Synergistic mechanism of disparate surface hydroxyls and oxygen vacancies towards peroxymonosulfate activation for durable water decontamination

Constructing oxygen vacancies (OVs) and surface hydroxyls are efficient methods in catalyst modification engineering. However, their unclear interrelationships limit the design of more efficient catalysts with dual active centers. For this purpose, we concurrently constructed the OVs and surface hydroxyls by Ni²⁺ doping α-FeOOH. The isomorphic substitution of Ni²⁺ to Fe³⁺ induced remarkable formation of OVs with the crystalline phase transformed to α-Fe₂O₃ and partial structure hydroxyls synchronously preserved. α-Fe_1.6Ni_0.4O₃H showed better performance in activating peroxymonosulfate (PMS) to degrade clothianidin (CLO) than α-FeOOH and α-Fe₂O₃, and almost complete removal of CLO within 30 min was achieved. Various in situ experiments and DFT calculations demonstrated that the hydrolyzed hydroxyls were prone to fill the OVs, leading to the poisoning of active sites, and replacing it is the necessary pathway for surface-complexation of PMS. The structure hydroxyls were verified to alleviate the occupation of hydrolyzed hydroxyls on OVs by hydrogen-bonding interaction, thus forming sustainable OVs and enhancing the PMS adsorption. Besides, OVs and structure hydroxyls synergistically enhanced the electron transfer ability of surrounding Fe sites, promoting PMS activation. Furthermore, α-Fe_1.6Ni_0.4O₃H/PMS system has good environmental tolerance, especially for common oxygenated anions (SO₄^2-, NO₃^– and HCO₃^–). Coupling with membrane filtration further improved its practicality, and the coupling system achieved excellent removal of CLO, UV₂₅₄ and DOM in raw water and filtered water. These findings lay a foundation for identifying the role of OVs and surface hydroxyls, as so to give new inspiration for developing efficient catalysts to facilitate PMS activation, and also propose a superior strategy for practical application of the catalytic system in water treatment.