Abstract
Laboratory advection-diffusion tests are performed on two regional soils-Brown Earth and Red Earth-in order to assess their capacity to control contaminant migration with synthetic contaminant solution of sodium sulphate with sodium concentration of 1000 mg/L. The test was designed to study the transport/attenuation behaviour of sodium in the presence of sulphate. Effective diffusion coefficient (De) that takes into consideration of attenuation processes is used. Cation exchange capacity is an important factor for the attenuation of cationic species. Monovalent sodium ion cannot usually replace other cations and the retention of sodium ion is very less. This is particularly true when chloride is anion is solution. However, sulphate is likely to play a role in the attenuation of sodium. Cation exchange capacity and type of exchangeable ions of soils are likely to play an important role. The effect of sulphate ions on the effective diffusion coefficient of sodium, in two different types of soils, of different cation exchange capacity has been studied. The effective diffusion coefficients of sodium ion for both the soils were calculated using Ogata Bank’s equation. It was shown that effective diffusion coefficient of sodium in the presence of sulphate is lower for Brown Earth than for Red Earth due to exchange of sodium with calcium ions from the exchangeable complex of clay. The soil with the higher cation exchange retained more sodium. Consequently, the breakthrough times and the number of pore volumes of sodium ion increase with the cation exchange capacity of soil.
Highlights
Natural clay deposits and compacted clay liners can act as barriers for contaminant migration and limit contamination of ground water resources
It was shown that effective diffusion coefficient of sodium in the presence of sulphate is lower for Brown Earth than for Red Earth due to exchange of sodium with calcium ions from the exchangeable complex of clay
This study describes results of laboratory column tests performed on two local soils to understand the processes controlling the migration of sodium ion and to assess their potential for liner application
Summary
Natural clay deposits and compacted clay liners can act as barriers for contaminant migration and limit contamination of ground water resources. The retardation of other cations will be more and is possible to design thickness of liners based on studies conducted on sodium ions. Transport of ions in porous media is controlled by a variety of physical, chemical and biological processes [1,2]. The physical processes include diffusion, advection and dispersion. The factors primarily affecting transport of species through porous media are hydraulic conductivity, k, dispersion coefficient D, and the retardation factor, R. Advection-dispersion flow occurs when seepage velocity is small and diffusion coefficient becomes an important factor. Dp D0WT De where WT is complex tortuosity factor, D0 is free solution diffusion coefficient; is volumetric water content;. The diffusion coefficient is even less than porous media diffusion coefficient, Dp, due to attenuation processes. The diffusion coefficient, which takes into account the various attenuation processes, is called effective diffusion coefficient, De
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