Immobilization of Mercury Using Iron Sulfide Minerals

Abstract

Mercury is a pervasive pollutant that has caused environmental and health problems throughout the world. Numerous industries including coal-fired power plants and chlor-alkali plants have discharged mercury into the environment. A common remediation technique at contaminated sites has been excavation and incineration of soils, which is costly and can emit harmful mercury vapor. An alternative approach is in situ immobilization of subsurface mercury using iron sulfide minerals. In this study, pyrite was chosen for mercury immobilization studies because it is the most abundant metal sulfide in nature. Iron monosulfide (FeS) was selected because the Fe2+ has been shown to readily exchange with Hg2+ to form HgS(s). Additionally, both of these minerals are known as scavengers of mercury in the environment. Batch experiments were conducted to investigate the kinetic and thermodynamic parameters involved in Hg(II) immobilization. Parameters such as pH, reaction time, and initial Hg(II) concentration were varied to determine optimal conditions. Batch studies revealed that both of these minerals can effectively remove Hg(II) from aqueous solution along a broad pH range. Additionally, the Hg(II) removal rates for both pyrite and FeS(s) increased with increasing pH. FeS(s) was found to be more efficient at removing Hg(II), most likely due to the formation of HgS(s). Column experiments were conducted to provide insight into the environmental behavior of Hg(II) under dynamic (flow) scenarios. Furthermore, models were generated using CXTFIT (version 1.0.001) to aid in the development of long-term barrier systems, such as permeable reactive barriers (PRBs). Column studies revealed that transport of the Hg(II) was significantly retarded in the presence of pyrite, indicating its ability as a potential barrier material. Due to nonequilibrium, the local linear equilibrium (LLE) model over predicted the BTCs; however, the presence of an irreversible fraction of Hg(II) on the pyrite acted to counteract the increased mobility. The asymmetric shape of the BTCs, which is indicative of rate-limited and/or non-linear adsorption, corresponded with the findings of the kinetic and equilibrium batch experiments.