Mercury Transformation and Release from Contaminated Soil Following Perturbations in Solution Chemistry and Application of Polysulfide

Abstract

Mercury (Hg) is a contaminant of concern due to the very high toxicity and bioaccumulating nature of organic Hg and the persistent leaching of Hg to water bodies from contaminated soils and sediments. The deleterious properties of Hg pose challenges for remediation as point source contamination can expand over time to affect much wider areas. Saturated, flow-through column experiments were conducted with riverbank sediment and floodplain soil collected from a contaminated reach of the South River near Waynesboro, VA. In one experiment, the composition of input solutions was varied to observe relationships among mobilized Hg, aqueous parameters and effluent constituents and identify dominant mechanisms and controls on Hg transport. Effluent Hg concentrations increased and remained elevated when a higher pH and alkalinity solution was input to the column. Effluent Hg and DOC concentrations were generally positively correlated. Increased effluent Hg concentrations broadly coincided with increased effluent iron (Fe) and manganese (Mn) concentrations and redox (Eh) minima. The lowest effluent Hg concentrations were observed upon decreasing the input solution pH from ~8.7 to ~6, whereas an increase in input pH from ~6 to ~12 coincided with the highest effluent Hg concentrations along with spikes in effluent Fe, Mn and DOC concentrations. Saturated flow-through column experiments with floodplain soil were conducted under both aerobic and anaerobic environments. Greater concentrations of effluent Hg were observed from the column operated in an aerobic environment as opposed to in an anaerobic environment. Two distinct effluent Hg concentration maxima were observed from the aerobic column with increased Hg concentrations observed together with a relatively high Eh (490 mV compared to average Eh of 360 mV) and low Fe and Mn concentrations, whereas the latter and greater Hg maximum broadly coincided with a sharp decrease in Eh (85 mV) and increased effluent Fe and Mn concentrations. The maximum effluent Hg concentration from the anaerobic column also broadly coincided with an increase in effluent Fe and Mn and a minimum Eh but the Hg release was of a much lower magnitude than from the aerobic column. Despite higher total effluent Hg concentrations from the aerobic column, methylmercury (MeHg) concentrations were consistently higher from the anaerobic column. A potassium polysulfide (KPS) solution (1 mM S) was applied to a fully-saturated, flow-through column of floodplain soil for approximately 10 pore volumes (PVs) under anaerobic conditions to assess the potential for polysulfide to stabilize Hg. Effluent Hg concentrations were very high during the application of the KPS solution and remained elevated above the control for the remainder of the experiment after the KPS application ceased; most other parameters were similar in the KPS and control column effluents for the duration of the experiment. An increase in effluent Hg from the KPS column was observed post-KPS application that broadly coincided with a decrease in Eh and increased effluent Fe and Mn. The relationship between increased Hg, Mn and Fe and decreased Eh was also observed in the control column, but the magnitude of Hg release was lower than from the KPS column. XANES sulfur spectra collected from the KPS-treated soil and the control were similar indicating that there was not an apparent change in solid-phase sulfur in the KPS-treated soil compared to the control soil. Dissolution of HgS and formation of highly mobile HgSx2- was likely the dominant mechanism for the Hg release. In situ immobilization of Hg in the floodplain soil was not achieved with the flow-through application of a polysulfide solution; contrary to past studies where immobilization was achieved by in situ formation of HgS via polysulfide application to elemental Hg0 in a glass bead medium.