Ion Concentration Caused by an External Solution into the Porewater of Compacted Bentonite

Transkriptio

1 Working Report Ion Concentration Caused by an External Solution into the Porewater of Compacted Bentonite Arto Muurinen November 2006 POSIVA OY FI OLKILUOTO, FINLAND Tel Fax

2 Working Report Ion Concentration Caused by an External Solution into the Porewater of Compacted Bentonite Arto Muurinen VTT Technical Research Centre of Finland November 2006 Working Reports contain information on work in progress or pending completion. The conclusions and viewpoints presented in the report are those of author(s) and do not necessarily coincide with those of Posiva.

3 ABSTRACT This study was part of a research project aimed at improving the understanding of swelling pressure in compacted bentonite as a function of density and ionic strength in the solution. In this part of the work, the concentration caused by an external solution into the porewater was studied. The tests were carried out with clay, from which easily dissolving impurities had been removed and the clay ion-exchanged to Na-form. NaCl solutions of different concentrations and deionized water were used to saturate the compacted bentonite samples through filter plates. The filter between the bentonite and saturating solution forms a semi-permeable membrane of the Donnan system. At test termination, the bentonite samples and saturation solutions were characterized. Modelling of the chloride concentrations in the porewater was done with the Donnan model assuming that chloride does not form neutral or cationic ion pairs or sorb in the bentonite. The first modelling of the porewater was based on the traditional Donnan model assuming a homogenous pore structure. At high NaCl concentrations, the model can predict the concentrations in the porewater rather well but at low concentrations, where the exclusion is stronger, the measured concentrations are clearly higher than the modelled values. In the second porewater modelling, it was assumed that there are two pore types, interlamellar and external (large) pores in bentonite. In this case, fitting is excellent with reasonable parameter selection. The pore structure and exclusion may, however, depend on the preparation of the bentonite sample. Effective homogenization of bentonite means smaller particles, smaller external pores and stronger anion exclusion. On the basis of modelling, the behaviour of different porewater types during the squeezing was evaluated. The evaluation concerned cases where the starting densities were 700 kg/m 3 and kg/m 3 and the squeezing ended with a dry density of kg/m 3. In the sample of the lower density, about 20 % was evaluated to come from the interlamellar water, while in the sample of the higher density about equal volumes came from the interlamellar and large pores. Keywords: bentonite, nuclear waste, porosity, exclusion, microstructure, Donnan model

4 ULKOPUOLISESTA LIUOKSESTA PURISTETUN BENTONIITIN HUOKOSVETEEN AIHEUTUVA IONIPITOISUUS TIIVISTELMÄ Tämä tutkimus oli osa tutkimusprojektia, jonka tarkoitus oli parantaa paisuntapaineen ymmärtämistä puristetussa bentoniitissa tiheyden ja liuoksen ionivahvuuden funktiona. Työn tässä osassa tutkittiin ulkoisen liuoksen aiheuttamaa pitoisuutta huokosvedessä. Testit tehtiin savella, josta oli poistettu helposti liukenevat epäpuhtaudet ja savi ionivaihdettu Na-muotoon. Puristetut näytteet kyllästettiin NaCl-liuoksella tai ionivaihdetulla vedellä suodatinlevyn läpi. Bentoniitin ja kyllästysliuoksen välissä oleva suodatin toimii Donnan puoliläpäisevänä kalvona. Testin lopussa bentoniittinäytteet ja kyllästysliuokset karakterisoitiin. Kloridipitoisuudet huokosvedessä mallinnettiin Donnanin mallilla olettaen, että kloridi ei muodosta neutraaleja tai kationisia ionipareja tai sorboidu bentoniittiin. Huokosveden ensimmäinen mallinnus perustui perinteiseen Donnanin malliin olettaen, että huokosrakenne oli homogeeninen. Korkeilla NaCl-pitoisuuksilla malli pystyy ennustamaan huokosveden pitoisuudet melko hyvin, mutta alhaisilla pitoisuuksilla, jossa ekskluusio on voimakkaampi, mitatut pitoisuudet ovat selvästi korkeampia kuin mallinnetut arvot. Toisessa mallinnuksessa oletettiin, että bentoniitissa on kahden tyyppisiä huokosia, interlamellaarisia (pieniä) ja ulkoisia (suuria). Tässä tapauksessa yhteensopivuus on erinomainen järkevillä parametrien arvoilla. Huokosrakenne ja ekskluusio saattavat kuitenkin riippua bentoniittinäytteen valmistustavasta. Bentoniitin tehokas homogenisointi merkitsee pienempiä partikkeleja, pienempiä ulkoisia huokosia ja voimakkaampaa anioniekskluusiota. Mallinnuksen pohjalta arvioitiin eri huokosvesityyppien käyttäytymistä puristuksen aikana. Arvio koski tapauksia, missä aloitustiheydet olivat 700 ja 1599 kg/m 3 ja puristus päättyi kuivatiheyteen 1900 kg/m 3. Alemman tiheyden näytteessä noin 20 % arvioitiin tulevan interlamellaarivedestä kun taas korkeamman tiheyden näytteessä suunnilleen yhtä suuret osuudet tulivat interlamellaarivedestä ja suurista huokosista. Avainsanat: bentoniitti ydinjäte, huokoisuus, ekskluusio, mikrorakenne, Donnan malli

5 FOREWORD This study was part of a co-operative research project financed by Posiva Oy and the Swedish Nuclear Fuel and Waste Management Co. (SKB) aimed at improving the understanding of swelling pressure in compacted bentonite as a function of density and ionic strength in the solution. The study was carried out at Clay Technology AB in Sweden and VTT Technical Research Centre of Finland.

