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Anion diffusion pathways in bentonite clay compacted to different dry densities
Applied Clay Science 23 (2003) 69 – 76 www.elsevier.com/locate/clay
Anion diffusion pathways in bentonite clay compacted to different dry densities Mireia Molera *, Trygve Eriksen, Mats Jansson Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology, 100 44 Stockholm, Sweden
Abstract Diffusion of the anions Cl and I in MX-80 compacted bentonite has been studied at different ionic strengths (0.01, 0.1 M NaClO4) and clay density (0.4, 0.8, 1.2, 1.8 g cm3) at the buffered pH of bentonite 8.2 using a through-diffusion technique with measurement of breakthrough curves and concentration profiles in the bentonite. Apparent diffusivities and capacity factors (a=e+qKd) are obtained from diffusion simulations using the computer code ANADIFF. Two diffusion processes, both with density and ionic strength dependent apparent diffusivities and capacity factors, were observed. The diffusion processes observed are ascribed to diffusion in intralayer and interparticle water. The experimental data indicate that intralayer water constitutes the dominating part of water in bentonite compacted to dry the densities 0.4 – 1.8 g cm3 studied in this work. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Chloride; Iodide; Diffusion; Bentonite; Anion exclusion; Intralayer water structure
1. Introduction The low hydraulic conductivity of water saturated compacted bentonite makes diffusion the dominating transport mechanism for radionuclides if released from a deep repository storage for radioactive wastes of KBS-3 type. In porous media like compacted bentonite, diffusion in the water phase is governed by several parameters, e.g. compaction, water content and water distribution and the interaction between the diffusants and the surface of the solid. Due to the negatively charged surface of montmorillonite, the dominating mineral in bentonite, cations are readily * Corresponding author. E-mail address: [email protected] (M. Molera).
sorbed and anions excluded from the diffuse layer. Whereas solutes, e.g. Co2+ sorbed by formation of inner sphere complexes are completely immobilized on adsorption, some cations like Na+, Cs+ and Sr2+ sorbed by ion exchange are to varying degree mobile in the diffuse layer (Molera and Eriksen, in press). Diffusion in bentonite of the long-lived radionuclides 129I and 99Tc, which exist predominantly as anions in oxidizing aqueous environments (Oscarson et al., 1992, 1994), has been of interest to many authors (Allard and Torstenfelt, 1983; Allard et al., 1980; Hayes et al., 1988; Kim et al., 1993; Muurinen and Lehikoinen, 1998). Furthermore, I diffusion and sorption on organophilic bentonites have been studied by Bors et al. (2000), Dultz and Bors (2000) and Riebe et al. (2001).
0169-1317/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-1317(03)00088-7
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M. Molera et al. / Applied Clay Science 23 (2003) 69–76
Often, in anion diffusion experiments, only the breakthrough curve is considered. At steady state, a plot of the accumulated amount diffused through a bentonite barrier versus time becomes a straight line. The apparent diffusivity, Da, and the effective diffusivity, De, can then be obtained from Eqs. (1) and (2), respectively. Da ¼
L2 6te
ð1Þ
De ¼
JL AðC0 Ct Þ
ð2Þ
where te is where the asymptote of the breakthrough curve intercepts the x-axis, L is the length of the diffusion cell, J is the flux through the diffusion cell, A is the cross-section area perpendicular to the diffusion direction, C0 and Ct are the concentrations in the inlet and outlet reservoir, respectively. In an early study on Cl and I diffusion in Erbslo¨h and Wyoming MX-80 bentonites using a throughdiffusion technique, Eriksen and Jacobsson (1981) noted that although steady state diffusion was indicated by the breakthrough curves, this was not the case when examining the concentration profiles in the
bentonite. The concentration distributions within the bentonite, compacted to 1.8 g cm3, far from varying linearly with the distance from the inlet, clearly indicates a complex diffusion behavior. To shed some light on the anion diffusion processes, a number of through-diffusion experiments have been carried out with Cl and I, different clay densities (0.4, 0.8, 1.2, 1.6 and 1.8 g cm3) and ionic strength (0.01 and 0.1 M) of the supporting electrolyte.
2. Experimental The bentonite used in this investigation is the American Colloid type MX-80 (Wyoming Na-bentonite). The bentonite (MX-80) has a clay content (<2 Am) of approximately 85% and a montmorillonite content of 80– 90 wt.% of this fraction. The remaining silt fraction contains quartz, feldspar and some micas, sulfides and oxides (Mu¨ller-Von Moos and Kahr, 1983). NaClO4 solutions were used to vary the ionic strength in the sorption and diffusion experiments. The solutions were prepared from analytical grade chemicals and Millipore deionized water. The
Fig. 1. Experimental setup for breakthrough diffusion through bentonite compacted to different density (0.4, 0.8, 1.2, 1.6 and 1.8 g cm3).
