Mobility and transformation of mercury in the sediments of Changjiang estuarine wetlands following the soluble ionic mercury inputs： A long-term microcosm study

To investigate the mobility and transformation of mercury (Hg) in the wetlands of Changjiang Estuary, microcosm incubation experiments were conducted under different redox conditions over a long period (252 days). Four sediments collected from different wetlands were added with dissolved Hg(NO₃)₂ to simulate recent Hg inputs to wetlands, resulting in sediment total Hg increased by 109.7%−275.1%. (1) The results showed that the concentrations of methylmercury (MeHg) in sediments increased by 1.9%−361.5% (on average 183.0%) over the course of incubation. Amendment of litterfall after 140 days incubation, anaerobic degradation of litter can significantly enhance MeHg production with a larger increase (on average 260.2%) compared to those in the control. These results suggest that soluble Hg is easily methylated to MeHg, especially with labile organic matter inputs, and the aging processes of Hg could be significantly influenced by labile organic matter. Furthermore, MeHg/THg (%), as an estimate of long-term MeHg production were significantly different among sediments for all sampling time points, which was most probably due to the differences of the Hg methylating bacteria in sediments. (2) During oxidation stage of the sediments, a significant negative correlation between the MeHg concentrations and the redox potential (Eh) was observed. The results indicate that MeHg demethylation occurred under oxic resuspension conditions, which was enhanced in the presence of labile organic matter. This was most probably due to the combination of bi otic demethylation with aerobic microorganisms and abiotic demethylation associated with reactive oxygen species from oxygenation of Fe(Ⅱ)-bearing particles. The role of the abiotic pathways and mechanisms in the degradation of methylmercury in estuaries, coasts and other natural aquatic systems needs to be further investigated. (3) Most of the Hg accumulated in the ＜8 μm fractions, probably due to the formation of Hg-organic matter complexes that was aggregated with metal (oxyhydr)oxides and clay minerals. Thus, very fine particles may be the main carriers of Hg in Changjiang Estuary.