6 1 TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ FOREWARD 1 INTRODUCTION EXPERIMENTAL Test material Studies on the swelling pressure samples Studies on the SAXS samples INTERPRETATION OF RESULTS Porewater chemistry Modelling of chloride concentration in porewater SUMMARY REFERENCES APPENDIX A APPENDIX B APPENDIX C APPENDIX D APPENDIX E APPENDIX F The water contents and densities of bentonite after swelling pressure measurement Chemical compositions of the squeezed porewaters The compositions of the porewaters based on dispersion method Concentrations of different components in the saturation solutions after swelling pressure measurement Concentrations of exchangeable cations in bentonite after swelling pressure measurement Concentrations caused by the external solution to the porewater of the SAXS samples.

7 2 1 INTRODUCTION In many countries compacted bentonite has been considered as a potential buffer material for the disposal of spent nuclear fuel. The expandability of bentonite caused by hydration of the exchangeable cations is a most important property for its use as a buffer. This study was part of a research project aimed at improving the understanding of swelling pressure in compacted bentonite as a function of density and ionic strength in the solution in general, and verifying the proposed thermodynamic model in particular. In this part of the project, the concentration caused by an external solution into the porewater was studied. The results of the swelling pressure studies have been reported in Karnland et al According to the Donnan model, polyelectrolytes, like bentonite, in contact with salt solutions through a membrane are expected to have a unique content of ions in the porewater. Equation (1), which is based on the Donnan model, presents the relationship between the concentration of a monovalent cation in the external water and porewater (Overbeek 1952, Karnland 1998, Stålberg 1999). ie C ie ccccc ccccc 4 * e Ce (1) 2 where C e is the cation concentration in the external water (mol/l) C ie is the cation concentration caused by the external water into the porewater (mol/l) C cc is the concentration caused by the exchangeable cations into the porewater (mol/l) ie, cc and e are activity coefficients in different conditions. C cc can be evaluated from the cation-exchange capacity and water content in the clay. In the case where chloride and sodium are the only ions in the system, the chloride concentration in the porewater is equal to C ie. By assuming that the activity coefficients in the porewater are equal ( ie = cc = c ) the concentration caused by the external solution into the porewater can be solved from Eq. (2). C ie Ccc Ccc 4 * ( e / c ) Ce (2) 2 In principle, the Donnan model is applicable for diluted solutions, where the activity coefficients are close to unity. In the case where high concentration is used in the external solution or the exchangeable and introduced cations cause a high concentration in the porewater, the activity coefficients should be considered. A clear correlation between the modelled and measured data for different physico-chemical conditions, e.g. densities and salt contents, would be a strong support for the model s correctness and usefulness. The most important result in the case of a clear correlation would be the conceptual model, i.e. an improved understanding of the clay water system in general.

8 3 2 EXPERIMENTAL 2.1 Test material The experiments were performed with purified MX-80 bentonite changed to sodium form. The purification procedure was based on that of Sposito et al. (1981) but included only ion exchange to sodium form with 1 M NaCl and removal of the excess NaCl by washing and dialysis. During this process, part of the dissolving accessory minerals was removed as well. Finally the bentonite was dried at 60 0 C In purification, 130 g of air-dry MX-80 bentonite was weighed into 18 one-litre centrifuge bottles. To each of the bottles, 500 ml of 1-molar NaCl solution was added. The bottles were shaken on a laboratory shaker for about 4 hours during the day and about 20 hours during the night followed by centrifugation and replacement of the solution by new one. The treatment was repeated twelve times with the 1-molar NaCl solution followed by three washings with deionized water. The dissolved components and ph of the separated solutions were determined (Figure 1). As can be seen, calcium and carbonate are present in the solution even at the end of treatment. This indicates that there is a solubility-limited source, which is not exhausted during the treatment. The bentonites of each bottle were then closed in dialysis tubing of a diameter of 50 mm, length of about 600 mm and pore size of to Daltons. The tubes were immersed in deionized water, two tubes in each vessel containing 7 litres of water Concentration (mg/l) Ca K Mg Si SO4 HCO3 ph ph Number of treatment Figure 1. The concentrations of the chemical components and ph in the purification solutions. Sodium and chloride are not shown.

9 4 In the beginning, the waters were changed daily, but towards the end of the dialysis the frequency was increased to three days. The water was changed 18 times all together, and the total dialysis time was 7 weeks. The electrical conductivity (Figure 2) and the chemical composition of the water (Figure 3) were determined in order to follow the progress of the process. The chloride concentration, which at the beginning was high, decreased after 11 water changes below the determination level. Sulphate and carbonate remained present in the water, which indicates the existence of continuously dissolving sources. The bentonite from the dialysis was spread on trays and dried for 10 days at 60 C Conductivity (ms/cm) Number of dialyse Figure 2. Electrical conductivities of the waters at the end of each dialysis. Concentration (mg/l) Cl Na HCO3 SO4 ph Number of dialyse ph Figure 3. Chemical compositions of the waters at the end of each dialysis.

