Abstract
The extent to which mining and landfilling activities in South Africa impact the environment and invariably have consequential effects on the health of inhabitants has remained insistently dire. Hence, for the study to assess the efficacy and potency of three subtropical clayey soils as buffers of contaminants, their compatibility with acid mine drainage (AMD) and municipal solid waste (MSW) landfill leachate was investigated through geochemical, mechanical and soil hydraulic conductivity testing, batch sorption, column diffusion, chemical and x-ray diffraction studies. The infusion of roughly 18-24 pore volumes of AMD through the soils triggered a dissolution of metals from soil grains. The soils adsorbed more potassium than sodium after 4-10 pore volumes of MSW landfill leachate intrusion. Generally, the effluent breakthrough curves of the respective soils showed early arrival and delayed desorption of magnesium and calcium while iron and nearly all heavy metals in the leachate were buffered. The effective diffusion coefficients for potassium and sodium were found to range between 1.5-1.9 × 10-10 m/s and 7.1-13 × 10-10 m/s respectively. The source solutions used as permeants in the study triggered desorption of chemical species from the exchangeable sites of the clayey soil minerals leading to the alteration, formation and dissolution of other soil minerals. Irrespective of the physicochemical and mineralogical transformations that occurred in the respective soils, the final hydraulic conductivity values satisfied the maximum soil acceptance criterion by roughly two order of magnitude lower than 1 × 10-9 m/s specified for clay liner construction in South Africa. Nonetheless, the three sampled natural subtropical soils were found to be incompatible with AMD and therefore, should not be solely used as naked natural buffers for AMD containment and related acid producing wastes, as they were mostly inadequate in buffering the potentially harmful AMD chemical species. Furthermore, it is not advisable to use the respective naked soils alone as natural buffers in MSW landfills with high concentrations of leachate chemical species as every soil system has a threshold. The soils can however, be used in composite barrier lining systems. This is such that, complex mechanisms provide the natural soils in-situ and ex-situ with their mechanical and physical behaviours including; adsorption, desorption, attenuation, complexation, pressure and transformations which contribute to the development of changes in net repulsion/attraction and natural bonding in the respective soils.
D.Phil. (Civil Engineering Science)