Environ. Sci. Technol. 2008, 42, 1600–1604
Mechanical Compaction of Smectite Clays Increases Ion Exchange Selectivity for Cesium LUC R. VAN LOON* AND MARTIN A. GLAUS Waste Management Laboratory, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
Received October 2, 2007. Revised manuscript received November 20, 2007. Accepted November 26, 2007.
A key discussion in radioactive waste disposal is the question of whether adsorption behavior of radionuclides in dispersed and in highly compacted materials is similar. It is common practice to measure sorption data on dispersed systems and to use these data to predict the sorption in compacted systems. The latter represent the situation in a real, deep geological repository system. The discussions focus mainly on items such as the effect of compaction on the accessibility of sorption sites, that is, on sterical effects, and not on the effect of compaction on the thermodynamics of the sorption processes. Here we show that in the case of sorption of cesium on sodium bentonite, material compaction indeed affects the thermodynamics of the sorption process such that sorption increases. This increase is due to a reduction of the interlayer space, leading to a lower ability of the interlayer water for cation hydration. Cations with a low hydration tendency, such as cesium, therefore accumulate in the interlayer space, whereas highly hydrated cations such as sodium tends to accumulate in the bulk water where water is easily available for hydration. The fact that mechanical compaction affects the thermodynamics of ion exchange processes in clay is an important finding and brings in a new aspect in the discussion on the transferability of thermodynamic data from diluted to compacted systems. The common practice of applying chemical and thermodynamic concepts valid for diluted systems to compacted systems must be basically scrutinized.
Introduction The sorption of radionuclides on repository and host rock materials is an important process retarding the transport of radionuclides from a deep geological repository toward the biosphere (1). Clay minerals such as bentonite are the most common materials used for backfilling emplacement tunnels. Moreover, argillaceous rocks are explored worldwide for their suitability to host radioactive waste repositories (2–4). Therefore, interaction of radionuclides with clay minerals is a central research activity for many radioactive waste management organizations. It is common practice to study sorption of radionuclides on loose, unconsolidated materials in so-called batch-type experiments because of convenience in changing and controlling physicochemical parameters such as pH, Eh, and ionic composition of the contacting solutions. In the real * Corresponding author: phone: +41-56-3102257; luc.vanloon@ psi.ch. 1600
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repository system, however, clay materials are only found in a highly compacted state: many host rocks have bulk dry densities (Fb) of 2000–2400 kg m-3, and the bentonite that will be used for backfilling is compacted to blocks of a bulk dry density of ca. 1600–1700 kg m-3 before emplacement in a repository. A key question is therefore whether it is justified to use sorption parameters measured on the dispersed (unconsolidated) material to predict sorption on the compacted system. The debate on this topic has been ongoing for many years, and the results obtained thus far are rather contradictionary, going from no effect of compaction on sorption (5, 6) up to a decrease in sorption (7, 8). In some cases an increase in sorption is observed (9, 10). The observed dependencies, however, depend on the type of element. Molera et al. (10) observed a decrease of the sorption of 85Sr2+ with increasing bulk dry density whereas the sorption of 134Cs+ decreased. For 60Co2+, no effect was observed. Also, chemical conditions seem to play a role. At an ionic strength of 0.1 M, the sorption of 85Sr2+ decreased, whereas at an ionic strength of 0.5 M, no effect was observed (10). The discrepancies are often explained by arguments based on site accessibility, that is, compaction of bentonite results in a decreased accessibility of sorption sites (7, 11). In their review on sorption and diffusion in compacted bentonite, Yu and Neretnieks (12) suggest that, where differences are found, these may be simply due to errors in measurement techniques. Amazingly no explanations based on changes in the thermodynamics of the sorption processes have been proposed. Thus, it is always assumed that compaction has no effect on thermodynamics. In this study, we revisit the sorption of cesium on dispersed and compacted bentonite and investigate the hypothesis that thermodynamics of the sorption process can be changed by mechanically compacting the bentonite.
