Transport and transformation of colloidal and particulate mercury in contaminated watershed

Abstract

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 (TEM) were mainly bound to particles with size fractions of <0.05 and >0.45 μm in the upstream. With increasing distance from the mine waste pile in the downstream, nano-colloidal Hg became dominant and remained constant throughout the whole river. The Hg isotope results also revealed that nano-colloidal Hg could migrate steadily for long distances into the downstream. Most importantly, a significantly positive correlation was observed between the proportion of nanocolloidal Hg to water total Hg (THg) and the proportion of methylmercury (MeHg) to water THg, indicating nano-colloidal Hg as an important substrate for Hg methylation in the river. These results highlighted the pivotal role of the nano-colloidal particles (<0.05 μm) as a significant reservoir for Hg in aquatic environment.