Wildfire-driven release of metal(loid)s from topsoils in a smelter-polluted semi-arid area: an experimental approach

Wildfires contribute to global emissions of trace elements. This study focuses on highly polluted areas near an operating copper smelter and old mine-tailing disposal sites in Tsumeb (semi-arid north of Namibia), where wildfires frequently occur. Capturing of particulates windblown from the ore processing and smelting areas by vegetation (trees, grass) leads to the topsoil enrichment with metal(loid) contaminants (up to 7090 mg/kg Cu, 2070 mg/kg As, 4820 mg/kg Pb, 3480 mg/kg Zn, 75 mg/kg Cd, 7.66 mg/kg Hg). Experimental samples corresponding to representative biomass-rich topsoils (bushland with acacia and marula trees, grassland) were investigated using a combination of mineralogical and geochemical methods. Wildfires were simulated using a thermodesorption (TD) technique (75-670 °C; Hg) and an experimental setup composed of a temperature-controlled furnace (250-850 °C), an aerosol-filtering unit and a gas-trapping device (As, Cd, Cu, Pb, Zn). The obtained ashes were investigated to depict any mineralogical and chemical transformations in order to understand temperature-dependent release of metal(loid) contaminants during the simulated wildfire.

Thermodesorption experiments indicated that more than 90% of Hg was released at ~340 °C, which corresponded to predominant grassland-fire conditions. A comparison with the TD curves of the Hg reference compounds confirmed that the Hg in the biomass-rich topsoils occurs as a mixture of Hg bound to the organic matter and metacinnabar (black HgS), which exhibited similarities with the TD pattern of smelter flue dust residue. Temperature-dependent release of other metal(loid)s (As, Cd, Cu, Pb, Zn) is dependent on their solid-state speciation. Cadmium is released at ~750 °C, corresponding to the thermal decomposition of carbonates, in which Cd is mainly bound. Arsenic exhibits first remobilization step at <350 °C (decomposition of arsenolite, As₂O₃) and the second step at >650 °C corresponding to the instability of arsenates and As-rich slag glass. Other contaminants (Cu, Pb, Zn) were mainly bound in carbonates, slag particles and sulfides/sulfosalts. During the simulated wildfire, they were mainly retained in the ash and were remobilized to a lesser degree at >650 °C. Calculations indicated that at 850 °C (worse-case wildfire scenario) 2-17 % of total As, Cu, Pb and Zn, 27-79 % of total Cd and 100 % of Hg can be volatilized from these biomass-rich contaminated topsoils.

This study was supported by the Czech Science Foundation (GACR project no. 19-18513S) and a student grant from the Grant Agency of Charles University (GAUK no. 1598218).