Efficient construction of bismuth vanadate-based Z-scheme photocatalyst for simultaneous Cr(VI) reduction and ciprofloxacin oxidation under visible light: Kinetics, degradation pathways and mechanism

Despite massive research efforts are devoted into wastewater treatment, the coexistence of organic compounds and heavy metals is an escalating environmental problem, especially in surface waters. In this work, a rapid reduction of Cr(VI) and oxidation of refractory ciprofloxacin (CIP) were achieved simultaneously by the Ag/AgBr decorated BiVO4 Z-scheme heterojunction under visible light irradiation. The ternary photocatalyst was fabricated by a facile hydrothermal method, followed by photoreduction process. Superior photocatalytic performance of Ag/AgBr/BiVO4 was not only reflected in single Cr(VI) reduction or CIP oxidation, but also displayed in simultaneous removal of two contaminants, which should be attributed to the enhanced visible light absorption, high charge carriers separation efficiency and redox of their photo-generated electrons and holes in the constructed Z-scheme heterostructure. Characterization methods including UV-vis absorption spectra (UV-vis DRS), transient photocurrent response (PC), photoluminescence spectra (PL) and electrochemical impedance spectra (EIS) were employed to confirm the mechanism. The active species trapping experiments and electron spin resonance (ESR) measurements demonstrated that h(+), center dot O-2(-) and center dot OH all participated in CIP degradation, while e(-) and center dot O-2(-) were the main active groups for Cr(VI) reduction. The presence of Ag facilitated a Z-scheme Ag/AgBr/BiVO4 photocatalyst due to its electron mediator role. The CIP mineralization was verified by three-dimensional excitation-emission matrix fluorescence spectra (3D EEMs) and total organic carbon (TOC) removal. At last, the possible CIP degradation pathway was also proposed. This work will provide a new route to design the novel Z-scheme photocatalysts for the simultaneous degradation of organic pollutants and reduction of heavy metal ions in water.