Life-sustaining of H+ in S(IV)/Fe(VI) system for efficient removal of dimethoate in water: Active species identification and mechanism

The present study developed a prolonged oxidizable sulfur(IV)/ferrate(VI) system for the generation of hydroxyl radical (OH) and sulfate radical (SO4−) within 120 min, and proposed a reaction mechanism. In the presence of H+ in solution, the S(IV)/Fe(IV) system provided excellent oxidation performance of refractory organics O,O-dimethyl-S-methylcarbamoyl methyl phosphorothioate (dimethoate, a widely used organophosphorus pesticide) within 120 min. The degradation efficiency of dimethoate in the bisulfite/Fe(VI) system (e.g., 90.06% dimethoate) was higher than that in the sulfite/Fe(VI) system (e.g., 51.03% dimethoate), while Fe(VI) alone degraded 27.81% of dimethoate. In the S(IV)/Fe(VI) system, the active species consisted of Fe(VI) (including the highly-reactive iron-based intermediate species, i.e. Fe(V) and Fe(IV)), OH and SO4−, with OH being identified as the main active species, contributing more than half of the total. The degradation mechanism of dimethoate in the S(IV)/Fe(VI) system mainly included the cleavage of the S-P bond, C-S bond and the oxidation of the P = S bond. Two plausible degradation pathways were proposed based on the detected transformation products. Dimethoate could be efficiently degraded when the [bisulfite]/[Fe(VI)] molar ratio was 4:1 with a low Fe(VI) dosage (218 µM), the changes in HA and HCO3− concentrations affected the transformation of dimethoate, the presence of Cl−, SO42− and NO3− had a negligible effect, and the S(IV)/Fe(VI) system performed well in real waters. The S(IV)/Fe(VI) system has excellent oxidation performance for the removal of organic pollutants in water, with a final pH (pH = 6.6) close to neutral and ultimate products of SO42− and Fe(OH)3, which has great potential for engineering applications.