smectite mixed clays

Physics and Chemistry of the Earth 33 (2008) S156–S162

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Sorption of strontium onto illite/smectite mixed clays T. Missana *, M. Garcia-Gutierrez, U. Alonso CIEMAT, Departamento de Medioambiente, Avenida Complutense, 22-28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Available online 14 October 2008 Keywords: Adsorption Ion exchange Surface complexation modelling Radioactive waste Clays

a b s t r a c t Radioactive strontium is a by-product of the fission of uranium and plutonium and its mobility in the environment is largely dominated by sorption onto soil minerals, especially clays. In this study, the sorption behaviour of Sr in illite/smectite mixtures was analyzed from a mechanistic point of view. Sorption of Sr onto smectite and illite, previously homoionised in Na, was studied under different pHs, ionic strengths and radionuclide concentrations. Sorption experiments were also carried out with smectite and illite, mixed in two different proportions: 50–50% and 75–25%. Sr sorption data that were obtained from the single mineral systems were modelled considering both ionic exchange and surface complexation and using a non-electrostatic model. The selectivity coefficients and complexation constants obtained for the individual minerals were incorporated into a model to predict the adsorption of Sr in binary adsorbent systems and the model predictions were consistent with Sr experimental adsorption data. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Strontium is a divalent cation without redox chemistry presenting several radioactive isotopes with the same physical properties. 85 Sr is used in industry and medicine, 90Sr can be found in nuclear reactors waste and it is one of the most hazardous radionuclides, posing a long term radiation hazard (Zu and Shaw, 2000) because of its long half-life (29 y) and high mobility. For its similarity to calcium, strontium is easily retained by living organisms, mainly in the bones and teeth (Froidevaux et al., 2006), and then converted into an internal long-lived source. The study of radionuclide retention in soils and in engineered barriers of nuclear waste repositories is important for safety assessment analyses. Clays are considered as suitable barriers in radioactive waste repositories, because of their low permeability and chemical buffering capacity (Pusch, 1992). Since they present high sorption capability for most cations, they are adequate materials for radionuclide retention and dispersion-retardation. Several studies on sorption of Sr onto illite, smectite or other clays exist but, in most of these studies, the experimental data are interpreted in a semi-empirical way (Bascetin and Atun, 2006; Bilgin et al., 2004; Dyer et al., 2000; Khan, 2004; Rafferty et al., 1981; Tsai et al., 2001; Wang and Staunton, 2005). Adsorption of Sr on kaolinite, illite and montmorillonite clays at high ionic strength was analyzed and modelled by Mahoney and Langmuir (1991). Rafferty et al. (1981) carried out a systematic experimental investigation of Sr adsorption onto montmorillonite, illite, kaolinite * Corresponding author. E-mail address: [email protected] (T. Missana). 1474-7065/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2008.10.020

and attapulgite. However, very few works dealing with sorption onto mixed clays exist (Tripathi et al., 1992; Zuyi and Wenming, 2003) and, in these publications, the interpretation of data is not mechanistic. The description of radionuclide retention by a mechanistic approach is more precise and appropriate than the usual distribution coefficient (Kd) or sorption isotherm approach, especially for safety assessment analysis. The former is based on a thermodynamic description of the radionuclide/solid interactions and it allows predicting sorption behaviour under a variety of conditions; the latter only allows obtaining semi-empirical parameters with a limited transferability. The application of a mechanistic description to sorption in binary systems represents the first step to understand natural clayey (mixed) systems. Since clay minerals may interact and modify their surface properties in certain mixtures, to establish if whether the sorption properties of clay mixtures are additive or not is a relevant issue. Smectite and illite are 2:1 layer type clays: their structure consists of two tetrahedral sheets that sandwich an octahedral sheet. In smectite, isomorphic substitutions take place both in the tetrahedral and octahedral sheets and these substitutions impart a permanent negative charge that is compensated by cation adsorption on the basal surfaces. The amount of these adsorbed cations corresponds to the cation exchange capacity (CEC) of the clay. In illite, substitutions predominantly take place in the tetrahedral sheets and potassium ions in the interlayer are not available for exchange with other ions in solutions (Newman, 1987; Van Holphen, 1977). Since only cations at the external surfaces are exchangeable, illite generally presents lower CEC than smectite. Another remarkable

