Kinetics of montmorillonite dissolution in granitic solutions

Applied Geochemistry 16 (2001) 397±407

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Kinetics of montmorillonite dissolution in granitic solutions F.J. Huertas *, E. Caballero, C. JimeÂnez de Cisneros, F. Huertas, J. Linares Department of Earth Sciences and Environmental Chemistry, EstacioÂn Experimental del ZaidõÂn, CSIC, Profesor Albareda 1, 18008 Granada, Spain Received 18 February 1999; accepted 4 April 2000 Editorial handling by R.L. Bassett

Abstract Experiments measuring smectite dissolution rates in granitic solutions were carried out in a semi-batch reactor at 20, 40, and 60 C. The pH conditions of the solutions range from 7.6 to 8.5. Solid samples were con®ned within a dialysis membrane and introduced in the solution. The solution was renewed every 7 days and the dissolution reaction was investigated by the variation of Si concentration in the solutions. The average rates at pH8 were 10ÿ14.13, 10ÿ13.70, and 10ÿ13.46 mol mÿ2 sÿ1, at 20, 40, and 60 C, respectively, and the activation energy for the dissolution reaction at pH 8 was 30.51.3 kJ molÿ1. Comparison of the present results with other studies reveals that the montmorillonite dissolution rate depends strongly on the pH of the solution, with a minimum value at pH 8±8.5. At room temperature, the dissolution rate was found to be linearly dependent on proton (acidic conditions) or hydroxyl (basic conditions) activity in solution: Rate ˆ 10ÿ11:39 a0:34 H‡ Rate ˆ 10ÿ12:31 a0:34 OHÿ

pH < 8 pH > 8:5

The comprehension of the dissolution mechanism can be improved by using surface complexation theory. Correlation between speciation of surface sites and kinetic results indicated that at room temperature the dissolution rate was ÿ directly proportional to the surface concentration of >AlOH+ 2 and >AlO surface complexes, under acidic or alkaline conditions, respectively. Rate ˆ 10ÿ8:0 f>AlOH‡ 2g Rate ˆ 10ÿ8:2 f>AlOÿ g

pH < 8 pH > 8:5

A multiple variable model is proposed to take into account simultaneously the e€ect of pH on dissolution rates and on activation energy. The rates estimated using the model are in good agreement with experimental dissolution rates. # 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction The storage of high level nuclear wastes (HLW) is an engineering problem with numerous political, social,

* Corresponding author. Tel.: +34-958-121011, ext. 209; Fax:+34-958-129600. E-mail address: [email protected] (F.J. Huertas).

and environmental implications. Principal types of underground repositories that are presently being investigated are sited in argillaceous rocks, crystalline formations, tu€s, and salt deposits (Brookins, 1984; Chapman and McKinley, 1989). Granite formations are one of the crystalline rocks tested. In this case, HLW are placed in galleries excavated in the host rock, and surrounded by bentonite as back®ll material. The engineered barrier must isolate the canister and protect the

0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(00)00049-4

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F.J. Huertas et al. / Applied Geochemistry 16 (2001) 397±407

