UC Merced

Addition of Mn(IV)-oxide phases pyrolusite or birnessite was investigated as remedial amendment for Hg-contaminated sediments. Because inorganic Hg methylation is a byproduct of bacterial iron and sulfate reduction, reaction of Mn(IV) oxides with pore water should poise sediment oxidation potential at a level higher than favorable for Hg methylation. Changes in Mn(IV)-oxide mineralogy and oxidation state over time were investigated in sediment tank mesocosm experiments in which Mn(IV)-oxide amendment was either mixed into Hg-contaminated sediment or applied as thin-layer sand cap on top of sediment. Mesocosms were sampled between 4 and 15 months of operation and solid phases were characterized by X-ray absorption spectroscopy (XAS). For pyrolusite-amended sediments, Mn(IV)-oxide was altered to a mixture of Mn(III)-oxyhydroxide and Mn,Fe(II,III)-oxide phases, with a progressive increase in the Mn(II)-carbonate fraction over time as mesocosm sediments became more reduced. For birnessite-amended sediments, both Mn(III) oxyhydroxide and Mn(II) carbonate were identified at 4 months, indicating a faster rate of Mn reduction compared to pyrolusite. After 15 months of reaction, birnessite was converted completely to Mn(II) carbonate, whereas residual Mn,Fe(II,III)-oxide phases were still present in addition to Mn(II) carbonate in the pyrolusite mesocosm. Mn(IV)-oxides in the thin layer sand cap showed no changes in XAS spectra after 10 months of reaction. Equilibrium phase relationships support the interpretation of mineral redox buffering by mixed-valent (Mn,Fe)(II,III) oxide phases. Results suggest that longevity of the amendment treatment for redox buffering can be controlled by adjustment of the mass and type of Mn(IV)-oxide applied, mineral crystallinity, surface area, and particle size.