Insights into the formation of environmentally persistent free radicals during photocatalytic degradation processes of ceftriaxone sodium by ZnO/ZnIn2S4

At present, the researches on photocatalysis were mainly focused on the design, improvement and development of catalysts, and less attention was paid to the existing characteristics of environmentally persistent free radicals (EPFRs) during the process of photocatalytic oxidation. In this study, A flower-like Z-type heterojunction ZnO/ZnIn2S4 (ZnO/ZIS) and typical antibiotic ceftriaxone sodium (CS) were taken as study objects, concentrating on the generation characteristics of EPFRs during the degradation of CS by ZnO/ZIS, and clarifying the degradation mechanism of CS in which EPFRs participated. The results showed that the degradation efficiency of 10 mg/L CS by 0.40 g/L ZnO/ZIS reached 85.3% in 150 min under the irradiation of 500 W xenon lamp. It was clear that ·O2- and h+ play major roles in CS degradation by ZnO/ZIS under visible light, and ·OH plays an auxiliary role. Furthermore, the formation mechanism of EPFRs during photocatalytic degradation processes of CS by ZnO/ZIS were first investigated thoroughly via experimental analysis and density functional theory (DFT) calculations. The concentration level of EPFRs centered on oxygen atoms is 1011 spin/mm3, which were generated in the process of degradation of CS by ZnO/ZIS under visible light. The production of EPFRs chiefly includes two procedures: chemical adsorption and transfer of electrons. The adsorption energy of precursor P8 on ZnIn2S4 side is -1.91 eV, the electrons transferred from precursor P8 and P11 to ZnO/ZnIn2S4 heterojunction. Surprisingly, EPFRs have little negative effects on the degradation process of CS by ZnO/ZIS. The study was not only a key field in the development of photocatalysis technology, but also a new way to study the removal mechanism of antibiotics.