New evidence for the involvement of superoxide and singlet oxygen in UV-activated peroxydisulfate system under acidic conditions

Sulfate (SO₄^∙-) and hydroxyl radicals (∙OH) are widely acknowledged as the primary reactive oxidizing species in ultraviolet irradiation/peroxydisulfate (UV/PDS) systems. However, if only SO₄^∙- and/or ∙OH are taken into account in such a system, some experimental data cannot be adequately explained, suggesting the presence of other significant and unknown oxidizing species. Here, the identity and origins of reactive species in the UV/PDS system at pH 4.5 were thoroughly investigated using the electron paramagnetic resonance (EPR) secondary radical spin-trapping method in conjunction with chemical probes, radical scavengers, 5,5-dimethyl-1-pyrrioline N-oxide (DMPO), 2,2,6,6-tetramethyl-4-poperidine monohydrate (TEMP), and various solvents. In addition to the dominant radical-SO₄^∙-, previously overlooked species such as the superoxide radical anion/hydroperoxyl radical (O₂^∙-/∙OOH) and singlet oxygen (¹O₂) contributed to ∼ 35 % and ∼ 17.5 % of the total radicals (SO₄^∙-, ∙OH, O₂^∙-/∙OOH, and ¹O₂), respectively. As a key byproduct of photolyzed PDS solution, H₂O₂ reacts with PDS to produce O₂^∙-/∙OOH, and indirectly contributes to the production of ¹O₂ that originates from the conversion of O₂^∙-/∙OOH. These discoveries advance our knowledge on the composition and origins of reactive species generated in the traditional UV/PDS system, which help to predict the varying efficacy of UV/PDS for wastewater treatment or environmental cleanup.