Insights into a CQD-SnNb2O6/BiOCl Z-scheme system for the degradation of benzocaine: Influence factors, intermediate toxicity and photocatalytic mechanism

The development of a novel Z-scheme system with a high separation efficiency of the charge carriers for remarkably enhanced photocatalytic activity is highly desirable. Herein, a carbon quantum dot-modified SnNb2O6/BiOCl (CQD-SNO/BOC) Z-scheme system, in which the CQDs acted as a bridge for the charge carriers, was successfully synthesized. As expected, the CQDs acted as a bridge for the charge carriers that could separate and accumulate electrons from SnNb2O6 and holes from BiOCl by promoting the transfer of the photoinduced charge carriers, which enabled the BiOCl holes to maintain their high oxidation capability. Additionally, the CQDs not only endowed the SNO/BOC Z-scheme system with a large specific surface area that provided a greater number of active sites but also noticeably improved its visible light absorption capacity. Therefore, the CQDSNO/BOC Z-scheme system showed an improved photocatalytic performance for the degradation of benzocaine (BC), which was over 20 times, 10 times and 2 times greater than those of BiOCl, SnNb2O6 and SNO/BOC Z-scheme systems, respectively. In addition, the effects of catalyst concentration, pH value, different anions, and humic acid (HA) concentration on the photodegradation of BC were investigated in depth. Simultaneously, a four-stage pathway for the degradation of BC was proposed based on the degradation products that were identified, and the accuracy of this pathway was further verified based on biotoxicity assessments and toxicity predictions of the products. According to the ESR spectra and active species trapping experiments, a mechanism for the CQD-SNO/BOC Z-scheme system was proposed, in which the CQD bridge accelerated charge transfer. Based on these results, the CQD-modified Z-scheme system could be an efficient and promising photocatalyst for the degradation of PPCPs in water.