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difference amongst the two clays is that smectite swells in presence of water because it hosts the water molecules in its interlayer sites, whereas illite is a non-expanding clay. Both characteristics are important for the performance of the clays as barriers for the radioactive waste confinement (Meunier et al., 1998) and make smectite illitization an interesting issue to be studied (Cuadros and Linares, 1996). This is an additional reason why it is of interest to study the sorption behaviour of illite/smectite mixtures. Besides the cation exchange properties, 2:1 layer clays present pH-dependent sorption properties because of the edge sites, similar to those occurring on the oxides surface, and that can be protonated or de-protonated (Van Holphen, 1977). Surface complexation may take place on these edge sites. To interpret and quantify the interactions occurring at clay/water interfaces that account for ionic exchange and surface complexation, the effects of the most important physico-chemical parameters such as pH, ionic strength and radionuclide concentration have to be studied independently. Sorption edges at constant ionic strength and sorption isotherms at a constant pH are both needed to determine surface complexation constants and selectivity coefficients for a quasi-thermodynamic description of the system. A methodology for a mechanistic description of sorption in clays and the determination of the above-mentioned parameters was developed for Ni and Zn in Na-montmorillonite (Bradbury and Baeyens, 1997) and successfully applied for sorption of other radionuclides either on montmorillonite or illite clays (Bradbury and Baeyens, 2002, 2005b). The aim of this study is to analyze the sorption behaviour of radioactive Sr on smectite, illite and on illite/smectite mixtures to develop a mechanistic model for interpreting the sorption data in the mixed system.

2. Materials and methods 2.1. Smectite The smectite clay (FEBEX bentonite) comes from the Cortijo de Archidona deposit (Almeria, Spain). This clay has a high smectite content (93 ± 2%), with quartz (2 ± 1%), plagioclase (3 ± 1%), cristobalite (2 ± 1%), potassic feldspar, calcite and tridymite as accessory minerals. Its chemical composition is: 58.71% SiO2; 17.99% Al2O3; 0.23% TiO2; 3.13% Fe2O3; 0.04% MnO; 4.21% MgO; 1.83% CaO; 1.31% Na2O; 1.04% K2O; 0.02% P2O5. The cation exchange capacity (CEC) is 102 ± 4 meq/100 g. The N2-BET surface area is 33 m2 g1 and the EGME surface area 725 m2 g1. A comprehensive characterization of the clay can be found elsewhere (Huertas et al., 2000). 2.2. Illite Silver Hill illite was obtained from the Source Clay Minerals Repository (The Clay Minerals Society). Its chemical composition is: 49.3% SiO2; 24.25% Al2O3; 0.55% TiO2; 7.32% Fe2O3; 0.55% FeO; 0.03% MnO; 2.56% MgO; 0.43% CaO; 0 Na2O; 7.83% K2O; 0.08% P2O5. The N2-BET surface area is 17 m2g1 and the EGME surface area 163 m2g1 (Elzinga and Sparks, 2001); the CEC is 17 meq/100 g. Other values for the CEC reported in the literature for the Silver Hill illite range from 10 to 40 meq/100 g (O’Loughlin et al., 2000). More details on this clay can be found elsewhere (Hower and Mowatt, 1966). 2.3. Radionuclide The radionuclide used in this work was a carrier-free 85Sr (strontium chloride in 0.5 M hydrochloric acid). The major radiation of

85

Sr comes from gamma emission at 514 KeV. of 64.85 days.

