Mercury in groundwater - Source, transport and remediation

Mercury (Hg) is one of the most toxic global pollutants of continuing concern, posing a severe threat to human health and wildlife. Due to its mobility, Hg is easily transported through the atmosphere and directly deposited onto water, sediments and soils or incorporated in biota. In groundwater, Hg concentrations can be influenced by either geogenic or anthropogenic sources, causing critical health effects such as damage to the respiratory and nervous systems. The geogenic sources of Hg include rocks and minerals containing Hg (cinnabar, organic-rich shales, and sulfide-rich volcanic) and geothermal fluids. The anthropogenic Hg sources include the combustion of fossil fuels, gold mining, chemical discharges from dental preparation, laboratory activities and legacy sites. In groundwater, the average background concentration of Hg is < 0.01 mu g/L. Mercury can be mobilized into groundwater from geogenic or anthropogenic sources due to changes in redox potential (Eh), with concentrations reaching above the WHO drinking water standard of 1 mu g/L. Under reducing conditions, microbial activity facilitates the reductive dissolution of FeOOH, causing the release of sorbed Hg2+ into groundwater. The released Hg2+ may be reduced to Hg-0 by either dissolved organic matter or Fe2+. The stability of Hg species (Hg-0, Hg-2(2+), Hg2+, MeHg) in groundwater is controlled by Eh and pH. While high Eh and low pH conditions can mobilize Hg from the solid into aqueous phases, the soil binding ability can sequestrate the mobilized Hg via adsorption of Hg2+ by goethite, hematite, manganese oxides, hydrous ferric oxides, or organic matter restricting it from leaching into groundwater. During groundwater contamination, remediation using nanomaterials such as pumice-supported nanocomposite zero-valent iron, brass shavings, polyaniline-Fe3O4-silver diethyldithiocarbamate, and CoMoO/gamma-Al2O3 has been documented. These promising emerging technologies utilize the principle of adsorption to remove up to 99.98 % of Hg from highly contaminated groundwater. This study presents an overview of groundwater contamination, remediation, complex biogeochemical processes, and a hydrogeochemical conceptual model concerning Hg's mobility, fate, and transport.