M. Molera et al. / Applied Clay Science 23 (2003) 69–76
radionuclides 36Cl and 125I (New England Nuclear, NEN) were purchased in aqueous solution. Tracer solutions were prepared by adding small aliquots of the stock solution to the solutions used in the experiments. The overall diffusant concentration was obtained by adding small volumes of standardized inactive solutions. 2.1. Diffusion experiments Bentonite was statically compacted in the diffusion cell, described in Eriksen and Jansson (1996) and Eriksen et al. (1999), to different dry densities of 0.4, 0.8, 1.2, 1.6 and 1.8 g cm3 as shown in Fig. 1. The diffusion cylinder and endplates containing in- and outlet channels fitted with metallic filters (0.82 mm thick) were assembled and the clay equilibrated with the aqueous solution for at least 3 weeks by pumping solution from reservoirs through the end plate channels. After water saturation, 25 cm3 of the saturated solution was transferred to each of the outlet reservoirs, and a small volume of diffusant solution, containing the tracer and inactive carrier required to give the chosen diffusant concentration, was added to the inlet reservoir. The volume of the inlet reservoir was sufficient to keep the concentration nearly constant (within a few percent) throughout the experiments. The activity of the solutions in inlet and outlet reservoirs was monitored on small sample volumes by h-scintillation (Cl-36, I-125). At the end of the experiments, the diffusion cell was dismantled and the bentonite sliced into thin sections. Each section was dried, weighed and analyzed for activity.
3. Evaluation of data The diffusive resistance of the filters is taken into account when evaluating the results, otherwise errors larger than 40% may arise (Put, 1991). An analysis of the system filter – clay – filter is required; hence, the computer code ANADIFF was used to evaluate the data. Analysis of the complete diffusion system is described in previous works (Eriksen and Jansson, 1996; Eriksen et al., 1999). The parameters used in ANADIFF code for anion diffusion can be seen in Table 1.
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Table 1 Parameters used in ANADIFF code for anion diffusion Parameter
Value or denoted
Unit
Filter thickness Filter diffusivity Filter porosity Soil’s length Soil’s diffusivity Soil’s capacity factor Inlet concentration Outlet concentration Total time of interest Cross-section area
0.082 Df 0.25 L Da (varied) a (varied) C0 0 t 0.785
cm cm2 s 1 – cm cm2 s 1 – cps cm 3 cps cm 3 days cm2
The ANADIFF code is based on the finite difference method and calculates the breakthrough curve as well as the concentration profile by varying Da and a, while keeping the remaining input data ef, Df, C0, A, F and L constant. (Da: apparent diffusivity, a: the diffusion capacity factor, e: porosity, C0: inlet concentration (constant), A: cross-section area of the diffuison cell, F: filter length and L: length of the diffusion cell. Subscript ‘‘f’’ denotes filter properties).
4. Results and discussion In a water saturated porous medium like compacted bentonite, the diffusion, assumed to take place in the pore water, is strongly influenced by interaction with the solid phase. The dominant component in bentonite, montmorillonite, carries a negative surface charge. Cation exchange and anion exclusion resulting in surface excess and deficit, respectively, of the diffusant are therefore important processes. Other sorption processes like surface complexation also results in surface excess. On a macroscopic scale, diffusion can therefore be described by apparent diffusivity and a capacity factor a. a ¼ e þ Kd q
ð3Þ
where e is the porosity given by the water content at the dry density q of the compacted bentonite. Kd is the distribution factor (cm3 g1), a generic term devoid of mechanism and used to describe the partitioning between the aqueous phase constituents on a solid phase. It is only valid when the concentration of free or unoccupied surface adsorption sites on a solid phase is in great excess with respect to the concen-
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M. Molera et al. / Applied Clay Science 23 (2003) 69–76
tration of the solute in the solution, and the activity of the solute is equal to unity (US_EPA, 1999). For nonsorbing neutral diffusants, a=e (i.e. Kd=0), for cations, a>e (i.e. Kd>0), and for non-sorbing anions, treating anion exclusion as negative sorption, a
Whereas the breakthrough curve indicates steady state through diffusion, this is clearly not the case according to the activity profile within the compacted bentonite. As can be seen, the profile in the clay cannot be modeled with only one diffusion process. The points between the inlet and 2 mm into the cell are far too high to be attributed to the fast diffusion process. On simulation of the breakthrough curve, we obtain apparent diffusivity, capacity factor and expected ac-
Fig. 2. (a) A typical breakthrough curve of iodide, carrier concentration is 106 M, dry density is 1.6 g cm3 and the ionic strength is 0.1 M (NaClO4). To reveal the sensitivity of the curve fitting, three different simulations are displayed. (b) Diffusion of iodide (I-125) in bentonite compacted to 1.6 g cm3 with supporting electrolyte NaClO4 0.1 M. (cell length=5 mm). To reveal the sensitivity of the curve fitting, four different simulations are displayed.