environment from any possible leakage. The properties of the bentonitic barrier have been extensively described elsewhere (i.e. Brookins, 1984; Miller et al., 1994). Interstitial solutions in a granite host rock repository percolate through the massif and reach the back®ll bentonite. Various kinds of chemical reactions may result from interaction between the granitic solution and the smectite material, thus changing chemical composition (i.e. ionic exchange reactions), mineralogy (i.e. loss of swelling properties and transformation to illite), and bentonite stability (i.e. dissolution and precipitation). Dissolution has been considered a source of released silica, but smectite dissolution has not been speci®cally tested. Although smectites are wide-spread in soils and sediment throughout the world, there are few studies dealing with smectite dissolution reaction under Earth surface or subsurface conditions. In addition, many of them are focused on narrow pH intervals, which prevent gaining a global vision of the reaction under the complete pH scale. Furrer et al. (1993) and Zysset and Schindler (1996) investigated the dissolution of K-montmorillonite at room temperature under acidic conditions (pH 1±5), obtaining a pH dependent rate in the order of 10ÿ13±10ÿ11.5 mol mÿ2 sÿ1. The mechanism was interpreted by using a surface complexation model and it was concluded that the dissolution is a surface controlled reaction, located predominantly at the crystal edge faces. Cama (1998) dissolved montmorillonite in a ¯ow-through reactor, at 80 C and pH=8.8. This author investigated the dependence of the dissolution rate on the solution saturation state. A rate of 10ÿ11.35 mol mÿ2 sÿ1 was calculated under far from equilibrium conditions. Hayasi and Yamada (1990) obtained a dissolution rate for Wyoming bentonite of 10ÿ11.49 mol mÿ2 sÿ1, in Na2CO3 solution (pH=10.5) at 80 C. On the other hand, Bauer and Berger (1998) studied the dissolution reaction in KOH solutions under highly alkaline conditions (pH>12), and used a model based on surface complexation theory. The rates calculated were 10ÿ11.8 and 10ÿ10.8 mol mÿ2 sÿ1, at 35 and 80 C, respectively. They also calculated a value of 52 kJ molÿ1 for the activation energy of the dissolution reaction. Smectite dissolution reaction has also been performed in very acidic solutions, because of the industrial interest in this process (i.e. NovaÂk and CõÂcel, 1978; TkaÂc et al., 1994; Komadel et al., 1996). However, the experimental conditions described for these treatments are neither frequent in a natural environment (i.e. mine drainage water), nor in nuclear waste repositories. In addition, the experiments conducted under higher temperature conditions (<100 C) could also be mentioned. Howard and Roy (1985) treated bentonite with synthetic basalt solution (pH 9.8) at 150 and 250 C and observed congruent dissolution at 150 C and precipitation of Albearing phases at 250 C. Cuadros and Linares (1996)

monitored the reaction of montmorillonite with solutions of KCl at di€erent hydrothermal temperatures (60±200  C). They observed a release of silica to solution, which was explained as a result of the initial dissolution of the smectite phase and the formation of illite-like layers. Nevertheless, certain conclusions cannot be easily derived from this type of study, because they are focused mainly on the smectite-to-illite conversion and it is dicult to distinguish the smectite-to-illite transformation from the smectite dissolution reaction. The results are dicult to compare a priori because of the varying experimental conditions under which they were obtained. Regarding their relevance to the repository, the study by Cama (1998) is the only one carried out under temperature or pH conditions which are in fact found in the back®ll bentonite (Miller et al., 1994; ENRESA, 1997b). The dissolution rate published in the Hayashi and Yamada (1990) study is comparable to that of Cama (1998), even though it was performed with a solution two units higher in pH. The experiments of Bauer and Berger (1998) concern the concrete plug which sealed the gallery in the repository, where highly alkaline solutions are possible. On the other hand, the solutions used in Furrer et al. (1993) and Zysset and Schindler (1996) were much more acidic than those found in the repository. The foregoing studies suggest that it would be useful to test the stability of smectite in contact with granite solutions. The present study deals with the kinetics of smectite dissolution under the pH, solution composition, and temperature in the repository. The results obtained, together with others reported in the literature have allowed the authors to gain a wider comprehension of the mechanism of smectite dissolution. 2. Materials and methods 2.1. Starting material A bentonite from Serrata de NõÂjar (AlmerõÂa, southeastern Spain) was selected as smectite material. This bentonite was formed by hydrothermal alteration of volcanic tu€ (for details regarding the deposit and the hydrothermal alteration process, Linares et al. 1973, 1993; Caballero et al., 1983, Leone et al, 1983; FernaÂndez Soler, 1992; Linares et al., 1993). The bulk bentonite was crushed and homogenised, then ground in an agate mortar, homogenised again and dried in an oven at 60 C. Mineralogical analysis was carried out by X-ray diffraction (XRD). Several XRD patterns were recorded on powder specimens, and on oriented and glycolated mounts (<2 mm size fraction). The semiquantitative analysis calculated using the relative peak areas, yielding the following average composition: 96% smectite, 2% cristobalite, 1% quartz, 1% calcite and K-feldspar in

