Co-disposal of wood ashes and mine wastes to control acid and metalliferous drainage

Remediating acid and metalliferous drainage (AMD) by co-disposing wood ashes with sulfidic mine wastes can be an environmentally sustainable option. This study evaluated the potential environmental performance of wood bed and fly ashes placed with six legacy mine waste samples from Tasmania. The wood ashes (6 g) were added to the mine wastes (14 g) as covers, mixed, and layers in kinetic leach columns irrigated with 30 mL of deionised water over 100 days. The bed and fly ash increased leachate pH by 0.7 and 0.1 units, respectively, for two low acid-generating (pH 4.4 and 5.3) samples but maintained or lowered the pH by ≤ 0.6 units for the remainder. The increase in pH was from acid buffering by calcite (0.14 % and 0.01 %) and reduced oxidation of sulfides, which was lowest under prolonged dry conditions. The wood ash covers reduced the concentrations of some metal(loid)s by up to 93 % by precipitation and sorption to Al and Fe oxyhydroxides and hydroxysulfates at pH > 3.5 and possibly pozzolanic reactions. The mixed and layered bed ashes increased pH by 0.4-0.6 units and immobilised metal(loid)s more than the cover system. The bed ash layers provided extra attenuation of metal(loid)s, possibly through filtration or adsorption. However, the water hazard risk was high despite the wood ash amelioration; therefore, mixing the wood ashes with more alkaline materials is recommended for the legacy mine wastes.