M. Molera et al. / Applied Clay Science 23 (2003) 69–76
tivity profile. Subtracting this activity profile from the experimentally determined activity profile in the bentonite, we obtain an ‘‘activity difference profile’’ close to the inlet of the diffusion cell. This activity profile is modeled by assuming a second slower diffusion process. The apparent diffusivities and capacities obtained by this modeling are given in Table 2. The apparent diffusivities for the two processes are plotted versus dry density of the compacted bentonite in Fig. 3. The apparent diffusivities for the fast process are in accordance with anion data from Kim et al. (1993), Sato et al. (1992) and Muurinen (1994). The capacity factors for the fast and slow processes are plotted versus dry density of the compacted bentonite in Fig. 4a,b. As can be seen from the plot, the capacity factor increases with ionic strength and decreases with increasing density of the compacted bentonite. The apparent diffusivity for the slow process is approximately two orders of magnitude lower than for
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the fast process at 0.4 g cm3 and nearly three orders of magnitude lower at 1.8 g cm3. In contrast to the fast process, the capacity factor for the slow process increases with increasing density of the compacted bentonite. The capacities for I at 0.01 ionic strength, being much higher than the porosity as measured by water content, clearly indicate sorption on the bentonite. Assuming diffusion in water and immobilization on sorption, the apparent diffusivity for the slow process, Das, is given by the equation Das ¼
Dw es 2 s es þ K d q
ð4Þ
Dw: diffusivity in water, s2: tortuosity, Kd: distribution coefficient, q: dry density of bentonite and es: porosity of diffusive pathway. The tortuosities at different compactions are not known, but assuming these to be similar to the tortuosities for the fast diffusion process, i.e. assuming
Table 2 Summary of the conditions and results from diffusion experiments (pH was buffered to 8.2) Anion
I I I I I I I I I I I I I Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl
Density (g cm 3) 0.4 0.4 0.4 0.8 0.8 0.8 1.2 1.2 1.2 1.6 1.8 1.8 1.8 0.4 0.4 0.4 0.8 0.8 0.8 1.2 1.2 1.2 1.6 1.8 1.8 1.8
Ionic strength (M) 0.01 0.1 0.1 0.01 0.1 0.1 0.01 0.1 0.1 0.1 0.01 0.1 0.1 0.01 0.1 0.1 0.01 0.1 0.1 0.01 0.1 0.1 0.1 0.01 0.1 0.1
Fast process Da (m2 s 1) 10
7 10 7 10 10 9 10 10 3.5 10 10 3 10 10 3.5 10 10 1.4 10 10 2.3 10 10 1.4 10 10 9 10 11 3.5 10 11 3 10 11 5 10 11 8 10 10 9 10 10 8 10 10 3.5 10 10 3.5 10 10 3.7 10 10 1.4 10 10 2.3 10 10 2.0 10 10 1.0 10 10 2 10 11 5 10 11 5.5 10 11
Slow process a 0.25 0.5 0.5 0.14 0.38 0.39 0.09 0.16 0.29 0.15 0.020 0.035 0.04 0.28 0.40 0.50 0.12 0.39 0.36 0.1 0.16 0.18 0.15 0.04 0.04 0.05
Da (m2 s 1) 12
5 10 – 1 10 11 2 10 13 3 10 13 4 10 13 1 10 13 1 10 13 1 10 13 6 10 14 6 10 14 8 10 14 1 10 13 1 10 11 – 1 10 11 5 10 13 3 10 13 5 10 13 1 10 13 5 10 14 8 10 14 2 10 13 2 10 14 2 10 14 1 10 13
a 1.06 – 0.74 2.9 0.2 1.22 4.3 0.25 1.3 1.12 3.4 0.28 1.63 0.172 – 0.12 0.29 0.22 0.22 0.43 0.19 0.34 0.20 0.57 0.28 0.17
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M. Molera et al. / Applied Clay Science 23 (2003) 69–76
outer-sphere and possibly ligand exchange reactions (Stumm and Sulzberger, 1992), replacing surface OH groups by Cl or I as probable sorption mechanisms. Returning to the fast diffusive process on equilibration of montmorillonite being a layered mineral with water or electrolytic solutions, the water forms up to
Fig. 3. Apparent diffusivities for the fast and the slow process versus dry density of the compacted clay.
the diffusive process to be limited by the sorption, we obtain the following results: es<0.01 at all densities, Kd (I) is 2.9F0.8, 0.36F0.04 cm3 g1 and Kd (Cl) is 1F0.5, 0.2F0.08 cm3 g1 in 0.01 and 0.1 M NaClO4 solution at pH 8.2. Measurements of negative sorption (ion exclusion) of Cl on montmorillonite in suspensions containing 1% clay by (Bolt and Warkentin, 1958) yielded negative sorption of 1 –11 cm3 g1 in 0.1 –0.001 M NaCl solution. The Kd values found in this work for I are in good agreement with data published by Bors (Pusch et al., 1999). Whereas various studies indicate that the edges of clay particles are positively charged at pH<7 – 8 (Hunter and Alexander, 1963; Nicol and Hunter, 1970), other data suggest neutralization at pH f6 (Goodwin, 1971). The preference of sorption on montmorillonite by anion exchange is reported to be I
Fig. 4. (a) Capacity factors for the fast diffusion process versus dry density of the compacted clay. (b) Capacity factors for the slow diffusion process versus clay dry density.
