Mechanistic insight into active species formation during Fenton-like processes by regulating dissimilar charged groups on Fe3O4 nanospheres

Iron-mediated activation of peroxymonosulfate (PMS) had long been of great interest for the effective oxidation of micropollutants. However, how the chemical environment of iron oxides influence active species formation in heterogeneous Fenton processes was poorly understood. Herein, Fe3O4-X nanospheres (X = –NH2, –COOH) were synthesized for coordination chemical environment regulation. The DFT calculation showed the models of Fe3O4-X modified by –NH2 and –COOH in four-coordination and six-coordination environments. Among them, Fe3O4-COOH in four-coordination environment had the lowest formation energy and highest adsorption energy combined with PMS, with a most stable system. In the adsorption model of four-coordination Fe3O4-COOH, the d-orbitals and s-orbitals energy levels of Fe atoms underwent drastic changes compared with those before adsorption, H atoms on the carboxyl group and O atoms on the PMS formed a new HO– that attached to Fe atoms, with a strong coupling between them. The simulation results confirmed the proposed activation mechanism of Fe3O4-COOH activating PMS to generate SO4·- and HO–, and then partial SO4·- reacted with HO– to generate ·OH, and finally the two radicals attacked tetracycline (TC) together. In Fe3O4-COOH/PMS system, Fe3O4-COOH/PMS system can convert more Fe2+ into Fe3+, which also means that more Fe2+ reacts with PMS to produce Fe3+ and SO4·-. The TC degradation efficiency of the Fe3O4-COOH/PMS system was 93%, higher than that of the Fe3O4-NH2/PMS system after 30 min of reaction. Consequently, this work contributes to the mechanistic insight into active species regulation during Fenton-like processes by manipulating surface groups.