Hydraulic, Diffusion, and Retention Characteristics of Inorganic Chemicals in Bentonite

Inorganic contaminants, while transported through the bentonite layer, are chemically adsorbed onto the particle surfaces and exhibit a delay in solute breakthrough in hydraulic barriers. Transport of inorganic leachate contaminants through bentonite occurs by advection, diffusion or a combination of these two mechanisms. During the process of chemical solute transport through low permeability bentonite, the amount of cation exchange on the clay particle surface is directly related to the cation exchange capacity (CEC) of montmorillonite and other mineral constituents. The process of diffusion and advection of various inorganic leachate contaminants through bentonite is thoroughly investigated in this study. Diffusion characteristics are of specific interest as they have a prominent effect on the long term properties of bentonite compared to advection. This is mostly true if the hydraulic conductivity of the material is less than 10-8 cm/s and if the thickness of the barrier is small. Chemical reactions in the form of cationic exchange on the clay particle surfaces has been incorporated in the analysis of the diffusion process. Adsorption-desorption (sorption) reactions of chemical compounds that influence the concentrations of inorganic leachates during transport in bentonite clay have been modeled using the Fick\u27s fundamental diffusion theory. Partition coefficients of the solutes in pore space, which affect the retardation factor of various individual ions of chemical solutions, have been investigated during transient diffusion and advection processes. Several objectives have been accomplished during this research study. An evaluation has been carried out of the hydraulic conductivity of bentonite with respect to single species salts and various combinations of electrolyte solutions. Diffusion properties of inorganic leachates through bentonite have been characterized in terms of apparent and effective diffusion coefficients. Time-dependent behavior of the diffusive ions has been analyzed in order to determine the total retention capacity of bentonite before electrical conductivity breakthrough and steady-state chemical stability are reached. An analytical solution of the attenuation of various inorganic ions concentrations through bentonite has been developed. Finally, recommendations were made for landfill liners exposed to highly concentrated inorganic leachates