Highly efficient photocatalytic degradation of microcystin-LR with permonosulfate activated by visible-light over Z-scheme ZnO/g-C3N4: Degradation pathways and mechanism

Abstract

Harmful algal blooms and the release of cyanotoxins pose a significant threat to aquatic life. This study aimed to develop a high efficient way to degrade microcystin-LR (MC-LR), a most common and highly toxic cyanotoxin in cyanobacterial blooms, by constructing a Z-scheme ZnO-g-C3N4 heterojunction for photocatalysis integrated with permonosulfate (PMS) activation process. The formation of the Z-scheme heterojunction facilitated the separation of photogenerated carriers and generated more active species with strong oxidative potential, resulting in more efficient degradation of MC-LR. Under visible light, the ZnO/g-C3N4/PMS photocatalyst successfully removed 97% of MC-LR (500 µg·L−1) within 120 min. In comparison, the ZnO/g-C3N4/PMS (k = 0.074 min−1) system exhibited degradation rate constants of 8.6 and 11.7 times higher than that of g-C3N4/PMS (k = 0.0086 min−1) and ZnO/PMS (k = 0.0063 min−1) systems, respectively. Chemical trapping experiments and EPR tests revealed that the predominant active species were •O2−, SO4•-, and •OH. The degradation route of MC-LR by ZnO/g-C3N4/PMS/Vis process was proposed based on the intermediate products analysis by UPLC-MS, wherein the addition of •OH and the oxidative cleavage by •O2− and SO4•- were the main degradation pathways. This study developed a prospective way to design Z-scheme heterojunction photocatalyst constructing with PMS activation for highly efficient prevention of harmful algal blooms.