Mobilization, Methylation, and Demethylation of Mercury in a Paddy Soil Under Systematic Redox Changes
Jianxu Wang, Sabry M. Shaheen, Min Jing, Christopher W. N. Anderson, Ann‐Christin Swertz, Shan‐Li Wang, Xinbin Feng, Jörg Rinklebe
Abstract
Methylmercury (MeHg) contamination in paddy fields is a significant environmental issue globally since over half of the population of our planet consumes rice. MeHg is a neurotoxin produced by microorganisms in oxygen-limited environments. Microbial effect on MeHg production is a hotspot of research; however, it has been largely ignored how the oxidation–reduction potential (Eh) shapes MeHg formation. Here, we elucidated Hg (de)-methylation in a contaminated soil by increasing Eh stepwise from −300 to +300 mV using a sophisticated biogeochemical microcosm. At the Eh range from −300 to −100 mV, high MeHg concentration and dissolved total Hg (THg) concentration were found due to a high relative abundance of Hg-methylation bacteria (e.g., Desulfitobacterium spp.), acidification, and reductive dissolution of Fe(oxyhydr)oxides. At the Eh range from 0 to +200 mV, the formation of colloids leads to adsorption of Hg and as a result colloidal Hg increased. MeHg reduction with Eh (−300 to +200 mV) increase was mainly attributed to a reduced Hg methylation, as dissolved THg and relative abundance of Desulfitobacterium spp. decreased by 50 and 96%, respectively, at Eh of +200 mV as compared to Eh of −300 mV. Mercury demethylation might be less important since the relative abundance of demethylation bacteria (Clostridium spp.) also decreased over 93% at Eh of +200 mV. These new results are crucial for predicting Hg risks in paddy fields.