Effects of ball milling on the photochemistry of biochar: Enrofloxacin degradation and possible mechanisms

Abstract Ball-milled biochar with enhanced physicochemical and sorption properties has been intensively investigated, but its photochemistry is far less studied. In this study, biochars produced at various pyrolysis temperatures were ball-milled and tested to determine enrofloxacin (EFA) photocatalytic degradation and its mechanisms. Ball-milled biochar could generate more •O2- under visible light irradiation, which favored the photocatalytic degradation of EFA. The ball-milled biochar pyrolyzed at 300 °C (BM300) exhibited the highest EFA degradation rate (80.2% compared with 13.9% for unmilled biochar) and mineralization ability (66.4% compared with 0% for unmilled biochar). The characterization results suggested that more oxygen-containing functional groups played a significant role in the enhancement of photocatalytic performance. Based on the characterization and experiment results, it was supposed that a semiconductor-like structure played a main role in photocatalysis of BM300. A new mechanism suggested that the electrons in carbon defects (as valence band) could be excited by visible light and move to oxygen-containing functional groups (as conduction band), then the electrons in conduction band was further transferred to dissolved oxygen to produce •O2-, and h+ could be formed in valence band at the same time. This work applied ball-milled biochar to the field of photocatalysis for the first time, and provided a new opportunity for the removal of EFA in aqueous solutions.