Impact of dissolved O2 on phenol oxidation by δ-MnO2

Abstract

Although redox reactions of organic contaminants with manganese oxides have been extensively studied, the role of dissolved O₂ in these processes has largely been overlooked. In this study, the oxidative degradation of phenol by δ-MnO₂ was investigated under both oxic and anoxic conditions. Dissolved O₂ inhibited phenol degradation due to its promoting role in the reoxidation and precipitation of reduced Mn(II) to Mn(III) on the δ-MnO₂ surface, resulting in partial transformation of δ-MnO₂ to “c-disordered” H⁺-birnessite at pH 5.5 and feitknechtite, manganite, and hausmannite at pH 7.0 and 8.5. The reformed Mn(III) phases could reduce phenol oxidation by blocking reactive sites of δ-MnO₂. In addition, dissolved O₂ caused a higher degree of particle agglomeration and a more severe specific surface area decrease, and hence lower reactivity of δ-MnO₂. These findings revealed that after reductive dissolution by phenol and reoxidation by dissolved O₂ throughout continuous redox cycling, δ-MnO₂ became less reactive rather than being regenerated. These results can provide new insights into the understanding of the oxidation of organic contaminants by manganese oxides in the natural environment.