S-scheme bismuth molybdate/tungstate heterojunction for efficient abatement of dyes and antibiotic and hexavalent chromium

Simultaneous removal of antibiotics and heavy metals via photocatalysis in wastewater treatment field are highly desirable but still challenging. Herein, a novel 3D hierarchical Bi2MoO6/Bi2WO6 (BMW-x, x = 5, 10, 20 and 50) S-scheme heterojunction was prepared via facile two-step hydrothermal strategy. The morphology, structure, composition and photoelectrical property of the as-prepared photocatalysts were investigated using multiple advanced techniques. The optimized BMW-20 heterostructure exhibited remarkable removal efficiencies for colored dye rhodamine B, colorless antibiotic metronidazole (MNZ), and hexavalent chromium Cr (VI) with percentages reaching 96.5%, 83.5%, and 80.3% respectively within 2 h, surpassing the individual component's performance. The removal rates of MNZ and Cr (VI) over BMW-20 in the coexisting system increased to 85.4% and 92.3%, respectively, and were higher than that of the single pollutant system, thus indicating a synergistic effect between simultaneous degradation of organic matter and reduction of heavy metal. The enhanced photocatalytic activity of BMW-20 can be attributed to its improved visible-light absorption and reduced electron-hole recombination. Free radical trapping experiments and electron spin resonance spectroscopy reveal that the photogenerated holes and hydroxyl radical play the crucial roles in the degradation of MNZ, while photoinduced electrons and superoxide anion are mainly responsible for the reduction of Cr (VI). Based on band-gap structure and active substance analysis, a detailed discussion was conducted on the possible mechanism of photocatalytic oxidation-reduction. This work provides a new insight into designing S-scheme heterojunction photocatalysts with high oxidation-reduction ability.