Oxidation of Substituted Anilines by Aqueous MnO2:  Effect of Co-Solutes on Initial and Quasi-Steady-State Kinetics

The effect of pH, Mn(II), and humic acid (HA) concentration as well as aromatic ring substituents on initial and quasi-steady-state rates of oxidation of monosubstituted anilines by MnO2 has been investigated in batch and completely mixed flow-through reactors. The reaction rates were strongly pH-dependent and increased with decreasing pH. Mn(II), Ca(II), HA, and other organic solutes inhibited the oxidation of substituted anilines. Inhibition by adsorbed Mn(II) was due to blocking of reactive Mn(IV) surface sites. The inhibitory effect of HA and some other organic solutes was in part due to reductive dissolution of MnO2 and, thus, due to formation of adsorbed Mn(II). Initial rates of oxidation of a series of substituted anilines were linearly correlated to polarographic half-wave oxidation potentials, E1/2, indicating that electron-transfer kinetics were relevant for the overall rate of oxidation. However, results from flow-through experiments demonstrate that such correlations may change or disappear, subject to changing solution composition. Quasi-steady-state rates were generally significantly smaller than initial rates, indicating that the ability of MnO2 to efficiently oxidize organic pollutants may be impaired by long-term exposure of the mineral surface to inorganic and organic co-solutes.