Iron–cobalt mixed oxide nanocatalysts: Heterogeneous peroxymonosulfate activation, cobalt leaching, and ferromagnetic properties for environmental applications

Sulfate radical-based advanced oxidation technologies (SR-AOTs) are attracting considerable attention due to the high oxidizing ability of SRs to degrade organic pollutants in aqueous environments. This study was carried out to respond to current concerns and challenges in SR-AOTs, including (i) need of heterogeneous activation of sulfate salts using transition metal oxides, (ii) nanoscaling of the metal oxide catalysts for high catalytic activity and promising properties with respect to leaching, and (iii) easy removal and recovery of the catalytic materials after their applications for water and wastewater treatments. In this study, we report a novel approach of using Fe–Co mixed oxide nanocatalysts for the heterogeneous activation of peroxymonosulfate (PMS) to generate SRs targeting the decomposition of 2,4-dichlorophenol, and especially focus on some synthesis parameters such as calcination temperature, Fe/Co contents, and TiO2 support. The physicochemical properties of the catalysts were investigated using porosimetry, XRD, HR-TEM, H2-TPR, and XPS. Ferromagnetic CoFe2O4 composites formed by thermal oxidation of a mixed phase of Fe and Co exhibited significant implications for the efficient and environmentally friendly activation of PMS, including (i) the cobalt species in CoFe2O4 are of Co(II), unlike Co3O4 showing some detrimental effects of Co(III) on the PMS activation, (ii) CoFe2O4 possesses suppressed Co leaching properties due to strong Fe–Co interactions (i.e. Fe–Co linkages), and (iii) Fe–Co catalysts in form of CoFe2O4 are easier to recover due to the unique ferromagnetic nature of CoFe2O4. In addition, the presence of Fe was found to be beneficial for enriching hydroxyl group content on the Fe–Co catalyst surface, which is believed to facilitate the formation of Co(II)-OH complexes that are vital for heterogeneous PMS activation.