Biogenic manganese oxides (BioMnOx) are among the strongest solid-phase oxidants in natural systems and thus are critical players in many geochemical cycles. These nanoparticles consist of Mn(IV)-rich octahedral layers in which vacancy sites occur as point defects and adsorb a range of metal cations. The relationship between structural properties and the redox reactivity of BioMnOx has been difficult to resolve due to the challenge of quantifying electron transfer reactions occurring at water-solid interfaces. In this study, we used 2,2’-Azino-bis(3-ethylbenzothiazoline-6sulfonic acid) (ABTS) as an organic reductant to assess the influence of Mn(III) content on Mn reduction rates and extents in the biogenic MnOx produced by Pseudomonas putida GB-1. We synthesized and characterized a set of BioMnOx precipitated in the absence of any organic buffers or exogenous organic compounds by adjusting the synthesis pH. Experiments were done using BioMnOx containing 0%, 36% and 67% Mn(III,II) as well as a purely-Mn(IV) abiotic analog, δ-MnO2 . Using a flow-through system coupled to a UV-vis spectrophotometer, we monitored ABTS speciation over time and determined the initial rate of ABTS oxidation (&lt; 20 sec). The apparent EH of the system was also calculated based on the speciation of ABTS after longer reaction times (4 to 19 days). We found that biogenic oxides showed higher initial oxidation rates (up to 149.7 μMmin−1 vs. 109.5 μMmin−1 for δ-MnO2 at pH 5.0) but lower reaction extent than δ-MnO2 (85% vs. 100% of total ABTS was oxidized to ABTS+ for δ-MnO2). In addition, our results showed a decrease in reactivity for BioMnOx as Mn(III,II) content increases (from 149.7 μMmin−1 to 70.8 μMmin−1, respectively for BioMnOx 0% and BioMnOx 67% Mn(III,II)). Long-term reaction extent highlighted the ability of BioMnOx to oxidize ABTS+ to ABTS2+ suggesting redox potential values above 1.1 V.

Assessing the reducibility of biogenic manganese oxides using ABTS as a redox probe

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.