Activation of peroxymonosulfate by nanoscaled NiFe2O4 magnetic particles for the degradation of 2,4-dichlorophenoxyacetic acid in water: Efficiency, mechanism and degradation pathways

Well-crystallized magnetic NiFe2O4 nanoparticles were applied to activate peroxymonosulfate (PMS) for degrading 2,4-dichlorophenoxyacctic acid (2,4-D) in water. The performance of NiFe2O4 synthesized with different metals (Ni and Fe) and citric acid ([Men+]T /CA) molar ratio was investigated. The synthesized NiFe2O4 with the molar ratio of [Men+]T /CA of 1:1 showed the best performance to activate PMS, exerting a positive effect on 2,4-D degradation. The removal efficiency of 2,4-D reached 97.5 % in the NiFe2O4(1:1)/PMS system. Various water matrix factors, including solution pH, reaction temperature and inorganic ions were considered during the removal of 2,4-D. The quenching tests and ESR spectra displayed that the generation of reactive oxygen species (ROS) in the NiFe2O4(1:1)/PMS system, ·OH and SO4·- played the predominant role during 2,4-D degradation. The cycles of Fe2+/Fe3+ and Ni2+/Ni3+ on the surface of the catalysts prompted the decomposition of HSO5- for ·OH and SO4·- generation. The degradation mechanism of 2,4-D was declared by the density functional theory (DFT) calculations. The Cl and O in the structure of 2,4-D were rich in electrons, which were possible reaction sites for ROS. And the degradation mechanism of 2,4-D mainly includes decarboxylation, –OH substitution of Cl and H, and opening ring of benzene structure.