Synergistic mechanism of surface oxygen vacancies and metal sites on Al-substituted NiFe2O4 during peroxymonosulfate activation in the solid-water interface for 2,4-D degradation

In this study, Al-doped NiFe 2 O 4 (NiAl x Fe 2-x O 4 ) was synthesized by the isomorphic substitution strategy, which was further employed for peroxymonosulfate (PMS) activation to remove 2,4-dichlorophenoxyacetic acid (2,4-D) in water. Doping of Al could construct oxygen vacancies (OVs) in the structure of NiAl x Fe 2-x O 4 , which further facilitates the exposure of metal sites for PMS activation. The removal rate of 2,4-D increased greatly compared to the NiFe 2 O 4 -activated process. By the density functional theory (DFT) process, it was verified that Al was more likely to replace Fe rather than Ni. OVs sites have the lowest adsorption energy of H 2 O ( E ads  = -0.95 eV), and easily formed surface hydroxyl groups (–OH), which were further replaced by PMS (HSO 5 - ). The formed surface complex Me-O-O-SO 3 - was readily activated via electron transfer between O-O bond, thus promoting the cleavage of it and inducing SO 4 · - generation. This process induced the high performance of the NiAl x Fe 2-x O 4 /PMS system for effective 2,4-D degradation. Furthermore, the purified water showed relatively low toxicity. The present study proposed new insights into the interface mechanism of PMS triggering.