Manganese oxides are considered to be one of the strongest naturally-occurring oxidants in near surface environments. These minerals can oxidize several classes of inorganic and organic contaminants, including those with phenolic functional groups. As a result, manganese oxides have been proposed for the passive, in-situ treatment of these pollutants in stormwater, landfill leachate, and other contaminated surface waters. This study evaluates the ability of synthetic δ-MnO<sub>2</sub>, a poorly crystalline Mn(IV) oxide similar to biogenic birnessite, to oxidize a series of 29 phenolic compounds by both sorption-limited and electron transfer-limited reaction pathways. These phenols range from simple mono- and bi-substituted model phenols to complex environmental contaminants including antimicrobial agents (4-cresol, triclosan), endocrine disruptors (bisphenol A, estrone, 17β-estradiol), and surfactants (4-n-nonylphenol, 4-tert-octylphenol). Quantitative structure-activity relationships (QSARs) between the target contaminant oxidation rates and their acid-dissociation constants, highest occupied molecular orbital energy, and oxidation energy provide insight into the rate-limiting steps and mechanisms of this reaction. This study shows QSARs are beneficial for comparing reactivity across literature studies and varying reaction conditions, as well as for predicting the relative rate constants for simple meta- and para-substituted phenolic compounds, however ortho-substituted phenols and more complex contaminants introduce predictive uncertainty. In addition, the reactivity of δ-MnO<sub>2</sub> is compared to manganese-rich reclaimed minerals collected from drinking water treatment filtration and passive acid mine drainage remediation sites. Although the reclaimed solids are 1 – 3 orders of magnitude less reactive, they are still capable of oxidizing the target phenolic compounds and result in similar quantitative structure activity relationships. The results of this study suggest that these waste minerals could be applied to treat contaminated water (e.g., stormwater) under environmentally-relevant conditions.

Kinetics and mechanisms of phenolic contaminant oxidation by environmentally-relevant manganese oxides

2019 Spring National Meeting

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