The potential wildfire effects on mercury remobilization from topsoils and biomass in a smelter-polluted semi-arid area

Abstract

Wildfires can be responsible for significant mercury (Hg) emissions especially in contaminated areas. Here, we investigated the Hg distribution in topsoils and vegetation samples and temperature-dependent Hg mobilization from biomass-rich topsoils collected near a copper (Cu) smelter in Tsumeb (semi-arid Namibia), where Hg-rich Cu concentrates are processed. The thermo-desorption (TD) experiments conducted on representative biomass-rich topsoils (3.9–7.7 mg Hg/kg) indicated that more than 91% of the Hg was released at ∼340 °C, which corresponds to the predominant grassland-fire conditions. The mineralogical investigation indicated that the Hg comes mainly from the deposited smelter emissions because no distinct Hg-rich microparticles corresponding to the windblown dust from the nearby disposal sites of the technological materials (concentrates, slags, tailings) were found. 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 exhibits similarities with the TD pattern of smelter flue dust residue. Despite the installation of a sulfuric acid plant in the smelter in 2015 and a calculated drop in the estimated Hg emissions (from 1301 ± 457 kg/y for the period 2004–2015 to 67 ± 5 kg/y after 2015), the Hg legacy pool in the smelter surroundings can potentially be re-emitted back to the atmosphere by wildfire. Using the Hg spatial distribution data in the area (184 km2), the estimates indicate that up to 303 kg and 1.3 kg can be remobilized from the topsoils and vegetation, respectively.