Sorption and Oxidative Degradation of Small Organic Molecules on Mn-OxidesEffects of pH and Mineral Structures

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