Mercury fractionation and methylation in Yangtze River Estuary wetland sediments: Insight from thermal decomposition analysis and microcosm incubation

Understanding mercury (Hg) compounds and methylating microbes in sediments is crucial for evaluating Hg risk in aquatic environments, and simple techniques need to be developed. Wetland sediments from the Yangtze River Estuary (collected in 2019 and 2024) were examined using 4-step thermal decomposition and sequential extraction techniques to determine Hg fractions, and microcosm experiments were performed to evaluate the contributions of microbial communities to methylmercury (MeHg) production. The results indicated that more than 65 % of the total Hg (THg: 39.9–106.5 ng/g) was released at 175–325 °C, which corresponds well to organic matter-bound Hg extracted by 1 M KOH. The ratios of Hg released at 225 °C to the total organic carbon (TOC) showed spatial patterns and declines similar to those of the THg/TOC ratios, suggesting that thermal decomposition is a potential tool for identifying anthropogenic Hg inputs in estuaries. Additionally, the levels of MeHg was 0.8 ± 0.4 ng/g, with 1.2 % of THg. Net MeHg production correlated more strongly with labile organic matter degradation processes than with Hg speciation, among which sulfate-reducing bacteria could play an important role in driving MeHg formation. These findings allow for a more effective assessment and management of Hg risk in estuarine ecosystems.