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Chapter 2
Metal Contaminant Oxidation Mediated by Manganese Redox Cycling in Subsurface Environment Zimeng Wang1,* and Daniel E. Giammar2 1Department
of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, United States 2Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States *E-mail:
[email protected] Mn oxides can oxidize a variety of metal contaminants in subsurface environments. Fundamental knowledge of the rates, mechanisms, and products of these oxidation reactions is critical to predicting the environmental fate and transport of these metals. This chapter reviews recent advances in characterizing the kinetics and molecular-scale mechanisms of metal oxidation by oxidized forms of Mn. Topics addressed include the dynamics of Mn oxide surface passivation, the mechanisms of solid-solid interactions, the potential importance of soluble Mn(III) species, and the catalytic role of Mn in some linked biotic-abiotic processes that result in metal oxidation. This review highlights the complexity of the coupling of the biogeochemical cycles of Mn and other elements. The Mn species responsible for contaminant metal oxidation goes beyond Mn oxides to also include reactive intermediate species. The chapter identifies future research that is needed to unravel the underlying mechanisms on a process level. Continuing research on metal oxidation by Mn oxides will support improved predictions of metal transport in the environment and aid in decision-making for treatment and remediation.
© 2015 American Chemical Society In Advances in the Environmental Biogeochemistry of Manganese Oxides; Sparks, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
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Introduction Soils have an average manganese composition of 440 mg/kg with a range of 7-9000 mg/kg (1). Dissolved manganese concentrations in anoxic groundwater at circumneutral pH are typically in the range of 1–20 μM (0.05-1.0 mg/L). Manganese oxide minerals are ubiquitous in natural environments, and there are more than 30 known Mn(III), Mn(IV), or mixed Mn(III,IV) oxide/hydroxide minerals with various layer or tunnel structures (2). Hexagonal phyllomanganates (birnessite-group minerals, MnO2-x, where x depends on the Mn(III) and Mn(II) fraction) are the dominant Mn oxides in natural aquatic systems (3). Mn oxides occur in subsurface environments with redox gradients where soluble Mn(II) encounters dissolved oxygen. The occurrence of Mn oxides in subsurface environments is largely driven by Mn-oxidizing microorganisms (23). Phylogenetically diverse microorganisms (bacteria and fungi) that are widespread in nature can oxidize Mn(II) to Mn(III/IV) oxides. Through reactive oxygen species or enzymatic reactions, these microorganisms accelerate Mn(II) oxidation at rates that are orders of magnitude higher than by homogenous oxidation in aqueous solution or by abiotic catalysis on mineral surfaces (23, 24). Even extremely low dissolved oxygen (