10 5 The purified bentonite still contained many accessory minerals. The amount of easily dissolving components was determined by shaking 0.7 g of bentonite and 7 g of water in a centrifuge tube for 13 days. The solution was separated from the bentonite by centrifuging. The solutions were then ultrafiltered with 12 k MWCO filters. The compositions of the solutions in these experiments are presented in Table 1. The chloride concentration in bentonite, calculated on the basis of a dissolution test, is about 1.7x10-4 g/g. Bicarbonate, sulphate and silica concentrations are most probably limited by the solubility or dissolution rate of the solid materials, and their total concentration in bentonite cannot be evaluated on the basis of this test. Sodium comes from montmorillonite through ion exchange with calcium and magnesium, and its concentration is thus determined by the dissolving of those components. Table 1. Concentration of dissolved components in the solution after an interaction of 13 days with bentonite and deionized water. The bentonite-to-water ratio in the experiment was 0.7 g to 7 g. Experiment Na Cl SO 4 HCO 3 Si (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) The exchangeable cations of the purified bentonite were determined by extracting the cations with 0.2 M NH 4 SCN in ethanol (Müller-Vonmoos & Kahr 1983). Ethanol instead of water was used to avoid dissolution of the accessory minerals. In the method, 0.7 g of bentonite was shaken with 7 ml of the NH 4 SCN solution in centrifuge tubes overnight and the solution was separated by centrifuging. The extraction was repeated five times. Representative samples from the solutions were combined and evaporated to dryness, dissolved in 10 ml of water and analyzed with ICP-AES. The results of the determinations are presented in Table 2. The bentonite is mostly (89 %) in sodium form. Calcium (7 %) and magnesium (3.5 %) are probably explained by dissolving carbonates, which maintain those cations in the dialyzing water. Table 2. Concentrations of exchangeable cations in purified bentonite before swelling pressure measurement. Sample Na K Ca Mg Total (meq/g) (meq/g) (meq/g) (meq/g) (meq/g) < < < < Average 0.74 <

11 6 2.2 Studies on the swelling pressure samples The samples studied in this work were obtained from a swelling pressure study reported in Karnland et al The swelling pressure of the compacted bentonite samples was studied experimentally as a function of the density and ionic strength of the solution used to saturate the clay. Altogether, 20 samples were studied. The clay densities and solutions used in the measurements are presented in Table 3. The tests were carried out with clay, from which easily dissolving impurities had been removed and the clay ionexchanged to Na-form. NaCl solutions of different concentrations and deionized water were used as the solution. At test termination, the bentonite samples and saturation solutions were characterized. Table 3. Experimental conditions in the swelling pressure measurements with purified MX-80 from where the samples used in this porewater study were obtained. Denomination Dry density (kg/m 3 ) Saturated density (kg/m 3 ) Saturation solution Conc. (mol/l) S Deionized w. 0 S NaCl 0.1 S Deionized w. 0 S NaCl 0.1 S NaCl 1 S NaCl 3 S Deionized w. 0 S NaCl 1 S NaCl 3 S Deionized w. 0 S NaCl 1 S NaCl 3 S NaCl 0.3 S NaCl 0.3 S NaCl 0.3 S NaCl 0.3 S NaCl 0.1 S NaCl 0.1 S Deionized w. 0 S Deionized w. 0

12 7 The bentonite sample cylinders obtained from the swelling pressure measurements were cut into smaller pieces according to Figure 4 in order to provide samples for different analyses and measurements. Half of the sample piece was used for the porewater studies while the other half (marked as CT s part in Figure 4) was left in Clay Technology AB for their studies. Figure 4. Cutting of bentonite samples into smaller pieces for different analyses and measurements after swelling pressure measurement.

13 8 The concentrations in the porewater were determined by two methods. The first method was based on the direct analysis of the squeezed porewaters. In the second method (dispersion method), bentonite was dispersed in deionized water from where the chloride was analyzed. The concentration in the porewater was calculated on the basis of the total chloride content in the bentonite and water content in bentonite. Sample A was cut from the bentonite used with a cylindrical knife and moved to the squeezing device for porewater squeezing. The squeezing was carried out at room temperature in normal atmosphere. The apparatus used for squeezing the porewaters is presented in Figure 5. It consisted of a pressing apparatus that was used to create the necessary long-term compression, and the compaction cell where the porewater was separated with a steel sinter from the bentonite and collected in a syringe. The long-term force was maintained with a strong spring. The pressure was increased stepwise up to 100 MPa, which took about one week (Figure 6). The sample size, 20 mm in diameter and 20 mm in height, used in squeezing gave porewater samples from about 0.5 ml to 3 ml, depending on the density of the bentonite. In some cases several subsequent fractions vs. increasing density were obtained. The samples C1, C2 and C3 were used to determine the water content of the bentonite before squeezing by drying the samples at C. The samples were rather homogeneously saturated, and the differences between the samples taken from different distances from the saturation surface were small. The densities, which were calculated on the basis of the water content assuming a full saturation, show the clay to be rather homogenous, too. The samples AC1, AC2 and AC3 were used to determine the water content of the bentonite after squeezing. The water content and density results are presented in Appendix A. Prior to the chemical analysis, the small squeezed porewater samples were ultra-filtered with 12k MWCO filters and diluted. The carbonate concentrations were determined by alkalinity titration and the other anions and cations by ion chromatography and ICP- AES. The results of the chemical analyses of the squeezed porewaters are presented in Appendix B. The samples B1, B2 and B3, representing different distances from the saturation surface, were used to determine the chloride concentration in the bentonite before squeezing by a dispersion method. In the measurement, the bentonite samples of about 1 g were dispersed into 100 to 500 ml of deionized water. After shaking for one week, the samples from the dispersion were centrifuged to separate the solution from the bentonite. The solutions were then ultrafiltered through 12k MWCO filters and their chloride concentrations determined by ion chromatography. On the basis of the result, the initial concentrations in bentonite porewater were calculated. The bentonite from which the porewater was squeezed was further cut into smaller pieces. The samples AB1, AB2 and AB3 were used to determine the chloride concentration in the bentonite after squeezing by the dispersion method. The chloride concentrations in bentonite porewater before and after squeezing are presented in Appendix C. The solutions used to saturate the bentonite samples during the swelling pressure measurements were analyzed after the test. Na, K, Ca, Mg and Si were determined by

