10−7 and5 × 10−3 mol/L. Furthermore, the matrix solution covers a wide range of conditions regarding the ionicstrength and the nature of the background electrolyte. Three selenite surface species had to be considered.Below pH 5, sorption is governed by a ligand exchange reaction with H2SeO3. Between pH 5 and 7, theexperimental data are well described considering the formation of a surface complex implying HSeO3

−.Finally, at pH above 7, we propose ternary surface complexes involving Ca2+ and Mg2+. The model, whichwe consider as operational, appears in agreement with spectroscopic data available in the literature andpredicts surprisingly well selenite sorption on MX-80 bentonite even in the compacted state at a drydensity of 1100 kg/m3. Based on the model, the solid phase montmorillonite is responsible for selenite

retention. Minor solid phases containing iron (pyrite, hematite) had not to be considered in the modelling.Interestingly, calcite (an important phase in MX-80 bentonite) has an indirect effect via the release of

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. Introduction

Bentonite is a clay-based natural material which is a candi-ate for the engineered barrier in deep geological repositories foradioactive waste (e.g. ref. [1]). The transport of contaminants inater-saturated porous clay systems is controlled by diffusion and

orption (e.g. refs. [2,3]). The literature on sorption of cations ontoispersed bentonite is voluminous (e.g. refs. [2,3]). Far fewer datare available for anions, and notably for selenite. Selenium is aadioelement of concern in nuclear waste repositories, not becausef its total activity but because of the long half life of its Se79 isotope6.5 × 105 years). Boult et al. [4] studied the sorption of Se(IV) onunigel V-1 bentonite and suggested that minor phases contain-

ng iron were responsible for sorption at pH > 7. The interaction ofelenite with montmorillonite, the main component of bentonite,as also investigated both at spectroscopic [5,6] and macroscopic

evels [7]. Selenite adsorption on montmorillonite decreases with

∗ Corresponding author.E-mail address: montavon@subatech.in2p3.fr (G. Montavon).

e(cc

927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2008.09.014

sequent contribution to Se(IV) sorption through a ternary surface complex.© 2008 Elsevier B.V. All rights reserved.

ncreasing pH, in agreement with the sorption behaviour of oxy-nions on oxides [7].

The goal of the present work is to develop an operational modelescribing the interaction of Se(IV) with the MX-80 bentonite, theeference material chosen for the French nuclear waste deep repos-tory concept [1]. The basic assumptions of the proposed modelre:

montmorillonite, the main component of MX-80 bentonite, is themajor reactive phase;sorption of selenite occurs predominantly at the edges of clayparticles;acid–base properties of edge surface sites can be described basedon the work of Tournassat et al. [8].

Further assumptions and simplifications will be detailed later.

Montmorillonite-Se(IV) interactions were studied in batch

xperiments using suspensions of synthetic montmorillonite5–40 kg/m) over a wide range of pH values (4–10), Se con-entrations (10−7 to 5 × 10−3 mol/L) and background electrolyteompositions (ionic strength I = 0.02–0.5 mol/L fixed with cations

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a+ or Ca2+ and anions NO3−, Cl− or SO4

2−). The model was thenpplied to selenite sorption onto MX-80 bentonite in the pres-nce of a synthetic groundwater (SGW) to evaluate the potentialole of minor phases. It was finally applied to predict experimentalata obtained for compacted MX-80 bentonite using the “capillary”ethod of Montavon et al. [9].

. Materials and methods

.1. Materials

All chemicals were of analytical grade. MX-80 bentonite (MX-0) was used without any purification. Its characteristics areescribed in detail elsewhere [1]. A small amount of Se(IV) originat-

ng from the clay was detected when the bentonite was in contactith water. In the pH range investigated (pH 4–10), this amount was

ound to be nearly constant ((2 ± 05) × 10−8 mol/L, solid-to-liquidatio of 20 kg/m3). This average amount was taken into accountn the overall quantity of Se(IV) introduced in the batch sorp-ion experiments. Synthetic montmorillonite (SM) was obtainedsing the method described in Reinholdt et al. [10]. Its formula is(Si3.8Al0.2)(Al1.67Mg0.33)O10(OH1.9F0.1)Na1.06). 75Se (including car-ier) was obtained from CERCA.

