Boosting the radical-induced reductive degradation of clofibric acid in water: Synergistic effect of SO32-/UV and hydrodynamic cavitation (HC).

Advanced reduction processes (ARPs) of SO32-/UV were combined with hydrodynamic cavitation (HC) for the first time to attempt a reductive degradation of clofibric acid (CLA) in water. At cavitation number (Cv) 0.2, rred 3.25, pH 6.5, the combined HC/SO32-/UV degraded 99.68 % of CLA in 90 min resulting in a synergistic effect of 1.42. Quenching experiments on SO32-/UV and HC/SO32-/UV revealed that HC promoted the formation of H• radicals, which were the primary species for CLA degradation. In contrast, the effect of oxidizing species, such as HO• and SO4•- radicals was negligible. HC/SO32-/UV remained high degradation of CLA in the presence of co-existing CO32-, Cl-, and SO42- anions. The presence of humic acids (HA) showed the strongest inhibitory effect, declining the degradation rate constant of CLA from 6.67 × 10-2 min-1 to 1.34 × 10-2 min-1. The presented findings offer mechanistic insights into the reductive degradation of CLA under the novel HC/SO32-/UV. Based on the detected intermediates by Gas Chromatography-Mass Spectrometry (GC-MS) and density functional theory (DFT), three pathways of CLA degradation were proposed. Specifically, the reductive degradation of CLA was initiated by nucleophilic attacks of H• radicals followed by cyclization, hydrogenation of the benzene ring, and fragmentation into smaller compounds, e.g., 3-methylbutanal. Importantly, developed process allows effective dehalogenation of the pollutant. Such approach can find wide applicability for degradation of other emerging organic pollutants, especially pharmaceuticals, pesticides and PFAS containing halogen atoms. This study brings a new strategy for the enhancement of SO32-/UV and demonstrates HC as a versatile and effective tool in water and wastewater treatment.