Analysis of mercury fractions in the fine grains of surface sediments in the Changjiang Estuary wetlands using thermodesorption and chemical extraction methods

Purpose The information about mercury (Hg) fractions obtained by simple thermodesorption screening techniques has not been compared with that from chemical extraction methods. Here, we examined Hg fractions in the surface sediments by a simple thermodesorption method and by a sequential selective extraction method to better understand the biogeochemical reactivity and mobility of Hg in the Changjiang Estuary wetlands. Materials and methods The concentrations of Hg fractions in sediments were analyzed by DMA-80 at different heating temperatures of 175, 225, 325 and 750 °C. Hg compounds in selected sediments were determined by sequential chemical extractions with 0.5 M HCl followed by 1 M KOH. Results and discussion The results showed that the proportions of labile Hg fractions released at 175 ℃ were less than 5% of the total Hg, which could correlate with highly reactive and poorly crystalline iron oxides that have high biogeochemical reactivity in sediments. Furthermore, more than 74% (of the total, on average) of Hg in the sediments was released at temperatures between 175 and 325 ℃, which corresponded well to the amount of organic-chelated Hg in the sediments as determined by extraction with 1 M KOH. These results suggested that Hg bound to humic substances or to organomineral complexes could play a key role in controlling the thermal release of different Hg species from the sediments. Conclusions The thermal decomposition method could serve as a fast and reliable screening method for the identification of different Hg compounds to help assess Hg mobility and bioavailability in aqueous (e.g., estuarine wetlands) sediments.