M. Molera et al. / Applied Clay Science 23 (2003) 69–76
three distinct layers of intralayer water (Torikai et al., 1995). This initial crystalline swelling is followed by further uptake of water (Norrish, 1954), the total amount of water in compacted bentonite being determined by volume restrictions. As seen by Luckham and Rossi (1999), in the structure of montmorillonite, only a relatively small proportion of the inorganic cations balancing the negative layer is located at external crystal surfaces. The majority of these cations are present in the intralayer space. The thin, negatively charged sheets are held together by the electrostatic forces between alternate layers of bridging cations (typically Na+ in sodium-montmorillonite). The distribution of charges compensating the permanent surface charge is divided into immobile ions in the Stern layer and mobile ions in the diffuse layer. Despite attractive electrostatic forces between the cations and the silicate layers, montmorillonite containing small, monovalent cations like Na+ can take up more water, and the intralayer spacing may increase abruptly up to 30– ˚ and may continue to increase until volume 40 A restrictions to several hundred Angstroms with water content. Due to this swelling of the clay particles, Cl ˚ , respectively) and I with ionic radii (1.67 and 2.06 A are believed to be not always excluded from the intralayer but very mobile in the diffuse layer contributing to the fast diffusion process. Assuming the bentonite compacted to 0.4 g cm3 to be freely expanded in 0.1 M solution, we can estimate the effect of the negative sorption. The bentonite used contains approximately 75% montmorillonite and the excluded volume should therefore be approximately 0.40.751=0.3 cm3. As the porosity for the slow diffusive pathway is low (<0.01), the capacity for the fast process at 0.1 M ionic strength can be calculated from the porosity as measured by water content (0.85) to be 0.55, which is in fair agreement with the experimentally found a-value 0.5. At higher densities and lower ionic strengths, overlapping of double layers will take place leading to decrease of the excluded volume (Haan, 1965) as compared to free expanded clay. Based on the experimental observations, we ascribe the fast diffusive process to intralayer diffusion and the slow process to diffusion in the interparticle water. The low porosity for the latter process indicates that intralayer porosity, due to osmotic expansion, is dominating even at 0.4 g cm3 dry density.
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5. Conclusions The combined information from diffusion breakthrough curves and concentration profiles for the anions I and Cl in compacted bentonite reveals two diffusion processes. Based on the effect of ionic strength and dry clay density on the apparent diffusivities and capacity factors, it is concluded that the diffusion processes are diffusion in intralayer and interparticle pore space. The intralayer diffusion is affected by ion exclusion and the interparticle diffusion is retarded by ionic strength dependent sorption.
Acknowledgements This work has been supported by the Swedish Nuclear Fuel and Waste Management (SKB).
References Allard, B., Torstenfelt, B., 1983. On the Solubility of Technetium in Geochemical Systems. (SKBF-KBS-TR-83-60) Swedish Nucl. Fuel Waste Manag. Co., Stockholm, Sweden. Allard, B., Torstenfelt, B., Andersson, K., Rydberg, J., 1980. Possible retention of iodine in the ground. MRS Symposium Proceedings: Scientific Basis for Nuclear Waste Management II. Materials Research Society, vol. 2. Plenum Press, New York, USA, pp. 673 – 680. Bolt, G.H., Warkentin, B.P., 1958. The negative adsorption of anions by clay suspension. Kolloid-Z. 156, 41 – 46. Bors, J., Dultz, S., Riebe, B., 2000. Organophilic bentonites as adsorbents for radionuclides: I. Adsorption of ionic fission products. Appl. Clay Sci. 16 (1 – 2), 1 – 13. Dultz, S., Bors, J., 2000. Organophilic bentonites as adsorbents for radionuclides: II. Chemical and mineralogical properties of HDPy-montmorillonite. Appl. Clay Sci. 16 (1 – 2), 15 – 29. Eriksen, T.E., Jacobsson, A., 1981. Ion Diffusion in Compacted Na and Ca bentonites. KBS Technical Report TR 91-12, Sweden Nucl. Fuel Waste Manag. Co., Stockholm, Sweden. Eriksen, T.E., Jansson, M., 1996. Diffusion of I, Cs+ and Sr2+ in Compacted bentonite—Anion exclusion and surface diffusion. SKB Technical Report TR-96-16, Swedish Nucl. Fuel Waste Manag. Co., Stockholm, Sweden. Eriksen, T.E., Jansson, M., Molera, M., 1999. Sorption effects on cation diffusion in compacted bentonite. Eng. Geol. 54, 231 – 236. Goodwin, J.W., 1971. Rheological studies on the dispersion of kaolinite suspensions. Trans. J. Br. Ceram. Soc. 70 (2), 65 – 70. Haan, F.A.M. de, 1965. Interactions of certain inorganic anions with clays and soils. Versl. Landbouwk. Onderz., No. 655. 167 pp. Hayes, K.F., Papelis, C., Leckie, J.O., 1988. Modeling ionic strength
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effects on anion adsorption at hydrous oxide/solution interfaces. J. Colloid Interface Sci. 125 (2), 717 – 726. Hunter, R.K., Alexander, A.E., 1963. Surface properties and flow behavior of kaolinite: I. Electrophoretic mobility and stability of kaolinite sols. J. Colloid Sci. 18 (9), 820 – 832. Kim, H.T., Suk, T.W., Park, S.H., Lee, C.S., 1993. Diffusivities for ions through compacted sodium-bentonite with varying dry bulk density. Waste Manage. 13 (4), 303 – 308. Luckham, P.F., Rossi, S., 1999. The colloidal and rheological properties of bentonite suspensions. Adv. Colloid Interface Sci. 82 (1 – 3), 43 – 92. Molera, M., Eriksen, T., 2002. Diffusion of 22Na, 85Sr, 134Cs and 57 Co in bentonite clay compacted to different densities: experiments and modeling. Radiochim. Acta 90 (9 – 11), 753 – 760. Mu¨ller-Von Moos, M., Kahr, G., 1983. Mineralogische Untersuchungen von Wyoming Bentonit MX-80 und Montigel. Nagra Technischer Bericht NTB 83-12, National Cooperative for the Disposal of Nuclear Waste, Baden, Switzerland. Muurinen, A., 1994. Diffusion of anions and cations in compacted sodium bentonite. Doctoral Thesis, Technical Centre of Finland, VTT Publication No. 168, Espoo. 158 pp. Muurinen, A., Lehikoinen, J., 1998. Evolution of the porewater chemistry in compacted bentonite. MRS Symposium Proceedings: Scientific Basis for Nuclear Waste Management XXI. Materials Research Society, Pittsburgh, Pennsylvania, USA, pp. 415 – 422. Nicol, S.K., Hunter, R.J., 1970. Rheological and electrokinetic properties of kaolinite suspensions. Aust. J. Chem. 23 (11), 2177 – 2186. Norrish, K., 1954. The swelling of montmorillonite. Discuss. Faraday Soc. 18, 120 – 134. Oscarson, D.W., Hume, H.B., Sawatsky, N.G., Cheung, S.C.H., 1992. Diffusion of iodide in compacted bentonite. Soil Sci. Soc. Am. J. 56 (5), 1400 – 1406.