14 9 ICP-AES, sulphate and chloride by ion chromatography, and carbonate by titration. The results for the external solutions are given in Appendix D. In the case of deionized water, the components in the saturation solution came from the bentonite. Sodium was the major cation, and bicarbonate, sulphate and chloride were the main anions. In the cases where sodium chloride solutions were used as saturation solutions, sodium and chloride remain as the main components in the solution. The charge balance is not complete, as seen in Appendix D, which indicates uncertainties ( 10 %) involved in the analysis methods. The dried bentonite samples C1, C2 and C3 from the swelling pressure measurement were used to determine the exchangeable cations and cation-exchange capacities of the bentonite. The exchangeable cations were determined by changing the cations from bentonite with 0.2 M NH 4 SCN in ethanol (Müller-Vonmoos & Kahr 1983). Five subsequent treatments with 0.7 g of bentonite and 7 ml of the solution were performed. Ethanol instead of water was used to avoid dissolution of the accessory minerals. The results are presented in Appendix E. It is obvious that the measured CEC depends on the concentration of the sodium chloride in the saturation solution. This suggests that part of the salt left in the sample during drying dissolves during the ion exchange with NH 4 SCN in ethanol. The analyses of the chloride concentration in the treatment solution support this assumption. The extra bentonite outside the A, B and C samples, marked as sample D, and the samples AD were left as back-up samples. 2.3 Studies on the SAXS samples In addition to the swelling pressure samples, a series of special samples were prepared for the SAXS (Small Angle X-ray Spectroscopy) measurements. The diameters of those samples were 25 mm and the heights 5 mm. The samples were compacted into the cells and saturated through a filter plate from one side with 0.1 or 0.3 M NaCl solutions for 12 days. The chloride concentrations in the samples were measured by the dispersion method. The results are presented in Appendix F.

15 10 Hydraulic cylinder Frame Piston Spring Piston Cylinder Syringe Cylinder Bentonite Sinter Syringe Figure 5. porewater. Pressing apparatus (left) and compaction cell (right) for squeezing of Squeezing S Pressure (MPa) Pressure Water Squeezed water (ml) Time (h) Figure 6. Typical pressure and squeezed porewater volume curves as a function of time (sample S2-3A).

16 11 3 INTERPRETATION OF RESULTS 3.1 Porewater chemistry The concentrations in the porewater were determined by two methods. The first one was based on the direct analysis of the squeezed porewaters. The results therefore give directly the real concentrations in the squeezed porewater sample. In the second method, the chloride concentration in the porewater was calculated on the basis of the total chloride content in the bentonite, determined by the dispersion method, and water content in bentonite determined by drying the sample. These values therefore give the average concentrations in total porewater. All the results are collected in Appendices C and D. Figure 7 presents as an example the anion concentrations in the squeezed porewater for an experiment carried out with deionized water. The concentrations are presented as a function of the density, which means the density over which the squeezing was performed. From the samples of low density, several subsequent fractions of the porewater could be obtained. This is seen as a step curve in the figure. Since pure water was used in the saturation, the ions come from the dissolving accessory minerals and ion exchange with montmorillonite. The most abundant anions are sulphate and bicarbonate, but some chloride and silicate can also be seen in the results. The chloride concentration is so low that it does not disturb the experiments carried out with NaCl solutions. The subsequent fractions of the squeezed porewaters show a decrease in concentration with increasing squeezing density. This suggests that such pores or parts of the pores, where the concentrations are higher, were emptied first. In those samples where sodium chloride solutions were used to saturate the samples, the main ions in the porewater are chloride and sodium, both coming from the saturation solution. Figure 8 presents an example of the chloride concentrations for these experiments. Only the chloride is presented because the concentrations of the other anions are much lower and thus less important in the interpretation of the results. The concentrations are presented for the porewaters before and after squeezing, for the squeezed porewaters and for the saturation solutions. The densities in the figure correspond with the same conditions, i.e. before (Disp 1), after (Disp 2) and during squeezing (Sq Pw). The concentration of the saturation solution is clearly higher than that in the porewater in all samples, indicating exclusion in the bentonite samples. Exclusion increases with increasing density and decreasing salt concentration, which is qualitatively in line with the Donnan exclusion model. The average concentration in the porewater is higher before squeezing than after it, i.e. Disp 1 > Disp 2. This suggests that the concentrations in the squeezed porewaters would be higher than the average concentration in the porewater. The analyses of the squeezed porewaters support this. The subsequent fractions of the squeezed porewaters show a decreasing concentration with increasing squeezing density, as in the samples of the deionized water.

