Synergistic mineralization of ofloxacin in electro-Fenton process with BDD anode: Reactivity and mechanism

This study aimed at exploring the performance of the electro-Fenton (EF) process with a boron-doped diamond (BDD) anode and an electrochemically exfoliated graphene-based cathode (EEGr), for removal of antibiotic ofloxacin (OFLO) from contaminated water. Compared with anodic oxidation (AO) process with BDD anode, the EF process with BDD anode (EF-BDD) could improve the decay kinetics of OFLO by 1.4-2.6 times and reduce the disposal cost by 28 %-41 %. Removal efficiency of TOC in EF-BDD process under 4.2 mA cm-2 was better than in AO process under 8.3 mA cm-2, pointing out that the cost effectiveness of the EF process for mineralization of OFLO under low current density conditions. This advantage is due to the simultaneous formation of homogeneous (center dot OH) and heterogeneous (BDD(center dot OH)) hydroxyl radicals in the process. The synergistic factor of center dot OH and BDD(center dot OH) for mineralization of OFLO decreased with the current density rising from 4.2 (0.28) to 16.6 mA cm-2 (-0.08). Decay kinetics of OFLO in the EF-BDD and AO were more converging under higher current densities owing to the production of large amount of BDD(center dot OH). In the EF-BDD, a trade-off should be considered between accumulated BDD(center dot OH) on the surface of BDD anode and H2O2 generation on the cathode. Absolute rate constant for oxidation of OFLO by center dot OH (kOFLO) was calculated to be 3.86 x 109 M-1 s-1. Evolution of carboxylic acids and inorganic ions as well as toxicity assessment were followed during the treatment of OFLO solutions. Results obtained proved that the EF-BDD process is outstanding for oxidative degradation of OFLO thanks to the synergistic effect of center dot OH and BDD(center dot OH). These very strong oxidants being non-selective, this result can be extended to the treatment of antibiotic organic pollutants.