Which route to take for diclofenac removal from water: Hydroxylation or direct photolysis?

Abstract

This study aimed at comparing the performance of hydroxylation and direct photolysis for removing diclofenac from water. Several parameters were assessed: degradation kinetics, mineralization degree, lipophilicity (log D)of the identified initial intermediates, acute ecotoxicity analyses (Daphnia similis and Lactuca sativa), and chronic ecotoxicity predictions (ECOSAR 1.11). Hydroxylation was achieved by TiO2-assisted photodegradation. Direct photolysis experiments were performed in the same reaction system, but with no TiO2 addition. At least initially (up to 10 min), both processes followed a zero order kinetics, but direct photolysis was faster than hydroxylation: k = (1.4 ± 0.038)× 10−1 μg L−1 min−1 (R2 = 0.991)and (8.7 ± 0.29)× 10−2 μg L−1 min−1 (R2 = 0.996), respectively. No mineralization was observed up to 30 min. During TiO2-assisted photodegradation, only hydroxylated intermediates were detected. Direct photolysis proceeded via carbazoles formation. Only direct photolysis was capable of forming hydrophilic products (log D < 0). Acute ecotoxicity analyses (Daphnia similis and Lactuca sativa)showed that DCF toxicity was successfully removed and no additional ecotoxicity was produced by the degradation products. Chronic ecotoxicity predictions (fish, daphnids, and algae)showed that direct photolysis would produce less toxic substances. In summary, the results point out that direct photolysis is a better choice for removing diclofenac from water, in comparison to TiO2-induced hydroxylation.