Attenuation of Heavy Metals by Geosynthetic Clay Liners

An issue of continuing environmental concern is the migration of potentially hazardous metals in waste containment facilities to the subsurface. Geosynthetic clay liners (GCLs) are now well recognised as an alternative to traditional compacted clay liners. Past research has established the use of clays for attenuating metallic contaminants; however the attenuation capacity of GCLs for metals remains in question. This paper describes research aimed at characterizing GCL behaviour in the presence of metal-rich permeants and assess their potential for metal attenuation in order to evaluate their potential use in different waste containment scenarios. To date, the migration of select metals (Al, Fe, Mn, Ni, Pb, Cd, Cu, Zn) have been investigated using continuous flow permeameters and diffusion testing in the presence of a municipal solid waste leachate and an acid mine drainage solution. The ability of the GCL to attenuate metals was measured by monitoring aqueous solutions before and after the experiment and by bulk analysis of the GCL itself. In predicting the long-term performance of the GCL, it is important to examine the mechanism of metal attenuation such as the role of adsorption onto bentonite, sorption onto Fe and Mn hydroxides and the effect of reducing conditions on the long-term stability of these hydroxides. Delayed breakthrough times and material analysis have shown evidence of metal attenuation and insights into adsorption mechanisms have been made. Material analysis to date has included constituent analysis of digested GCL soil, soil pore water analysis and X-ray diffraction. Future research will use material analysis methods of micro-imaging (electron microprobe, scanning electron microscopy) to further explain retention mechanisms.