Oscarson, D.W., Hume, H.B., Choi, J.W., 1994. Diffusive transport in compacted mixtures of clay and crushed granite. Radiochim. Acta 65 (3), 189 – 194. Pusch, R., Muurinen, A., Lehikoinen, J., Bors, J., Eriksen, T., 1999. Microstructural and Chemical Parameters of Bentonite as Determinants of Waste Isolation Efficiency. European Commission, Nuclear Science and Technology, Final Report of Contract No. FI4W-CT95-0012, EUR 18950 EN. Put, M.J., 1991. An improved mathematical model for the interpretation of the flow-through type diffusion tests with influence of filterplates. Radioact. Waste Manag. Nucl. Fuel Cycle 16 (1), 69 – 81. Riebe, B., Bors, J., Dultz, S., 2001. Retardation capacity of organophilic bentonite for anionic fission products. J. Contam. Hydrol. 47 (2 – 4), 255 – 264. Sato, H., Ashida, T., Kohara, Y., Yui, M., Sasaki, N., 1992. Effect of dry density on diffusion of some radionuclides in compacted sodium bentonite. J. Nucl. Sci. Technol. 29 (9), 873 – 882. Stumm, W., Sulzberger, B., 1992. The cycling of iron in natural environments: considerations based on laboratory studies of heterogeneous redox processes. Geochim. Cosmochim. Acta 56 (8), 3233 – 3257. Swartzen-Allen, S.L., Matijevic, E., 1974. Surface and colloid chemistry of clays. Chem. Rev. 74 (3), 385 – 400. Torikai, Y., Sato, S., Ohashi, H., 1995. Thermodynamic properties of water in compacted bentonite under external pressure-free conditions. MRS Symposium Proceedings: Scientific Basis for Nuclear Waste Management XVIII, vol. 353, Pt. 1. Materials Research Society, Pittsburgh, Pennsylvania, USA, pp. 321 – 328. US_EPA, 1999. Understanding Variation in Partition Coefficient, Kd, Values, US Environmental Protection Agency, EPA 402-R99-004A and B.
Anion diffusion pathways in bentonite clay compacted to different dry densities Mireia Molera *, Trygve Eriksen, Mats Jansson Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology, 100 44 Stockholm, Sweden
Abstract Diffusion of the anions Cl and I in MX-80 compacted bentonite has been studied at different ionic strengths (0.01, 0.1 M NaClO4) and clay density (0.4, 0.8, 1.2, 1.8 g cm3) at the buffered pH of bentonite 8.2 using a through-diffusion technique with measurement of breakthrough curves and concentration profiles in the bentonite. Apparent diffusivities and capacity factors (a=e+qKd) are obtained from diffusion simulations using the computer code ANADIFF. Two diffusion processes, both with density and ionic strength dependent apparent diffusivities and capacity factors, were observed. The diffusion processes observed are ascribed to diffusion in intralayer and interparticle water. The experimental data indicate that intralayer water constitutes the dominating part of water in bentonite compacted to dry the densities 0.4 – 1.8 g cm3 studied in this work. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Chloride; Iodide; Diffusion; Bentonite; Anion exclusion; Intralayer water structure
1. Introduction The low hydraulic conductivity of water saturated compacted bentonite makes diffusion the dominating transport mechanism for radionuclides if released from a deep repository storage for radioactive wastes of KBS-3 type. In porous media like compacted bentonite, diffusion in the water phase is governed by several parameters, e.g. compaction, water content and water distribution and the interaction between the diffusants and the surface of the solid. Due to the negatively charged surface of montmorillonite, the dominating mineral in bentonite, cations are readily * Corresponding author. E-mail address: [email protected] (M. Molera).
sorbed and anions excluded from the diffuse layer. Whereas solutes, e.g. Co2+ sorbed by formation of inner sphere complexes are completely immobilized on adsorption, some cations like Na+, Cs+ and Sr2+ sorbed by ion exchange are to varying degree mobile in the diffuse layer (Molera and Eriksen, in press). Diffusion in bentonite of the long-lived radionuclides 129I and 99Tc, which exist predominantly as anions in oxidizing aqueous environments (Oscarson et al., 1992, 1994), has been of interest to many authors (Allard and Torstenfelt, 1983; Allard et al., 1980; Hayes et al., 1988; Kim et al., 1993; Muurinen and Lehikoinen, 1998). Furthermore, I diffusion and sorption on organophilic bentonites have been studied by Bors et al. (2000), Dultz and Bors (2000) and Riebe et al. (2001).