Materials and Methods Bentonite. The bentonite used was Volclay KWK (Südchemie, Germany). The clay was used as received without further treatment. The chemical and mineral composition is given in Van Loon et al. (13). The cation occupancy of the bentonite was determined by extracting the exchangeable cations with a 0.5 M CsNO3 solution as described in Fernández et al. (14). Cs+ acts as a highly selective cation and displaces all exchangeable cations from the montmorillonite if its concentration is high enough. Sorption on Compacted Clay. A given amount of bentonite (5.25 g) was compacted in a sample holder to bulk dry densities between 1300 and 1900 kg m-3 and sandwiched between two stainless steel filters (stainless steel 316L, 2.56 × 10-2 m diameter, 1.58 × 10-3 m thickness, 30% porosity, 10 µm pore size). Two end plates were placed on both sides of the filter-clay-filter sandwich and screwed together by a set of four bolts. A cross-section picture of the equipment (diffusion cell) is given in Figure 1. More details can be found in Van Loon et al. (15). The clay samples were saturated with artificial pore water (Table 1). For each bulk dry density, a pore water composition was calculated based on the mineral phases present in the bentonite, the loading of the cations on the surface, and the presence of anions in the clay. The procedure is explained in detail in Fernández et al. (14). After a resaturating period of 4 weeks, the samples were contacted with 500 mL of the same artificial pore solution labeled with radioactive 134Cs+ (activity concentration: 109 Bq m-3). The experimental setup is shown in Figure 2. The starting concentration of Cs+ was ca. 1.5 × 10-10 M. The evolution of the activity in the solution was monitored by γ-counting 10.1021/es702487m CCC: $40.75
2008 American Chemical Society
Published on Web 01/31/2008
FIGURE 1. Cross-section picture of the diffusion cell used for the sorption measurements on compacted bentonite.
FIGURE 3. Change of the reservoir concentration of 134Cs+ for a solution in contact with bentonite at a solid-to-liquid ratio of 0.01 g cm-3 and compacted to different bulk dry densities. repeated five times. After the fifth treatment, 30 cm3 were replaced by only 10 cm3 of solution containing carrier-free 134Cs+. The suspensions were shaken end-over-end for 24 h. After phase separation by centrifugation (98 000g, 30 min), the activity in the supernatant solution was measured by γ-counting (Minaxi-γ, Autogamma 5000 series, Packard). The distribution coefficient was calculated from the difference in radioactivity before and after equilibrium using eq 1.
Results and Discussion
FIGURE 2. Schematic overview of the experimental setup used in the sorption experiments on compacted bentonite. (Minaxi-γ, Autogamma 5000 series, Packard), and equilibrium was assumed from the moment that no change in radioactivity could be observed any longer. From the difference in activity at the beginning and at the end of the experiment, the distribution ratio, Rd was calculated according to eq 1, Rd )
Ain - Aeq V × Aeq m
(1)
where Ain and Aeq are the amount of radioactivity (Bq) at the beginning and the end of the experiment, respectively, V is the volume of the solution in contact with the clay (m3), and m represents the dry mass of bentonite (kg). The sorption of Cs+ on the equipment was found to be negligibly small. Sorption on Dispersed Clay. In a first step, bentonite was conditioned with the artificial pore water. A given amount of air-dried bentonite (0.2 g) was weighted in a centrifuge vial, and 40 cm3 of artificial porewater were added. The mixtures were shaken end-over-end for 24 h, after which a phase separation was performed by centrifugation (98 000g, 30 min). At the high ionic strength conditions of the measurements (I ≈ 0.3 M) and mainly due to the presence of bivalent cations at a concentration of 20–30 mM, montmorillonite flocculates (16) and centrifugation at 98 000g for 30 min was sufficient to remove the colloids from solution. A 30 cm3 portion of the supernatant liquid was removed and replaced by 30 cm3 of fresh solution. This procedure was
