IONIC MIGRATION IN FROZEN SOILS AND ICE

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1 IONIC MIGRATION IN FROZEN SOILS AND ICE E.M. Chuvilin, E.D. Ershov, O.G. Smirnova Department of Geocryology, Faculty of Geology, Vorobyevy Gory, Moscow State University, Moscow, Russia, Abstract An experimental study of the migration of ions of chemical elements in frozen soils and ice is described. The factors and conditions determining the intensity of migration of ions at negative temperatures are revealed. The acquired experimental data is evidence of the relatively high migration ability of ions in frozen soils and ice. The values of the effective diffusion coefficient (Def.) are in the order of cm 2 /s. The larger values of Def. are characteristic of ice. In this paper we look into the trends of the ionic permeability of frozen soils and ice changes in relation to their temperature, structure and composition. Introduction IMPORTANCE OF THE PROBLEM The study of ionic migration in frozen soils and ice is important. Frozen soils are often used as a suitable environment for the purposes of disposing of the industrial wastes and of highly concentrated solutions. Lately, consideration is being given to the idea of disposing of toxic and radioactive wastes by burial in frozen soils. Migration of ions in frozen soil must be better understood before this disposal option can be safely used. ANALYSIS OF EXISTING DATA At present, the migration of the chemical elements in frozen media is confirmed by field studies. The existence of salt haloes around deposits in the cold regions is widely accepted (Melnikov et al., 1988). Moreover, a qualitative assessment of the migration of chemical elements in frozen media has been made. However, field observations around a certain deposit cannot be generalized as they are defined by the unique interaction of natural conditions. Thus, laboratory studies of the migration of the chemical elements, the determination of the mechanism of this process and the role of the different factors are particularly important. Data concerning the diffusion transfer of ions in polycrystalline ice is almost non-existent. At present, field observations suggest that ice is permeable to ions. A geochemical diffusive halo in glacier ice above an ore deposit is an example (Makarov, 1991). However, processes governing the migration and accumulation of ions in ice are still unknown. There is only a small amount of data available concerning the quantitative assessment of the ionic permeability of the frozen soils and ice. It shows that the diffusion velocity of ions in frozen soils can be equal to the velocity in unfrozen soils (Table 1). To explain the migration of ions of chemical elements at negative temperatures, the following experimental program was carried out. Testing procedure The primary method used for the experiments was the so-called contact method whereby frozen soil and ice samples come in contact with heavy metal solutions under constant temperature. Sand, sandy loam, and two clays were studied. The characteristics of there soils are described in more detail in Chuvilin and Smirnova (1997). The soil samples were prepared using the technique described in Chuvilin, et al. (1996). Artificially prepared polycrystalline ice was used. Special attention was given to the structure of the ice. A series of experiments was conducted to develop recommendations for the preparation of the ice samples. A technique allowing the preparation of identical ice samples with a uniform structure was selected. The ice and frozen soil samples were brought to thermal equilibrium at the experimental temperature and then brought into contact with salt solutions. The duration of the experiments varied from several days to 3 months. The temperature chosen for these experiments ranged between -2 and -20 C. The solute concentration of such salts as Sr(NO 3 ) 2, Co(NO 3 ) 2, Pb(NO 3 ) 2, Zn(NO 3 ) 2 was 0.1 N. E.M. Chuvilin, et al. 167

