Manganese (III, IV) oxides can oxidize natural organic matter (NOM) and thereby accelerate its decomposition in ecosystems. However, the types of oxidizable NOM compounds and their oxidation products have not been identified. We characterized the macromolecular composition of fulvic acid (FA) before and after oxidation by d-MnO<sub>2</sub> for 21 day at pH 4 or 6 using 21-Tesla ESI-FT-ICR mass spectrometry. Among 12,968 initial compounds, 4,962 were oxidized at pH 4, producing 4,308 new ones that were absent in the initial FA, while only 3,471 compounds were oxidized at pH 6, producing 2,843 new ones. Regardless of pH, the oxidized compounds were mainly polycyclic aromatics (22–28%), aromatics (17–20%) and lignin/phenolic formulas (49–64%). They were relatively rich in O and reducing C as well as N, S and P, compared to the non-oxidizable ones. The compounds oxidized at pH 6 were more reducing, larger in molecular size, but relatively poor in O than those at pH 4. The oxidation broke larger molecules into smaller ones and increased the C oxidation state. The newly produced compounds were dominated by lignin/phenolic (58–64%) and aliphatic compounds (14–21%). This study indicates that all major groups of organic compounds in NOM are oxidizable, albeit a portion of each. The oxidation may change the degradability of NOM by microorganisms and its adsorption affinity on mineral surfaces.

Oxidative decomposition of natural organic matter by manganese oxide

ACS Fall 2020 Virtual Meeting

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