Degradation of hydroxychloroquine in aqueous solution by UVC-activated peroxymonosulfate (UVC/PMS): degradation kinetics, mechanisms and pathways, identification of transformation products, and toxicity assessment

This study explored the UVC-activated peroxymonosulfate (UVC/PMS) process for eliminating hydroxychloroquine (HCQ), a newly recognized environmental contaminant, with emphasis on identifying degradation by-products, elucidating transformation mechanisms, and assessing ecotoxicity. Key operational factors such as initial levels of HCQ and PMS, ultraviolet intensity, pH, and prevalent anions were systematically examined for their influence on the degradation performance. The degradation kinetics of HCQ was effectively modeled using a pseudo-first-order framework, demonstrating accurate prediction of removal behavior under UVC/PMS treatment conditions. At initial [PMS] = 0.132 mM, initial [HCQ] = 0.023 mM, pH = 7, UVC intensity of 22.5 mW·cm −2 , 93 % removal of HCQ was achieved in 120 min reaction time. Scavenging tests using ethanol and tert-butanol indicated that hydroxyl radical was the dominant radical species. Radical scavenging experiments involving ethanol and tert-butanol revealed that hydroxyl radicals ( • OH) were the primary reactive species driving HCQ degradation. In terms of water chemistry, carbonate (CO₃ 2− ) and phosphate (PO₄ 3− ) anions enhanced the degradation efficiency, whereas chloride (Cl − ), sulfate (SO₄ 2− ), and nitrate (NO₃ − ) exhibited inhibitory effects on the process performance. Despite the high rate of HCQ removal in the studied process, 19.35 % TOC removal was achieved in 180 min. Furthermore, to comprehensively evaluate the process efficiency, LC-MS analysis was applied and 22 TPs were identified based on which the degradation pathways were proposed. Last but not least, phytotoxicity test was done and indicated that some TPs formed during the UVC/PMS process had equivalent or even higher toxicity than HCQ which requires further investigations. • Hydroxychloroquine (HCQ) was effectively degraded by UVC-activated peroxymonosulfate (UVC/PMS). • The degradation of HCQ followed a pseudo-first-order kinetic model under all tested conditions. • The combination of UVC and PMS significantly enhanced both degradation rate and efficiency. • Common inorganic anions exhibited variable effects on the performance of the UVC/PMS system. • A total of 22 transformation products were identified by LC-MS, and major degradation pathways were proposed.