17 12 S2-1 (785 kg/m 3, d.i.w ater) Concentration (mg/l) Cl SO4 HCO Dry density (kg/m 3 ) Figure 7. Anion concentrations in the squeezed porewater fractions in a sample saturated with deionized water S2-4 (0.1 M ) Chloride (mg/l) Disp 1 Disp 2 Sq Pw Ext w Dry density (kg/m 3 ) Figure 8. Chloride concentrations in the porewater and saturation solution. (Disp 1 and Disp 2 are porewaters before and after squeezing by dispersion method, SqPw is squeezed porewater, Ext w is saturation solution).

18 Modelling of chloride concentration in porewater Modelling of the chloride concentrations in the porewater was done with the Donnan model assuming that chloride does not form neutral or cationic ion pairs or sorb in the bentonite. The first modelling of the porewater was based on the traditional Donnan model presented in Equation (2), which allows evaluation of the relationship between the concentration in external water and porewater. The sinter between the bentonite and saturating solution forms a semi-permeable membrane of the Donnan system. The method assumes that the bentonite layers and porewater form a homogenous mixture and that there is only one type of water, the interlamellar water. The total water content and cation-exchange capacity (CEC 83 meq/g), needed in the calculations, were based on our own measurements. The total surface area (620 m 2 /g) was evaluated on the basis of the CEC of the purified material and the surface area and CEC given by Müller- Vonmoos and Kahr (1983) for Wyoming bentonite. The results of the modelling are presented in Figure 9. It is obvious that at high concentrations the model can predict the concentrations in the porewater rather well. At low concentrations however, where the exclusion is stronger, the measured concentrations are clearly higher than the modelling values. The difference between the measured and modelling values clearly increases with decreasing concentration. It was concluded that the microstructure of bentonite is more complex than the homogenous structure, assumed in the simple Donnan model, and that the effects of the microstructure should be included in the model. According to the review by Sacci et al. (2000), many studies suggest that there are different types of water in bentonite (interlamellar, intraparticle, free). Such a structure would cause decreased exclusion in 1.E+01 3 M 1.E+00 1 M Chloride (mol/l) 1.E-01 1.E M 0.1 M 1.E Dry density (kg/m 3 ) Figure 9. Chloride concentrations in the porewater as a function of the dry density of the sample and the concentration of the saturation solution. The concentrations were determined by the dispersion method (points). The solid points mean the large swelling pressure samples and the open ones the small SAXS samples. The modelling curves are based on the Donnan theory.

19 14 large pores where the average concentration of the exchangeable cations would stay low. When the concentration of the exchangeable cations decreases, the concentration in the pore will approach the concentration of the saturation solution. In the second modelling, it was assumed that there are two pore types, interlamellar and external (large) pores. It was also assumed that there are two Donnan membranes in the system. The first one is the sinter, which forms a membrane between the outer solution and the large pores. The second one is at the end of the montmorillonite stacks, which form a membrane between the large pores and the interlamellar pores. Figure 10 illustrates the idea of the system. This type of modelling requires parameter selection, which allows evaluation of the surface areas and water volumes related to each pore type. When the volumes and the surface areas are known, the concentrations of the exchangeable cations in each pore type can be calculated. The CEC, total surface area and water content were received as in the first modelling. The size of the interlamellar pores was evaluated on the basis of the SAXS measurements performed for the samples saturated with 0.3 M NaCl solution. Similar dependence of the interlamellar space on the dry density has been reported by Kozaki et al Figure 11 shows the measured pore sizes as a function of density together with the fitting curve that was used in the calculations. One important parameter to be selected is the surface area of the external pores. Its direct measurement is difficult in the water-saturated clay. However, it is often proposed that bentonite while being saturated by water will keep its microstructural features. A value for the external surface was then selected on the basis of the BET surface area measurements on freeze-dried clay samples of dry density kg/m 3 and saturated with 0.3 M NaCl. The measured value, 35 m 2 /g, was adjusted in modelling to 20 m 2 /g to obtain the best fitting for the curves. Bentonite Outer solution 4 Sinter End of stack Figure 10. Schematic structures used in the modelling, where two pore sizes were used together with the Donnan model. In the figure: 1) stack of montmorillonite, 2) external water, 3 interlamellar water.

20 Interlamellar space (nm) Muurinen et al Kozaki Dry density (kg/m 3 ) Figure 11. Statistical interlamellar space as a function of the clay dry density used in modelling. The curve is based on SAXS measurements for samples saturated with 0.3 M NaCl (Muurinen et al. 2004) and the values by Kozaki When the total water content, total surface area, the interlamellar space and the surface area of the external pores are known, the other microstructural parameters can be calculated. Figure 12 presents the pore spaces for the interlamellar and external pores as a function of the density, and Figure 13 shows different types of porosities as a function of the density. The average chloride concentrations in the porewater calculated by coupling the microstructure and the Donnan model are compared in Figure 14 with the values determined by the dispersion method. It is obvious that fitting is excellent, with very reasonable parameter selection. The coupled model allows also calculation of the concentrations in different types of pores. Figure15 compares the measured concentrations of the squeezed porewaters with the calculated concentrations in the external pores (E) and interlamellar pores (I). The measured values are slightly lower than the curves of the external pores, indicating dilution caused by the interlamellar water during squeezing. The systematic behaviour of the measured points in Figure 15 suggests that during squeezing the clay goes through similar states, which would be obtained by saturating clay of the same density.

