INVESTIGATION INTO THE ATTENUATION OF METALS IN GCLS INTENDED FOR MINE WASTE CONTAINMENT
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This research evaluated the use of geosynthetic clay liners (GCLs) as a potential barrier material to the migration of metals that are leached from mine waste. This thesis consists of two parts. In the first part, micro analytical methods including µXRD with synchrotron-generated µXRF elemental mapping and synchrotron-based µXRD (S-µXRD) were used to characterize the GCL bentonite and distinguish how mechanisms of metal attenuation could be identified. These analytical methods were of particular use for the clay material as they offer non-destructive, in situ investigation of various soil characteristics with microspatial resolution. The combination of the analytical methods allowed for identification of minerals such as gypsum and pyrite, not accessible by conventional methods. In particular, distinguishing accessory crystalline phases present in the “starting material” bentonite from those formed as a result of interaction with metal-bearing leachates is critical, as the development of metal-attenuating crystalline phases can have a significant long-term impact on metal mobility. In the second part of the thesis, the migration behaviour of metals (As, Al, Cd, Cu, Fe, Mn, Ni, Sr, and Zn) was investigated by means of diffusion tests and permeation experiments using four metal-containing waters: acidic rock drainage (ARD), lime-treated ARD, water from gold mine tailings, and landfill leachate with metal loading. Effective diffusion coefficients of the metals were calculated by modelling laboratory diffusion and sorption data. Water pre-hydrated GCLs were permeated with 15-21 pore volumes (PVs) of solution and their interaction with these solutions was examined in terms of both the hydraulic conductivity and the change in the geochemical characteristics of the permeant over time. The greatest increase in hydraulic conductivity occurred for the acidic rock drainage, where it increased from 1.6x10-11 m/s to 1.3x10-10 m/s following 21 PVs of permeation; still a very low value when compared to regulatory standards for clay barrier materials. Observed delayed breakthrough curves were indicative of the GCL’s strong attenuation capacity for a number of metals. An understanding of mechanisms of metal retention at both the micro and macro-scale levels will facilitate effective pollution prevention using GCLs.