Dyeing wastewater is one of environmental pollutions, having a serious threat to the environment and human-being health. Therefore, the purification of dyeing wastewater becomes a challenge. Among various methods for treating dyeing wastewater, photocatalysis is developed as an efficient environmental pollution control technology due to its mild reaction conditions, sustainable driving energy sources, and no secondary pollution after the reaction. Zinc tungstate (ZnWO₄) as a semiconductor photocatalyst material with stable physical and chemical properties has a promising application potential in photocatalytic removal of environmental pollutants. However, ZnWO₄ has a wide bandgap and a low migration efficiency of carriers, resulting in unsatisfactory photocatalytic intrinsic activity. Therefore, researchers attempted to optimize the photocatalytic performance of ZnWO₄ via regulating its morphology, size and crystallinity. Sonochemical method is a highly concerned method for synthesizing nanomaterials. Under ultrasonic cavitation conditions, the physical and chemical environment of the reaction system will undergo significant changes, i.e., the instantaneous generation of high temperature, high pressure, and strong shock waves, which are beneficial for strengthening the fracture of chemical bonds and the generation of new chemical bonds, thereby regulating the morphology and structure of material. In this work, ZnWO₄ with oxygen defects nanomaterials were prepared by a sonochemistry-assisted hydrothermal method, and the influence of sonochemical treatment time on the photocatalytic activity of ZnWO₄ was investigated as methylene blue was used as a target pollutant.

ZnWO₄ with oxygen defects nanomaterials were prepared by a sonochemistry-assisted hydrothermal method. In the preparation, 0.297 g Zn(NO₃)₃·6H₂O was dissolved in 8 mL distilled water under constant stirring, to obtain solution A. Also, 7 mL distilled water containing 0.392 g Na₂WO₄·2H₂O was added in dropwise to solution A and mixed by stirring for 30 min. Afterwards, LC-UP-400 ultrasonic signal connected Φ10 luffing rod (selecting ultrasonic power was 50%, ultrasonic treatment for 2 s, and stay for 2 s) was used to irradiate the white suspension for 1h, and then the suspensions were put into Teflon-lined autoclave, sealed and maintained at 140 ℃ for 20 h. After cooling to room temperature, the product was washed with distilled water and ethanol for several times. The final products were obtained after drying at 60 ℃ for 12 h. The samples of ZnWO₄ nanomaterials treated at different ultrasonic treatment time (X min) were named as ZnWO₄-X.

The photocatalytic activity of as-prepared samples was evaluated by MB photocatalytic removal in 10 mg/L of MB dyeing wastewater. Herein, MB dyeing wastewater under light illumination without the addition of photocatalysts as a blank test for comparison, having an unsatisfactory photoreduction effect. After 40 min of illumination, ZnWO₄ has a weak photocatalytic activity for MB photodegradation, i.e., 52% of MB is photocatalytically degradated by pure ZnWO₄. However, the induction of oxygen vacancy defects results in an enhanced photocatalytic performance for MB removal over ZnWO₄-5 min. The optimized ZnWO₄-5 min displays a superior photocatalytic activity for MB removal, which can remove 95.0% of MB after 40 min of illumination. The kinetic of MB photodegradation was further analyzed by plotting the -ln(c/c₀) as a function of irradiation time. The calculated rate constant k of ZnWO₄-5min is 0.067 min^-1, which is 4.5 times greater than that of ZnWO₄. The reusability of ZnWO₄-5min was investigated by performing 5 consecutive cycles for MB photodegradation. The results show that the photoreduction efficiency maintains at a high level (i.e., >90%) after five cycling experiments, indicating the superior stability and huge potential value of ZnWO₄-5 min for the treatment of dyeing wastewater. The oxygen vacancy defects induced by ultrasound cause the lattice contraction within ZnWO₄, which is beneficial for optimizing electronic structure, exposing more catalytic active sites, broadening the photocatalytic response range, and improving the migration efficiency of photogenerated carriers, thereby significantly enhancing the photocatalytic degradation performance of ZnWO₄ in dyeing wastewater.

ZnWO₄ with oxygen defects nanomaterials were prepared by a sonochemistry-assisted hydrothermal method, and the influence of sonochemical treatment time on the photocatalytic activity of ZnWO₄ was investigated when MB was used as a target pollutant. The results showed that sonochemical treatment could induce the formation of oxygen vacancy defects in ZnWO₄ nanoparticles, thereby optimizing electronic structure, exposing more active sites, broadening the light response range and improving the transfer efficiency of photogenerated carriers, thus enhancing the photocatalytic degradation performance of ZnWO₄ for dyeing wastewater. The optimized ZnWO₄-5 min displayed a superior photocatalytic activity for MB removal, which removed 95.0% of MB after 40 min of illumination. The calculated rate constant k of ZnWO₄-5 min was 0.067 min^-1, which was 4.5 times greater than that of ZnWO₄. Furthermore, ZnWO₄-5min displayed a superior photocatalytic stability, and the photoreduction efficiency maintained at a high level (>90%) after five cycling experiments. This indicated that sonochemically-induced oxygen vacancy defects be an effective method to improve the photocatalytic performance of ZnWO₄. This work can provide some ideas for the preparation of high-performance semiconductor photocatalytic materials for the treatment of dyeing wastewater.