Enhanced activation of peroxymonosulfate using oxygen vacancy-enriched FeCo2O4−x spinel for 2,4-dichlorophenol removal: Singlet oxygen-dominated nonradical process

The introduction of oxygen vacancy (OV) is a promising strategy to boost performance for the removal of recalcitrant organic contaminants through sulfate radical-based advanced oxidation processes. Herein, a novel catalyst, namely, FeCo2O4−x nanoparticles (NPs) with abundant OV (FCOVNPs) was successfully obtained by a simple hydrothermal synthesis method, and its catalytic performance toward 2,4-dichlorophenol (2,4-DCP) removal was evaluated by coupling it with peroxymonosulfate (PMS). Results of X-ray photoelectron spectroscopy demonstrated the existence of 14.9 % of OV on the surface of FCOVNPs, while 25.9 and 37.8 % of metal vacancy were observed for a mixture of Fe2O3 and Co3O4 NPs (FCONPs) and CoFe2O4 NPs (CFONPs), respectively. Raman spectroscopy, electron paramagnetic resonance, and trapping experiments indicated that the introduction of abundant OV was conducive to the regulation of the band structure and charge movement behaviors of FCOVNPs, which accelerated the cycle of redox pairs from Co3+/Fe3+ to Co2+/Fe2+, while also generating singlet oxygen (1O2), which was the primary radical responsible for PMS activation. Moreover, sulfate (SO4−) and hydroxyl (OH) radicals were also generated on the surface of FCOVNPs through synergistic interactions among OV, transition-metals, and surface hydroxyl groups. Therefore, degradation occurred via direct and indirect radical mechanisms toward 2,4-DCP removal in the FCOVNPs/PMS system due to the efficient optimization of OV, resulting in highly-efficient degradation of 97.3 % compared to that in the FCONPs (46.8 %) and CFONPs (77.9 %). Moreover, FCOVNPs exhibited superior durability after five cycles. The catalytic mechanism and 2,4-DCP degradation pathways were proposed when the FCOVNPs/PMS system was used as the catalytic system. Thus, the FCOVNPs/PMS system can provide some novel inspirations for the development of crystal defects on the surface of spinel catalysts in the environmental field.