Sorption\nand Oxidative Degradation of Small Organic\nMolecules on Mn-OxidesEffects of pH and Mineral Structures

Manganese (Mn)-oxides regulate carbon (C) cycling in\nsoils by sorbing\nand oxidizing organic compounds. The composition of soil organic matter\nvaries widely, and little is known about the reactivity of individual\norganic compounds with structurally diverse Mn-oxides under various\nenvironmentally relevant pH conditions. Here, we examined the affinity\nof six organic compounds for three Mn-oxides, comprised of layer (birnessite\nand hydrous Mn oxide HMO) or tunnel (cryptomelane) structures, at\nacidic (pH 4), slightly acidic (pH 6), and slightly alkaline (pH 8)\nconditions. Cryptomelane, with a higher specific surface area and\npoint of zero charge, showed higher reactivity than that of HMO and\nbirnessite. Interestingly, these Mn-oxides, although different in\nstructures, decomposed each organic compound to form the same products.\nCitrate, pyruvate, ascorbate, and catechol induced reduction and dissolution\nof Mn-oxides. After the reaction, the average oxidation state of Mn\nin the solids was much lower at pH 4 than at pH 6 and 8, suggesting\nmore reduction under more acidic conditions. Even when reacting with\nphthalate and propanol, which only sorbed to Mn-oxides but did not\ndegrade, there was proton-promoted Mn dissolution under acidic conditions.\nThese results suggest the significance of environmental pH and mineral\nstructures in affecting the Mn–organic interactions and provide\nfundamental insights into a better understanding of the roles of Mn-oxides\nin regulating soil C cycling.