85

Sr has a half-life

2.4. Preparation of the clay suspensions Previous to sorption experiments, the clays were purified and homoionised in Na. The ‘‘natural” clays were washed three times with 1 M NaClO4 to eliminate all the soluble salts and to obtain the homoionic Na-form. In the case of the illite, is defined ‘‘Na-form” the one where the exchangeable cations at the external surfaces are substituted by Na (obviously, ions in the interlayer cannot be substituted). After eliminating the supernatant of the last washing, the Naclay was placed in centrifuge tubes, with deionised water, and a clay size fraction lower than 0.5 lm was obtained by centrifuging (600g, 10 min). This fine fraction was precipitated in a glass container with 1 M NaClO4 in order to prevent extensive clay dissolution. The clay washing/centrifuging procedure was repeated approximately 20 times. When enough fine fraction was collected, the clay suspension was introduced in dialysis bags, which were sealed and placed in 3 L containers filled with the electrolyte (NaClO4) to condition the suspension at selected ionic strength. The electrolyte was changed two or three times per day until the conductivity of the suspension in the bags and of the external electrolyte was the same. The homoionisation process did not affect the main properties of the clay (CEC or surface area). The concentration of the clay material in the suspension to be used for sorption experiments was determined by gravimetry. The solid to liquid ratio (S), used in these experiments, typically ranged from 0.5 to 2 g L1. Apart from pure smectite (S100-I0) and pure illite (S0-I100) suspensions, two additional suspensions were prepared by mixing the two original clays to a known ratio: (a) 75% smectite and 25% illite (S75-I25) and (b) 50% smectite and 50% illite (S50-I50). The suspensions of the mixed clays were prepared to a solid to liquid ratio of approximately 1 g L1. 2.5. Sorption experiments The experiments were carried out under atmospheric conditions. Na-clays conditioned to different ionic strengths were used (from 2103 to 2101 M in NaClO4). The kinetics of the sorption process was first investigated to determine the time needed to reach the sorption steady-state. Kinetic tests were carried out at pH 6.5-7 and ionic strength of 1101 M. The Sr uptake was complete after few hours in the smectite, but it took longer in the case of illite. The contact times selected for sorption experiments were 2 and 4 days for smectite and illite, respectively. Four days of contact times were also taken in the case of the smectite/illite mixtures. Sorption edges were carried out by changing the pH of the suspensions from approximately pH 3–10 with NaOH or HCl 0.1 M. Three different aliquots of the suspension, at the selected pH, were introduced in 12.5 ml ultracentrifuge tubes and then the radionuclide was added. After radionuclide addition, the tubes were sealed and maintained in continuous stirring during the selected equilibrium time and afterwards ultra-centrifuged (645,000g, 30 min). After the solid separation, two aliquots of the supernatant were extracted from each tube for the analysis of the final activity that was measured by means of a NaI c-counter (Packard Autogamma). Sorption isotherms were carried out by varying the radionuclide concentration at a fixed pH and fixed background electrolyte concentrations. To perform the experiment with high Sr concentrations (>1106 M), non radioactive chemical (SrCl2) of high purity (Mercks) was added apart from radiotracer. The separation and counting procedure was the same described for the sorption edges.

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The distribution coefficient amongst the solid and the liquid phase, Kd, was calculated by means of the formula:

Kd ¼

Ci  Cf V  m Cf

ðE:1Þ

Ci and Cf are the initial and final activity in the liquid phase, m the mass of the clay (g) and V the volume of the liquid (ml). The amount of Sr adsorbed onto the ultracentrifuge tubes was checked at different pH, with and without the presence of the solid: it was always lower than 5% of the initial activity. 3. Modelling Since both ionic exchange and surface complexation can contribute to the sorption of radionuclides in clays, it is important to differentiate these contributions. The ionic exchange reactions between a cation B, with charge zB, which exists in the aqueous phase, and a cation A with charge zA at the clay surface (X) is defined by

zB A  X þ zA B () zA B  X þ zB A:

ðE:2Þ

The cation exchange reactions can be described in terms of selectivity coefficients. Following the Gaines and Thomas (1953) definition, the selectivity coefficient is expressed by A

B

K SEL ¼

ðNB ÞzA ðaA ÞzB ðNA ÞzB ðaB ÞzA

ðE:3Þ

where aA and aB are the activities of the cations A and B and NA y NB are the equivalent fractional occupancies. The equivalent fractional occupancy of a cation Y, NY, is defined as the equivalents of Y adsorbed per unit mass divided by the CEC (expressed in eq/g):

NY ¼

eqðYÞads =g solid : CEC

ðE:4Þ

Bradbury and Baeyens (1994) described a method to determine selectivity coefficients of a cation present in the solution at trace concentrations with sorption measurements. In a bi-ionic system, where the cation B is present at trace levels, NA is approximately 1 (E.3). When ionic exchange is the main sorption mechanism and the distribution coefficient (Kd(ex)) is known, the selectivity coefficient can be determined with: A

B

K SEL ¼

 z K d ðexÞ  zB A czAB ðAÞzB CEC czBA

ðE:5Þ

cA and cB are the solution activity coefficients of cations A and B. In the case of Na-clays, the electrolyte A is monovalent (Na) and, Sr, the cation to be sorbed (B) is divalent. The Eq. (E.5), in this case, can be written as Na

