Radical generation in thermally activated peroxydisulfate process with temperature threshold as low as 45 °C

Although thermally activated peroxydisulfate (PDS) is often regarded as a radical-dominant oxidation process, the accurate temperature threshold necessary for substantial radical production remains a subject of debate, despite its critical relevance for mechanism interpretations and process design optimization. Through a series of radical identification experiments (i.e., electron paramagnetic resonance (EPR) and fluorescence assays), we demonstrate that hydroxyl radical (radOH) and sulfate radical (SO₄rad⁻) are not genuinely produced at lower temperatures (25–47 °C). The detection of the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) adduct of radCH₂OH (indicative of the H abstraction of methanol by free radicals) and hydroxylated product of the fluorescence probe coumarin, provides preliminary evidence that the temperature threshold for radical generation in the heat/PDS system is 48 °C. This threshold is further refined to 45 °C, as evidenced by the detection of hydrogen signals corresponding to the heptet of 5,5-dimethylpyrrolidin-2-one-1-oxyl (DMPOX) at this temperature, using ¹H nuclear magnetic resonance (NMR) technique. At a temperature of 80 °C, SO₄rad⁻ is initially generated in the heat/PDS system, resulting in the formation of the heptet DMPOX. In conclusion, this study clarifies the identity of SO₄rad⁻ and establishes the temperature threshold for its generation, which has significant implications for the process optimization and cost saving.