Abiotic and biotic processes of diclofenac in enriched nitrifying sludge: Kinetics, transformation products and reactions.

Diclofenac (DCF), as an emerging contaminant in aquatic environments, has sparked increasing concerns about its impact on the environment. Nitrification in wastewater treatment processing has removed DCF to a large extent. However, the removal characteristics and mechanisms of DCF in the nitrification process are still poorly understood. In this study, enriched nitrifying sludge was used to investigate the transformation of DCF during the nitrification process. Elimination of DCF caused by volatilization, hydrolyzation and adsorption was limited. Abiotic nitration removal was confirmed as significant in enriched nitrifying sludge at a low pH and high nitrite concentration. Free nitrite acid was proposed as the reaction species participating in the DCF transformation process, and a regression equation was developed to predict the contribution of abiotic nitration on DCF removal in enriched nitrifying sludge. By slowly and continuously adding an ammonia stock solution and controlling the pH, we avoided the effect of abiotic nitration removal, and DCF biodegradation was positively correlated to specific ammonium oxidation rates (SAORs). The removal of DCF fit the first order kinetic model (R2  = 0.8285, p < 0.05) with an SAOR of 0.25 mg NH4 + -N/(gMLSS·min). The high removal rate constant of k (0.1286 L/(gMLSS·h)) and short half-life (2.48 h) revealed the strong capability of nitrifying bacteria to transform DCF. Nine DCF transformation products were identified and three of them were quantified in the transformation process. The formation of kinetic profile 4-OH-DCF, 5-OH-DCF and DCF-Benzoic acid (DCF-BA) implied that hydroxylation may be the first reaction of DCF and DCF-BA may be a terminal product that resists further degradation. The postulated reactions concerning the transformation of DCF were hydroxylation, lactam formation and oxidation. Accordingly, a detailed degradation pathway was presented.