Formation and transformation of reactive species in the Fe²⁺/peroxydisulfate/Cl⁻ system

The influence of Cl⁻ on the formation mechanism of active components is often neglected in the Fe²⁺/peroxydisulfate (PDS) system containing a large amount of ferryl ion reactive specie (Fe(Ⅳ)). In the current investigation, the effects of Cl⁻ concentration on the removal of methyl phenyl sulfoxide (PMSO), the formation of methyl phenyl sulfone (PMSO₂), the transformation of reactive species and oxidation products were investigated under different reaction conditions that included Fe²⁺ dosage, PDS dosage, and pH₀. The results showed that Cl⁻ complexing Fe²⁺ increased the formation path of sulfate radical (SO₄^·−) in the Fe²⁺/PDS system. Fe²⁺ dosage and pH₀ value affected the content and morphology of Fe²⁺-Cl⁻ complex, thus affecting the composition of reactive species. According to the experiment of free radical steady-state concentration, it was found that low concentration of Cl⁻ reacted with SO₄^·− and increased the steady-state concentration of chlorine radicals (8.09 × 10⁻¹³ M [·Cl]_ss at 1.41 mM Cl⁻), while at high concentration of Cl⁻, the contents of SO₄^·−, hydroxyl radical (·OH) and dichloride anion radicals (Cl₂^·−) increased and the contents of Fe(Ⅳ) and ·Cl decreased. ·Cl had strong reactivity with PMSO, and PMSO and its oxidation products were chlorinated under the combined action of ·Cl and Cl₂^·−. This work reveals the reaction mechanism and environmental application risks of Fe²⁺/PDS technology and lays the groundwork for subsequent industrial application of Fe²⁺/PDS system.