0169-1317/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-1317(03)00088-7
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M. Molera et al. / Applied Clay Science 23 (2003) 69–76
Often, in anion diffusion experiments, only the breakthrough curve is considered. At steady state, a plot of the accumulated amount diffused through a bentonite barrier versus time becomes a straight line. The apparent diffusivity, Da, and the effective diffusivity, De, can then be obtained from Eqs. (1) and (2), respectively. Da ¼
L2 6te
ð1Þ
De ¼
JL AðC0 Ct Þ
ð2Þ
where te is where the asymptote of the breakthrough curve intercepts the x-axis, L is the length of the diffusion cell, J is the flux through the diffusion cell, A is the cross-section area perpendicular to the diffusion direction, C0 and Ct are the concentrations in the inlet and outlet reservoir, respectively. In an early study on Cl and I diffusion in Erbslo¨h and Wyoming MX-80 bentonites using a throughdiffusion technique, Eriksen and Jacobsson (1981) noted that although steady state diffusion was indicated by the breakthrough curves, this was not the case when examining the concentration profiles in the
bentonite. The concentration distributions within the bentonite, compacted to 1.8 g cm3, far from varying linearly with the distance from the inlet, clearly indicates a complex diffusion behavior. To shed some light on the anion diffusion processes, a number of through-diffusion experiments have been carried out with Cl and I, different clay densities (0.4, 0.8, 1.2, 1.6 and 1.8 g cm3) and ionic strength (0.01 and 0.1 M) of the supporting electrolyte.
2. Experimental The bentonite used in this investigation is the American Colloid type MX-80 (Wyoming Na-bentonite). The bentonite (MX-80) has a clay content (<2 Am) of approximately 85% and a montmorillonite content of 80– 90 wt.% of this fraction. The remaining silt fraction contains quartz, feldspar and some micas, sulfides and oxides (Mu¨ller-Von Moos and Kahr, 1983). NaClO4 solutions were used to vary the ionic strength in the sorption and diffusion experiments. The solutions were prepared from analytical grade chemicals and Millipore deionized water. The
Fig. 1. Experimental setup for breakthrough diffusion through bentonite compacted to different density (0.4, 0.8, 1.2, 1.6 and 1.8 g cm3).
M. Molera et al. / Applied Clay Science 23 (2003) 69–76
radionuclides 36Cl and 125I (New England Nuclear, NEN) were purchased in aqueous solution. Tracer solutions were prepared by adding small aliquots of the stock solution to the solutions used in the experiments. The overall diffusant concentration was obtained by adding small volumes of standardized inactive solutions. 2.1. Diffusion experiments Bentonite was statically compacted in the diffusion cell, described in Eriksen and Jansson (1996) and Eriksen et al. (1999), to different dry densities of 0.4, 0.8, 1.2, 1.6 and 1.8 g cm3 as shown in Fig. 1. The diffusion cylinder and endplates containing in- and outlet channels fitted with metallic filters (0.82 mm thick) were assembled and the clay equilibrated with the aqueous solution for at least 3 weeks by pumping solution from reservoirs through the end plate channels. After water saturation, 25 cm3 of the saturated solution was transferred to each of the outlet reservoirs, and a small volume of diffusant solution, containing the tracer and inactive carrier required to give the chosen diffusant concentration, was added to the inlet reservoir. The volume of the inlet reservoir was sufficient to keep the concentration nearly constant (within a few percent) throughout the experiments. The activity of the solutions in inlet and outlet reservoirs was monitored on small sample volumes by h-scintillation (Cl-36, I-125). At the end of the experiments, the diffusion cell was dismantled and the bentonite sliced into thin sections. Each section was dried, weighed and analyzed for activity.
3. Evaluation of data The diffusive resistance of the filters is taken into account when evaluating the results, otherwise errors larger than 40% may arise (Put, 1991). An analysis of the system filter – clay – filter is required; hence, the computer code ANADIFF was used to evaluate the data. Analysis of the complete diffusion system is described in previous works (Eriksen and Jansson, 1996; Eriksen et al., 1999). The parameters used in ANADIFF code for anion diffusion can be seen in Table 1.
