Influence of [sulfite]/[Fe(VI)] molar ratio on the active oxidants generation in Fe(VI)/sulfite process

Although several groups have made efforts to study micropollutants degradation by Fe(VI)/sulfite process, the mechanism is far from clear and warrants further investigation. Herein, the degradation kinetics and mechanism of selected micropollutants by sulfite (SO₃²⁻)-activated Fe(VI) oxidation were systematically investigated. The oxidation rates of enrofloxacin (ENR) and phenol in Fe(VI)/sulfite process ranged from 0.151 s⁻¹ to 6.18 s⁻¹ at pH 6.5 and 8.0. Sulfite applied in multiple-addition mode improved the degradation efficiency of micropollutants with electron-rich moieties compared to the single-addition mode. Based on results of the quenching experiments and kinetic simulation, Fe(V) was identified as the predominant active oxidant at [SO₃²⁻]/[Fe(VI)] molar ratio of 0.1 to 0.3. However, both Fe(V) and SO₄^[rad]−/^[rad]OH contributed to micropollutants oxidation at [SO₃²⁻]/[Fe(VI)] molar ratio ≥ 0.4 and their contributions were strongly dependent on the properties of micropollutants. The different degradation products of ENR in Fe(VI)/sulfite process at different sulfite dosages further supported the contribution of different active oxidants at different [SO₃²⁻]/[Fe(VI)] molar ratios. The toxicity of the reaction products of ENR towards Vibrio qinghaiensis sp.-Q67 decreased dramatically after Fe(VI)/sulfite treatment. The results of this work may promote the application of sulfite-activated Fe(VI) oxidation in water treatment.