Mn(II) oxidation by the UV/chlorine system under near-neutral pH conditions: The important role of ClO· and ClO2

The oxidation kinetics of Mn(II) by free chlorine is relatively low under near-neutral pH conditions which limits the Mn removal efficiency in drinking water treatment. Therefore, this study investigated the oxidation efficiency of Mn(II) by the UV-enhanced chlorination (UV/chlorine) system and identified the responsible reactive radical species. The results show that the oxidation kinetic of Mn(II) was greatly enhanced by the UV/chlorine system under near-neutral pH or even acidic conditions. The pseudo-first-order reaction rate of Mn(II) at pH 8.0 (within the first 20 min) increased from 2.60 × 10-5 s-1 to 3.41 × 10-4 s-1. Based on the scavenging experiments and the steady-state kinetic modeling, ClO· and ClO2, whose steady-state concentration (∼10-10 M and ∼10-9 M, respectively at pH 8.0) was at least 4 orders of magnitude higher than that of HO· and Cl·, were recognized as the dominant reactive species contributing to the oxidation of Mn(II). Kinetic model calculations indicate that the contribution of ClO· to the oxidation of Mn(II) was consistently maintained above 70 %, and ClO2 also played an important role in the oxidation of Mn(II) especially under acidic and alkaline conditions. In addition, the background components of HCO3- and Cl- had negligible influence on the oxidation efficiency because they barely changed the concentration of the ClO· and ClO2. This study first demonstrates the important role of ClO2 in the oxidation of Mn(II) in the UV/chlorine system, and the possible role of ClO2 in the degradation of some organic pollutants needs to be carefully evaluated in the future.