21 16 30 Pore space (nm) External pores Interlamellar pores Dry density (kg/m 3 ) Figure 12. Pore spaces of the interlamellar and external pores as a function of density Total Porosity External pores Interlamellar pores Dry density (kg/m 3 ) Figure 13. Porosity (total, external, interlamellar) as a function of the clay density.

22 17 1.0E+01 3 M Chloride (mol/l) 1.0E E M 0.1 M 1 M 1.0E Dry density (kg/m 3 ) Figure 14. Chloride concentrations in the porewater as a function of the dry density of the sample and the concentration of the saturation solution. The concentrations were determined by the dispersion method. The solid points represent the swelling pressure samples and the open ones SAXS samples. The modelling curves are based on coupling of the microstructure and the Donnan model. 1.0E+01 E-3M Chloride (mol/l) 1.0E E E E-03 E-0.3M E-0.1M E-1M IL-3M IL-1M IL-0.3M IL-0.1M 1.0E Dry density (kg/m 3 ) Figure 15. The chloride concentrations in the squeezed porewater fractions (points) as a function of the dry density of the sample and the concentration of the saturation solution. The modelling curves are based on coupling of the microstructure and the Donnan model. The solid lines represent the calculated concentrations in the large pores (E) and the dotted lines in the interlamellar pores (IL).

23 18 Figure 16 presents, on the basis of the model, from where the squeezed porewater comes. The figure shows the behaviour of the porewater of one cubic centimetre of saturated clay when the density increases during the squeezing. The curves concern the starting densities of 700 kg/m 3 and kg/m 3. In both cases, the squeezing ends at the dry density of kg/m 3, where squeezing of the samples typically ended. In the first case, where the total volume of water in the beginning is ml, ml is squeezed out, of which ml comes from the external pores and ml from the interlamellar pores. In the second case, the amount of water at the beginning is ml. During squeezing 0.21 ml of the water is removed, of which ml comes from the external pores and ml from the interlamellar pores. In the sample of the lower density, about 20 % comes from the interlamellar water, while in the sample of the higher density about equal volumes come from the interlamellar and large pores. The effect of the mixing depends also on the concentration difference between the water types. In the experiment with the 3 M solution, the difference between the interlamellar and external water is not large, but it increases strongly when the concentration of the saturation solution decreases Interlam. External Total Squeezed water (ml) Dry density (kg/m 3 ) Figure 16. Evaluation of the volumes of different water types, when the samples of 1 cubic centimetre are squeezed. The starting densities are 700 kg/m 3 and kg/m 3. In the model of this study, we used only two pore sizes for each density. In reality there is probably some kind of pore size distribution. We also used only one external surface area for all the densities and salt concentrations, although it can well depend on the density and salt concentration. However, the assumption of different pore sizes remarkably improves the fitting of the Donnan model with the measured data, which supports this type of structure in bentonite.

24 19 In the paper by Muurinen et al. (2006) the effect of homogenization on the exclusion of chloride in compacted bentonite was studied. The basic idea of the study was to prepare homogenized samples and compare their properties with non-homogenized samples. Dispersion with ultra sound or saturation with deionized water before the equilibration with NaCl solution was used for homogenization. The concentrations in the porewater of the samples saturated first with deionized water were clearly lower than the samples which were saturated directly with 0.1 M NaCl solution. This probably reflects the effect of homogenization and smaller size of the external pores on the chloride concentrations. The experimentally determined chloride concentrations in Muurinen et al are compared in Figure 17 with the porewater concentrations calculated assuming only one pore size (curve 1) or by coupling the microstructure and the Donnan model (curves 2 and 3). In curve 2, the external surface area is assumed to vary from 15 m 2 /g at kg/m 3 to 140 m 2 /g at 500 kg/m 3. In curve 3, a constant external surface area of 20 m 2 /g was used. When the Donnan model is used in assuming a fully homogenous sample, meaning that there is only interlamellar water, the curve is below all the measured points. The assumption of two pore sizes and varying external surface area gives an excellent agreement with the measured values for the samples saturated first with deionized water (points A, B, C). The modelling assuming two pore sizes and a constant external surface area better explains the measured values of the samples saturated directly with 0.1 M NaCl (points D and E), where less homogenization was expected. The points D and E in Figure 17 represent the experimental values of this study. The pore structure and exclusion may thus depend on the preparation of the bentonite sample. Effective homogenization means smaller particles, smaller external pores and stronger anion exclusion. Figure 18 presents different pore types in bentonite in equilibrium with 0.1 M NaCl as a function of dry density. The values are based on the porewater studies of this report and those presented in Muurinen et al Curve 1 presents the water porosity (total porosity) in the clay. It can be determined by drying a water-saturated sample or calculated on the basis of the specific density of bentonite particles. Curve 2 presents the chloride porosity. The measured points represent a case where the sample is saturated with deionized water before equilibration with 0.1 M NaCl solution. Curve 3 presents the porosity which is not accessible for chloride. The curve is obtained by subtracting the chloride porosity from the total porosity. It includes the interlamellar porosity and the double layers in the external pores. Curve number 4 presents the interlamellar porosity. It has been evaluated on the basis of the Donnan modelling assuming two pore types in the clay.