B

K SEL ¼

  K d ðexÞ  2 c2Na ðNaÞ2 : CEC cB

SOH () SO þ Hþ

K a1 ; K a2

ðE:7Þ ðE:8Þ

ðE:11Þ

The acid-base properties of the edge sites are generally determined by potentiometric titrations. Several methods can be used to carry out titration of clays from fast titrations to batch backtitration methods accounting for dissolution of the solid phase (Bradbury and Baeyens, 1997; Schultess and Sparks, 1986), which can be an important factor for clays. The acid-base properties of smectites have been largely studied, but fewer studies are available for illite (Beene et al., 1991; Du et al., 1997; Liu et al., 1999; Sinitsyn et al., 2000). Smectite titration curves are almost independent on the ionic strength (Avena and de Pauli, 1998; Bradbury and Baeyens, 1997; Missana and Adell, 2000) and illite (Beene et al., 1991; Bradbury and Baeyens, 2005b; Du et al., 1997; Sondi et al., 1996). To simulate the titration curves and the cation surface complexation in smectite, Bradbury and Baeyens (1997) used a non-electrostatic model, which was later successfully applied for illite (Bradbury and Baeyens, 2005a, 2005b, 2005d). In the present study, a non-electrostatic approach was used as well. The acid-base characteristics of the Na-smectite used here were previously analyzed (Missana et al., 2002). Protonation/ deprotonation constants for the Silver Hill illite obtained by a non-electrostatic model were not available in the literature thus, for the illite, the parameters found in Bradbury and Baeyens (2005b) were considered. In smectite, the density of the edge sites (SOH) is approximately 5–10% of the total CEC, whereas in illite the value is usually significantly higher (Bradbury and Baeyens, 2005c; Sinitsyn et al., 2000; Sondi et al., 1996) leading to a more pronounced dependence of the charge on the pH. The SOH sites density for the smectite was 1.82 lmolm2 (Missana et al., 2002) which corresponds approximately to 6% of the CEC. The SOH sites density for illite is generally much higher and values between 20 and 50% of the CEC are reported. Here, a density of 4 lmolm2 was used, a factor 0.4 lower than the CEC, as reported in Bradbury and Baeyens (2005a). The Sr speciation calculations and sorption modelling were done with the CHESS v 2.4 code (van der Lee and De Windt, 1999). Best fits of the experimental curves were obtained with a trial and error procedure, minimizing the v2 function defined as

v2 ¼

 where SOHþ 2 , SOH and SO represents, respectively, the positively charged, neutral and negatively charged surface sites, and Ka1 and Ka2 are the intrinsic equilibrium acidity constants. The mass law equations corresponding to the reactions Eqs. (E.7) and (E.8) are:

  ðSOHÞfH g FW ¼ ; exp  RT ðSOHþ2 Þ   ðSO ÞfHþ g FW exp  ¼ ðSOHÞ RT

SOH þ Mzþ () SOMz1 þ Hþ :

ðE:6Þ

At the edge sites (SOH), the pH-dependent charge is determined by the following protonation/de-protonation reactions:

SOHþ2 () SOH þ Hþ

where {} represents the ion activity and () the ion concentrations. Since the activity coefficients for all the surface species are assumed to be equal, the activity of these species can be substituted by their concentration (). The exponential represents the coulombic term that accounts for the electrostatic effects (Dzombak and Morel, 1990). Non-electrostatic models, as the one used in this work, do not consider this correction. The specific adsorption of cations at these surface functional groups can be described by reactions as:

N 1X ðyf  yi Þ2

m

1

r2i

ðE:12Þ

where yf is the fit estimate, yi the experimental data. ri is the uncertainty in yi, and m represents the degrees of freedom. (m = Np1 where N is the number of observations, and p is the number of coefficients or parameters used in the regression fit).

4. Results and discussion 4.1. Sr speciation

þ

K a1 K a2

ðE:9Þ ðE:10Þ

Before starting the sorption study, geochemical calculations were done to determine the speciation of Sr in the system, considering a radionuclide concentration [RN] = 1108 M, atmospheric conditions, and an ionic strength of 0.1 M in NaClO4. The predom-

