Ultrasound-assisted heterogeneous degradation of tetracycline over flower-like rGO/CdWO4 hierarchical structures as robust solar-light-responsive photocatalysts: Optimization, kinetics, and mechanism

Novel hybrid decontamination processes based on visible-light-responsive photocatalysts (VLPHS) have recently become a focus of environmental research. In this study, the sonophotocatalytic degradation of the antibiotic tetracycline (TC) using a binary rGO/CdWO4 composite under simulated visible-light irradiation was demonstrated for the first time. Structural characterization confirmed that flower-like CdWO4 particles with a wolframite phase structure were successfully immobilized on the surface of the graphene oxide (GO). Subsequently, reduced GO/CdWO4 (rGO/CdWO4) VLPHS were prepared via the facile photocatalytic reduction of GO/CdWO4. A Box-Behnken design based on response surface methodology (RSM) was employed to determine the maximum efficiency of sonophotocatalytic degradation by optimizing the process parameters (pH, initial TC concentration, treatment time, and catalyst dosage). The high determination coefficients (R-2 = 0.9818 and adjusted R-2 = 0.9636) indicated that the experimental values fitted the proposed RSM model well. Compared with CdWO4, rGO/CdWO4 VLPHS exhibited significant photoelectrochemical (PEC) performance, superior sonophotocatalytic activity, and mineralization efficiency. The enhanced catalytic activity was mainly due to the larger optical adsorption range, greater photo-induced charge carrier transfer, and higher surface area. In addition, rGO/CdWO4 exhibited a catalytic activity that was 1.5 and 3 times higher than that of commercial nano-ZnO and -TiO2, respectively. The kinetic analysis indicated that the degradation rate of TC follows the Langmuir-Hinshelwood kinetic model. The possible photocatalytic mechanism behind the degradation of TC by rGO/CdWO4 was also tentatively proposed. This study provides a simple and scalable pathway to produce highly efficient rGO-based VLPHS for photocatalytic and PEC applications.