Degradation of Diclofenac Sodium by the UV/Chlorine Process: Reaction Mechanism, Influencing Factors and Toxicity Evaluation

Abstract This study examined the reaction mechanism, influencing factors and toxicity of diclofenac sodium (DS) degradation by ultraviolet (UV)/chlorine process. The UV/chlorine was capable of eliminating DS from water. The DS degradation during the UV/chlorine process followed a pseudo-first-order kinetic model that was influenced by chlorine dosage, solution pH, humic acid and bicarbonate concentrations. The free chlorine affects not only DS elimination, but the contribution of various active species as well. Increasing free chlorine dosage from 1 to 7 mg·L-1 increased the first-order rate constant of NaClO, ·OH and reactive chlorine species (RCS) from 0.00063, 0.00328 and 0.002030 min−1 to 0.00233, 0.01010 and 0.09740 min−1, respectively, and increased the contribution of RCS from 8.20% to 75.71%, while the contributions of UV, NaClO, and ·OH were declined from 76.02%, 2.54% and 13.24% to 14.63%, 1.81%, and 7.85%, respectively. The contribution of RCS became increasing prominence with the increment of free chlorine dosage. The kobs,UV/chlorine,DS value decreased from 0.07970 to 0.04450 min-1 as pH increased from 5.0 to 8.0. The presence of bicarbonate and natural organic matter both exerted an inhibitory effect on DS degradation. Eleven intermediate products were identified and the degradation pathway involved C-N cleavage, condensation, hydroxylation, and decarboxylation was proposed. The UV/chlorine process effectively reduced acute toxicity and was superior to chlorination. The genotoxicity induced by a chlorinated solution treated by the UV/chlorine process exhibited negative genotoxicity. These results show that the UV/chlorine process is capable for the degradation and detoxification of DS.