Surface Diffusion: Is It an Important Transport Mechanism in Compacted Clays?
Surface diffusion, or migration within the electrical double layer next to mineral surfaces, is often invoked as a significant contributor to the overall diffusion coefficient in compacted clays, particularly where model predictions underestimate measured diffusion coefficients. The potential for surface diffusion of Sr2+, Ca2+ and Na+ on three clays compacted to dry bulk densities of 1.25 and 1.60 Mg/m3 was examined. The clays were a bentonite, an illite/smectite, and a glacial lake clay (composed mainly of smectite, illite, kaolinite and quartz). The clays were saturated with a Na-Ca-Cl-dominated synthetic groundwater solution with an effective ionic strength of 220 mol/m3. Total intrinsic diffusion coefficients for the cations were determined from their steady-state flux through compacted clays, and apparent diffusion coefficients were obtained from the time-lag technique. Models of diffusive transport in compacted clays, based only on diffusion in the pore solution, adequately described the diffusion data for all clays and diffusants, and there was no need to invoke other transport mechanisms, like surface diffusion. The data indicate that surface diffusion is not a significant transport mechanism in compacted clays at least to a clay density of 1.60 Mg/m3.
Key WordsCompacted clays Surface diffusion Transport mechanism
Unable to display preview. Download preview PDF.
- Alberts, J. J., J. E. Pinder, E. Wurtz, and S. Lesnek. 1986. The effect of pH, solid phase, particle concentration and equilibrium time on the partition coefficients of curium on natural sediments. In Application of Distribution Coefficients to Radiological Assessment Models. T. H. Sibley and C. Myttenaere, eds. London: Elsevier Applied Science Publishers, 72–82.Google Scholar
- Bradbury, M. H., A. Green, D. Lever, and I. G. Stephen. 1986. Diffusion and permeability based on sorption measurements in sandstone, anhydrite and upper magnesian limestone samples. UKAEA Report AERE-. 11995. United Kingdom Atomic Energy Agency, Oxfordshire, U.K.Google Scholar
- Carter, D. L., M. M. Mortland, and W. D. Kemper. 1986. Specific surface. In Methods of Soil Analysis, Par. 1. 2nd Ed. A. Klute, ed. Madison, Wisconsin: ASA and SSSA, 413–423.Google Scholar
- Chandratillake, M., W. E. Falck, and D. Read. 1992. CHEMVAL Project: Guide to the CHEMVAL Thermodynamic Database. U.K. DOE Report No. DOE/HMIP/ RR/92/094. 2. pp.Google Scholar
- Crank, J. 1975. The Mathematics of Diffusion, 2nd Ed. Oxford, England: Clarendon Press.Google Scholar
- Hume, H. B. 1993. Apparatus and procedures for measuring diffusion and distribution coefficients in compacted clays. AECL Research Report, AECL-10981, COG-93-446. AECL Research, Chalk River, Ontario.Google Scholar
- Jackson, M. L. 1975. Soil Chemical Analysis-Advanced Course, 2nd Ed. Madison, Wisconsin: Published by the Author, University of Wisconsin.Google Scholar
- Kemper, W. D. 1986. Solute diffusivity. In Methods of Soil Analysis, Par. 1. 2nd Ed. A. Klute, ed. Madison, Wisconsin: ASA and SSSA, 1007–1024.Google Scholar
- Miyahara, K., T. Ashida, Y. Kohara, Y. Yusa, and N. Sasaki. 1991. Effect of bulk density on diffusion of cesium in compacted sodium bentonite. Radiochim. Acta 52/53: 293–297.Google Scholar
- Nelson, D. W., and L. E. Sommers. 1982. Total carbon, organic carbon, and organic matter. In Methods of Soil Analysis, Part 2. A. L. Page, R. H. Miller, and D. R. Keeney, eds. Madison Wisconsin: ASA and SSSA, 539–579.Google Scholar
- Oscarson, D. W., and D. A. Dixon. 1989. Elemental, min-eralogical, and pore-solution composition of selected Canadian clays. AECL Research Report, AECL-9891. AECL Research, Chalk River, Ontario, Canada.Google Scholar
- Oscarson, D. W., and H. B. Hume. 1993. On the smectite to illite reaction. AECL Research Report, AECL-10842. AECL Research, Chalk River, Ontario, Canada.Google Scholar
- Oscarson, D. W., H. B. Hume, and F. King. 1994. Sorption of cesium on compacted bentonite. Clays & Clay Miner. (in press).Google Scholar
- Rasmuson, A., and I. Neretnieks. 1983. Surface migration in sorption processes. SKB Technical Report 83-37, Swedish Nuclear Fuel and Waste Management Company, Stockholm, Sweden.Google Scholar
- Skagius, K., and I. Neretnieks. 1985. Diffusion measurements of caesium and strontium in biotite gniess. SKB Technical Report 85-15, Swedish Nuclear Fuel and Waste Management Company, Stockholm, Sweden.Google Scholar