.2. Sorption study

All experiments were performed at T = 23 ± 3 ◦C in an inert gaslove box (N2). We studied the sorption of Se(IV) on model (SM)nd natural (MX-80) solids. The nature of the electrolyte dependedn the specific objective of the experiment: to mimic Se(IV)orption in “natural” systems, a synthetic water (SGW) typicalf the composition of pore water in the claystone of the Bureite was used ([K+] = 4 × 10−4 mol/L, [Ca2+] = 6 × 10−3 mol/L,Mg2+] = 3 × 10−3 mol/L, [Na+] = 4 × 10−2 mol/L, [Cl−] = 5.8 ×0−2 mol/L) [1]. To develop the sorption model, a range ofonic strengths (0.02–0.5 mol/L) and electrolyte compositionsegarding the nature of both the anion (Cl−, SO4

2−, NO3−) and of

he cation (Na+, Ca2+) were investigated. For both compacted andispersed states, a pre-equilibration of solid and liquid phases waserformed before addition of Se(IV) (i.e. the solution compositionsf interest are expected not to vary in the presence of the soliduring the actual adsorption experiments). For the MX-80/SGWystem, Se(IV) did not change its redox state under the experi-ental conditions of the study. Selenite in the aqueous phase was

nalyzed by HPLC-ICP–MS with the methodology described inore detail elsewhere [11]. Based on these data, it can be safely

ssumed that Se(IV) is stable in the model systems (i.e., with SM).For the different systems studied, two types of data were

ollected: sorption was studied (a) at initial, fixed selenite concen-ration as a function of pH (pH sorption envelopes) and (b) as aunction of total selenite concentrations at constant pH (sorptionsotherms).

.2.1. Dispersed stateThe classical batch method was used as previously described [9].

olid mass to liquid volume ratios (S/L) ranged from 5 to 40 kg/m3.olid–liquid separation was performed by filtration through 0.2 �mlters. No sorption of Se(IV) was observed on the filters. The reten-ion of Se was expressed according to the equation:

d(D) = Atot − AL · L(1)

AL S

ith Kd(D) the distribution coefficient measured in the dispersedtate, Atot the overall activity added to the system (Bq), AL the equi-ibrium activity measured in the liquid phase (Bq), and S/L the solido liquid ratio in 10−3 kg/m3.

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sicochem. Eng. Aspects 332 (2009) 71–77

.2.2. Compacted stateThe “capillary” method [9] was used to study the MX-80/SGW

ystem. In summary, capillaries in PEEK (1 mm internal diameter)re used as supports to compact the material. The mass of materialompacted was (2.5 ± 1.0) × 10−6 kg. These capillaries containinghe compacted material were then connected to an external solu-ion to saturate the pores (exterior volume of 0.5 × 10−6 m3 perapillary). After addition of selenite to the external solution (tracerlus stable selenium, if necessary), the system was equilibratedntil the total activity in the water-saturated compacted mate-ial, which represents the sum of the activity in the pore waternd the activity adsorbed on the material, was constant with time.ssuming that the volumic activities in the external solution and

n the pore water are identical, a distribution coefficient for theompacted state, Kd(C) (in 10−3 kg/m3), can be defined as follows:

d(C) = Atot − Ap

Ap· �p (2)

ith Atot and Ap the activities in the water-saturated compactedaterial and in the pore water, respectively (in Bq) and �p the poreater porosity defined as the volume of water (in 10−3 m3) per

ilogram of dry sample. The latter refers to the bulk water andas estimated using I− [9] under conditions where its sorption

ould be neglected (pH ∼ 8; [NaI] = 10−3 mol/L). �p amounts to 0.24±0.04) × 10−3 m3/kg [9].

