The role of the direct and indirect mechanism in the advanced oxidation process induced degradation of ciprofloxacin

The fluoroquinolone ciprofloxacin occurs worldwide in the aquatic environment and has therefore been listed on the 3rd European union watchlist for monitoring. To eliminate sewage and wastewater treatment plants as entry pathways, advanced oxidation processes have been intensely researched. Hence, photolysis at different pH ranges was studied for its capacity to eradicate ciprofloxacin. High-performance liquid chromatography coupled to high-resolution mass spectrometry was used to identify the degradation products and to monitor all compounds. Ecotoxicity was assessed using quantitative structure-activity relationship analysis comprising the Ecological Structure Activity Relationships tool. Two degradation mechanisms were found active: the direct mechanism, i.e. degradation through ultraviolet absorption by the substance, and the indirect mechanism caused by hydroxyl radicals from water photolysis. The radical scavenger tert-butanol and pH variations revealed that the indirect mechanism predominated in general. The direct photo-induced degradation proceeded about 10 times slower. Products could be attributed to the mechanisms. Based on their chemical structures, all identified products were predicted less toxic by quantitative structure-activity relationship than ciprofloxacin. Mechanistic insight suggested that photo-induced advanced oxidation processes proved efficient for ciprofloxacin elimination when generating hydroxyl radicals. Compounds and pH range hampering their occurrence diminish the efficacy of elimination.