F.J. Huertas et al. / Applied Geochemistry 16 (2001) 397±407

trace amount. The XRD patterns of the oriented mounts showed the presence of randomly interstrati®ed illite (15%), calculated according to Moore and Reynolds (1989). The original solid sample was analysed for major elements by the wet methods of Shapiro (1975). The cation exchange capacity (CEC) and the exchangeable cations were independently analysed by saturation with NH+ 4 at pH 7 and subsequent displacement of NH+ 4 with NaCl (Soil Conservation Service, 1972). The chemical analysis results are summarised in Table 1. After correction for the presence of other phases identi®ed by XRD, the structural formula of the smectite was the following: Ca0:18 Mg0:13 Na0:20 K0:20 ‰Al2:83 Fe0:37 Mg0:89 Š ‰Si7:58 Al0:42 Š O20 …OH†4 The external surface area of the bentonite was determined by 5-point N2 adsorption isotherms (BET), yielding an average value of 57.0 m2 gÿ1. Total surface area was determined by water vapour adsorption (Keeling, 1961), resulting in 649 m2 gÿ1. 2.2. Composition of the solutions Three groups of arti®cial granitic solutions were prepared, with 3 di€erent concentrations for each group, in order to cover a wide range of compositions. The ®rst one (type A) corresponds to the interstitial solution of the bentonite deposit (ENRESA, 1997b). The bentonite interstitial solution (solution A1), obtained by squeezing at 70 MPa, can be described as a Na±Mg±Cl solution, with an ionic strength of 0.29 mol lÿ1. The other two are granitic solutions (type B and C), corresponding to that proposed by Moody (1982) (solution B1) and the percolating solution of the Grimsel granite formation, in Switzerland (solution C1) (ENRESA, 1997b). Compositions of the 3 solutions are in Table 2. 2.3. Dissolution experiments 5 g of bentonite were placed in a dialysis bag, in order to con®ne the solid while also permitting circulation of the dissolved species. This bag was immersed in 125 ml of granitic solution in a per¯uoroalcoxy (PFA) batch reactor. The experiments were performed at 20, 40, and

399

60 C. Temperature of the experiments at 20 C was maintained constant by environment temperature control (2 C). In the other sets, it was maintained constant by means of ovens (3 C). After 7 days of reaction, the solution was collected and replaced by 125 ml of fresh solution. This procedure was repeated every 7 days for 6 months. The reactors were hand-agitated once per day. The bentonite underwent no pre-treatment, with the exception of grinding. The ®ne particles present in the bulk sample dissolve faster than the bigger ones and can give rise to an initial dissolution stage previous to the linear dissolution stage. This will not be considered in the evaluation of the smectite dissolution rate, which is derived exclusively from the linear part. Solutions were analysed for pH, Si, Na, K, Ca, and Mg. Only data for pH and Si are presented in this paper. The changes in Na, K, Ca, and Mg are controlled mainly by ionic exchange reactions, and will be discussed in another paper. Silica was analysed by spectrophotometry using the molybdate-blue method (Shapiro, 1975). The accuracy was better than 3% for individual analyses. At the end of the dissolution experiment, the dialysis bag was removed and the solid sample was washed with water:acetone 1:1 in centrifuge tubes 3 times. Solids were oven-dried at 60 C. BET measurement of the samples altered during the dissolution experiments indicated no change in surface area within the accuracy of the method, estimated to be 10% (Davis and Kent, 1990). 3. Results The bentonite was progressively dissolved by the granitic solutions, thus releasing Si into solution. Silica quantities were higher in the ®rst renewals, but later came down to a constant concentration (Fig. 1). Higher Si concentrations corresponded to higher temperature. However, the experimental set-up prevented the solutions from reaching saturation with respect to silica. It should be noted that the solutions are always undersaturated with respect to amorphous silica, (i.e. 120 ppm at 25 C, Iler, 1979), irrespective of temperature. Similar concentrations were observed for the 9 series of solution

Table 1 Chemical analysis of the smectite samplea

Total (%) Exchangeable cations (meq/100 g) CEC (meq/100 g) a

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

H2O+

Total

58.92

19.48

3.48

2.51 47.0

4.83 35.5

2.28 25.8

1.21 2.3

7.09

99.8 110.6 106

Weight percentages refer to oxides and exchangeable cations are expressed in meq/100 g.

