Degradation kinetics and mechanism of diclofenac by UV/peracetic acid

In this work, the degradation kinetics and mechanism of diclofenac (DCF) by UV/peracetic acid (PAA) was investigated. The effects of pH, PAA dose and common water components such as inorganic ions and dissolved organic matter (DOM) on DCF degradation by UV/PAA were also evaluated. It was observed that the addition of PAA promoted the photodegradation of DCF due to the generation of reactive radicals in the photolysis of PAA, which was also confirmed by the radical scavenging experiment. The best degradation efficiency of DCF was obtained at pH 8.5. The removal of DCF was enhanced gradually with increasing PAA dose. Since NO₃ ^- is a photosensitive substance which can generate HO˙ under UV irradiation, its existence promoted the degradation of DCF. The presence of CO₃ ^2- could slightly improve DCF degradation, which might be due to the role of generated carbonate radicals. Cl^-, SO₄ ^2- and Fe³⁺ had little effect on DCF removal, while Cu²⁺ could enhance DCF degradation because of its catalytic ability for PAA decomposition. An inhibition effect on DCF removal was observed in the presence of DOM, and it was more obvious in higher concentration of DOM. The elimination of total organic carbon (TOC) was low. According to the twelve reaction products detected in the UV/PAA system, the probable transformation mechanism of DCF was proposed exhibiting eight reaction pathways, i.e., hydroxylation, decarboxylation, formylation, dehydrogenation, dechlorination-hydrogenation, dechlorination-cyclization, dechlorination-hydroxylation and amidation. This study indicates that UV/PAA is a promising method for DCF removal from contaminated water.