Modeling degradation kinetics of gemfibrozil and naproxen in the UV/chlorine system: Roles of reactive species and effects of water matrix

The UV/chlorine system has been regarded as an efficient oxidation technology for the removal of aqueous micropollutants. However, the roles of the possible radical species for this system on the elimination under environmentally relevant conditions/real waters were still largely unknown. Herein, the specific roles of radical species in the UV/chlorine oxidation degradation of gemfibrozil and naproxen as representative micropollutants were quantified by a steady-state kinetic prediction model considering the effects of water matrices. Overall, the model predicted results are consistent with the experimental data well. •OH and reactive chlorine species (RCS, such as Cl•, ClO•, and Cl2•−) contributions to gemfibrozil and naproxen degradation were water matrix specific. In pure water, both primary reactive species (i.e., •OH and Cl•) and secondary species ClO• dominated gemfibrozil and naproxen degradation, and their individual and the sum of the contributions to degradation rates reduced with pH increase of from 5 to 9. In the presence of Cl−, we found that Cl2•− and in particular ClO• were responsible for the enhanced degradation with increasing Cl− concentrations due to the considerable ClO• reactivity of gemfibrozil (1.93 × 109 M−1 s−1) and naproxen (9.24 × 109 M−1 s−1) and the rapid transformation of Cl2•− to ClO•. The presence of HCO3− notably facilitated the degradation in the UV/chlorine process because of the generation of CO3•−. CO3•− showed high reactivity with gemfibrozil and naproxen corresponding to respective second-order reaction rate constants of 2.45 × 107 and 3.50 × 107 M−1 s−1. Dissolved organic matter induced obvious scavenging for •OH, Cl•, and ClO• and greatly retarded the degradation. The constructed model considering the effects of above water matrix has successfully predicted the oxidation degradation kinetics in real waters, and both •OH and CO3•− are the predominant reactive species in the degradation. This study is helpful for comprehensive understanding the roles of possible radical species in micropollutant removal by UV/chlorine oxidation under real water matrix.