Oxidation-reduction reactions govern the transformation and mobilization of many heavy metal, metalloid, and organic contaminants in environmental systems. Manganese oxides (MnOx), ubiquitous in environmental systems, are among the strongest naturally occurring oxidants and greatly influence elemental biogeochemical cycles through microbial redox poising. Solid Mn(IV)Ox can be exploited as a soil remediation technology by creating a redox buffer for contaminants such as mercury (Hg), which can be converted into neurotoxic methylmercury (MeHg) under reducing conditions by microbial methylation. To enhance MnOx redox buffering, this study attempts to increase the surface area available for electron transfer by dispersing MnOx onto a high surface activated carbon and synthesizing a manganese (IV) oxide modified activated carbon (MOMAC). However, both natural and synthetic MnOx solids contain small fractions of Mn(II) or Mn(III) which can also affect oxidizing capacity and reactivity. Therefore, elucidating the average oxidation state (AOS) of MOMAC can provide valuable information about the efficiency of MOMAC synthesis products as a redox buffer amendment for sediments or soils. Determination of AOS by iodometric titration was compared with results from X-ray photoelectron spectroscopy of Mn2p spectra. Spectra were analyzed to determine Mn oxidation states by fitting the Mn2p<sub>3/2</sub> peak from unknown samples with multiplet structure peaks derived from peak fits of Mn(II, III, or IV) reference materials. The distribution of Mn oxidation states in MnOx on activated carbon was investigated with microfocus X-ray absorption spectroscopy (XAS) and imaging. The AOS determined for MOMAC suggests a presence of mixed Mn(II, III, and IV) oxidation states. These products were tested in parallel flow-through column experiments where Hg-contaminated creek sediments were amended with MOMAC to evaluate the ability to reduce net mercury methylation. Bulk XAS measurements of sediment extracted from the column will be used to interrogate changes in Mn oxidation state and evaluate the transformation of MOMAC amendments.

Manganese(IV) oxide modified activated carbon amendments for mercury remediation in contaminated sediments

ACS Spring 2022

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