The activity in the pore water is defined as:

p = mtot�pAex

1 + �H2O�HTO(3)

ith mtot the mass of the water-saturated compacted material (ing), Aex the volumic activity measured in the external solutionin 103 Bq/m3) and �HTO the total porosity. The pore volume, 0.49±0.03) 10−3 m3/kg, was determined using HTO [9].

.3. Analytical

Se (75Se) was analyzed by liquid scintillation, with a Packard550 TR/AB Liquid Scintillation analyzer, or by ICP MS, using a PQ-xel ICP-MS (VG-Elemental).

.4. Modelling

The sorption model was constructed using synthetic montmoril-onite as a model solid. The resulting parameters were then appliedo data obtained with MX-80 bentonite suspensions to assess theffect of secondary phases on the retention. Finally, the model wassed to predict the retention in the compacted state using MX-80entonite.

By analogy with sorption on solid oxides, Se(IV) species aressumed to interact with surface sites located at the edges of mont-orillonite particles (e.g. ref. [7]). Published models describing

e(VI) sorption on clay materials are built based on sorption prop-rties of oxides for Se(IV) [7,12]. The goal of the work was to definemore “realistic” model in the sense that properties of montmo-

illonite surfaces are considered. In this context we note that thecid–base behaviour of edge sites is still a matter of discussion inhe literature (e.g. ref. [12]) considering in particular the numberf surface sites [8] and the necessity and nature of electrostaticerms [14–16]. Our approach was therefore to define an operational

odel, i.e., as simple as possible, with no contradictions with rel-

vant spectroscopic data available [5,6]. The reactivity of surfacedge sites was described based on the work of Tournassat et al. [8].t is a non-electrostatic model based on the MUSIC approach [17].he major simplification brought here to this model consisted ineducing the number of sites. This is possible in a non-electrostatic

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odel since the concomitant effect of electrostatics from all sitesn individual sites, which would be operative in an electrostaticodel, is not an issue. The following assumptions were made:

(i) Se(IV) sorption occurs predominantly on reactive surface edgealuminol groups; this is a reasonable assumption according tothe literature [5,7,12].

(ii) Only the most abundant sites are considered. It implies in par-ticular that all sites involving a bond with a substituted cationare neglected.

iii) Only the proton-active sites (i.e. reactive with respect to pro-tons and hydroxyls within the pH range generally consideredfor sorption experiments [3–14]) are taken into account.

According to the above assumptions, three sites remain for reac-ions with selenite. They are called in the terminology of Tournassatt al. Aloh–O–Sitd, Aloh–O–Aloh and AlohO and will be referred to inhe following as sites of type 1, 2 and 3, respectively (see Table 1).he first two sites are bridging, the latter is a terminal site. Inter-stingly, these sites are in close agreement with those suggestedy Bourg et al. [19] in their model. All site densities for our simpleodel were taken from Tournassat et al. [8]. It will be discussed

ater that some adjustments became necessary. To further simplifyhe system, the exchange reactions with cations were not explic-tly considered in the model. The concentrations of the major ionsoming from the dissolution of montmorillonite and relevant forhe present study (Si, Mg) were taken from ref. [8].

As indicated above, electrostatic terms are not explicitly takennto account. Bradbury and Baeyens [20] as well as Tournassat et al.8] argued that electrostatics are of minor importance since theiritration curves were not sensitive to changes in ionic strength.ourg et al. [19] recently found that the “spillover” of electro-tatic potential between montmorillonite basal and edge surfacess important to understanding the ionic-strength-dependence of

ontmorillonite acid–base titration curves, but noted that the mostppropriate way to model this spillover is not known. Bourg et al.19] also showed that experimental acid–base titration data cannote quantitatively predicted using edge site pKa values calculatedith the bond-valence models advocated by Bickmore et al. [18] or

y Tournassat et al. [8] i.e., at least one pKa value must be fitted.

