REDUCTIVE DISSOLUTION OF MANGANESE(III,IV) (HYDR)OXIDES BY EXTRACELLULAR BIOCHEMICALS: STRUCTURE-REACTIVITY INVESTIGATIONS

Manganese (hydr)oxides are among the most reactive oxidants in soils and sediments, and accumulate near redox gradients. Our hypothesis is that reactions with manganese (hydr)oxides serve as important sinks for biochemicals exuded by organisms. Our studies began with the synthesis of the pure Mn(III) phase MnOOH(manganite), the pure Mn(IV) phase MnO₂(pyrolusite), and the mixed phase MnO₂(birnessite). To provide the basis for predicting pathways and rates of oxidation, reactions of biochemicals within and across compound classes have been explored. Malonic acid and malonic acid dimethyl ester are far more reactive towards MnO₂(birnessite) than dimethylmalonic acid, indicating that the presence of hydrogen in the alpha-position relative to carboxylate groups is important. At pH 5.0, malonic acid, acetoacetic acid, and acetylacetone yield reaction rates with MnO₂(birnessite) that are within an order of magnitude of one another. The drop off in reaction rate with increasing pH for acetylacetone is considerably less than observed with malonic acid and acetoacetic acid, and is likely due to its much higher pK_a. Consideration of tautomerization equilibria indicates that the concentration of enol tautomer at fixed pH increases in the order: malonic acid < acetoacetic acid < acetylacetone. Aldehydes generally react more rapidly than beta-diketones and simple ketones. Beta-unsaturated carboxylic acids, such as fumaric acid and maleic acid, are also subject to oxidation. Citric acid is both a reductant and a moderately strong chelating agent. Autocatalysis, driven by the oxidation of Mn(II) complexes by surface-bound Mn(III) and Mn(IV), has been observed with citric acid and a half dozen structurally-related compounds. Mn(III) complexes in solution, however transitory, are potentially quite toxic to organisms, and therefore merit special attention.