71
Table 1 Parameters used in ANADIFF code for anion diffusion Parameter
Value or denoted
Unit
Filter thickness Filter diffusivity Filter porosity Soil’s length Soil’s diffusivity Soil’s capacity factor Inlet concentration Outlet concentration Total time of interest Cross-section area
0.082 Df 0.25 L Da (varied) a (varied) C0 0 t 0.785
cm cm2 s 1 – cm cm2 s 1 – cps cm 3 cps cm 3 days cm2
The ANADIFF code is based on the finite difference method and calculates the breakthrough curve as well as the concentration profile by varying Da and a, while keeping the remaining input data ef, Df, C0, A, F and L constant. (Da: apparent diffusivity, a: the diffusion capacity factor, e: porosity, C0: inlet concentration (constant), A: cross-section area of the diffuison cell, F: filter length and L: length of the diffusion cell. Subscript ‘‘f’’ denotes filter properties).
4. Results and discussion In a water saturated porous medium like compacted bentonite, the diffusion, assumed to take place in the pore water, is strongly influenced by interaction with the solid phase. The dominant component in bentonite, montmorillonite, carries a negative surface charge. Cation exchange and anion exclusion resulting in surface excess and deficit, respectively, of the diffusant are therefore important processes. Other sorption processes like surface complexation also results in surface excess. On a macroscopic scale, diffusion can therefore be described by apparent diffusivity and a capacity factor a. a ¼ e þ Kd q
ð3Þ
where e is the porosity given by the water content at the dry density q of the compacted bentonite. Kd is the distribution factor (cm3 g1), a generic term devoid of mechanism and used to describe the partitioning between the aqueous phase constituents on a solid phase. It is only valid when the concentration of free or unoccupied surface adsorption sites on a solid phase is in great excess with respect to the concen-
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tration of the solute in the solution, and the activity of the solute is equal to unity (US_EPA, 1999). For nonsorbing neutral diffusants, a=e (i.e. Kd=0), for cations, a>e (i.e. Kd>0), and for non-sorbing anions, treating anion exclusion as negative sorption, a
Whereas the breakthrough curve indicates steady state through diffusion, this is clearly not the case according to the activity profile within the compacted bentonite. As can be seen, the profile in the clay cannot be modeled with only one diffusion process. The points between the inlet and 2 mm into the cell are far too high to be attributed to the fast diffusion process. On simulation of the breakthrough curve, we obtain apparent diffusivity, capacity factor and expected ac-
Fig. 2. (a) A typical breakthrough curve of iodide, carrier concentration is 106 M, dry density is 1.6 g cm3 and the ionic strength is 0.1 M (NaClO4). To reveal the sensitivity of the curve fitting, three different simulations are displayed. (b) Diffusion of iodide (I-125) in bentonite compacted to 1.6 g cm3 with supporting electrolyte NaClO4 0.1 M. (cell length=5 mm). To reveal the sensitivity of the curve fitting, four different simulations are displayed.
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tivity profile. Subtracting this activity profile from the experimentally determined activity profile in the bentonite, we obtain an ‘‘activity difference profile’’ close to the inlet of the diffusion cell. This activity profile is modeled by assuming a second slower diffusion process. The apparent diffusivities and capacities obtained by this modeling are given in Table 2. The apparent diffusivities for the two processes are plotted versus dry density of the compacted bentonite in Fig. 3. The apparent diffusivities for the fast process are in accordance with anion data from Kim et al. (1993), Sato et al. (1992) and Muurinen (1994). The capacity factors for the fast and slow processes are plotted versus dry density of the compacted bentonite in Fig. 4a,b. As can be seen from the plot, the capacity factor increases with ionic strength and decreases with increasing density of the compacted bentonite. The apparent diffusivity for the slow process is approximately two orders of magnitude lower than for
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the fast process at 0.4 g cm3 and nearly three orders of magnitude lower at 1.8 g cm3. In contrast to the fast process, the capacity factor for the slow process increases with increasing density of the compacted bentonite. The capacities for I at 0.01 ionic strength, being much higher than the porosity as measured by water content, clearly indicate sorption on the bentonite. Assuming diffusion in water and immobilization on sorption, the apparent diffusivity for the slow process, Das, is given by the equation Das ¼
Dw es 2 s es þ K d q
ð4Þ
Dw: diffusivity in water, s2: tortuosity, Kd: distribution coefficient, q: dry density of bentonite and es: porosity of diffusive pathway. The tortuosities at different compactions are not known, but assuming these to be similar to the tortuosities for the fast diffusion process, i.e. assuming
Table 2 Summary of the conditions and results from diffusion experiments (pH was buffered to 8.2) Anion
I I I I I I I I I I I I I Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl
Density (g cm 3) 0.4 0.4 0.4 0.8 0.8 0.8 1.2 1.2 1.2 1.6 1.8 1.8 1.8 0.4 0.4 0.4 0.8 0.8 0.8 1.2 1.2 1.2 1.6 1.8 1.8 1.8
Ionic strength (M) 0.01 0.1 0.1 0.01 0.1 0.1 0.01 0.1 0.1 0.1 0.01 0.1 0.1 0.01 0.1 0.1 0.01 0.1 0.1 0.01 0.1 0.1 0.1 0.01 0.1 0.1
Fast process Da (m2 s 1) 10
7 10 7 10 10 9 10 10 3.5 10 10 3 10 10 3.5 10 10 1.4 10 10 2.3 10 10 1.4 10 10 9 10 11 3.5 10 11 3 10 11 5 10 11 8 10 10 9 10 10 8 10 10 3.5 10 10 3.5 10 10 3.7 10 10 1.4 10 10 2.3 10 10 2.0 10 10 1.0 10 10 2 10 11 5 10 11 5.5 10 11
Slow process a 0.25 0.5 0.5 0.14 0.38 0.39 0.09 0.16 0.29 0.15 0.020 0.035 0.04 0.28 0.40 0.50 0.12 0.39 0.36 0.1 0.16 0.18 0.15 0.04 0.04 0.05
Da (m2 s 1) 12
5 10 – 1 10 11 2 10 13 3 10 13 4 10 13 1 10 13 1 10 13 1 10 13 6 10 14 6 10 14 8 10 14 1 10 13 1 10 11 – 1 10 11 5 10 13 3 10 13 5 10 13 1 10 13 5 10 14 8 10 14 2 10 13 2 10 14 2 10 14 1 10 13
a 1.06 – 0.74 2.9 0.2 1.22 4.3 0.25 1.3 1.12 3.4 0.28 1.63 0.172 – 0.12 0.29 0.22 0.22 0.43 0.19 0.34 0.20 0.57 0.28 0.17
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outer-sphere and possibly ligand exchange reactions (Stumm and Sulzberger, 1992), replacing surface OH groups by Cl or I as probable sorption mechanisms. Returning to the fast diffusive process on equilibration of montmorillonite being a layered mineral with water or electrolytic solutions, the water forms up to
Fig. 3. Apparent diffusivities for the fast and the slow process versus dry density of the compacted clay.
