Homogeneous activation of bisulfite by transition metals for micro-pollutant degradation: Mn(VII) versus Cr(VI)

• K of the Mn(VII)/S(IV) process was 4–5 orders of magnitude higher than the Cr(VI)/S(IV) process. • The reaction stoichiometry of [Cr(VI)] 0 :[S(IV)] 0 was determined to be 1:30–1:40. • SO 4 − was major active oxidant in Mn(VII)/S(IV) and Cr(VI)/S(IV) systems. • Mn(VII)/S(IV) process exhibits a stronger adaptation to the water matrices. In this study, the oxidation kinetics, mechanisms, effects and products of metalaxyl (MTL) by two different S(IV)-based processes (i.e., Mn(VII)/S(IV) and Cr(VI)/S(IV)) were systematically investigated and compared. The results showed that the concentration of S(IV) was primarily responsible for driving Mn(VII) and Cr(VI) consumption with a measured second order rate constant ( k ), and the k of Mn(VII) was less dependent on pH than that of Cr(VI), ranging from 10 4.7 M −1 s −1 to 10 4.9 M −1 s −1 and 10 −0.4 M −1 s −1 to 10 1.6 M −1 s −1 , respectively. Moreover, the reaction stoichiometry of [Cr(VI)] 0 :[S(IV)] 0 was estimated to about 1:30–1:40 in the presence of O 2 , which was significantly different from the molar ratio of [Mn(VII)] 0 to [S(IV] 0 (1:5–1:10). Sulfur radical (SO 4 − ) was determined to be a major active oxidant in both the systems based on the chemical probes and radical scavenging experiments. Besides, carbonate and chloride have greater inhibitory effects than other coexisting solutes (e.g., nitrate and organic matter) because of the higher reactivity with radical species. In addition, the proposed degradation of MTL mainly proceeded through hydroxylation, demethylation, and carboxylation reactions. All these results indicated that Mn(VII) outperforms Cr(VI) in the activation of S(IV) auto-oxidation for the degradation of MTL. An extremely fast degradation rate, high mineralization and strong adaptation to the water matrices were observed in the process, which were attributed to ultra-fast production of high concentrations of radical species such as SO 4 − , HO and high valent manganese intermediates (e.g., Mn(III) and Mn(V)).