Novel advances using sewage sludge in engineered dry covers for sulphide mine tailings remediation

Effective remediation of sulphide-bearing mine tailings is a fundamental aspect in mine plan development. Novel solutions to replace virgin materials in engineered dry cover amendments using sewage sludge biosolids (SS) were evaluated in laboratory-, pilot-, and field-scale experiments. The aim of this thesis is to be able to identify the applicability of SS as a long-term (10 000 year) dry cover amendment for sulphide mine tailings remediation. The utilisation of SS could provide a low-cost approach to mitigate sulphide mine tailings oxidation, acid rock drainage (ARD) formation, and offer a solution towards the co-disposal of two wastes simultaneously. Three experiments evaluated five different SS applications at pilot- and field-scales. The experiments were innovative as they stepped outside of the laboratory domain, and evaluated time periods spanning 0-8 years. The first study addressed a novel solution to replace common sealing layer barrier materials within a composite dry cover with SS. In the 8-year experimental period, it was found that the SS acted as an effective physical and organic reactive barrier to oxygen diffusion. This created a reductive, high alkalinity, near-neutral pH environment in the underlying tailings. Sludge-derived Cd, Cu, Pb, and Zn accumulated in the sludge-to-tailings interface, and the effluent drainage exhibited low (<10 µg L-1) dissolved Cd, Cu, Pb, and Zn concentrations. However, aerobic and anaerobic degradation processes exhausted 85 % of the organic fraction of the SS, and combined with the leaching of metals, exhibited a 19.6 % sludge mass reduction. The second study focused upon using a field-scale application of SS as a vegetation substrate applied onto a formerly remediated tailings impoundment. The objective delineated the magnitude, duration and fate of sludge-borne constituents by groundwater well monitoring methods and geochemical modelling. The findings indicated that ~ 17 % of the SS was degraded after 2 years of surface weathering, due to nitrification, aerobic degradation, and the leaching of Cu, Ni, Pb, and Zn. The metals formed highly mobile organo-metallic complexes that were readily transported though the groundwater. Stoichiometric ratios indicated that nitrate acted as an oxidant to pyrite in the underlying tailings. The effects were temporary due to vegetation establishment, which suppressed nitrate release within 2 years. The third field-scale study investigated the use of a single SS layer onto bare sulphide mine tailings, and when combined with a fly-ash layer. The aim of the study was to identify if a single layer would mitigate oxygen diffusion, and if not, if a combined sludge/fly-ash cover would be more effective. The findings of this study indicated that a single layer of SS above the tailings was ineffective at preventing oxygen diffusion and ARD formation, and merely slowed the process by 20 % in 2 years. In addition, sludge-derived metals (Cu, Fe, Ni, Zn) accumulated in the underlying tailings. The combined sludge/fly-ash cover was effective at immobilising the sludge-derived metals, and acted as an effective barrier to oxygen. Nevertheless, the SS continued to perpetuate the application, as high concentrations of nitrate provided an oxidant to pyrite. Two laboratory experiments were set-up to quantify aerobic and anaerobic biodegradation rates of the organic fraction of the SS and to calibrate them with the field data to provide a model for estimating the life-time of the cover. Within 156 and 230 day experiments at 20-22°C, aerobic and anaerobic degradation rates each resulted in a 14.8 % and 27.8 % loss of the organic fraction respectively. Field biodegradation rates were modelled to be slower, due to colder field temperatures and a high degree of water-saturation. Predictions indicate aerobic biodegradation rates will result in a 20 % loss of total organic matter degradation in 2 years of field conditions. Anaerobic biodegradation prediction rates indicate an exhaustion of readily-degradable organic fractions within 2 years. However, a large residual organic fraction left is more recalcitrant to biodegradation. The findings of this thesis indicate that SS may be effective in limited application types for sulphide mine tailings remediation. However, long-term engineered cover applications may be compromised by high rates of biodegradation in both surface and sub-surface amendments