For the calculations, the Phreeqc code was used [21]. Calcite dis-

olution (systems with MX-80 bentonite) was taken into accountsing the solubility product given in the Llnl.dat database. Selenitend silicium speciation were described using the OECD thermo-ynamic data bases [22,25]. For the kinetic experiments in the

Mct((

able 1odelling parameters

ite types Sorbing capac

ontmorillonite descriptionAlOh–O–SiTd, S1 31.3AlOh–O–AlOh, S2 33.7 (0.4b)AlOh–O, S3 19.3

eaction

ater/solid equilibriaS1OH1/2 � S1O−1/2 + H+

S2OH2+ � S2OH + H+

S3OH2+1/2 � S3OH−1/2 + H+

orption reactionsH2SeO3 + S2OH+

2 + S2OH � ( S O)2SeOH+ + 2H2O (4)HSeO3

− + S1OH1/2 � S1OHHSeO3−1/2 (5)

Ca2+ (Mg2+) + SeO32− + S3OH2

1/2 � S3OH2Ca(Mg)SeO31/2(6)

a For MX-80 bentonite, the number of sites is adjusted considering a montmorillonite cb Number of sites considered in the modelling to react with Se(IV) (see text).

sicochem. Eng. Aspects 332 (2009) 71–77 73

ompacted state, the sorption model was coupled with a diffusionodel. Charge effects on the transport were not considered.

. Results and discussion

.1. Kinetic experiments

For the dispersed state and for the different systems studied,orption is rapid, i.e. steady-state conditions are obtained afterbout 2 days of contact (data not shown). This is consistent withecent work on the kinetics of sorption of selenite on montmoril-onite [26].

In the compacted state, the transfer of selenite from the externalolution to the water-saturated porous medium is diffusion con-rolled [1]. A longer contact time is therefore necessary to reachquilibrium conditions. The equilibration process was followed bynalyzing the transient evolution of the radiotracer content in theompacted system (i.e. pore volume + bentonite). Sorption equilib-ium is considered to be reached when this concentration does notary with time. Experimental data are presented in Fig. 1 for twoH values.

The lines correspond to simulations with Phreeqc. The increasen concentration is well described by diffusion of Se from the exter-al solution into the compacted clay medium with an effectiveiffusion coefficient of 4 × 10−11 m2/s. The diffusion coefficient isimilar to that measured for halide ions under comparable condi-ions [1]. Based on the kinetic experiments the equilibration timeas fixed to 2 months.

.2. The model system

.2.1. Experimental dataThe results are presented in Fig. 2. Fig. 2A and B show the results

or the simple electrolyte systems. The isotherm was measured athe pH of maximum selenite sorption, i.e. at pH around 4 (Fig. 2B).he pH-adsorption envelopes obtained for several simple elec-rolyte solutions (NaCl, NaNO3, Na2SO4) and SGW are presented inig. 2A and C, respectively. As observed with oxides (e.g. ref. [27]),he Kd values decrease with increasing pH. In the 4–7 pH-range,he composition of the liquid phase does not affect Se(IV) sorption,either by the nature of the anion nor the major cation (Na, K, Ca,

g) of the supporting electrolyte. However, above pH 7, the solution

omposition does influence Se(IV) sorption. The Kd value continueso decrease with increasing pH for solutions of simple compositionFig. 2A) whereas it remains nearly constant in the presence of SGWFig. 2C).

itiesa (mmol/kg) Reference

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ontent of 79.2% [1].

74 G. Montavon et al. / Colloids and Surfaces A: Phy

Fig. 1. Sorption kinetics in the compacted state for MX-80/SGW system. The dataare presented as the amount of Se in the consolidated medium saturated with water(in 10−12 mole) per mm of compacted material; the lines correspond to a calculationmade with De = 4 × 10−11 m2/s and Kd = 3 (pH 7.8) and 57 (pH 3) mL/g.

