New insights into dual metals boost PMS activation over iron tungstate: The role of surface hydroxyl

The activity of iron oxychloride (FeOCl) in advanced oxidation processes (AOPs) was limited by the redox cycle of Fe(III)/Fe(II). To resolve this problem, FeOCl loading on WS₂ (FW-X) was successfully synthesized with the different ratios of Fe/W (X = 0.5, 1, 2, 3) to active peroxymonosulfate (PMS). Among them, FW-3 displayed excellent catalytic performance which activated 0.5 mM PMS to degrade 97 % thiamphenicol (TAP) in 45 min. FW-3/PMS performed well in a wide pH range (3–10). Besides, the anions (${Cl}^{-},{NO}_3^{-},H_{2}P{O}_4^{-},HP{O}_4^{2-}\text{and}{HCO}_3^{-})$ and humic acid inhibited the oxidation of TAP in varying degrees. During the catalytic process, H₂O and PMS were adsorbed on the surface of FW-3 with the formation of surface hydroxyl groups (Fe–OH) and Fe–HOOSO₃. Surface hydroxyls as a bridge promoted the electron transfer between O–O bond and Fe sites. Then O–O bond broke to generate reactive oxygen species (ROS, $H{O}^{·}$, ${SO}_4^{\bullet -}$, Fe(IV), $O_2^{\bullet -}$ and ${}^1{O}_2$), and ${SO}_4^{\bullet -}$ was the dominant ROS for the degradation of TAP. Additionally, WS₂ as a co-catalyst broke the limitation of the iron redox cycle. Under the attack of these ROS, TAP produced eighteen intermediates through four possible degradation pathways. According to ecotoxicity assessment, toxicity of some intermediates produced from path of S-C bond breakage was less harmful than TAP. After three consecutive cycles, FW-3 maintained outstanding catalytic performance. Additionally, FW-3/PMS possessed a good non-selectivity that could remove different contaminants. In a word, this paper proved the potential of FW-3/PMS in practical wastewater remediation.