25 A B C D E 0.06 Cl (mol/l) Dry density (kg/m 3 ) Figure 17. Measured chloride concentrations in the porewaters of the bentonite and model curves. In the case of A, B and C the samples were saturated first with deionized water followed by equilibration with 0.1 M NaCl. In D and E direct saturation with NaCl solution was done. The curves have been calculated with the Donnan model assuming 1) homogeneous bentonite, 2) two pore sizes and changing external surface area, 3) two pore sizes and constant external surface area Porosity (%) Dry density (kg/m3) Figure 18. Different pore types in bentonite saturated first with deionized water followed by equilibration with 0.1 M NaCl. 1) total porosity, 2) external porosity, 3) interlamellar porosity, 4) chloride porosity and 5) porosity not accessible for chloride.

26 21 4 SUMMARY This study was part of a research project aimed at improving the understanding of swelling pressure in compacted bentonite as a function of density and ionic strength in the solution. According to the Donnan model, polyelectrolytes, like bentonite, in contact with salt solutions through a membrane are expected to have a unique content of ions in the porewater. In this part of the work, the concentration caused by an external solution into the porewater was studied. The results concerning the swelling pressure have been reported in Karnland et al The tests were carried out with clay, from which easily dissolving impurities had been removed and the clay ion-exchanged to Na-form. NaCl solutions of different concentrations and deionized water were used to saturate the compacted bentonite samples through filter plates. The filter between the bentonite and saturating solution forms a semi-permeable membrane of the Donnan system. At test termination, the bentonite samples and saturation solutions were characterized. The concentrations in the porewater were determined by two methods. The first one was based on the direct analysis of the squeezed porewaters giving directly the real concentrations in the squeezed porewater sample. In the second method, the chloride concentration in the porewater was calculated on the basis of the total chloride content in the bentonite, determined by dispersing bentonite first in deionized water from where chloride was analyzed. These values give the average concentrations in total porewater. Modelling of the chloride concentrations in the porewater was done with the Donnan model assuming that chloride does not form neutral or cationic ion pairs or sorb in the bentonite. The first modelling of the porewater was based on the traditional Donnan model assuming a homogenous pore structure. At high NaCl concentrations, the model can predict the concentrations in the porewater rather well but at low concentrations, where the exclusion is stronger the measured concentrations are clearly higher than the modelling values. In the second porewater modelling, it was assumed that there are two pore types, interlamellar and external (large) pores in bentonite. It was also assumed that there are two Donnan membranes in the system. The first one is the sinter, which forms a membrane between the outer solution and the large pores. The second one is at the end of the montmorillonite stacks, which form a membrane between the large pores and the interlamellar pores. In this case, fitting is excellent with very reasonable parameter selection. The pore structure and exclusion, however, depend on the preparation of the bentonite sample. Effective homogenization of bentonite means smaller particles, smaller external pores and stronger anion exclusion. On the basis of modelling, the behaviour of different porewater types, when the density increases during the squeezing was evaluated. The evaluation concerned cases where the starting densities were 700 kg/m 3 and kg/m 3 and the squeezing ended in the dry density of kg/m 3. In the sample of the lower density, about 20 % was evaluated to come from the interlamellar water, while in the sample of the higher density about equal volumes came from the interlamellar and large pores.

27 22 REFERENCES Karnland, O Bentonite swelling pressure in strong NaCl solutions. Helsinki: Posiva Oy. Report POSIVA 98-01, 36 p. ISBN Karnland, O., Muurinen, A., Karlsson, F., Bentonite swelling pressure in NaCl solutions Experimentally determined data and model calculations. Advances in Understanding Engineered Clay Barriers, Alonso & Ledesma (eds), 2005 Taylor & Francis Group, London, pp ISBN Kozaki, T., Sato, H., Fujishima, A., Saito, N., Sato, S., Ohashi, H Effect of dry density on activation energy for diffusion of strontium in compacted sodium montmorillonite. In: Mat. Res. Soc. Symp. Proc., vol Pittsburgh, Pennsylvania, Materials Research Society, pp Muurinen, A., Karnland, O., Lehikoinen, J Ion concentration caused by an external solution into the porewater of compacted bentonite. Physics and Chemistry of the Earth 29 (2004) Muurinen A., Karnland, O., Lehikoinen, J Effect of homogenization on the microstructure and exclusion of chloride in compacted bentonite. Physics and Chemistry of the Earth xx (2006) xxx-xxx (Article in press), doi: /j.pce Müller-Vonmoos, M. And Kahr, G Mineralogische Untersuchungen von Wyoming Bentonit MX-80 und Montigel. Baden/Switzerland, NAGRA, Technisher Bericht 83-12, 15 p. + app. 13 p. Overbeek, J Electrochemistry of doublelayer. In: Kruyt, H Collois Science. Amsterdam: Elsevier Publishing Company, pp Sacci, E., Michelot, J-L. & Pitsch, H Porewater extraction from argillaceous rocks for geochemical characterization. Methods and interpretation. France: AEN/OECD, 185 p. ISBN Sposito, G., Holtzclaw, K. M., Johnston, C. T., Le Vesque, C. S Thermodynamics of sodium-copper exchange on Wyoming bentonite at 298 K. Soil Sci. Soc. Am. J., vol. 43, pp Ståhlberg, J Retention model for ions in chromatography. Journal of Chromatography A, vol. 855, 3-55.

