Remediation of mercury contaminated soil and biological mercury methylation in the landscape

Accumulation of mercury (Hg) in soil originating from both natural and anthropogenic sources poses a major hazard to environmental and human health. Inorganic Hg(II) in soil can be transformed to highly toxic methylmercury (MeHg) mainly via methylating microorganisms. Although MeHg constitutes less than 2% of total Hg in soil, it enters aquatic systems through runoff and can be subsequently bioaccumulated along the food chain, thereby causing severe harm to humans. Current major remediation techniques to control soil Hg contamination were reviewed. Organic matter, clay/minerals and complexation ligands within soil are principal factors influencing Hg mobility that is crucial for evaluating and optimising remedial techniques. The potential of soil washing to treat soil Hg contamination was evaluated. The studied soil was fractionated from fine to coarse particles to assess the effectiveness of physical separation. Batch leaching and pH-static titration tests were performed using (1) water, (2) EDTA, (3) NaOH, (4) HCl, (5) acidic leachates from biodegradable wastes, and (6) alkaline leachates from fly/bottom ashes, to estimate the efficiency of chemical extraction. Less than 1.5% of the total Hg could be mobilised after combined treatments, implying very tight binding of Hg to soil particles, thereby hampering soil washing as a strategy for the studied soil. Hg(II) methylation in boreal soils and lake sediments can have major consequences for MeHg inputs to downstream aquatic systems. It is therefore important to understand the biogeochemical mechanisms involved in MeHg formation in these landscapes. The microbes involved in Hg(II) methylation in sediments and boreal forests and wetlands were investigated by high-throughput 16S rRNA and hgcA sequencing with molecular barcoding. In all three environments, hgcA sequences were distributed among Proteobacteria, Firmicutes and Euryarchaeota, and Deltaproteobacteria, particularly Geobacteraceae, appeared to play a predominant role. Ruminococcaceae were also abundant Hg(II) methylators in soils from one forest and all the wetlands. The boreal forest survey provided some first insights about the possible link between MeHg formation and non-Hg(II) methylating bacterial communities that likely support the growth and activity of Hg(II) methylating members. Results from wetlands pointed out nutrient status as an important factor shaping Hg(II) methylating communities across the four wetlands, and highlighted a significant role of water content and iron in controlling the distribution of Hg(II) methylators within individual wetlands. Furthermore, the interactions between Hg(II) methylating groups revealed that the more anaerobic and productive conditions seemed to favour the activity of Methanoregulaceae and hamper the growth of Ruminococcaceae. Results from lake sediments supported that Geobacteraceae have an important role in Hg(II) methylation under ferruginous geochemical conditions. Our findings provide a better understanding of Hg(II) methylating communities in the landscape