Acid mine drainage and metal(loid) risk potential of South African coal processing wastes

Abstract

The beneficiation of coal reduces its ash and sulfur content but generates a waste stream that can have adverse environmental impacts. This study evaluated the acid mine drainage (AMD) and metal(loid) risk potential of waste generated from the processing of South African Witbank and Waterberg coals. Geochemical static and kinetic tests classified the two Witbank samples as potentially acid-forming (3.2–30 kg H2SO4/t) and the Waterberg sample as non-acid-forming (−46 kg H2SO4/t). The AMD of pH 1.8–3.2 generated by the Witbank samples can pose a severe danger to human health. All the coal wastes were more enriched in minor and trace elements than average global coal. Sequential chemical extractions of the elements revealed the organic and sulfide phases to be the major carriers of As (37–72 %), Cu (51–60 %), Mo (34–54 %), and Se (31–69 %). However, >50 % of Ba, Cr, Sn, P, Sn, Sr, Th, U, and V remained in the environmentally inert residue phase. Risk assessments determined that only Fe can be leached in concentrations that pose a high risk, whereas Al, As, Mn, Pb, SO4, and Sb can pose a moderate risk to water quality. The Waterberg sample had the highest metal(loid) risk. The outcomes of this study highlight that an AMD treatment method is required to mitigate the environmental impacts of the investigated coal processing wastes.