Table 2 gives an overview of the cation loading of the bentonite; 71% of the surface charge is compensated by Na+. Ca2+ and Mg2+ each compensate 13% of the charge. Sodium is thus the dominating cation on the surface of the KWK bentonite. The depletion of 134Cs+ from the contacting solution as a function of time is shown in Figure 3. It can be seen that 134Cs+ is more depleted in the case of the more compacted bentonite. The corresponding sorption values (Rd), defined as the ratio of the amount of 134Cs+ sorbed per mass unit of dry clay (mol kg-1) and the amount of 134Cs+ per volume unit of the equilibrium solution (mol dm-3), are given in Table 4 together with those measured for the dispersed bentonite (Table 3). For a bulk dry density of 1300 kg m-3, sorption on compacted bentonite is in excellent agreement with that measured on dispersed bentonite (Table 3) for similar water chemistry conditions. Compacting the bentonite to densities larger than 1300 kg m-3 increased the sorption. The Rd (dm3 kg-1) values increase almost by a factor of 10 from 69 dm3 kg-1 at Fb ) 1300 kg m-3 to 620 dm3 kg-1 at Fb ) 1900 kg m-3 at a nearly constant water chemistry. The dominant clay mineral in bentonite responsible for sorption is montmorillonite (Figure 4), a swelling 2:1 clay mineral buildup from sheets composed of an Al-octahedral layer (O-layer) sandwiched between two Si-tetrahedral layers (T-layer). Because of isomorphic substitution of Al by mainly Mg in the octahedral layer, montmorillonite has a permanent negative charge. This charge is compensated by cations. In the case of Na-montmorillonite, sodium is the dominant charge compensating cation. Different sheets (TOT) are stacked together to form a clay platelet. The space between the sheets is filled with water, the so-called interlayer water. The charge compensating cations (Na+ in our case) are located mainly in this interlayer water. The sorption of Cs+ VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Composition of the Bentonite Pore Water for Bentonite at Different Bulk Dry Densities dry density (kg m-3) porosity
1300 0.122
1600 0.044
1900 0.019
-3.42 8.00 0.26
-3.47 8.00 0.29
-3.65 8.00 0.33
log PCO2 (bar) pH ionic strength (M) Na (M) K (M) Mg (M) Ca (M) Sr (M) Cl (M) SO4 (M) Cinorg. (M) F (M) Si (M)
1.83 × 10-1 2.7 × 10-3 1.0 × 10-2 9.2 × 10-3 8.1 × 10-5 1.81 × 10-2 1.02 × 10-1 8.9 × 10-4 2.2 × 10-4 1.8 × 10-4
2.07 × 10-1 3.1 × 10-3 1.2 × 10-2 9.8 × 10-3 8.6 × 10-5 6.18 × 10-2 9.5 × 10-2 8.0 × 10-4 2.2 × 10-4 1.8 × 10-4
2.54 × 10-1 3.7 × 10-3 1.5 × 10-2 1.2 × 10-2 1.1 × 10-4 1.70 × 10-1 7.1 × 10-2 5.5 × 10-4 1.9 × 10-4 1.8 × 10-4
sumcations (meq dm-3) sumanions (meq dm-3)
2.24 × 10-1 2.23 × 10-1
2.54 × 10-1 2.53 × 10-1
3.12 × 10-1 3.13 × 10-1
TABLE 3. Rd, Ke, and ∆Gex Values for Sorption of Cs+ on Dispersed Bentonite for Different Concentrations of Na+ in the Equilibrium Solutiona
TABLE 2. Cation Composition of the Bentonite Exchanger as Determined by Extraction with 0.5 M CsNO3a cation
loading (meq kg-1)
NX b
Na K Mg Ca
585 20 106 111
0.71 0.02 0.13 0.14
sum
822
1.00
[Na+] [M] [dm3
Rd Ke ∆Gex [kJ mol-1]
on bentonite is a cation exchange process and can be written according to eq 2 (17), +
X - Na + Cs S X - Cs + Na
(2)
where X represents the negatively charged clay surface. The equilibrium constant (Ke) for this reaction is given by eq 3, Ke )
(X - Cs)(Na+) (X - Na)(Cs+)
(3)
where (X - Na) and (X - Cs) represent the sodium and cesium activity in the clay phase, and (Cs+) and (Na+) are the corresponding activities in the contacting bulk liquid phase. We assume that the activity coefficient of Na+ and Cs+ in the bulk liquid are identical, as well as the activity coefficient of Na+ and Cs+ in the clay phase, so that equation 3 can be replaced by eq 4, Ke )
[X - Cs][Na+] [X - Na][Cs+]
(4)
where the box brackets denote concentrations of both elements in the clay (mol kg-1) and in the bulk liquid (mol dm-3) phase. Because [X - Cs]/[Cs] represents the distribution coefficient Rd, eq 4 can be written as eq 5: Ke )
Rd[Na+] [X - Na]
(5)
In the case of Volclay bentonite, [X - Na] ) 0.585 mol kg-1 (Table 2). 1602
9
0.183
0.207
0.254
62.7 ( 2.4 19.6 -7.4
51.3 ( 2.1 18.2 -7.2
42.3 ( 1.0 18.4 -7.2
a Rd was calculated by eq 1, Ke was calculated by eq 5, and ∆G ex is defined by eq 6.
