Characterization of Manganese Oxide Modified Activated Carbon for Remediation of Redox-Sensitive Contaminants

Manganese oxide modified activated carbon (MOMAC) wassynthesizedas a novel in situ sediment and soil amendment fortreatment of redox-sensitive contaminants, such as mercury (Hg), throughbuffering of reduction-oxidation (redox) potential and sorption.This study characterized MOMAC synthesis products at three differentMn concentrations on activated carbon (AC) surfaces and compared themwith homogeneously precipitated Mn oxide (MnOx) and unmodified ACfor properties influencing redox buffering and sorption capacity.Bulk spectroscopic analyses (XAS and XPS), XRD, and electron microscopyshowed that homogeneous MnOx matched the local structure of vernadite(delta-Mn(IV)O-2), while MOMAC formed aggregates on theAC surface composed mostly of vernadite with fractions of manganite(gamma-Mn(III)OOH) (17-46%) and sorbed Mn(II) (11-21%).Higher bulk surface area and lower Mn average oxidation state wereassociated with MOMAC and are attributed to the reduction of Mn(IV)by Mn(II) adsorbed on AC or diffused into AC pores. Cation exchangereactions of Na+ and Ca2+ also contributed toMn oxidation state changes by driving disproportionation of Mn(III)to Mn(II) and Mn(IV). In batch slurry experiments with and withoutHg-contaminated sediment from Oak Ridge National Laboratory (TN, U.S.A.),addition of MOMAC and MnOx resulted in higher solution redox potentialand lower pH compared to AC and no-amendment controls. MnOx poisedsolution redox at higher potential than MOMAC, but MOMAC was moreeffective at sorbing Hg released by the oxidation of sediment HgS(s),Hg-0, and/or organic-associated Hg. By combining redox bufferingwith sorption, MOMAC is a promising in situ amendmentthat may efficiently target redox-sensitive contaminants in aquaticsediments.