BIOTIC/ABIOTIC PATHWAYS OF HG(II) REDUCTION BY DISSIMILATORY IRON REDUCING BACTERIA

Elevated levels of mercury (Hg) have been found in groundwater aquifers in the United States. Whereas the subsurface transport of mercuric Hg [Hg(II)] is retarded by sorption, elemental Hg [Hg(0)] is highly mobile. Redox processes in saturated sediments that reduce Hg(II) to Hg(0) may contribute to mercury mobilization in groundwater. In this project, we conducted laboratory studies to investigate the mechanisms of Hg(II) reduction by dissimilatory iron reducing bacteria (DIRB). The experimental data indicated that DIRB grown on poorly crystalline ferrihydrite can rapidly reduce Hg(II) to Hg(0). The formation of secondary iron minerals was observed during bacterial growth suggesting the possibility of a biotic/abiotic pathway for Hg(II) reduction by biogenic Fe(II) minerals. Abiotic Hg(II) reduction experiments performed with Fe(II)-containing minerals demonstrated that magnetite [Fe₃O₄] can reduce Hg(II), but siderite [FeCO₃], vivianite [Fe₃(PO₄)₂ · 8H₂O], and Fe(II) adsorbed onto goethite[α-FeOOH] are poorly reactive towards mercuric ions. Hg(II) reduction by magnetite occurs within minutes, with reaction rates varying as a function of magnetite surface area, pH, and chloride concentration. Spectroscopic analysis of mercury-reacted magnetite samples revealed a decrease in Fe(II) content, corresponding to the oxidation of Fe(II) to Fe(III) in the magnetite structure. These results suggest that Hg(II) reaction with biogenic magnetite is a kinetically favorable pathway for Hg(II) reduction by DIRB.