The oxidative degradation of Caffeine in UV/Fe(II)/persulfate system—Reaction kinetics and decay pathways

• Published in: Water environment research 2021-04, Vol.93 (4), p.559-569
• Main Authors: Rao, Yongfang, Long, Huimin, Hao, Jingchen
• Format: Article
• Language: English
• Subjects: Anions; Caffeine; Chloride ions; Decay; decay pathways; Degradation; Demethylation; Ferrous Compounds; Ferrous ions; Free radicals; Hydroxyl radicals; Hydroxylation; Imidazole; Intermediates; Ions; Iron; Kinetics; Liquid chromatography; Mass spectrometry; Mass spectroscopy; Oxidation; Oxidation-Reduction; Oxidative Stress; Pyrimidines; Reaction kinetics; Solid phase methods; sulfate radicals; Sulfates; Ultraviolet radiation; Water Pollutants, Chemical - analysis
• ISSN: 1061-4303; 1554-7531
• DOI: 10.1002/wer.1458

In this study, the degradation of caffeine was investigated by UV/Fe2+/persulfate (PS) process. Caffeine (CAF) degradation in sole‐UV, UV/Fe2+, UV/PS, and Fe2+/PS systems was also conducted to examine the contribution of isolated processes to CAF degradation. The effects of pH levels, the concentration of Fe2+ and PS, inorganic anions, and initial concentration of CAF on the performance of UV/Fe2+/PS process were evaluated. Radical competitive reactions indicated both hydroxyl radicals and sulfate radicals played important roles in CAF degradation in UV/Fe2+/PS system. Nine intermediates, among which three were detected for the first time, were identified by ultra‐performance liquid chromatography/electrospray‐time‐of‐flight mass spectrometry (UPLC/ESI‐TOF‐MS) and SPME (solid‐phase microextraction)/GC/MS. The possible degradation pathways of CAF were proposed, among which demethylation, hydroxylation, the oxidation of olefinic double bond, and the cleavage of pyrimidine ring and imidazole ring were involved in the degradation of CAF in UV/Fe2+/PS system. Practitioner points Caffeine degradation by UV/Fe2+/PS process was investigated. Caffeine degradation did not follow a simple pseudo‐first order kinetics Chloride ions promoted CAF degradation. The anions NO3−, SO42−, and H2PO4− exerted a negative influence on caffeine degradation. Nine intermediates were detected, and decay pathways were proposed. UV‐promoted Fe(II)/Fe(III) redox cycle and CAF degradation pathways.