Insight into UV-LED/chlorine/Fe3+ process for reducing ibuprofen and caffeine: Kinetics, mechanism and DBP formation

The UV-LED/chlorine process is considered an emerging technology to effectively remove pollutants from water. The introduction of FeCl3 can significantly improve the yields of reactive species in UV-LED/chlorine process, thereby achieving full utilization of chlorine and efficient removal of pollutants. In this research, ibuprofen (IBP) and caffeine (CAF) with different activities towards various radicals were selected as target pollutants to investigate the degradation performance of UV-LED/chlorine/Fe3+ systems. The introduction of Fe3+ dramatically accelerated IBP and CAF degradation and the enhancement effect of the latter was more pronounced in UVLED/chlorine process. The hydroxyl radical (center dot OH) produced by UV-LED/chlorine/Fe3+ system was the major contributor to IBP degradation (89.9 %), while both center dot OH (51.3 %) and reactive chlorine species (38.2 %) generated by UV-LED/chlorine/Fe3+ system under the same operating conditions played the important role on CAF degradation. As the dosages of Fe3+ and chlorine increased, the removal of CAF and IBP both showed varying degrees of improvement. The presence of HCO3- , HA and Br- restrained the removal of IBP and CAF, while the addition of Cl- posed a slight effect on their removal. Based on HPLC/MS analysis and DFT calculations, the reactive sites and possible decay pathways of IBP and CAF were proposed. Additionally, the chlorination of IBP and CAF pretreated with UV-LED/chlorine/Fe3+ increased the toxicity risk caused by the production of disinfection by-products (DBPs), which was significantly reduced after adsorption by adjusting the solution pH.