Activity and mechanism of vanadium sulfide for organic contaminants oxidation with peroxymonosulfate

Transition metal sulfides have been demonstrated to be effective for peroxymonosulfate (PMS) activation towards wastewater treatment. However, the activity of vanadium sulfide (VS4) and the role of the chemical state of V have not been revealed. Here, three types of VS4 with various morphologies and chemical states of V were synthesized by using methanol (M-VS4, nanosphere composed of nanosheets), ethanol (E-VS4, sea urchin like nanosphere) and ultrapure water (U-VS4, compact nanosphere) as hydrothermal solvent, respectively, and used as heterogeneous catalysts to activate PMS for the degradation of refractory organic pollutants. The effects of PMS concentration, temperature, pH, inorganic ions, and humic acid (HA) on the degradation efficiency of VS4/PMS system were investigated systematically. The results indicated that the highest specific surface area and lowest ratio of V5+ enable E-VS4/PMS system possessed the highest performance in degrading tetracycline hydrochloride (TCH), in which 100% TCH was removed after operating 10 min (0.805 min-1) under a relatively low concentration of PMS (1 mM) and catalyst (100 mg/L). It also revealed that the system exhibited a typical radical process in TCH degradation, which could be attributed to the redox cycles between V5+, V4+ and V3+ in the presence of PMS to generate various radicals. This radical process enabled the E-VS4/PMS system with a high activity in wide reaction conditions and high mineralization ratios in degrading various refractory organic pollutants within 10 min. In addition, the E-VS4/PMS system exhibited favorable reusability and stability with very less V and S ions leaching, and showed excellent performance in real water purification.