[Bisulfite Promoted Minute Fe²⁺-Activated Peroxydisulfate for Paracetamol Degradation]

Fe²⁺ has been commonly selected to activate peroxydisulfate(PDS) for sulfate radical(SO₄^-·) generation because of its eco-friendly, cost-effective, and high activity characteristics. However, Fe²⁺ can be rapidly oxidized to Fe³⁺ in the reaction, leading to poor utilization of iron for PDS activation. Further, a fairly high concentration of Fe²⁺ is generally required and may cause iron sludge production and secondary pollution. In this study, a minute Fe²⁺-activated PDS system induced by bisulfite(BS) was used to degrade paracetamol(APAP) in water. The results showed that the Fe²⁺-PDS system could be enhanced by the circulation of Fe²⁺-Fe³⁺ with the injection of BS and by keeping Fe²⁺ at a high concentration. Under the optimal conditions(PDS=0.6 mol·L^-1; BS=0.4 mol·L^-1; Fe²⁺=10 μmol·L^-1; pH=4), 100% APAP(4 μmol·L^-1) was removed within 180 s. The degradation rate of APAP increased with the increase in BS(0-0.6 mmol·L^-1) and PDS(0.2-1.5 mmol·L^-1) concentration, and a modest Fe²⁺ concentration could accelerate APAP removal. Co-existing substances inhibited the APAP removal and followed the order of HCO₃^->HPO₄^2->Cl^->NO₃^->humic acid(HA). Based on the quenching experiments and electron paramagnetic resonance spectroscopy test, SO₄^-· was shown to be the primary reactive species for APAP decomposition in the BS-Fe²⁺-PDS process. Three-dimensional fluorescence spectroscopy revealed that APAP intermediates had fluorescence characteristics. Moreover, five intermediates were identified, and the probable APAP degradation pathways were proposed. The removal efficiencies of APAP were lower in real waters than that in ultrapure water. Nevertheless, the removal effect was greatly improved after a prolonged reaction time. All results indicated that the BS-Fe²⁺-PDS system could be a promising method for organic pollutant treatment.