Photocatalytic degradation of chlorinated organic pollutants by ZnS@ZIF-8 composite through hydrogen peroxide generation by activating dioxygen under simulated sunlight irradiation.

Photocatalytic hydrogen peroxide (HO) production and its application in advanced oxidation processes (AOPs) are regarded as low-cost and environmentally friendly wastewater treatment processes. Herein, by modifying a small amount of sulphide on the zeolitic imidazolate framework-8 (ZIF-8), a ZnS@ZIF-8 composite and used for photocatalytic HO production to degrade chlorinated organic pollutants under simulated sunlight (SSL). ZnS@ZIF-8 composite could enhance the separation of photo-induced charge carriers, promote electron transfer from zinc sulphide (ZnS) to ZIF-8, which exhibited good selectivity for the two-electron oxygen reduction reaction (2e-ORR) and two-electron water oxidation (2e-WOR) pathways. Based on oxygen (O) activation, the developed ZnS@ZIF-8/O/SSL system could achieve 6.43 mmol/L HO production within 150 min, which was approximately 8.66 and 10.36 times higher than that of the ZnS/O/SSL and ZIF-8/O/SSL systems, respectively. In the ZnS@ZIF-8/O/SSL system, the ORR, WOR and HO photolysis led to the generation of hydroxyl radical (•OH), while the photochemical behavior of ZnS in ZnS@ZIF-8 composite resulted in the generation of active hydrogen (*H). Benefitting from the high concentration of HO and the coexistence of redox species in the ZnS@ZIF-8/O/SSL system, various chlorinated organic pollutants could be dechlorinated and mineralized. In addition, a possible mechanism for photocatalytic HO production was also proposed. Importantly, the proposed process did not involve an additional sacrificial agent or Fenton-like catalysts. This work provides insights into the potential application of ZnS@ZIF-8 composite for HO production and wastewater treatment.