Macromolecular Characterization of Compound Selectivity for Oxidation and Oxidative Alterations of Dissolved Organic Matter by Manganese Oxide

Abstract

Manganese (Mn) oxides can oxidize dissolved organic matter (DOM) and alter its chemical properties and microbial degradability, but the compound selectivity for oxidation and oxidative alterations remain to be determined. We applied ultrahigh mass spectrometry to catalog the macromolecular composition of Suwannee River fulvic acid (SRFA) before and after oxidation by a Mn oxide (δ-MnO 2 ) at pH 4 or 6. Polycyclic aromatic hydrocarbons, polyphenols, and carbohydrates were more reactive in reducing δ-MnO 2 than highly unsaturated and phenolic (HuPh) compounds and aliphatics, but highly abundant HuPh contributed the most (∼50%) to the overall reduction of δ-MnO 2 . On average, oxidized species had higher molecular weights, aromaticity, carbon unsaturation degree, 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, with stronger changes at the lower pH. This DOM oxidation on polar mineral surfaces was more selective but shared similar selectivity rules to adsorption. The abiotic oxidation resembles microbial oxidative degradation of organic matter, and Mn oxide−oxidizable carbon may be a useful index for detection and identification of labile organic carbon.

Selective Reactivity and Oxidation of Dissolved Organic Matter by Manganese Oxides

“…Furthermore, the formulas that increase in relative intensity after reaction span a greater range of the van Krevelen diagram than the identified oxidation products, which are typically confined to the lignin-like region of the plots and are consistent with selective phenol-like reactions within the pool of DOM molecules (Figures and S11). The scattered product formulas support the observed differences in aromaticity and molecular weight measures using both bulk measurements and FT-ICR MS analysis (Table S6) as well as evidence of non-oxidative transformations occurring in the DOM pool and are supported by observations made after reacting high concentrations (∼300 mg-C L -1 ) of Suwannee River fulvic acid with δ-MnO 2 at pH 4 . Although there are differences in bulk DOM compositional changes and a wide range of products are generated by the reaction of DOM and acid birnessite, these results demonstrate that MnO 2 reacts selectively with electron-rich, lignin-like formulas even in waters containing highly diverse DOM.…”

“…Thus, DOM remaining in solution is slightly reduced relative to the initial DOM, contradicting FT-IR observations of reacted DOM, ,, as well as measures of Mn reduction in this data set and previous literature studies. ,,, However, weighted values encompass all transformations to the DOM pool including non-oxidative reactions and fractionation due to selective sorption and colloidal organic matter–Mn­(II/III) formation. Studies using a fulvic acid isolate or DOM derived from biochar have similarly observed that O:C w can either increase or decrease after reaction with Mn oxides. , While decreases in O:C w indicate reduction of C in the bulk DOM pool, this result does not negate other measures of aromaticity and molecular weight which suggest that oxidation occurs. Rather, more specific evidence supporting the formation of oxidized product formulas is necessary.…”

“…Studies of DOM isolates, model compounds, and single DOM samples reacted with Mn oxides provide contradictory results. For instance, some studies report little sorption to the Mn oxide surface, , while others report extensive sorption and fractionation, ,,,,, with varying transformation products , and extent of reductive Mn oxide dissolution. ,,,,− Based on these studies, DOM composition is expected to influence the extent and manner of bulk DOM changes among the 30 waters. Specifically, we hypothesize that aromatic DOM from bogs and rivers will have greater changes in [DOC], SUVA 254 , E 2 :E 3 , and Mn chemistry than less aromatic, microbially processed DOM from lakes because aromatic moieties (e.g., phenols) have greater sorption capacities and can be redox-active.…”

“…Despite the possibility for DOM fractionation and multiple reaction mechanisms, there are a lack of studies comparing transformations across different DOM sources to determine how composition influences reactivity with Mn oxides. Most research on this system focuses either on the changes in Mn structure and DOM sorption ,, or on comparing Mn oxides reacted with DOM isolates and/or model compounds. ,, Studies that include whole water DOM commonly seek to understand interactions with dissolved trace metals (e.g., Fe, Al). − Due to the nature of these studies, changes in organic carbon are typically attributed to oxidation based on evidence of Mn reduction or small organic acid product formation. ,, However, these studies do not provide conclusive proof that DOM oxidation occurs; therefore, it is challenging to apply their results to more complex systems or different DOM sources.…”

Redox-induced transformation of potentially toxic elements with organic carbon in soil

“…Manganese (Mn) minerals are ubiquitous oxidants in the soil systems, and the redox reactions between organic carbon and Mn oxide occur extensively (Fig. 3) (Ma et al 2020;Zhang et al 2021). Manganese could be the dominant redox moiety in soil, especially in the surface layer, even when compared with iron minerals (Jones et al 2020).…”