Reinvestigation of the Role of Humic Acid in the Oxidation of Phenols by Permanganate

Humic acid (HA) affects the oxidation of phenolic compounds by permanganate, but the role of HA in the oxidation of phenols by permanganate is far from clear. The mechanisms by which HA influences the oxidation of phenols by permanganate at pH 5.0–9.0 were systematically examined in this study. The presence of HA enhanced the oxidation of phenolic compounds by permanganate at pH ≤7.0, with greater enhancement at lower pH values. The presence of HA facilitated the <i>in situ</i> formation of MnO<sub>2</sub>, implying the importance of reductive moieties of HA in this reaction. This was supported by the finding that HA preoxidized by ozone showed enhancements in the oxidation of phenols by permanganate at pH 5.0–6.0 smaller than those seen with pristine HA. The good correlation between HA-induced improvement in the oxidation rates of phenols by permanganate and those by preformed colloidal MnO<sub>2</sub> at pH 5.0 confirmed that contribution of MnO<sub>2</sub> formed <i>in situ</i> for the oxidation of phenols under this condition. The differences in the influence of Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub> and HA on the oxidation of phenol by permanganate revealed the fact that the continuous generation of fresh MnO<sub>2</sub> and stabilization of the MnO<sub>2</sub> formed <i>in situ</i> by HA were crucial for the HA-induced enhancement of the oxidation of phenols by permanganate at pH ≤7.0. The consumption of permanganate by HA and the poor oxidation ability of <i>in situ</i>-generated MnO<sub>2</sub> under alkaline conditions resulted in the slightly negative effect of HA on the degradation rates of phenols by permanganate at pH >7.0.