Formation of mercuric sulfide in soil

Analysis of mercury-contaminated soil from the flood plain of East Fork Poplar Creek (EFPC) in Oak Ridge, TN, using a scanning electron microscope (SEM) with energy-and wavelength X-ray dispersive spectroscopy (EDS/WDS) and a transmission electron microscope (TEM) with select area electron diffraction (SAED) revealed the presence of submicron, crystalline mercuric sulfide (HgS) in the form of metacinnabar. The HgS formed in place after the deposition and burial of mercury-contaminated soils. A reaction path model developed to describe the geochemical evolution of the soil redox conditions during flooding predicted that the resultant pe and pH of the soil would be within the stability range of HgS. The reaction of mercury with other metal sulfides or sulfhydryl groups in the soil may have also contributed to the formation of HgS. The formation of HSS is significant to the remediation efforts at EFPC because the toxicity, leachability, and volatility of mercury in soils is dependent on the solid phase speciation. Because the local hydrogeochemical conditions are not unique, the formation of HgS at this site has implications to other environments as well.