Enhancement and Inhibition of Oxidation in Phenolic Compound Mixtures with Manganese Oxides

Abstract

Phenolic compounds can be oxidized by manganese oxides in the subsurface or during passive treatment of stormwater runoff and wastewater effluent. Contaminated waters may contain a complex mixture of phenolic compounds, and these mixtures may have additive, synergistic, or antagonistic toxicological effects. However, the effects of such mixtures on the rates and mechanisms of phenol oxidation by manganese oxides are not well understood. This study addresses this knowledge gap by determining pseudo-first-order oxidation rates and oxidation mechanisms for four phenols reacted with δ-MnO2 in varying mixtures. Oxidation rates of electron transfer-limited phenols (i.e., triclosan, resorcinol, bisphenol A) are inhibited in mixtures. This inhibition, as well as the observed decrease in sorption, is consistent with competition between phenols occurring near the Mn surface. In contrast, the oxidation rate and extent of sorption for sorption-limited 4,4′-biphenol are enhanced in mixtures. The rate enhancement is partially driven by indirect oxidation through radical-mediated reactions, likely from phenoxy radicals produced when electron transfer-limited phenols react with δ-MnO2. These findings demonstrate that mixtures have a large impact on phenol oxidation by manganese oxides, as the electron transfer-limited phenols are inhibited due to competition interactions and the sorption-limited phenol has increased oxidation rates due to indirect oxidation by radicals and enhanced sorption.