Clay landfill liners subject to variable interfacial redox and pH conditions: Heavy metal and clay interactions

Municipal landfill leachate contains high levels of metal species, including Cu, Zn, Cd and Pb generating competitive interactions between these metal species and the landfill clay liner materials. Characterization of the clay fraction, physical and chemical characteristics of landfill clay liner materials, the competitive interaction of metal species with clay liner materials, the kinetic modeling of metal species transport in clay liner, and the effect of cyclic interfacial redox potential on the structural changes to the clay liner and leachate metal transport were investigated in this study. Competitive Freundlich isotherms were utilized to describe the interaction of metal species with the clay liner materials. The affinities of the three clay materials with metal species decreased in the order of: bentonite > Amite Soil > kaolin while the relative affinities with the three clays followed a consistent trend of Pb > Cu > Zn > Cd. A surface complexation model (the diffuse-double layer model) was applied to model the interaction of Cu, Zn, Cd and Pb with the three clay liner materials. The introduction of Mg or Ca in rainfall-runoff residual leachate decreased the metal species adsorption capacities by up to 50%. A shrinking-core model and pore and surface diffusion model were also tested to model the binding of metal species onto compacted clays. The clay liners were exposed to cyclic redox potentials (-200 mV, 0 mV and +300 mV) and the experimental results indicated that pore size distribution and hydraulic conductivity were changed for bentonite and Amite soil clay liners. However, kaolin clay liner showed little changes either in porosity or hydraulic conductivity.