Manganese (Mn) redox cycling is coupled to the decomposition of natural organic matter in the environment. On one hand, the reduction of Mn(III, IV) by organic matter (OM) produces Mn(II) and changes the chemical composition and phases of Mn (III, IV) oxides. On the other hand, OM is oxidized by Mn(III,IV) to produce chemically modified macromolecules and new condensed compounds in addition to low-molecular-weight (LMW) organic molecules. The coupled Mn-OM processes not only alter the chemical reactivity of Mn oxides but also accelerate OM decomposition and carbon cycling. In this presentation, I will overview recent studies on the formation and transformation of Mn(III) and Mn(IV) oxides and the accompanied oxidative alterations of dissolved OM (DOM). These studies showed that Mn(IV) oxides underwent partial reduction DOM, which enriched the structure with Mn(III) and Mn(II), and even transformed it to Mn(III) oxyhydroxides (e.g., MnOOH) and Mn(II, III) oxide (e.g., Mn3O4). The above changes depended on DOM to Mn(IV) oxide ratios, solution pH, and the types of co-existing metal cations. At the same time, significant alterations occurred to the chemical composition of DOM. The oxidation produced formic and acetic acids and pyruvate, and likely other unidentified LMW compounds as well. At the macromolecular level, Mn(IV) oxide selectively oxidized certain organic molecules in DOM. For Suwannee River fulvic acid, polycyclic aromatic hydrocarbons, polyphenols, and carbohydrates were more reactive than highly unsaturated and phenolic compounds and aliphatics. On average, oxidized species had higher molecular weights, aromaticity, carbon unsaturation degree, the nominal oxidation state of carbon, and oxygen and nitrogen contents but were lower in hydrogen content compared to unoxidized species. The oxidation decreased these molecular indices and oxygen and nitrogen contents but increased the hydrogen content. This DOM oxidation on polar mineral surfaces was more selective but shared similar selectivity rules to adsorption. The above-observed alterations of DOM seemed to depend on the source of DOM. Moreover, iron (III) oxides can act as a catalyst, mediating oxidation of Mn(II) by O2 to form MnOOH that in turn oxidized DOM and got reduced to Mn(II). Overall, the abiotic oxidation of OM by Mn oxides resembles microbial oxidative degradation of OM in many aspects.

Coupled formation and transformation of Manganese Oxides to decomposition of dissolved organic matter

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