3

twasearbdTrwi(nFusmotdsa

Fig. 2. Sorption of Se(IV) on synthetic montmorillonite in the dispersed state. (A) pH-en0.05 mol/L, NaNO3; �: 0.016 mol/L, Na2SO4; ♦: 0.05 mol/L, NaCl). (B) Effect of selenite conc(C) pH-envelope measured in the presence of SGW ([Se]tot = 4 × 10−6 mol/L) (D) Effect of Clines correspond to the calculations made with the parameters given in Table 1.

sicochem. Eng. Aspects 332 (2009) 71–77

.2.2. ModellingWe first accounted for the experimental data obtained for solu-

ions of simple composition (Fig. 2A and B). The decrease of Kdith increasing pH may be explained, in analogy to oxides, by a lig-

nd exchange mechanism (e.g. ref. [7]). An EXAFS study on Se(IV)orbed on synthetic montmorillonite at pH 6 did not show thexistence of an inner sphere surface species [6]. The formation ofn outer-sphere complex should rather be considered in this pHange. The formation of outer-sphere complexes is often explainedy electrostatic interactions. As a consequence, sorption is stronglyependent on the presence of competing ions and on ionic strength.hus, the presence of outer-sphere complexes is often inferred indi-ectly from competitive effects. In the present system, no such effectas observed: Kd values were not affected by neither consider-

ng the nature of the counter anion (Fig. 2A) nor its concentrationFig. 2B). We also note that the appearance of an ionic strength effecteed not necessarily be related to the adsorption mechanism [23].urthermore, the note by Wiesner et al. [24] suggests that reportedptake curves at high ionic strength might involve errors on the pHcale which are on the order of the reported ionic strength effects oray at least make them less important. From a quantitative point

f view, the present non-electrostatic model is not capable of dis-inguishing between inner- and outer-sphere complexes. The Kdecrease with the pH for 9 > pH > 6 can be well explained by theorption of the singly protonated selenite ion on the type 1 site inmonodentate fashion (Eq. (5) in Table 1).

velope measured in the presence of different anions with [Se]tot = 10−6 mol/L (�:entration; pH 4.1 and [NaCl] = 0.1 (triangle), 0.05 (squares) and 0.01 mol/L (circles).

a concentration on Kd value; pH 7.5, [NaCl] = 0.05 mol/L and [Se]tot = 10−6 mol/L. The

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Eq. (5) could not explain selenite sorption below pH 6 (dottedine in Fig. 2A). In this pH range other surface species should beonsidered. This appears consistent with the work of Peak et al. [5].sing XAS methods, they showed at the surface of montmorillonitet pH 4.5 the formation of a bidentate binuclear surface speciesased on Se–Al distance, number of backscatters and geometricalspects. Among the sites considered in our simplified model, onlyhe site of type 3 can lead to the formation of such a surface com-lex. However, the Kd dependence with pH cannot be quantitativelyeproduced. The calculation can explain the Kd increase from pHto 5 (considering the sorption of H2SeO3) but not the slight Kd

ncrease observed from pH 5 to 3 (a strong increase is predicted).Among the different possibilities tested (nature of the species

dsorbed and nature of the site considered), a good agreementetween the experiment and the calculation was obtained con-idering the interaction of H2SO3 with sites of type 2 (Eq. (4) inable 1 and lines in Fig. 2A). While this allows a good descriptionf the pH dependence, the sorption isotherm cannot be recov-red. To improve this, the number of available sites for the ligandxchange mechanism would need to be decreased to 0.4 mmol/kg.he operational model of Bradbury and Baeyens for describing theorption of metal ions on montmorillonite (e.g. ref. [14]) had to beodified similarly, i.e. the capacity of the sites of type 1 (deter-ined by potentiometric titration) had to be decreased from 45

o 2 mmol/kg to explain the Kd dependence with metal ion load-ng. This decrease may be explained for example by a competition

ith H4SiO4. The concentration of Si in the pH range 4–9 is nearlyonstant and amounts to 8 × 10−5 mol/L [8]. To explain the data,i sorption must be described by a constant Kd value of 416 mL/gn the pH range where Eq. (4) in Table 1 contributes significantlyo Se(IV) sorption (3–6). In this case, 33.3 mmol/kg of the sites areblocked” and are not available for Se(IV) sorption. This Kd value cane quantitatively explained by the sorption of Si on S2OH2

