Aquatic sediments are important sources of toxic methylmercury (MeHg) to the environment because they support anaerobic bacteria that methylate inorganic mercury (Hg). Common Hg remediation strategies such as sediment removal or capping can be prohibitively expensive or impractical when compared to in situ treatment methods designed to disfavor Hg methylation and increase sorption of MeHg to solids. Using profundal sediments from a Hgcontaminated reservoir, we assessed the effectiveness of Mn(IV) oxide (MnOx) amendments in decreasing porewater and sediment Hg and MeHg concentrations relative to sediments treated with granular activated carbon (AC) and unamended controls. We conducted two sediment slurry incubation experiments (0−20 days and 0−5 days) using sediments amended with MnOx, AC, or a mixture of the two. X-ray absorption spectroscopy data showed that the MnOx amendments were rapidly reduced to Mn 2+ over the course of the incubation. In both experiments, MnOx addition resulted in elevated oxidation−reduction potential and lower MeHg concentrations in porewater relative to unamended controls after 2−3 days. The MnOx amendments decreased porewater MeHg with similar or greater effectiveness as AC. However, sediments amended with MnOx released inorganic Hg into porewater in some experiments. Coamendment of MnOx with sorbents could curtail diffusion of MeHg from aquatic sediments and slow MnOx reduction while limiting the release of potentially problematic byproducts such as Mn 2+ and Hg(II).

Manganese oxide modified activated carbon (MOMAC) was synthesized as a novel in situ sediment and soil amendment for treatment of redox-sensitive contaminants, such as mercury (Hg), through buffering of reduction−oxidation (redox) potential and sorption. This study characterized MOMAC synthesis products at three different Mn concentrations on activated carbon (AC) surfaces and compared them with homogeneously precipitated Mn oxide (MnOx) and unmodified AC for properties influencing redox buffering and sorption capacity. Bulk spectroscopic analyses (XAS and XPS), XRD, and electron microscopy showed that homogeneous MnOx matched the local structure of vernadite (δ-Mn(IV)O 2 ), while MOMAC formed aggregates on the AC surface composed mostly of vernadite with fractions of manganite (γ-Mn(III)OOH) (17−46%) and sorbed Mn(II) (11−21%). Higher bulk surface area and lower Mn average oxidation state were associated with MOMAC and are attributed to the reduction of Mn(IV) by Mn(II) adsorbed on AC or diffused into AC pores. Cation exchange reactions of Na + and Ca 2+ also contributed to Mn oxidation state changes by driving disproportionation of Mn(III) to Mn(II) and Mn(IV). In batch slurry experiments with and without Hg-contaminated sediment from Oak Ridge National Laboratory (TN, U.S.A.), addition of MOMAC and MnOx resulted in higher solution redox potential and lower pH compared to AC and no-amendment controls. MnOx poised solution redox at higher potential than MOMAC, but MOMAC was more effective at sorbing Hg released by the oxidation of sediment HgS(s), Hg 0 , and/or organic-associated Hg. By combining redox buffering with sorption, MOMAC is a promising in situ amendment that may efficiently target redox-sensitive contaminants in aquatic sediments.

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