Laboratory Determination of Multicomponent Effective Diffusion Coefficients for Heavy Metals in a Compacted Clay


Abstract: Diffusion and sorption of heavy-metal ions (Cd, Cr, Cu, Fe, K, Mn, Ni, Pb, and Zn) through an Ankara clay liner were studied experimentally. The Ankara clay consists of calcite, quartz, smectite and illite, and is a highly plastic inorganic clay with optimum weight water content of 32% and corresponding dry density of 1.413 g/cm3. Under optimum compaction conditions, effective porosity (volumetric water content) and hydraulic conductivity in the material were determined as 0.45 and 1.3 to 2.6x10-9 cm/s, respectively. The Ankara clay adsorbed Cd, Cr, Cu, Fe, K, Mn, Ni, Pb, and Zn metals, but desorbed Ca, Mg and Na metals for the solution concentration range used. Sorption isotherms of these ions were experimentally established. Effective diffusion coefficients of Cd, Cl, Cr, Cu, Fe, K, Mn, Ni, Pb, and Zn metals, determined through column experiments are, respectively, 2.5, 9.5, 2.2, 2.9, 2.2, 7.9, 3.1, 1.6, 3.2 and 2.5 (x10-6) cm2/s. The tortuosity factor of non-reactive chloride is approximately 0.47. The factor changes between 0.34 and 0.45 for the other ions. The results show that diffusion is the dominant mass transport mechanism in an Ankara clay liner. Due to sorption processes, transported heavy metals studied in this research will be significantly attenuated in the liner. Because diffusion and sorption processes depend on solution composition, mass transport determinations in clay liners require case studies using multicomponent systems similar to those presented in this study.

Keywords: contaminant transport, diffusion coefficients, heavy metals, clay liner, Ankara clay

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