Electro-Fenton degradation of antibiotic ciprofloxacin (CIP): Formation of Fe^sub 3+^-CIP chelate and its effect on catalytic behavior of Fe^sub 2+^/Fe^sub 3+^ and CIP mineralization

This work investigated the effect of the chelation of Fe3+ with a fluoroquinolone (FQ) antibiotic ciprofloxacin (CIP) on the catalytic behavior of Fe3+/Fe2+ and the degradation and mineralization of CIP in electro-Fenton (EF) process using activated carbon fiber (ACF) felt cathode. First of all, effect of initial Fe2+ and Fe3+ concentrations on the degradation and mineralization of CIP in EF process was compared, while the optimal initial Fe2+ and Fe3+ contents were both verified to be 1.50 mmol dm-3, in which 73% and 72% of initial TOC were eliminated after 360 min electrolysis, respectively. Secondly, Fe3+-CIP chelate was produced during CIP degradation in EF process, whose predominant form was confirmed as 1:2 metal-ligand stoichiometry. The formation of Fe3+-CIP chelate had no obvious inhibition on the degradation and mineralization of CIP due to the efficient Fe3+ reduction on the ACF cathode with excellent catalytic feature. About 17% and 35% of initial 1.50 mmol dm-3 Fe3+ were reduced to Fe2+ at 18 mA cm-2 in ultrapure water and 200 mg dm-3 CIP solutions during 30 min pre-aeration of EF process, respectively, while higher Fe2+ concentration was maintained for initial 1.50 mmol dm-3 Fe3+ during CIP degradation. Finally, the evolution of F-, NH4+ and NO3- ions released to the acidic medium were determined, as well as the formation of short-chain carboxylic acid (oxalic, oxamic and formic acids). Eight aromatic intermediates were identified using UPLC-QTOF-MS/MS, and a feasible mineralization pathway was proposed.