+ and2OH surface sites. However, when Se and Si are both present, noarameters allowed to explain both pH edge and sorption isothermshile maintaining a constant Kd value for Si. When the concentra-

ion of Se(IV) becomes important, an efficient competition with Sias always predicted. This assumption appears therefore not reli-

ble. This results is not surprising considering the weak effect ofhe nature of the anions (notably SO4

2−) on Se(IV) adsorption (seeig. 2A). Such a decrease of the number of sites can be phenomeno-ogically explained by the low amount of geometrical dispositionsn the edges allowing the formation of the bidentate coordina-ion.

According to this description, the ligand exchange mechanismorms the dominating Se-surface species below pH 5 for traceoncentrations of Se(IV). The bidentate tetranuclear surface com-lex formed does not account for the number of Al backscatters1.7 ± 0.3) determined by Peak et al. [5]. However, the parametersf our model indicate that both outer-sphere (46%, Eq. (5) in Table 1)nd inner-sphere (54%, Eq. (4) in Table 1) complexes coexist inhe sample prepared for EXAFS measurements (1 kg/m3, 0.01 mol/LaNO3, 25 �mol/L Se(IV)). The EXAFS spectrum thus represents

he sum of different contributions (Se–O, Se–Al) associated to twoifferent complexes, and the parameters obtained represent anverage. An underestimation of the number of Al atoms by a factorf about two is therefore expected as the presence of an outer-phere complex leads to a weak Se–Al contribution considering themportant Se–Al distance.

In conclusion, our model, although it remains operational, is in

ood agreement with spectroscopic results given in the literature5,8].

The difference observed for pH > 7 between Fig. 2A and C coin-ides with the occurrence of SeO3

2− in solution. Furthermore, thencrease of Se(IV) sorption in the SM/SGW system is explained by

its

w

sicochem. Eng. Aspects 332 (2009) 71–77 75

he presence of divalent cations Mg2+ and Ca2+ in solution (theature of the anion having no effect on the sorption). This is alsoonfirmed in Fig. 2D: the Kd values slightly increase with Ca con-entration.

According to the NEA-OECD thermodynamic database, SeO32−

nteracts with Mg and Ca ions to form SeO3X precipitates (X = Ca,g) [22]. Although the formation of precipitates would increase

he apparent sorption, no precipitation is expected considering thexperimental conditions chosen and the Ksp values reported byompean et al. [22]. If the solid compounds SeO3X exist, one would

lso expect the formation of soluble ion pairs SeO3X. Log K valuesor the formation constants of SeO3X complexes can actually beound [28]. The formation of such complexes is also expected sinceomplexes of SO3

2− with divalent cations Ni2+, Cd2+, Mn2+, Co2+

29] and Ca2+ have been reported [30].Based on these considerations, the effect observed above pH 7

ould be described by considering the adsorption of SeO3X com-lex on SM surface. However, this latter is not referenced in theEA/OECD database [22] and its consideration might require aew thermodynamic evaluation of selenite in aqueous solution.he effect on Se(IV) sorption behaviour in Ca, Mg-rich systems washerefore described by considering the formation of a ternary sur-ace complex involving Ca, Mg (Eq. (6) in Table 1). The nature andhe speciation of the surface site(s) interacting with SeO3X need note resolved within an operational model. As for equilibria (4) and5), the experimental observation allowing to constrain the modelarameterization is the Kd dependence with pH. The sorption pro-ess is pH-independent for 7 < pH < 9. As shown in the next part forhe “natural” system, this insensitivity to pH can be extended toH 10. To take this observation into account, the interaction wasescribed considering an interaction with the site of type 3 (Eq. (6),able 1).