T. Missana et al. / Physics and Chemistry of the Earth 33 (2008) S156–S162

inant species present in solutions was Sr2+ for pH values lower than 8. Carbonated species appeared only at a very alkaline pH. 4.2. Sr sorption onto smectite (S100-I0) Fig. 1 shows the sorption results on Na-smectite. Fig. 1a shows Sr sorption edges at different ionic strengths (0.1, 0.05, 0.01 and 0.002 M in NaClO4). The Sr concentration was 1.6106 M. The experimental data are expressed as the logarithm of the distribution coefficient (Kd) as a function of the pH. Sr adsorption was strongly dependent on the ionic strength and practically independent on the pH, according to an ionic exchange mechanism. At pH higher than 9, very small increase of sorption was observed when increasing the pH, this effect being more pronounced at higher ionic strengths. This indicates that sorption by surface complexation becomes non-negligible at higher pH and ionic strength. Fig. 1b shows Sr sorption isotherms obtained at pH 6.5. The experimental data are expressed as the logarithm of the adsorbed Sr concentration vs. the logarithm of the Sr concentration in solution, at equilibrium. The initial Sr concentration varied approximately from 1109 to 1104 M. Sorption was linear over the range of concentrations considered, and saturation of the sorption sites was reached only at the highest Sr concentrations. Additional sorption data on the Na-smectite system can be found in Missana and Garcia-Gutierrez, 2007.

a

6.0 5.5

0.002 M

Log Kd (ml/g)

5.0 4.5 0.01 M 4.0 3.5 3.0 0.05 M 2.5 0.1 M 2.0 2

3

4

5

6

7

8

9

10

11

pH

b

-3

Log[Sr]ads (mol/g)

-4 -5 -6 0.01 M

-7

0.05 M

-8 0.1 M

-9 -11

-10

-9

-8

-7

-6

-5

-4

-3

Log[Sr]eq (M) Fig. 1. (a) Sorption edges of Sr onto Na-smectite at different ionic strengths in NaClO4. [Sr] = 1.6106 M; (b) sorption isotherms Sr Na-smectite at pH 6.5 and four different ionic strengths in NaClO4. (j) 0.1 M; (d) 0.05 M; (N) 0.01 M and (.) 0.002 M. The continuous lines correspond to the data fit with the modelling parameters included in Table 1.

S159

Calculation of the selectivity coefficients for Sr-Na exchange was made considering the mean Kd obtained at different ionic strengths and using Eq. (E.6). The calculated mean logarithm of the selectivity coefficient with respect to Na for Sr ðLogSr Na K SEL Þ was 0.66 ± 0.06 (Missana and Garcia-Gutierrez, 2007). To account for the small effect produced by surface complexation observed at high pH, the formation of the following surface complex was hypothesized Eq. (E.11):

SOH þ Sr2þ () SOSrþ þ Hþ The parameters used for the modelling of the Na-smectite sorption data are summarized in Table 1 and the model curves were drafted as continuous lines in Fig. 1. 4.3. Sr sorption onto illite (S0-I100) Fig. 2 shows the sorption experiments performed with the Naillite. Fig. 2a shows the Sr sorption edges performed at two ionic strengths (I = 0.1 M and I = 0.2 M in NaClO4) and with a tracer concentration of [Sr] = 1.8109 M. As already observed for smectite, sorption was independent on pH (for pH lower than 8) and dependent on the ionic strength. At pH higher than 8, a non-negligible increase in sorption was observed and this effect was more evident than that observed in Na-smectite. The contributions to sorption of ion exchange and of surface complexation are indicated in Fig. 2a. Fig. 2b shows the sorption isotherms, at the same ionic strengths as those used in sorption edges (I = 0.1 and I = 0.2) and pH 6.5. The initial Sr concentration varied approximately from 1011 to 109 M. Sorption was linear in the range of concentrations studied with a slope of the curve very close to 1. Since the range of tracer concentration was smaller than the one analyzed in the S100-I0 case, saturation of sorption sites was not reached. At pH 6.5, where ion exchange was predominant, additional single Kd determinations ([Sr] = 1106 M) were carried out at different ionic strengths to calculate, using Eq. (E.6), the selectivity coefficient for Sr relative to Na ðLogSr Na K SEL Þ in the illite. All the sorption data obtained (included those presented in Fig. 2) are summarized in Table 2. When Eq. (E.6) is expressed in a logarithmic form, the relation between the logarithm of the distribution coefficient Kd(ex) and the logarithm of the electrolyte concentration [Na] is linear and the slope of the straight line is 2 Eq. (E.6). Fig. 3 shows the logarithm of the distribution coefficient vs. the logarithm of the Na concentration, obtained with the data presented in Table 2, and the applied linear fit. The experimental behaviour is quite in agreement with the theoretical one, since the relation is linear (r = 0.988) with a slope of 1.62. The deviation from theoretical value (2) is caused by the fact that the calculated selectivity coefficients actually show a small dependence on the ionic strength. A similar behaviour (linear relation between logarithm of distribution coefficients and logarithm of Na concentration with a slope lower than 2) was also observed, for example, by Rafferty et al. (1981). One possible reason explaining why selectivity coefficients decrease when the ionic strength decreases is the fact that the system is not completely bi-ionic. Due to small dissolution of the clay particles, ions different from Na can be found in the solution and these ions can compete with Sr for sorption (Missana et al., submitted for publication). The competitive effect for ionic exchange is higher at the lower ionic strengths. Other possible theoretical reason for the variability of the selectivity coefficients as a function of the ionic strength was discussed by Mc Bride (1980); as clays are materials with a ‘‘non-rigid structure” the formation of tactoids strongly depends on the cation composition at the surface and ionic strength, so that ionic exchange can be affected by differences in the structural organisation