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Table 2 Composition (in ppm) of the initial solutions used for the smectite dissolution Type A

Na+ K+ Mg2+ Ca2+ Clÿ Brÿ NOÿ 3 SO2ÿ 4 HCOÿ 3 CO2ÿ 3 SiO2 pH Ia a

Type B

A1

A2

A3

B1

B2

B3

C1

C2

C3

241 2.97 93.6 65.9 638 1.34 9.39 140 17.0 ± 1.28 7.78 0.029

541 5.54 192 121 1292 2.68 18.8 347 34.0 ± 2.56 7.71 0.058

2536 27.1 1018 479 6452 13.39 93.9 1475 170 ± 12.80 7.81 0.291

132 1.16 0.51 59.0 271 ± ± 19.30 4.30 ± ± 7.39 0.010

609 5.46 2.53 288 1305 ± ± 96.5 21.5 ± ± 7.44 0.048

1256 10.61 5.07 579 2652 ± ± 193 43.0 ± ± 7.62 0.097

17.38 0.228 0.01 6.00 16.5 ± ± 5.48 17.1 2.36 ± 8.01 0.001

182 2.02 0.13 19.7 165 ± ± 54.8 171 23.6 ± 8.19 0.012

324 3.88 0.26 37.0 330 ± ± 110 342 47.2 ± 8.13 0.023

I; ionic strength in mol lÿ1.

Fig. 1. Concentration of SiO2 (ppm) in the renewal solutions during the experiments. Only data for solution C1 are shown, at the 3 experiment temperatures. The other solutions displayed a similar behaviour.

compositions and 3 temperatures. The dissolution rate of smectite can be expressed as the following:

ÿ

Type C

1 dc…Si† ˆ SAk n dt

…1†

where n is the stoichiometric coecient for Si in the smectite formula calculated for O20(OH)4, c(Si) is the concentration of Si in mol lÿ1, S is the speci®c surface area in m2 gÿ1, A is the solid solution ratio in g lÿ1, and k stands for the rate constant in mol mÿ2 sÿ1. The speci®c dissolution rate is given by the value of k, if S and

A are constant during the dissolution experiment. The minus sign indicates Si release. In order to calculate the dissolution rate, total released Si can be plotted against time (Fig. 2). It can be observed that Si concentration versus time follows a curve which progressively becomes a straight line. The high initial dissolution rate is likely to be the result of various processes, such as dissolution of ®ne-grained materials, highly strained areas on large grains or defects (i.e. Holdren and Berner, 1979; Schott et al., 1981; Chou and Wollast, 1984, 1985; Knauss and Wolery, 1988, 1989; Stillings and Brantley, 1995; Huertas et al., 1999). As time progresses, all ®ne particles will be dissolved given sucient time, and they will no longer contribute to the overall dissolution rate. Smectite dissolution rate has been calculated as the slope of the linear part of the curves in Fig. 2. Results are summarised in Table 3. The pH of the solution was measured during the experiment, immediately after the withdrawn solution was cooled. The pH values reported in Table 3 correspond to the average pH value measured during the experiment; the error is the standard deviation of these values. The error for dissolution rates is estimated to be lower than 12%, and is dominated by the uncertainty on BET surface area measurement in the solids. The contribution of accessory mineral present in the bentonite sample to overall Si release is negligible during the linear dissolution stage and it would be included in the error of the dissolution rate. Under the experimental conditions (pH 8 and 20 C, for example), quartz dissolves at a rate of 10ÿ12 mol mÿ2sÿ1 (i.e. Dove and Elston, 1992), and smectite at 10ÿ14 mol mÿ2 sÿ1 (see Table 3), whereas the smectite:quartz ratio in the solid sample is 96:1. Surface areas of this smectite and quartz are, respectively, 57 and 0.1±1 m2 gÿ1 (i.e. Drees et al.,

F.J. Huertas et al. / Applied Geochemistry 16 (2001) 397±407

1989; Dove and Elston, 1992). Consequently, the contribution of smectite to silica release is 2 or 3 orders of magnitude higher than the contribution of quartz. Similar arguments can be proposed for the other accessory minerals, based on their low surface area (White and Brantley, 1995). Results in Table 3 show similar dissolution rates for the 9 solutions at the same temperature. Within the

401

short pH interval studied, the di€ering chemistry of each of the solutions (relative abundance of cations, pH, ionic strength) does not appear to a€ect the reaction and temperature is the variable which controls the dissolution reaction rate. 4. Discussion 4.1. E€ect of pH

Fig. 2. Total SiO2 released (ppm) by bentonite dissolution in contact with solution C1. Straight lines correspond to the regression of the points, which followed a linear trend.