As shown by the solid lines in Fig. 2C and D, relatively goodgreement is found between the experimental data and the modelsing the parameters given in Table 1.

.3. The bentonite system

The final objective had been to develop an operational modelescribing Se(IV) sorption on MX-80 bentonite in SGW. The modelescribed in the previous section was first tested with experimen-al data measured in the dispersed state in the presence of NaNO3Fig. 3A and B). The basic assumptions of the model are:

the most important interacting phase is montmorillonite and theparameters for the interaction with Se are derived from the sim-ple system with SM.calcite is controlling Ca concentration in solution. The concentra-tion of Mg due to the dissolution of montmorillonite is relativelylow [8] and can therefore be neglected.

The results are presented as solid (overall sorption) and dashedcontribution of the different surface species) lines in Fig. 3A and. The experimental results are rather well predicted. The modelescribes both the pH-dependence of Kd and the effect of Se(IV)oncentration.

The model for MX-80 was finally tested in the “real system”,.e. for MX-80 bentonite in the presence of SGW both in the dis-ersed and compacted states. The experimental data are presented

n Fig. 3C and D. The model yields a good description of Se(IV) sorp-ion (solid lines in Fig. 3). This indicates that compaction had noignificant effect on Se(IV) retention.

The measured data and the model predictions are consistentith our earlier assumption that montmorillonite is the mineral

76 G. Montavon et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 332 (2009) 71–77

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ig. 3. Sorption of Se(IV) on MX-80. (A) Sorption pH-envelope obtained in the preset pH 4 in the presence of NaNO3 0.05 mol/L. Sorption data obtained as a functionispersed (open symbols) and compacted (filled symbols) states. The lines correspo

hase controlling Se(IV) sorption. The contribution of iron-basedineral phases (hematite and pyrite), which represent about 5.5%

f the total surface area [1], to the overall sorption can obvi-usly be neglected within the present model. Calcite appears asminor phase that influences Se(IV) sorption above pH 7. The out-

ome is in qualitative agreement with previous work by Goldbergnd Glaubig [7]. They proposed that calcite was responsible forhe sorption of selenite in a calcareous montmorillonite systemor pH > 7. However, they ascribed this effect to a strong affin-ty of selenite for calcite. The effect of calcite on the retention ofelenite in our system was found to be indirect: calcite dissolu-ion releases Ca in solution which interacts with SeO3

2−, to formotentially both aqueous and ternary surface complex on mont-orillonite.

. Conclusion

A model is proposed to simulate Se(IV) sorption on clay mate-ials. The surface reactivity (number of surface sites, acid–baseeactions) is described based on the work of Tournassat et al.8]. Experimental data measured on a pure montmorillonite weresed to parameterize the model. In the absence of divalent metal

ons, in agreement with relevant spectroscopic data [5,6], twoelenite surface species were considered. To explain experimen-

al data measured at pH above 7 in the presence of Ca2+ and

g2+, a ternary surface complex had to be included. The modelas successfully applied to describe Se(IV) sorption on MX-80entonite considering that montmorillonite was responsible forelenite retention.

[

NaNO3 0.05 mol/L with [Se(IV]tot = 5 × 10−7 mol/L. (B) Sorption isotherm measured(C) and Se(IV) concentration (pH 4) (D) for the “natural system” MX-80/SGW in

the prediction made with the parameters given in Table 1.

cknowledgements

We thank ANDRA (French National Agency for Radioactiveaste Management) and the European Commission through the

ntegrated project FUNMIG for their financial support and ANDRAor providing the bentonite sample. We also thank J. Brendle of theniversity of Mulhouse (France) for providing the synthetic mont-orillonite and I. Bourg from Lawrence Berkeley Lab (California)

or editorial comments.

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