pH/Temperature tuning selective generation of SO₄^•−/HO^•/Fe^Ⅳ in Fe²⁺-activated peroxide water systems

Active species (AS) are key in Fenton/Fenton-like reactions. The unclear AS category and generation order of SO₄^•− and HO^• in the reaction of peroxymonosulfate (PMS) and Fe²⁺, as well as the quantity of SO₄^•− produced in the Fe²⁺/peroxydisulfate (PDS) system, limiting the practical application of selective AS generation for targeted pollutant degradation. Understanding the pathway of PMS/PDS/H₂O₂ activated by Fe²⁺ for SO₄^•−, HO^•, and Fe^Ⅳ generation is critical for the selective generation of AS by adjusting reaction conditions of pH or temperature. Results suggested that SO₄^•− was the sole PMS active product at T < 340 K and pH < 12, subsequently driving HO^• generation from H₂O, while Fe^ⅣO²⁺ was rapidly generated due to the chemical interaction between Fe²⁺ and PMS. In Fe²⁺/PDS system, one SO₄^•− instead of reputed two SO₄^•− was generated since the coactions of Fe²⁺ and SO₄ moiety, while Fe^ⅣO²⁺ is generated when H₂O acts as reactant at pH 0 −7. In Fe²⁺/H₂O₂ system, Fe^ⅣO²⁺ can only be formed stem from the pre-reaction of HO^• generation. Furthermore, Tuning the reactant concentration could convert the AS category. This work advances the cognition of Fenton/Fenton-like microcosmic reactions, and is positive to the future design of experimental and industrial processes.