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T. Missana et al. / Physics and Chemistry of the Earth 33 (2008) S156–S162

Table 1 Summary of the main parameters of the Na-smectite and Na-Illite needed for modelling purposes. Parameters

Smectite

Illite

CEC (meq/100 g) BET area (m2 g1) Density of edge sites (SOH) eqm2 Species

Composition

102.0 Huertas et al. (2000) 33.0 Huertas et al. (2000) 1.8 Missana et al. (2002) Log K

17.0 O’Loughlin et al. (2000) 17.0 Elzinga and Sparks (2001) 4.0 Log K

SO[-] (deprotonation) SOH2[+] (protonation) X2Sr (exchange) SOHSr[+] (complexation)

1 H[+], 1 SOH +1 H[+], 1 SOH 2 XNa, 2 Na[+], 1 Sr[2+] 1 H[+], 1 SOH, 1 Sr[2+]

8.4 Missana et al. (2002) 5.3 Missana et al. (2002) 0.7 Missana and Garcia-Gutierrez (2007) 5.2 Missana and Garcia-Gutierrez (2007)

6.2 Bradbury and Baeyens (2005a) 5.5 Bradbury and Baeyens (2005a) 1.4 this work 5.9 this work

a

Table 2 Logarithm of Kd of Sr in the Na-illite vs. logarithm of Na concentration pH = 6.5.

2.8

LogKd (ml/g)

2.6

Surface Complexation

2.4 0.1 M 2.2

NaClO4 (M)

Log Kd (±0.1)

S (g/L)

LogSr Na K SEL

0.003 0.01 0.02 0.045 0.1 0.1 0.2 0.2

4.90 3.87 3.34 3.16 2.11 2.20 1.93 1.96

1.0 0.4 1.0 0.5 1.0 1.0 1.4 1.0 Mean Std (r)

0.98 1.04 1.14 1.70 1.40 1.49 1.86 1.88 1.44 0.36

2.0 1.8 0.2 M 2

3

Ionic Exchange

4

5

6

7

8

9

10

11

pH -8.5

4.5

LogKd (ml/g)

b

5.0

Log (Sr)ads (mol/g)

-9.0

-9.5

4.0 3.5 3.0 2.5

-10.0

2.0

0.1 M

-10.5

1.5 -2.5

0.2 M

-2.0

-1.5

-1.0

-0.5

Log(Na) (M)

-11.0 -10.0

-9.5

-9.0

-8.5

-8.0

Log (Sr)eq (mol/L) Fig. 2. (a) Sorption edges of Sr onto Na-illite at two different ionic strengths in NaClO4. [Sr] = 1.8109 M; (b) sorption isotherms Sr Na-illite at pH 6.5 and two different ionic strengths in NaClO4. (j) 0.1 M; () 0.2 M. The continuous lines correspond to the data fit with the modelling parameters included in Table 1.

of the clay platelets. Other authors related the variation of illite selectivity coefficient to heterogeneity of surface exchange sites. To account for this heterogeneity, a multi-site exchange model was used (Tournassat et al., 2007). However, for sake of simplicity, a single exchange site model was used in this study. Thus, the calculated mean LogSr Na K SEL was 1.44 ± 0.36. The calculated error is slightly higher than the experimental one (±0.10). This means that the sorption data at very low or very high ionic strengths might not be perfectly matched by the model. As previously done for smectite, the formation of SOSr[+] complex at the illite edge sites was taken into account to model the results Eq. (E.11). The parameters