Table 3 Smectite dissolution rates for the di€erent experimental conditionsa T (C) A1 pHb 20 40 60

A2 log k

pHb

A3 log k

pHb

log k

7.640.17 ÿ14.04 7.760.17 ÿ14.00 ± ± 7.990.11 ÿ13.69 7.930.17 ÿ13.46 7.930.19 ÿ13.66 7.890.12 ÿ13.43 7.830.21 ÿ13.44 7.790.17 ÿ13.40 B1 pH

20 40 60

B2 log k

pH

The dissolution rates calculated cover a range of approximately one pH unit, near pH=8. Fig. 3 is a plot of the logarithm of the rate versus the pH of the solution. The values obtained for the 3 temperatures shows a slow decrease as pH increases. The pH does not strongly a€ect the dissolution rate within the interval investigated. Ionic strength was suciently low, thus showing no clear in¯uence. The dissolution rates yielded for the granitic solutions at each temperature are very similar, irrespective of the granitic solution. Therefore, an average rate can be calculated for each temperature and pH condition (pH 8) (Table 4). The dissolution rates calculated in the present investigation at 20 C are 1 (at pH=1) or 2 (pH=5) orders of magnitude lower than those of Zysset and Schindler (1996) at 25 C. They are also 2 orders of magnitude lower than the values proposed by Bauer and Berger (1998) for pH values <12 at 35 C. Such di€erences are likely to be due to the di€erent pH ranges studied. However, with respect to the value obtained by Cama (1998), there is a substantial discrepancy which may be due to the use of a ¯ow-through reactor. Some dissolution rates reported in the literature and those obtained in the present study are plotted in Fig. 4. Dissolution rates under highly alkaline conditions at 20 C have been estimated using the activation energy of

B3 log k

pH

log k

7.800.27 ÿ14.06 7.650.27 ÿ14.09 7.740.27 ÿ14.14 8.000.17 ÿ13.66 7.670.17 ÿ13.66 7.760.17 ÿ13.71 7.860.16 ÿ13.49 7.530.16 ÿ13.41 7.620.16 ÿ13.45 C1 pH

20 40 60

C2 log k

pH

C3 log k

pH

log k

8.460.27 ÿ14.19 8.570.27 ÿ14.15 8.610.27 ÿ14.16 8.480.17 ÿ13.80 8.590.17 ÿ13.68 8.630.17 ÿ13.70 8.340.16 ÿ13.44 8.450.16 ÿ13.48 8.490.16 ÿ13.50 a b

Rates are in mol mÿ2 sÿ1. pH corresponds to the average pH during the experiment.

Fig. 3. Plot of the logarithm for the dissolution rates against the average pH of the solution.

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Table 4 Average dissolution rates of smectite at pH 8 Temperature ( C)

Log rate (mol mÿ2 sÿ1)

20 40 60

ÿ14.13 ÿ13.70 ÿ13.46

Fig. 4. Variation in the smectite dissolution rate as a function of the pH. Data from other studies are also included (Hayashi and Yamada, 1990; Zysset and Schindler, 1996; Bauer and Berger, 1998). Dissolution rates under alkaline conditions at 20 C estimated from Bauer and Berger (1998).

Bauer and Berger (1998). No research has addressed the behaviour of smectite during the dissolution reaction within the complete pH interval and, therefore, partial studies should be put together to obtain a general overview. The 3 groups of rates under room temperature conditions can be used to obtain a pattern of dependence of dissolution rate on pH. It shows a classical Ushape curve. The present values around pH 8 would correspond to the minimum dissolution rate of smectite. The pH value for the minimum rate should be close to the point of zero net proton charge (PZNPC), which was not determined for the smectite specimen. However, it is in agreement with the PZNPC value of 7.7 obtained for Wyoming montmorillonite (Stadler and Schindler, 1993). Using the rates from Zysset and Schindler (1996) and Bauer and Berger (1998), in addition to the present data, the slopes for the acidic and basic branch of the pH-dependent dissolution rate at room temperature were ÿ0.34 and +0.34, respectively. The rate laws under acidic and basic conditions were calculated by ®tting the dissolution rate (log rate) to the pH value, and yielded the following equations:

Rate ˆ 10ÿ11:39 a0:34 H‡

pH < 8

…2a†

Rate ˆ 10ÿ12:31 a0:34 OH-

pH > 8:5

…2b†

These reaction orders with respect to protons and hydroxyls (i.e. the exponents) are similar to other values obtained for phyllosilicates (Nagy, 1995), feldspars (Blum and Stillings, 1995), pyroxene and amphiboles (Brantley and Chen, 1995). The dissolution mechanism can be interpreted in terms of surface complexation theory. Stadler and Schindler (1993) studied the surface properties of montmorillonite (Wyoming), obtaining surface parameters and calculating the distribution of sites as a function of pH. They model the surface behaviour on the basis of 3 surface sites: aluminols (>AlOH), silanols (>SiOH), and ionic exchange positions. The proton adsorption/ desorption properties of aluminols and silanols depend on the pH of the solution (Fig. 5). The ion exchange sites should be taken into account only under moderate or high acidic conditions (Wieland and Stumm, 1992; Stadler and Schindler, 1993; Zysset and Schindler, 1996). Fig. 5a shows that at a pH value of 8.5, approximately, positive and negative charge on aluminol sites at the smectite surface are balanced. Using the Stadler and Schindler (1993) data, the following relations can be calculated between surface charge associated to aluminols and proton (or hydroxyl) activity in solution: ÿ3:41 0:33 f>AlOH‡ aH‡ 2 g ˆ 10

f>AlOÿ g ˆ 10ÿ4:15 a0:36 OHÿ

pH < 8:5 pH > 8:5

…3a† …3b†

where, {} stands for concentration of surface sites, in mol mÿ2. On the other hand, the surface density of negatively charged silanol sites increases from pH 4 to pH 8 (Fig. 5b) and remains constant for higher pH values. Under acidic conditions it is generally admitted that dissolution is governed by the presence of >AlOH+ 2 groups (i.e. Brady and Walther, 1992; Wieland and Stumm, 1992; Walther, 1996; Devidal et al., 1997). Combining Eqs. (2a) and (3a), one obtains: Rate ˆ 10ÿ8:0 f>AlOH‡ 2 g pH < 8

…4a†

The dissolution rate is proportional to the surface site density of positively charged aluminol sites and it is consistent with the dissolution rate law proposed by Zysset and Schindler (1996) for K-montmorillonite under acidic conditions. Under basic conditions, both aluminol and silanol sites are negatively charged (Fig. 5), and might contribute to the dissolution mechanism. Huertas et al. (1999), correlating dissolution experiments and surface properties of kaolinite, inferred that the process of

F.J. Huertas et al. / Applied Geochemistry 16 (2001) 397±407

Rate ˆ 10ÿ8:2 f>AlOÿ g pH > 8:5

Fig. 5. Surface speciation of aluminol and silanol sites in Wyoming montmorillonite, experimentally measured by Stadler and Schindler (1993). Surface site densities of aluminol and silanol groups are, respectively, 2.8 and 1.7 mmol mÿ2.

kaolinite dissolution under alkaline conditions is not promoted by negative Si sites. Comparison of Figs. 4 and 5 suggests a similar conclusion. Dissolution proceeds at a minimum rate at a pH value of 8±8.5, when positive and negative aluminol sites are balanced. Dissolution rate increases as surface density of >AlOÿ sites increases. Apparently, the deprotonation of silanol sites for pH >4 may not directly contribute to the dissolution process, as also observed by Huertas et al. (1999) in kaolinite. The main di€erence between the kaolinite and montmorillonite structure is that the former is a 1:1 type, and the latter a 2:1. Isomorphic substitution of octahedral cations in kaolinite is very limited, whereas it may be considerable in the case of montmorillonite. The behaviour of the ``aluminol sites'' in dioctahedral smectites is thus a simpli®cation of the real situation and should be considered the average behaviour of sites linked to the octahedral cations in the structure. Using the surface acidity constants of the oxides (Sverjensky and Sahai, 1996) for purposes of comparison, Al and Fe(III) sites display similar character, but Mg sites may be slightly more basic than Al sites. Therefore, isomorphic substitutions of AlVI for MgVI in montmorillonite may slightly shift the PZNPC and the speci®c charge distribution as a function of pH, but not the role played by the existing surface sites. The isomorphic substitution of dioctahedral by trioctahedral cations will a€ect the permanent charge related to ion exchange positions. These positions only contribute to surface charge at low pH values (i.e. Stadler and Schindler, 1993; Zysset and Schindler, 1996). Assuming this hypothesis is valid, surface speciation of Wyoming bentonite can be applied to the montmorillonite under study. Combination of Eqs. (2b) and (3b) renders the following rate law under alkaline conditions:

