Transport and transformation of colloidal and particulate mercury in contaminated watershed

Submicron colloids ubiquitously present in aquatic environments and can facilitate long transport of absorbed contaminants. Impact of particle size distribution on mercury (Hg) mobility and transformation in the complex aqueous matrices is still unclear. In this study, we considered Hg mine wastes as a natural Hg releasing source to local rivers, and collected water samples from the source to the downstream during high and low flow periods. The water samples were analyzed for Hg morphology, concentration, speciation, and isotope to understand transport and transformation dynamics along the river flows. We found that visible Hg compounds observed by transmission electron microscopy were mainly bound to particles with size fractions of <0.05 and >0.45 μm in the upstream, while the proportion of Hg bound to particles with 0.05-0.45 μm only accounted for 20.0 ± 17.1 % of the total Hg (THg). With increasing distance from the mine waste pile in the downstream, the <50 nm size fraction Hg became the dominant from due to settling of large particles and remained constant throughout the whole river. The Hg isotope results also revealed that the <50 nm size fraction Hg could migrate steadily for long distances into the downstream. Most importantly, a significantly positive correlation was observed between the proportion of the <50 nm size fraction Hg to water THg and the proportion of methylmercury (MeHg) to water THg, indicating the <50 nm size fraction Hg as an important substrate for Hg methylation in the river. These results highlighted the pivotal role of the the <50 nm size particles as a significant reservoir for Hg in aquatic environment.