2 Table 1. Values of effective diffusion coefficient (D) at positive and negative temperatures Medium Contaminant T, 0 C D, m 2 /s Author sand, Wvol.= Sr Prohorov, clay, Wvol.=0.57 Sr Prohorov, (kaolinite) Cl soil (black earth) 90 Sr Prohorov, 1982 Wvol.=0.35 clay Na Barone et al., 1989 clay (monmorillonite) Na Murrman, 1973 clay, Wvol.=0.5 (kaolinite) Na Lebedenko, 1989 ice (monocrystal) HF Barnaal and Slotfeild- Ellingsen, 1983 ice (monocrystal) NaCl, Barnaal and Slotfeild- HNO 3 Ellingsen, 1983 Table 2. Value of effective diffusion coefficient in frozen media resulting from their contact with frozen salt solution (concentration 0.1 N) Conditions Effective diffusion Frozen medium t, o C Period, days Contact solution coefficient, cm 2 /s sand Sr(NO 3 ) sandy loam Sr(NO 3 ) loam Sr(NO 3 ) clay (W=35%) Sr(NO 3 ) clay (W=42%) Sr(NO 3 ) clay (W=47%) Sr(NO 3 ) clay (W=35%) Sr(NO 3 ) clay (W=35%) Sr(NO 3 ) ice (d=6 mm) -2 3 Sr(NO 3 ) ice (d=6 mm) -6 7 Sr(NO 3 ) ice (d=6 mm) Sr(NO 3 ) ice (d=6 mm) Zn(NO 3 ) ice (d=6 mm) Co(NO 3 ) ice (d=6 mm) Pb(NO 3 ) Samples for moisture content and chemical composition and for the investigation of cryogenic structure of the soil were taken both before and after the experiment. The data obtained was used for the calculation of the ion and moisture fluxes in the soil samples and ion fluxes in ice. Then using Fick's law of diffusion, the diffusion coefficients in soils and ice were determined. Results and discussion Figure 1. Accumulation of ions (Sr) through frozen samples (kaolinite clay) interacting with water Sr(NO 3 ) 2 solution of 0.1 N for 3 month: 1- t=-20 C, 2 - t=-70 C, t=-200 C. ROLE OF UNFROZEN WATER IN THE TRANSFER OF IONS Transfer of ions in frozen soils takes place in a film of unfrozen water, which exists on the surface of mineral particles. The unfrozen moisture content in frozen soils is controlled by thermodynamic conditions and also the composition and degree of dispersion of soils. A decrease in temperature results in gradual freezing of the unfrozen water, reduction of the thickness of water films on the surface of mineral particles, and a decrease of communication between them. Thus, a decrease 168 The 7th International Permafrost Conference

3 occurs in the total accumulation of ions and the ionic permeability of frozen soils (Figure 1, Table 2). The experimental data on the interaction of samples of frozen kaolinite clay with Sr(NO 3 ) 2 solution of 0.1 N for 3 months show that lowering the temperature from -2 C to -20 C decreases the distance travelled by the ions from 8 to 1.5 cm. The values of the effective diffusion coefficient are decreased by almost one order of magnitude (Table 2). In case of negative temperatures from -0.5 to -1.5 C for fine-grained soils, active transfer of unfrozen water occurs. This results in intensive processes of soil structure development and changed soil properties (Ershov et al., 1995). ICE AS A MEDIUM OF IONIC TRANSFER In polycrystalline ice, ion migration occurs primarily along the borders of crystals, where there are high concentrations of dislocations and superficial defects and where a water film exists. The thickness of the film is increased in the presence of dissolved salts and other impurities. The experimental research confirms the existence of unfrozen water in polycrystalline ice. According to the data, received by the NMR method (Xu Xiaozu et al., 1993), there is unfrozen water in samples of polycrystalline ice. The amount is determined not only by temperature, but also by crystal size (Figure 2). The greater the crystal size, the smaller will be the diffusion, as the specific surface of the crystal becomes smaller. Correspondingly, the permeability resulting from permeability between crystal, where unfrozen water films exists, which is the most important component of the ions transfer, also becomes smaller. Pore ice in frozen soils also allows the movement of ions along the borders of ice crystals. Figure 2. Unfrozen water content in multicrystaline ice with different grain size vs. temperature. ION MIGRATION IN ICE As experimental research has shown, polycrystalline ice is permeable to ions (Chuvilin and Smirnova, 1997). Ionic permeability of ice can exceed the ionic permeability of frozen soils. This is due to the permeability between crystals, the active role of water films on the surface of the ice crystals, and also, to the weak effect of sorption processes. The ionic permeability of ice depends on its composition and structure (density, air inclusions, crystal size, presence and composition of