403

…4b†

Smectite dissolution is proportional to the density of charged aluminol groups, under acidic or alkaline conditions. Zysset and Schindler (1996) suggested that, in acidic solution, the dissolution process is located at the crystal edge faces. This mechanism may also be extended to smectite dissolution in basic solutions, because aluminol sites are located on the crystal edges of the phyllosilicate structure (Bleam et al., 1993). Under acidic and alkaline conditions, the formation of a charged surface complex at an Al centre would induce the detachment of Al and the formation of the precursor of the activated complex involved in the limiting step for the dissolution reaction (Devidal et al., 1997; Huertas et al., 1999). 4.2. E€ect of temperature The temperature increases the Si concentration in the granitic solution, and consequently it increases the smectite dissolution rate, as shown in Fig. 3. The e€ect of the temperature on the dissolution rates is a function of the activation energy, Ea, and is expressed by the Arrhenius equation: k / eÿEa =RT

…5†

where k is the rate constant, R stands for gas constant and T corresponds to absolute temperature. The logarithm of the dissolution rate is plotted against the inverse of the absolute temperature in Fig. 6. According to the aforementioned, all the rates have been considered to make a unique group, irrespective of temperature or solution. The results are distributed along a straight line, whose slope yields an activation energy of 30.51.3 kJ molÿ1. This value is lower than the one calculated by Bauer and Berger (1998) for smectite, but is within the activation energy range for the dissolution reaction of other phyllosilicates under various conditions (Nagy, 1995). The dissolution process is a complex reaction, composed of several elementary steps. The dissolution rate will be controlled by the slowest step (limiting step), but the in¯uence of temperature on the di€erent parameters is unclear. Lasaga et al. (1994) suggested the use of the term ``apparent activation energy'' when referring to a complex process, because the temperature dependence of the overall dissolution and precipitation reactions may be more complex than the temperature dependence for an elementary reaction. No data presently allow for an unambiguous evaluation of the e€ects of pH on the activation energy for the smectite dissolution reaction. Bauer and Berger (1998) observed that the activation energy of kaolinite dissolution varied between 33 and 51 kJ molÿ1 for a solution composition between 0.1 and 3 M in KOH. The Ea is

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doubled within a pH interval of approximately one unit. On the other hand, Carroll and Walther (1990) calculated activation energies for kaolinite dissolution in the pH range of 1±12, within a temperature interval of 25± 80 C. The minimum was obtained for a pH value of 7, and Ea increased with both increasing and decreasing pH. Similar behaviours have been reported for feldspars (Helgeson et al., 1984; Tole et al., 1986). The surface chemistry of the smectite may be responsible for the in¯uence of the pH on the activation energy. The dissolution process is a surface reaction at the mineral/solution interface and its mechanism involves

Fig. 6. Arrhenius plot for the smectite dissolution rates at pH 8. The solutions were treated as a single group of data (see text).

Fig. 7. Multiple variable model for the smectite dissolution rate. Both dissolution rate and activation energy are pHdependent. (see text). Data from other studies are also included (Hayashi and Yamada, 1990; Zysset and Schindler, 1996; Bauer and Berger, 1998).

the adsorption of soluble species and the formation of surface complexes. The surface complex which is predominant within a certain pH interval will determine the dissolution rate under such conditions. Several dissolution mechanisms may coexist within the complete pH interval, due to the presence of positive, neutral, and negative surface complexes. In addition, one might expect some in¯uence of temperature on surface charge. Nevertheless, surface charge is only slightly a€ected by temperature between 25 and 60 C for the case of alumina, silica or kaolinite (i.e. Brady, 1994; Ward and Brady, 1998), and a similar behaviour may be assumed for other clay minerals. It is reasonable to suppose that the dissolution reaction at high temperature is controlled by the same mechanism that operates at 25 C (Brady and Walther, 1992). In fact, calculations of activation energy using dissolution rates at constant pH already include the e€ect of the change in surface charge with temperature, since temperature only slightly a€ects surface charge and the derived values of Ea (Brady and Walther, 1990). Furthermore, the activation energy will be in¯uenced by the contribution of the enthalpy of adsorption phenomena. These circumstances may result in the variation of the activation energy as a function of the pH. Using the activation energy from Bauer and Berger (1998) (Ea=52 kJ molÿ1 at pH 13), as well as the presat one (Ea=30.5 kJ molÿ1 at pH 8), the variation of the activation energy with pH for alkaline conditions (pH>8) can be estimated at Ea=4.3.pHÿ3.9, for pH >8. A linear dependence between these variables is consistent with the one observed by Carroll and Walther (1990) in kaolinite, although they obtained a steeper slope (Ea / 6.94 pH). No data allow the estimation of how the activation energy varies with changing pH values under acidic conditions. In a preliminary approximation, Ea can be assumed to be also proportional to 4.3 pH. These hypotheses allow the calculation of a multiple variable model, which includes simultaneously the e€ect of pH (proton activity in solution) and temperature. The results are shown in Fig. 7. A reasonably strong agreement between experimental and calculated rates can be observed. The dissolution rates predicted at 80 C are lower than the experimental ones, which may indicate that the activation energy of Bauer and Berger (1998) is lower than could be expected. However, it is necessary to carry out additional investigations to check this point and to ®ll the gaps existing under acidic conditions at temperatures higher than 25 C, and under weak basic conditions. 5. Conclusions The dissolution rate of dioctahedral smectite in contact with granitic solutions has been calculated at 20, 40,

