Interactions between biogenic selenium nanoparticles and goethite colloids and consequence for remediation of elemental mercury contaminated groundwater

Ubiquitous colloidal minerals such as goethite can have a significant impact on the performance of nanoparticles-based groundwater remediation due to aggregation. Heteroaggregation and retention of Se nanoparticles (SeNPs) by goethite in groundwater, and its impact on Hg(0) remediation by SeNPs were investigated in this study. In order to mitigate the adverse effects of aggregation, the effects of bacterial extracellular polymeric substances (EPS) on the stability of SeNPs and Hg(0) sequestration using SeNPs were also evaluated. Heteroaggregation of SeNPs with goethite in groundwater was stronger than homoaggregation of SeNPs or goethite. Addition of EPS could slightly decrease homoaggregation of SeNPs and significantly reduce heteroaggregation. Column transport experiments showed that goethite coated quartz sand could retain 1.36 times a higher amount of SeNPs than uncoated quartz sand. Hg(0) remediation by SeNPs was significantly inhibited by heteroaggregation of SeNPs with goethite and EPS could effectively mitigate this inhibitory effect. The Hg(0) removal efficiency decreased to 71.6% and 66.9%, respectively in the presence of 20 and 100mgL(-1) goethite. When 200mgL(-1) EPS was added together with 100mgL(-1) goethite, 81.2% of the supplied Hg(0) was removed from the groundwater. This study demonstrates that the widespread presence of goethite could significantly reduce the remediation efficiency of Hg(0) contaminated groundwater and that EPS is a promising amendment for mitigating the adverse effects of heteroaggregation. This research also contributes to a further understanding of the environmental behaviour of nanoparticles.