Smelter-derived soil contamination in Luanshya, Zambia

Abstract

Extensive mining and smelting contributed to the declining quality of Luanshya soils. The local smelter was the epicenter of contamination as shown by a spatial distribution analysis. Closeby soil profiles smelter exhibit extremely high Cu concentrations (up to 46,000 mg kg^-1 Cu) relative to deeper layers where only background levels of trace elements were observed. A remote profile did not exhibit significant contamination. Lead isotopic ratios revealed that Pb contamination in the Luanshya soils was not smelter-derived. It was shown in this way that the historical usage of leaded gasoline was the main source of this metal. Although the Luanshya smelter also produced Co, this metal was not an important contaminant. Copper leaching was a concern in Luanshya. Upwards of 52 % of Cu was extractable in the exchangeable step of a sequential extraction procedure (SEP), but only for samples where Cu concentrations were high, suggesting that Cu was released exclusively from anthropogenic particles. This was supported by the SEP results for similar depths at the remote soil, where only a small fraction of Cu was labile (5.6 %). Lead and Co were strongly bound in the soils throughout. The excess of Cu in the topsoils was mostly bound in smelter-derived particles. These appeared as spherical fast-cooled droplets composed mostly of sulfides, oxides, and glass. X-ray diffraction and electron probe microanalysis of those particles allowed for a phase classification. Compositions were regularly not stoichiometric so most particles were classified as intermediate solid solutions. However, molecular proportions often closely resembled those of bornite, chalcanthite, cuprospinel, covellite, delafossite, diginite, or hydrous ferric oxides. Concentrations of Cu were often 100 % near the center of the particles indicating an inefficient smelting process. Weathering to some degree was common, which in conjunction with the susceptibility of Cu leaching was highly alarming.

Keywords: Contamination; Environmental; Mineralogy; Pb isotopes; Profile; Soils.