a The sum of the exchangeable cations represents the cation exchange capacity (CEC). b Nx: fractional cation occupancy
+
kg-1]
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The free exchange energy (∆Gex) can then be calculated by eq 6: ∆Gex ) -RT ln Ke
(6)
Table 3 and Table 4 summarize the Rd, Ke, and ∆Gex values for the dispersed and compacted systems, respectively. For the dispersed systems, the average equilibrium constant, Ke, is 18.7 ( 0.8 resulting in a ∆Gex ) -7.3 ( 0.1 kJ/mol at 298 K. The value Ke ) 18.7 is in the lower range of values reported in Benson (18) (log K ) 1.5 ( 0.3). In the case where the equilibrium constant equals unity, the free exchange energy (∆Gex) is zero, or none of the cations is selected by the clay phase. In the case of the Na/Cs system, the free exchange energy is negative, and Cs+ is clearly selected by the clay phase. There exist different theories to explain the observed selectivity of Na-montmorillonite for cesium (19–22). An extensive overview is given in a recent publication by Teppen and Miller (22). All theories explaining cation selectivity by clays only consider the energies of the cation-clay interactions: given two cations of equal valence, the more weakly hydrated one will undergo a stronger Coulombic attraction by the charged surface. This explains why the selectivity of clays for alkali cations follows the so-called lyotropic series (eq 7). Cs+ > Rb+ > K+ > Na+ > Li+
(7)
For the compacted systems, identical values of Ke and ∆Gex as those for the dispersed systems were observed only for a bulk dry density of 1300 kg m-3. For bulk dry densities Fb > 1300 kg m-3, a gradual increase of both Ke and -∆Gex could be observed. Mechanical compaction of Na-montmorillonite thus results in an increased selectivity for cesium. This effect was surprising to us and could not be explained by the classical ion exchange theory. Therefore, we were
TABLE 4. Rd, Ke and ∆Gex Values for Sorption of Cs+ on Compacted Bentonite for Different Bulk Dry Densities (Gb)a Gb [kg m-3] [Na+]
[M] Rd [dm3 kg-1] Ke ∆Gex [kJ mol-1]
1300 0.183 69.0 ( 2.0 21.6 -7.6
1600 0.207 109 ( 4 38.6 -9.1
1900 0.254 620 ( 4 269 -13.9
a Rd was calculated by eq (1), Ke was calculated by eq (5) and ∆G ex is defined by eq (6).
TABLE 5. Overview of Differences in Hydration Energies in Bulk Water and Interlayer Water Gb [kg m-3] mol-1]
∆Gex [kJ ∆Gh-b [kJ mol-1] ∆Gh-il [kJ mol-1]
disperse
1300
1600
1900
-7.3 -117 +109.7
-7.6 -117 + 109.4
-9.1 -117 + 107.9
-13.9 -117 + 103.1
two reactions equals the overall free exchange energy. We applied this energy cycle proposed by Teppen and Miller (22) to our measurements (Table 5). The overall free exchange energy is calculated from the sorption measurements by applying eq 6. The free energy for the exchange of Na+ for Cs+ in the bulk water (∆Gh-b) is given by the difference in the hydration energies (eq 8), ∆Gh-b ) NaGh-b - CsGh-b
(8)
where NaGh-b and CsGh-b are the hydration energies of Na+ and Cs+ in the bulk water, respectively. We used here the values published by Markus (26). The free energy of exchange of Na+ by Cs+ in the interlayer (∆G h-il) cannot be measured, but can be inferred from the difference between the overall free exchange energy (∆Gex) and the free hydration energy in the bulk water (∆Gh-b): ∆Gh-il ) ∆Gex - ∆Gh-b
FIGURE 4. Schematic presentation of the structure of Na-montmorillonite. The interlayer contains cations (Na+ and Cs+) that compensate the excess negative charge of the clay and hydration water (interlayer water). The h region represents the interlayer distance, and d is the basal distance that can be measured by X-ray diffraction.
