Simultaneous sulfamethazine oxidation and bromate reduction by Pd-mediated Z-scheme Bi2MoO6/g-C3N4 photocatalysts: Synergetic mechanism and degradative pathway

The electron mediator is a feasible tool to promote electron-hole separation and transfer in Z-scheme photocatalysts, offering a promising opportunity to enhance the redox capacity. Pd-mediated Z-scheme Bi2MoO6/g-C3N4 (Pd/Bi2MoO6/g-C3N4) were synthesized for simultaneously oxidizing sulfamethazine (SMZ) and reducing BrO3– under LED-simulated daylight (380 ≤ λ ≤ 780 nm). Pd/Bi2MoO6/g-C3N4 exhibited higher photoactivities as compared to Bi2MoO6/g-C3N4, which was attributed to the favorable interfacial charge transfer induced by Pd electron mediator. Enhanced removal kinetics of SMZ and BrO3– were observed in the binary system compared to those achieved when only one contaminant was present. Synergetic mechanisms were proposed as follows: i) simultaneous SMZ oxidation by holes and BrO3– reduction by electrons resulted in less charge recombination and more efficient consumption of reactive species (RSs), ii) BrO3– played multiple roles in SMZ oxidation, altering the RSs speciation and forming bromine radicals, which not only accelerated the SMZ oxidation but formed bromated byproducts. Pathways for simultaneous removal of SMZ and BrO3– were proposed based on the formed byproducts and Br mass balance. The practical potential of Pd/Bi2MoO6/g-C3N4 for simultaneously removing SMZ and BrO3– was investigated in actual water matrixes. After treating for 10 h, no SMZ was detected and the residual BrO3– was below 10 μg·L–1. This work provides an illustrative strategy for the applications of noble-metal mediated Z-scheme photocatalysts in water treatment by simultaneous oxidation and reduction, taking advantages of the oxidative and reductive RSs generated during photocatalytic process.