Impact of dissolved O2 on phenol oxidation by δ-MnO2

Although redox reactions of organic contaminants with manganese oxides have been extensively studied, the role of dissolved O-2 in these processes has largely been overlooked. In this study, the oxidative degradation of phenol by delta-MnO2 was investigated under both oxic and anoxic conditions. Dissolved O-2 inhibited phenol degradation due to its promoting role in the reoxidation and precipitation of reduced Mn(ii) to Mn(iii) on the delta-MnO2 surface, resulting in partial transformation of delta-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 delta-MnO2. In addition, dissolved O-2 caused a higher degree of particle agglomeration and a more severe specific surface area decrease, and hence lower reactivity of delta-MnO2. These findings revealed that after reductive dissolution by phenol and reoxidation by dissolved O-2 throughout continuous redox cycling, delta-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.