28 23 APPENDIX A THE WATER CONTENTS AND DENSITIES OF BENTONITE AFTER SWELLING PRESSURE MEASUREMENT Dry and wet densities of bentonites before squeezing calculated on the basis of water content. Sample Water/ Density Sample Water/ Density Sample Water/ Density bentonite (dry) bentonite (dry) bentonite (dry) (g/g) (kg/m 3 ) (g/g) (kg/m 3 ) (g/g) (kg/m 3 ) S2-1-C S2-1-C S2-1-C S2-2-C S2-2-C S2-2-C S2-3-C S2-3-C S2-3-C S2-4-C S2-4-C S2-4-C S2-5-C S2-5-C S2-5-C S2-6-C S2-6-C S2-6-C S2-7-C S2-7-C S2-7-C S2-8-C S2-8-C S2-8-C S2-9-C S2-9-C S2-9-C S2-10-C S2-10-C S2-10-C S2-11-C S2-11-C S2-11-C S2-12-C S2-12-C S2-12-C S2-13-C S2-13-C S2-13-C S2-14-C S2-14-C S2-14-C S2-15-C S2-15-C S2-15-C S2-16-C S2-16-C S2-16-C S2-17-C S2-17-C S2-17-C S2-18-C S2-18-C S2-18-C S2-19-C S2-19-C S2-19-C S2-21-C S2-21-C S2-21-C

29 24 APPENDIX A, continued Dry and wet densities of bentonites after squeezing calculated on the basis of water content measurement. Sample Water/ Density Sample Water/ Density Sample Water/ Density bentonite (dry) bentonite (dry) bentonite (dry) (g/g) (kg/m 3 ) (g/g) (kg/m 3 ) (g/g) (kg/m 3 ) S2-1-AC S2-1-AC S2-1-AC3 n.s. n.s. S2-2-AC S2-2-AC2 n.s. n.s. S2-2-AC3 n.s. n.s. S2-3-AC S2-3-AC S2-3-AC S2-4-AC S2-4-AC S2-4-AC S2-5-AC S2-5-AC S2-5-AC S2-6-AC S2-6-AC S2-6-AC S2-7-AC S2-7-AC S2-7-AC S2-8-AC S2-8-AC S2-8-AC S2-9-AC S2-9-AC S2-9-AC S2-10-AC S2-10-AC S2-10-AC S2-11-AC S2-11-AC S2-11-AC S2-12-AC S2-12-AC S2-12-AC S2-13-AC S2-13-AC1 n.s. n.s. S2-13-AC1 n.s. n.s. S2-14-AC S2-14-AC S2-14-AC1 n.s. n.s. S2-15-AC S2-15-AC S2-15-AC S2-16-AC S2-16-AC S2-16-AC S2-17-AC S2-17-AC S2-17-AC S2-18-AC S2-18-AC S2-18-AC S2-19-AC S2-19-AC2 n.s. n.s. S2-19-AC3 n.s. n.s. S2-21-AC S2-21-AC2 n.s. n.s. S2-21-AC3 n.s. n.s. n.s. = no sample

30 25 APPENDIX B CHEMICAL COMPOSITIONS OF THE SQUEEZED POREWATERS Sample Na Ca Mg Si Cl SO 4 HCO 3 TDS (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) S2-1A/1 ( * S2-1A/ S2-1A/ S2-2A/ n.a. n.a. n.a n.a S2-2A/ n.a. n.a. n.a n.a S2-2A/ n.a. n.a. n.a n.a S2-3A/ S2-3A/ S2-3A/ S2-3A/ S2-4A/ S2-4A/2 543 < S2-4A/3 436 < S2-4A/ S2-5A/ n.a. n.a. n.a n.a S2-5A/ n.a. n.a. n.a n.a S2-6A/ n.a. n.a. n.a n.a S2-6A/ n.a. n.a. n.a n.a S2-7A S2-8A n.a. n.a. n.a n.a S2-9A n.a. n.a. n.a n.a S2-10A S2-11A S2-12A n.a. n.a. n.a n.a S2-13A/ n.a. n.a. n.a n.a S2-13A/ n.a. n.a. n.a n.a S2-13A/ n.a. n.a. n.a n.a S2-14A/ n.a. n.a. n.a n.a S2-14A/ n.a. n.a. n.a n.a S2-15A 1960 n.a. n.a. n.a n.a S2-16A 2247 n.a. n.a. n.a n.a S2-17A 574 n.a. n.a. n.a n.a S2-18A 979 n.a. n.a. n.a n.a S2-19A/1 106 n.a. n.a. n.a. n.a. 71 n.a. 176 S2-19A/2 70 n.a. n.a. n.a. n.a. 57 n.a. 127 S2-19A/3 58 n.a. n.a. n.a. n.a. 49 n.a. 107 S2-19A/4 54 n.a. n.a. n.a. n.a. 56 n.a. 99 S2-19A/5 39 n.a. n.a. n.a. n.a. 39 n.a. 79 S2-21A/ n.a. n.a. n.a n.a S2-21A/ n.a. n.a. n.a n.a S2-21A/3 642 n.a. n.a. n.a n.a * The last numbers mean the subsequent fractions of the squeezed porewater n.a. = not analysed