admixtures). An increase in the size of ice crystals results in a decrease of the total accumulation of ions (Figure 3). The values of the ion fluxes decreases from mg eq/cm 2 s (in a sample with crystals of 1 mm) to mg(eq/cm 2 s (in a sample with crystals of 6 mm) Figure 3. Accumulation of ions (Sr) through ice samples of various crystal size (d) interacting with water Sr(NO 3 ) 2 solution of 0.1 N and t=-6 C for 7 days. With an increase in crystal size, there is a reduction of the specific surface of ice samples and a decrease in the quantity of water existing on crystal borders. This results in a reduction of the conducting area of a sample and of the amount of transfer. The effective coefficients of diffusion calculated were cm 2 /s in a sample of ice with a crystal size of 1 mm and cm 2 /s in a sample of ice with crystals of 6 mm. The reduction of ionic permeability of ice with an increase of crystals size can be connected to the increase of migration pathways for ions in samples, which have less unfrozen water. Migration of different ions in ice is defined by two sets of characteristics: on the one hand, by the ion parameters (charge, size, etc.), and on the other hand, by the peculiarities of the physic-chemical interference. Research on the transfer of different ions (Pb, Sr, Co, Zn) in ice has not shown any definite correlation. Their E.M. Chuvilin, et al. 169

4 coefficients of diffusion were found to be of the same magnitude, around 10-6 cm 2 /s (Table 2). The influence of temperature on migration of chemical elements in ice was considered for samples interacting with an Sr(NO 3 ) 2 solution of 0.1 N. The experiments were conducted at temperatures -20 C, -6 C, and -20 C. The results of the experiments show that, with a decrease in temperature, the intensity of migration of ions in ice drops (Figure 4). The density of migration fluxes of ions is reduced from mg eq/cm 2 s at t=-2 C to mg eq/cm 2 s at t=-6 C to mg eq/cm 2 s at t=-20 C. The distance travelled by the ions (Figure 4), as well as the ionic permeability of ice, decrease at lower temperatures (Table 2). Figure 5. Accumulation of ions (Sr) through frozen samples (kaolinite clay) of various ice content interacting with water Sr(NO 3 ) 2 solution of 0.1 N and t=-70 C for 3 months. frozen soils rises significantly. This can be explained by the rise in the quantity of the pore ice, which is an active medium for the transfer (Table 2). It once again testifies to the important role of pore ice of frozen soils in the transfer of ions. We have also obtained data on the character of ionic migration depending on the degree to which frozen soil pores are filled by ice. Figure 4. Accumulation of ions (Sr) through ice samples interacting with water Sr(NO 3 ) 2 solution of 0.1 N vs temperature. It appears that the presence of salts dissolved in ice increases the ionic permeability, as the water film on the ice crystals thickens and facilitates transfer of ions. Air inclusions present in ice reduce its ionic permeability. The influence of ice crystal orientation requires further study. ION MIGRATION IN FROZEN SOILS Experimental research has shown that the higher the degree of dispersion (from sand to loam) the greater the ion and water accumulation. However, the distance over which ions are carried decreases. Thus, the ionic permeability of frozen soils falls with an increase in degree of dispersion (Table 2). This aspect is considered in more detail elsewhere (Chuvilin and Smirnova, 1997). INFLUENCE OF ICE CONTENT AND CRYOGENIC STRUCTURE The ice content of frozen soils influences ion transfer. It has been noted, that with the rise in ice content, the total accumulation of ions, the depth of their penetration in a sample (Figure 5) and the ionic permeability of With a reduction of the degree of pore filling in frozen kaolinite clay by frozen moisture from 1 to 0.2, total accumulation of ions, and also the depth of their penetration, falls. This is explained by a reduction in the quantity of pore ice and the interruption in the pathways of ionic migration. The intensity of ion migration and the ionic permeability of frozen soils are in many respects determined by their cryogenic structure. The results of research on samples of kaolinite clay with horizontal layers