Fig. 3. Logarithm of the distribution coefficient of Sr in Na-illite as a function of the logarithm of the Na concentration in solution, and linear fit of data.

used for the modelling of the Na-illite sorption data are summarized in Table 1 and the model curves were plotted as continuous lines in Fig. 2. Very similar Sr sorption behaviour was observed for the studied 2:1 clays. Sorption is clearly dominated by ionic exchange, but a small sorption contribution at the edge sites must be considered. The surface complexation contribution was observed at pH higher than 8 and it was more pronounced in the illite. 4.4. Sr sorption onto smectite/illite mixtures (S50-I50 and S75-I25) Sr sorption edges were carried out at ionic strength of 0.1 M using two smectite/illite mixtures: 50% smectite and 50% illite (S50-I50) and 75% smectite 25% illite (S75-I25). Data of the mixtures were compared with those of Na-smectite (S100-I0) and Na-illite (S0-I100) obtained in the same experimental conditions.

T. Missana et al. / Physics and Chemistry of the Earth 33 (2008) S156–S162

Fig. 4a shows the experimental Sr sorption data obtained for the different clay mixtures. The sorption behaviour was very similar in all the samples analyzed and all the experimental data were within the experimental uncertainty. The main features observed for the single minerals were reproduced also in the mixtures. To model the sorption data of the mixtures the given proportion of clay particles in suspension was considered, to determine the proportion of smectite/illite sorption sites available. The selectivity coefficients and complexation constants were those derived from the mechanistic sorption model developed in the single mineral experiments (Table 1). Fig. 4b shows the simulations obtained for Sr sorption for the two smectite/illite mixtures and the two single minerals: the simulation matches well the experimental data confirming the very small variation of the Sr uptake (within the experimental error) when the illite/smectite proportion changes. Furthermore, the model illustrates that sorption due to ionic exchange is slightly higher when the smectite content increases. At pH higher than 9, when surface complexation starts to make the difference, the higher the illite content, the higher the sorption. This behaviour predicted by the model could not be clearly observed in the experimental data. 5. Conclusions The sorption behaviour of radioactive Sr on smectite/illite clays mixed in different proportions was experimentally studied. A mechanistic model based on the information on adsorption on single minerals was used for interpreting the sorption data in the mixed systems.

a

3.0

(mL/g) LogKd

Acknowledgments

References

2.0 100 % Smectite (S100-I0) 75 % " (S75-I25) 50 % " (S50-I50) 0% " (S0-I100)

1.5

1.0

3.0 S0-I100 S50-I50

2.5

LogKd (ml/g)

In both single clays, Sr sorption included a mayor contribution due to ionic exchange and a smaller contribution due to surface complexation. The selectivity coefficients and complexation constants were derived from the single mineral experiments considering one type of exchange site and one type of complexation site and, with these parameters, both sorption edges and sorption isotherms were satisfactorily fit. The obtained selectivity coefficients for illite show certain dependence with the ionic strength, but the error on the mean value was not very different from the experimental error, allowing considering the simple one –exchange site model as reasonably adequate. The adsorption behaviour of the illite/smectite mixtures was modelled considering the relative concentration of the two adsorbents and their affinity to Sr given by the parameters previously determined from the single sorbent systems. The whole sorption data set, obtained in the mixtures of the studied 2:1 clay materials, could be successfully modelled using these selectivity coefficients and complexation constants. The experimental effort to create a sorption parameter database based on a mechanistic approach is worth to improve the safety assessment analyses of radioactive repository and for predicting the migration behaviour of a radionuclide accidentally introduced in the geosphere. This approach has a clear advantage over empirical methods because it allows explaining and justifying the sorption values chosen in model predictions.

This work has been partially financed by the EU within the FUNMIG project (Ref. FP6-516514) and the ENRESA-CIEMAT framework. The support of Victor Fernandez and Petri Gil to laboratory activities is acknowledged. Professor N. Clauer is thanked for his valuable comments and corrections to the manuscript.

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pH Fig. 4. (a) Sorption edges of Sr onto Na-smectite and Na-illite mixed in different proportions. The ionic strength is 0.1 M in NaClO4 and [Sr] = 1.8109 M. (b) Modelling of sorption data.

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