Session: Reactivity & Transformation of Manganese Oxides in Natural and Engineering Systems:

Production of neurotoxic methylmercury (MeHg) occurs primarily under reducing conditions through microbial iron- and sulfate- reduction pathways. Redox buffers, such as solid manganese(IV) oxide (MnOx), can promote oxidizing conditions to thermodynamically disfavor mercury methylation. The ability of MeHg to bioaccumulate in higher organisms creates an environmental hazard at low (ng/g) concentrations in sediment. Solid Mn(IV)-oxide and commercial activated carbon (AC) were investigated as cost-effective, in-situ amendments for reducing concentrations of dissolved inorganic mercury (Hg) and MeHg in contaminated aquatic sediments. Mercury-contaminated sediment slurries were amended with MnOx, AC, or a combination thereof, and incubated for short (0-5 days) and long (0-20 days) time periods. Sulfate reduction and MeHg production were stimulated with dissolved sulfate and organic carbon additions in short-term experiments to further assess the ability of MnOx to inhibit MeHg production. Incubations amended with MnOx showed high oxidation-reduction potential (ORP) with a corresponding decrease in the ratio of porewater MeHg to Hg. However, an increase in total dissolved Hg suggested oxidative dissolution of Hg-sulfide minerals. Manganese K-edge X-ray Absorption Spectroscopy (XAS) linear combination fit analysis of amended sediments with reference Mn-compound spectra revealed rapid reductive dissolution of Mn(IV)-oxide to aqueous or adsorbed Mn(II). Spectroscopic data indicated no formation of Mn(III) intermediate phases during dissolution of the amendment that could provide additional redox buffering. Improvements in amendment longevity is a crucial next step to increase the effectiveness of the treatment. Surface modification of AC with MnOx, to create manganese oxide modified activated carbon (MOMAC), may improve longevity by slowing MnOx dissolution. Amended sediment slurry incubations suggest that MnOx is a promising treatment for Hg-remediation in aquatic systems.

Peggy O'Day

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