Metal(loid)s remobilization and mineralogical transformations in smelter-polluted savanna soils under simulated wildfire conditions

Abstract

The surroundings of mines and smelters may be exposed to wildfires, especially in semi-arid areas. The temperature-dependent releases of metal(loid)s (As, Cd, Cu, Pb, Zn) from biomass-rich savanna soils collected near a Cu smelter in Namibia have been studied under simulated wildfire conditions. Laboratory single-step combustion experiments (250–850 °C) and experiments with a continuous temperature increase (25–750 °C) were coupled with mineralogical investigations of the soils, ashes, and aerosols. Metals (Cd, Cu, Pb, Zn) were released at >550–600 °C, mostly at the highest temperatures, where complex aerosol particles, predominantly composed of slag-like aggregates, formed. In contrast, As exhibited several emission peaks at ~275 °C, ~370–410 °C, and ~580 °C, reflecting its complex speciation in the solid phase and indicating its remobilization, even during wildfires with moderate soil heating. At <500 °C, As was successively released via the transformation of As-bearing hydrous ferric oxides, arsenolite (As2O3) grains attached to the organic matter fragments, metal arsenates, and/or As-bearing apatite, followed by the thermal decomposition of enargite (Cu3AsS4) at >500 °C. The results indicate that the active and abandoned mining and smelting sites, especially those highly enriched in As, should be protected against wildfires, which can be responsible for substantial As re-emissions.