Xuecong Qian,Shilong Wang,Haijun Cheng,Ziye Jia,Da Wang,Yandong Xie,Jinhao Duan,Yu Tian,Jun Ma
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 ClO 2 , 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 ClO 2 also played an important role in the oxidation of Mn(II) especially under acidic and alkaline conditions. In addition, the background components of HCO 3 − and Cl − had negligible influence on the oxidation efficiency because they barely changed the concentration of the ClO · and ClO 2 . This study first demonstrates the important role of ClO 2 in the oxidation of Mn(II) in the UV/chlorine system, and the possible role of ClO 2 in the degradation of some organic pollutants needs to be carefully evaluated in the future.
Degradation of sulfapyridine as a representative sulfonamide by the UV/chlorine process was investigated. Kinetic experiments revealed that aniline and sulfonamide moieties were sulfapyridine’s main reaction sites with •OH, and the aniline moiety was the preferable site for Cl• attack. Effects of four influence factors (pH; humic acid, HA; HCO3–; Cl–) on the degradation kinetics were explored by response surface methodology, and it was found that pH and HA have the highest level influence (81%) among all influence variables (linear, quadratic, and interactive terms), and low pH and concentrations of HA were beneficial for degradation. Approximately 70% of sulfapyridine degradation was observed within 60 min in the UV/chlorine system (pH = 5) without HA. A kinetic model considering steady-state concentrations of reactive radicals and their second-order rate constants is suitable for predicting sulfapyridine degradation, elucidating that •OH and Cl• are major radicals for sulfapyridine degradation in freshwater, and •OH and Cl2•– are responsible for the degradation in coastal seawater.
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