FIGURE 5. Thermodynamic cycle for the exchange of Na+ by Cs+ for dispersed bentonite (after Teppen and Miller (22)). forced to look for alternative approaches to explain the observed data. Teppen and Miller (22) recently developed an interesting alternative approach to explain selectivity of a clay for alkali cations based on a thermodynamic cycle for cation exchange (Figure 5). In a compacted bentonite sample, there are two types of water: interlayer water and interparticle water (22–25), the latter having properties similar to bulk liquid water. Because the interlayer is only 3-9 Å thick, there may not be enough room for complete hydration shells to form around cations. Further the close proximity of two siloxane surfaces causes the water to have less orientational freedom, resulting in water with a different structure than bulk water. An ion exchange process can be seen as a liquid/liquid partitioning of cations between the interlayer water and the bulk water. The overall ion exchange reaction can be subdivided into two separate reactions, one occurring in the interlayer water (clay phase) and one in the interparticle water (aqueous phase). The sum of the free energies of these
(9)
The positive value of ∆G h-il indicates that putting Cs+ in the interlayer of the clay is energetically unfavorable. This is because the hydration energy of Na+ in the interlayer water is larger than that of Cs+; the absolute difference, however, is smaller than in bulk water. The energy gained by having Na+ in the bulk water instead of Cs+, however, is larger and compensates the energy penalty of having Cs+ in the interlayer; therefore, Cs+ is “selected” by the clay. In the case were the interlayer is infinitely hydrated, that is, the interlayer water has similar properties as the bulk water, the hydration energy of Cs+ and Na+ in the interlayer water would be the same as in the bulk water, and the free energy change for replacing a Na+ by a Cs+ ion in the interlayer would be +117 kJ/mol, resulting in a zero free exchange energy. In the latter case, Cs+ would not be selected by the clay. This mechanism, developed by Teppen and Miller (22), is not only applicable to clay systems but also explains ion selectivity in nanopores in general, and particularly in channels in biological membranes (27). A gain of ion selectivity was observed by pore volume reduction of the bacterial Porin OmpF (28). Because the interlayer is less hydrated, the left-hand term in Figure 5 is less positive, and Cs+ is selected. Up to a bulk dry density of 1300 kg m-3, the interlayer has three layers, similar to the clay in the dispersed system. This explains the similar values for the free exchange energy and the resulting selectivity for cesium for the dispersed systems and the compacted bentonite with Fb ) 1300 kg m-3. Increasing the bulk dry density by mechanical compaction beyond 1300 kg m-3 results in a reduction of the interlayer space (h in Figure 4). Van Loon et al. (13) recently showed by diffusion measurements with 36Cl– in bentonite KWK that, at Fb ) 1600 kg m-3, ca. 40% of the interlayer water forms three layers of water and 60% is present as two layers of water. At densities beyond 1900 kg m-3, ca. 50% of the interlayer water formed two layers of water, and 50% only formed one layer of water. Evidence for interlayer compression in Na-montmorillonite by compaction was also given by Kozaki et al. (29) from measurements of the basal distance (d in Figure 4) by X-ray diffraction as a function of the bulk dry density. Thus, above Fb ) 1300 kg m-3, the interlayer contains less water, and the VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ability to hydrate Na+ effectively in the interlayer decreases, so there is more energetic reason for Na+ to enter the bulk solution. The interlayer has to have cations, so it takes the ones where the penalty for poor hydration is least, and this is the case for Cs+. The explanation for the effect of mechanical compaction on cation selectivity has similarities with the theoretical model of Laird and Shang (21) describing the interaction between crystalline swelling and cation exchange selectivity in 2:1 phyllosilicates. Laird and Shang (21) demonstrated that an increase in selectivity is correlated with a decrease of the basal distance (interlayer space) of the clays. The fixation of, for example, Cs+, K+ and Rb+ in the interlayers of 2:1 phyllosilicates is an extreme case wherein the interlayer has collapsed to 10 Å and, as a consequence, selectivity for weakly hydrated cations increases several orders of magnitude. The fact that mechanical compaction affects the thermodynamics of ion exchange processes in clay is an important finding and brings in a new aspect in the discussion on the transferability of thermodynamic data from loose onto compacted systems. The common practice of applying chemical and thermodynamic concepts valid for diluted systems to compacted systems must be basically scrutinized.
Acknowledgments This work has been performed in the framework of the European Integrated Project NF-PRO (Contract No. FI6WCT-2003-02389) and was financially supported by the Nation al Cooperative for the Disposal of Radioactive Waste (NAGRA) and the Swiss State Secretary for Education and Research (SBF). We thank W. Müller and R. Carmine for the technical assistance.
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ES702487M