F.J. Huertas et al. / Applied Geochemistry 16 (2001) 397±407

and 60 C. The activation energy for the dissolution reaction under these conditions (pH 8) yielded a value of 30.51.3 kJ molÿ1. These results, together with other reported dissolution rates, have permitted an examination of the smectite dissolution process based on surface complexation theory. This examination has revealed that the dissolution rate at room temperature is proportional to the 0.34 power of the activity of protons or hydroxyls. If the rates are correlated with data on surface site density, it can be concluded that the limiting step involves the aluminol sites located at the crystal edge faces. Dissolution rate is directly proportional to surface concentration of the charged aluminol groups, ÿ >AlOH+ under 2 under acidic conditions and >AlO basic ones. The e€ect of the temperature on the dissolution rate is likely to be dependent on the pH, since pH values also a€ect the activation energy. Assuming that the activation energy is proportional to pH, the authors have elaborated a simple, multiple variable model including the e€ect of pH and temperature on the dissolution rate. The agreement between experimental and calculated rates is quite good, and thus supports the validity of the model. These results can be applied to the in situ test at the Grimsel underground research laboratory (Switzerland), where a full-scale experiment of a nuclear waste repository is presently underway (ENRESA, 1997a,b). Fig. 8 is a simultaneous plot of the temperature pro®le (ENRESA, 1997b) and the smectite dissolution rates perpendicular to the axis of the heater (canister) at a pH value 8. Within the clay barrier, the temperature decreased from 100 C at the heater wall to 50 C in the point of contact with the granite host rock. The quantity of smectite dissolved per year for the latter temperatures and rate pro®le is plotted in Fig. 9. In the granite/bentonite interface, it represents 0.02 g of smectite per g of bentonite per year. However, it increases exponentially close to the heater, reaching approximately 0.17 g smectite per g bentonite per year at the canister/bentonite contact. This result is not negligible. In addition, microbial activity may modify the conditions, acidifying the solution by the release of organic acids from their metabolism (Ehrlich, 1990) and may enhance the dissolution rate by one or two orders of magnitude (i.e. Kummer and Stumm, 1980; Wieland and Stumm, 1992). The real situation may not be so alarming. According to the foregoing, the dissolution rate at the pH conditions of the percolating solution in the host rock is the minimum. The e€ect of dissolution on the back®ll material would be reduced by the low ¯ux of solution in granite formations and by the isolation properties of the barrier to the outer part. This ¯ux may produce dissolution in conditions close to equilibrium and thus at a much lower rate. More research should be done in order to properly evaluate the e€ect of dissolution and microbial activity under the conditions existing in the back®ll bentonite.

405

Fig. 8. Temperature pro®le at the bentonite barrier (Grimsel test site), and corresponding dissolution rates estimated on the basis of laboratory data.

Fig. 9. Number of grams of smectite dissolved per grams of bentonite and per year, and silica released, in grams per grams of bentonite and per year, for the dissolution rate pro®le show in Fig. 8. See text for comments.

Acknowledgements Financial support was provided by ENRESA and EU through FEBEX contract (703227, and FI4W-CT950006). The authors are grateful to J. Cuevas for BET analysis and J. Cuadros for helpful discussions. Technical assistance of M.J. Civantos and M.A. Guerrero are recognised. This paper was substantially improved based upon reviews by two anonymous reviewers and the Associated Editor.

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