Photo-catalytic degradation of triclosan with UV/iodide/ZnO process: Performance, kinetic, degradation pathway, energy consumption and toxicology

This study develops UV/Iodide/ZnO (UIZ) photo reactor to employs it for degradation, dechlorination and mineralization of triclosan (TCS) content of wastewaters. Degradation efficiency of the UV-only, UV/ZnO, UV/Iodide and UIZ processes were determined as 12.3%, 37.3%, 50.43% and 89.83% respectively, at 20 min reaction time, 10 mg L-1 TCS concentration and pH of 7. Findings indicated that the TCS completely degraded within 20 min treatment time. According to Weight ratio evaluations, the 1.26:2:10 ration is the optimal Weight ratio of Iodide/ZnO/TCS in the UIZ process. Analyzing effluents of the UIZ reactor by liquid chromatography/mass spectroscopy after 5, 10, 15 and 20 min reaction time revealed that TCS decomposes to ring-shaped compounds in less than 10 min and all TCS and its metabolites convert to linear compounds within 15 min reaction time. Investigating kinetic of the process through a pseudo first-order model using 1 to 10 mg L-1 TCS over 1 to 15 min reaction time demonstrated that the observed rate constant (k(obs)) decreases from 0.3405 to 0.0687 min(-1) and the observed TCS degradation rate (r(obs)) increases from 3.405 to 13.74 mg L-1 min(-1) with increasing the TCS concentration. Furthermore, electrical energy consumption (E-EO) of the UIZ process was calculated with kinetic model 2.48 to 12.29 and with merit-figure model 2.87 (2.87-8.95) to 14.88 (14.88-33.02) kW h m(-3) from 1 to 10 mg L-1, respectively. In addition, the effect of co-existing water anions on degradation of 10 mg L-1 TCS was explored during 20 min processing. While 100% degradation was achieved in the absence of any anion, adding nitrate, as the most effective anion, decreased the TCS degradation to 72.88%. The highest dechlorination was observed at Iodide/ZnO weight ratio 1.26:1 and pH 9. Mineralization of TCS by UIZ process was achieved 38.6% after 15 min reaction time. In presence of the effluents extracted after 10 min TCS degradation (even after long incubation periods), no growth inhibition was observed using Analysis of the effluent dilution effect and toxicity assessment on Escherichia coli. Finally, feeding effluents of the UIZ photoreactor to cyclic rotating-bed biological reactor (CRBR) helped to reduce COD nearly completely after 1.5 h aeration, respectively. Therefore, the proposed UIZ/CRBR process is a promising technology for efficient treatment of the wastewaters containing TCS.