OXIDATION OF SUBSTITUTED ANILINES BY AQUEOUS MNO2 - EFFECT OF CO-SOLUTES ON INITIAL AND QUASI-STEADY-STATE KINETICS

The effect of pH, Mn(ll), 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 i n batch and completely mixed flow-through reactors. The reaction rates were strongly pH-dependent and increased with decreasing pH. Mn(ll), Ca(ll), HA, and other organic solutes inhibited the oxidation of subst ituted anilines. Inhibition by adsorbed Mn(ll) was due to blocking of reactive Mn(IV) surface sites. The inhibitory effect of HA and some ot her organic solutes was in part due to reductive dissolution of MnO2 a nd, thus, due to formation of adsorbed Mn(ll). Initial rates of oxidat ion of a series of substituted anilines were linearly correlated to po larographic half-wave oxidation potentials, E-1/2, indicating that ele ctron-transfer kinetics were relevant for the overall rate of oxidatio n. However, results from flow-through experiments demonstrate that suc h correlations may change or disappear, subject to changing solution c omposition. Quasi-steady-state rates were generally significantly smal ler than initial rates, indicating that the ability of MnO2 to efficie ntly oxidize organic pollutants may be impaired by long-term exposure of the mineral surface to inorganic and organic co-solutes.