Exploration of biomass ashes (BA) to decontaminate highly metal-rich acid mine drainages (AMDs): Column and batch experiments

This work investigates the suitability of biomass ash (BA), a waste generated after biomass burning, as alkaline material to treat highly acidic (pH 1.9–2.0) and metal rich acid mine drainages (AMD). To address this issue, batch (at solid:liquid ratios of 1:2, 1:5; 1:10, 1:20, 1:100 and 1:200) and column experiments were performed. During batch experiments, the contact of AMD with BA provoked an intense increase of pH values, especially at higher S:L ratios (1:2 and 1:5), i.e., from 1.9 to 7.3 and 6.4, respectively, due to the alkalinity provided by BA, which led to strong removal of dissolved metal/loids (e.g., 89–99 % of Fe, 99% of Al, Cu and As, 75–99% of Pb) and sulfate (66–68%) for both ratios. The removal efficiency obtained using intermediate and low S:L ratios was remarkably lower for most metal/loids except for As, with values above 95% at the end of the experiment for all S:L ratios. On the other hand, the dissolution of metal oxides, initially contained in BA, led to the release of elements commonly found associated to these oxides in BA (e.g., Al, Ca, Mg, K, Na, Sr, or P). The removal rates obtained in column experiments were lower, due to the fast depletion of alkalinity during the first days of the experiment, which make columns less suitable for AMD treatment than batch reactors. A removal of 100% of Cu, As, V and Ga, 99% of Pb, 73% of Fe and Cd, 64% of Zn and Co, and 60% of sulfate was achieved after 24 h. However, the efficiency of the column decreased progressively to the end of the experiment, reaching similar values than in input waters, except in the case of As (around 91% of removal), due to the preferential sorption of oxyanions (H₂AsO₄⁻ and HAsO₄²⁻). The precipitation of schwertmannite and to a lesser extent jarosite, and sorption processes on these minerals, are the main process controlling metal retention in both batch and column experiments. Despite the low alkalinity of the BA used, the removal rates of metal (loid)s were significant, and hence, it constitutes a promising option to treat AMD in mining areas worldwide where this waste is generated.