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 x 10-5 s-1 to 3.41 x 10-4 s-1. Based on the scavenging experiments and the steady-state kinetic modeling, ClO & sdot; 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 & sdot; and Cl & sdot;, were recognized as the dominant reactive species contributing to the oxidation of Mn(II). Kinetic model calculations indicate that the contribution of ClO & sdot; 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 & sdot; 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.