David Kossoff

Cycling of As, P, Pb and Sb during weathering of mine tailings: implications for fluvial environments

Abstract The weathering and oxidation of mine tailings has the potential to contaminate water and soil with toxic elements. To understand the mechanisms, extent and products of the long-term weathering of complex Bolivian tailings from the Cerro Rico de Potosí, and their effects on As, Pb, P and Sb cycling, three-year long laboratory column experiments were carried out to model 20 years of dry- and wet-season conditions in the Pilcomayo basin. Chemical analysis of the leachate and column solids, optical mineralogy, X-ray diffraction, scanning electron microscopy, electron probe microanalysis, micro-scale X-ray absorption near edge structure spectroscopy, Bureau Commun de Référence sequential extraction and water-soluble chemical extractions, and speciation modelling have shown that the weathering of As-bearing pyrite and arsenopyrite, resulted in a loss of 13-29% of the original mass of As. By contrast, Pb and Sb showed much lower mass losses (0.1-1.1% and 0.6-1.9%, respectively) due to the formation of insoluble Pb- and Sb(V)-rich phases, which were stable at the low pH (~2) conditions that prevailed by the end of the experiment. The experiment also demonstrated a link between the cycling of As, Sb, and the oxidation of Fe(II)-bearing sphalerite, which acted as a nucleation point for an Fe-As-Sb-O phase. Phosphorus was relatively immobile in the tailings columns (up to 0.3% mass loss) but was more mobile in the soil-bearing columns (up to 10% mass loss), due to the formation of soluble P-bearing minerals or mobilization by organic matter. These results demonstrate the influence of mine tailings on the mobility of P from soils and on the potential contamination of ecosystems with As, and strongly suggest that these materials should be isolated from fluvial environments.

Incongruent weathering of Cd and Zn from mine tailings

Abstract Weathering of is charged mine tailings contaminates streams, rivers and floodplains with toxic metals on a vast scale. The magnitude of the problem depends on input tailings mineralogy, storage and dispersal, and climatic conditions. To better understand the mechanisms of long-term tailings weathering, a leaching column study was established, incorporating tailings and soil from Potosí, Bolivia, with the aim of modelling a 25 year field period. The Zn/Cd molar ratio of the tailings leachate water, initially 738 for the unaltered tailings, is highly variable over 15 model years of leaching, particularly in the mixed tailings-soil columns. Columns with soil have ratios as high as 2563, while pure tailings columns reach ratios of <376. We employ complementary techniques, involving atomistic computational modelling, leachate analysis and mineralogical characterization, to elucidate the mechanisms governing these incongruent Cd and Zn weathering dynamics.