the diffusive process to be limited by the sorption, we obtain the following results: es<0.01 at all densities, Kd (I) is 2.9F0.8, 0.36F0.04 cm3 g1 and Kd (Cl) is 1F0.5, 0.2F0.08 cm3 g1 in 0.01 and 0.1 M NaClO4 solution at pH 8.2. Measurements of negative sorption (ion exclusion) of Cl on montmorillonite in suspensions containing 1% clay by (Bolt and Warkentin, 1958) yielded negative sorption of 1 –11 cm3 g1 in 0.1 –0.001 M NaCl solution. The Kd values found in this work for I are in good agreement with data published by Bors (Pusch et al., 1999). Whereas various studies indicate that the edges of clay particles are positively charged at pH<7 – 8 (Hunter and Alexander, 1963; Nicol and Hunter, 1970), other data suggest neutralization at pH f6 (Goodwin, 1971). The preference of sorption on montmorillonite by anion exchange is reported to be I
Fig. 4. (a) Capacity factors for the fast diffusion process versus dry density of the compacted clay. (b) Capacity factors for the slow diffusion process versus clay dry density.
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three distinct layers of intralayer water (Torikai et al., 1995). This initial crystalline swelling is followed by further uptake of water (Norrish, 1954), the total amount of water in compacted bentonite being determined by volume restrictions. As seen by Luckham and Rossi (1999), in the structure of montmorillonite, only a relatively small proportion of the inorganic cations balancing the negative layer is located at external crystal surfaces. The majority of these cations are present in the intralayer space. The thin, negatively charged sheets are held together by the electrostatic forces between alternate layers of bridging cations (typically Na+ in sodium-montmorillonite). The distribution of charges compensating the permanent surface charge is divided into immobile ions in the Stern layer and mobile ions in the diffuse layer. Despite attractive electrostatic forces between the cations and the silicate layers, montmorillonite containing small, monovalent cations like Na+ can take up more water, and the intralayer spacing may increase abruptly up to 30– ˚ and may continue to increase until volume 40 A restrictions to several hundred Angstroms with water content. Due to this swelling of the clay particles, Cl ˚ , respectively) and I with ionic radii (1.67 and 2.06 A are believed to be not always excluded from the intralayer but very mobile in the diffuse layer contributing to the fast diffusion process. Assuming the bentonite compacted to 0.4 g cm3 to be freely expanded in 0.1 M solution, we can estimate the effect of the negative sorption. The bentonite used contains approximately 75% montmorillonite and the excluded volume should therefore be approximately 0.40.751=0.3 cm3. As the porosity for the slow diffusive pathway is low (<0.01), the capacity for the fast process at 0.1 M ionic strength can be calculated from the porosity as measured by water content (0.85) to be 0.55, which is in fair agreement with the experimentally found a-value 0.5. At higher densities and lower ionic strengths, overlapping of double layers will take place leading to decrease of the excluded volume (Haan, 1965) as compared to free expanded clay. Based on the experimental observations, we ascribe the fast diffusive process to intralayer diffusion and the slow process to diffusion in the interparticle water. The low porosity for the latter process indicates that intralayer porosity, due to osmotic expansion, is dominating even at 0.4 g cm3 dry density.
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5. Conclusions The combined information from diffusion breakthrough curves and concentration profiles for the anions I and Cl in compacted bentonite reveals two diffusion processes. Based on the effect of ionic strength and dry clay density on the apparent diffusivities and capacity factors, it is concluded that the diffusion processes are diffusion in intralayer and interparticle pore space. The intralayer diffusion is affected by ion exclusion and the interparticle diffusion is retarded by ionic strength dependent sorption.
Acknowledgements This work has been supported by the Swedish Nuclear Fuel and Waste Management (SKB).
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