Antimony and arsenic behaviour in lead/zinc mine tailings during storage under vegetation cover

Abstract

The high-volume, fine grained tailings produced from Pb/Zn ore processing need to be carefully managed. Metalloid elements, As and Sb, are present in tailings at ∼800 and ∼80 mg kg−1 respectively, and in neutral pH leachates at 5–50 μg L−1. Despite these relatively low leachate concentrations, As and Sb can cause regulatory concern due to their high toxicity and propensity for bioaccumulation. As and Sb mobility in tailings are controlled by their chemical speciation and associations with mineral phases. Changes in As and Sb speciation were, therefore, determined in depth profile samples taken from an active tailings management facility during waste storage up to 8 years since deposition. At this site, primarily to prevent dust formation, a vegetation cover was established by addition of organic compost to surface layers and seeding grasses. Over time a robust vegetation cover was established consisting of perennial grasses, clovers, and after 8 years, small trees and shrubs. The surface layer of the tailings also became progressively more oxidised over time producing a substrate more suitable for plant growth enabling the establishment of beneficial vegetation cover and the development of a thin soil-like surface layer. As and Sb were both present in predominately reduced 3+ forms in freshly deposited tailings but were converted to oxidised 5+ forms in older samples. Oxidation of Fe(II) in pyrite also occurred, producing increased amounts of weak acid leachable Fe(III)-oxides in the tailings. Sorption of As to neoformed iron oxides in leachate drains reduced concentrations in leachates to below regulatory limits, but Sb sorption was relatively ineffective resulting in higher Sb concentration in site drainage waters, which may require specific treatments to reduce Sb concentrations prior to discharge.