Study on the synergistic mechanism, apparent kinetics with reaction order, and toxicity of degradation products of norfloxacin by zinc oxygen vacancies and Z-scheme heterojunction photocatalysis

In this paper, ZnCdS/ZnCr-LDHs Z-scheme heterojunction containing zinc-oxygen bi-vacancy (V_Zn + O) were constructed and applied to photodegradation of norfloxacin (NOR) in water. A comprehensive investigation was conducted into the effects of concentration of V_Zn in the composite, amount of catalyst, concentration of norfloxacin, temperature of the solution, and pH on the degradation activity. It comprehensively investigates the reaction kinetics, establishing a complete kinetic equation and determining the activation energy. Key parameters including catalyst dosage, pH, and reaction orders are systematically analyzed to elucidate their influence on the reaction process. The reaction mechanism of photodegradation of NOR is deeply analyzed from the perspectives of thermodynamics and kinetics. Z-scheme heterojunction not only facilitates the separation of photogenerated charges, but also preserves photogenerated electrons with strong reducing ability and photogenerated holes with high oxidizing ability; The existence of double vacancies not only provides more active sites for the material, but also regulates the band structure of the material, which contributes to the generation of free radicals. The synergistic effect of Z-scheme heterojunction and V_Zn + O resulted in a removal rate of 89 % for NOR with a concentration of 20 mg/L after 90 min of visible light irradiation. In addition, based on ESR and XPS test results as well as DFT calculations, the synergistic mechanism of Z-scheme heterojunction and V_Zn + O in the photodegradation process was analyzed in detail from the aspects of band structure, electron transfer, free radical species, and built-in electric field. The pathway of NOR photodegradation and the toxicity analysis of intermediate products were also provided.