of ice have shown that the presence of ice layers, oriented perpendicular to flow, results in a reduction of ion transfer. Use of this property opens an opportunity for the creation of artificial geochemical barriers around deposits of chemically active, toxic (including radioactive) wastes in permafrost. ESTIMATION OF IONIC PERMEABILITY OF FROZEN SOILS AND ICE Our research has allowed calculation of effective diffusion coefficients in frozen soils and ice of various structure and composition. The results are shown in Table 2. Conclusions The ionic permeability of ice can exceed the ionic permeability of frozen soils, which can be attributed to the unique mechanism of transfer in ice and to the absence 170 The 7th International Permafrost Conference

5 of the processes of adsorption hindering the ionic migration. A reduction in temperature results in a substantial drop in the total accumulation of ions and in the ionic permeability of frozen soils and ice. It is due to the freezing of some fraction of unfrozen water films (which are the medium for ion migration) and a decrease in the ion transferring porosity. The larger the ice crystals, the smaller the accumulation of and permeability to ions, as the specific surface of the crystals becomes smaller. At the same time, the permeability caused by the permeability between crystals, which is the most important component of the ions transfer, also becomes smaller. The effective diffusion coefficient decreases with a decrease in the ice content of frozen soils, as well as with a reduction in the degree of pore filling by frozen moisture. This is explained by a reduction in the quantity of pore ice and therefore water films around ice crystals, that it is medium for ion migration. The ionic permeability of frozen soils depends on their cryogenic structure. The existence of ice layers, oriented perpendicularly to a migration flow results in significant reduction of ion transfer. Unequivocal dependence between the type of ions and the ionic permeability of frozen soils and ice was not observed. The ionic permeability of frozen soils decreases with an increase in the degree of dispersion, as well as by a change from kaolinite clay to montmorillonite clay, that is mainly due to an increase in the cation exchange capacity of the soils and their adsorbtion characteristics. References Barone, F.S., Yanful, E.K., Quigley, R.M., and Rowe, R.K. (1989). Effect of multiple contaminant migration on diffusion and adsorption of some domestic waste contaminants in a natural clayey soil. Canadian Geotechnical Journal, 26, Chuvilin, E.M., Smirnova, O.G., and Kochetkova, N.U. (1996). Evaluating the ionic permeability of frozen soils and ice. In Proceedings 5th Chinese Conference on Glaciology and Geocryology, Lanzhou, China, pp Chuvilin, E.M. and Smirnova, O.G. (1997). The study of the ionic permeability of frozen soils and ice. In Proceedings of International Symposium on Ground Freezing and Frost Action In Soils, Lulea, Sweden, pp Ershov, E.D., Lebedenko, Y.P., and Chuvilin, E.M. (1995). Processes in frozen soils interacting with solutes. In Ershov, E.D.(ed), Basis of Geocryology. Part 1. Physico-chemical laws of geocryology. Moscow, pp (in Russian). Lebedenko, Y.P. (1989). Cryogenic migration of ions and moisture in ice-saturated disperse soils. Engineering Geology, 4, (in Russian). Melnikov, P.I., Ivanov, O.P., Makarov, V.N., Pitulko, B.M., and Shvarcev, S.L. (1988). Phenomenon of the chemical element migration and its significance in prospecting the deposits in the regions of perennially frozen ground. Doklads of AS USSR, 4, v.303, (in Russian). Makarov, V.N. (1996) Geochemical fields in cold region. In Proceedings 1st Conference of Russian Geocryologists, Moscow, Book 2, pp (in Russian). Murrman R.P. (1973). Ionic mobility in permafrost. In Proceedings 2nd International. Conference on Permafrost, National Academy of Science Press, Washington, D.C. pp Prohorov, V.M. (1982). Migration radioactive contaminates in soils. Nauka, Moscow, (195 pp). (in Russian). Xu Xiaozu, Zhang Lixin, Deng Yousheng, Wang Jiacheng, Lebedenko, Y.P. and Chuvilin E.M. (1993). Unfrozen water content in multi-crystal ice. In Proceedings 6th InternationalPermafrost Conference, V. 2, pp E.M. Chuvilin, et al. 171