Challenging established norms: The unanticipated role of alcohols in UV/PDS radical quenching.

UV/peroxydisulfate (UV/PDS) process is known to be highly efficient for degrading micropollutants from water by generating sulfate (SO₄^•-) and hydroxyl radicals (HO^•). Reliable analyses of short-lived SO₄^•- and HO^• are therefore critical for understanding reaction mechanisms and optimizing operating conditions. Currently, alcohols are commonly used as quenchers to distinguish radicals based on the assumption that they exclusively react with target radicals without other influences. However, this study for the first time reveals a series of unexpected effects that challenge this conventional wisdom because: 1) adding alcohols altered the decomposition rates of PDS by replacing the reactions between SO₄^•- and HO^• with PDS by the reactions between secondary reactive species and PDS; and 2) SO₄^•- preferably reacted with alcohols to generate nonnegligible level of hydrogen peroxide (H₂O₂) under oxygen-rich conditions, which subsequently led to indirect formation of HO^•. Additionally, the formation of H₂O₂ was substantially impacted by the types of alcohols, dosages, dissolved oxygen, and solution pH. Using probe tests as tools, we found that the actual SO₄^•- levels after dosing alcohols were only slightly different from assumed/expected levels, whereas the actually HO^• levels were 43.7, 3364.9, and 12.5 times higher than assumed/expected conditions for samples dosed with methanol, iso-propanol, and tert-butanol, respectively. These unanticipated effects thus suggest that cautions are needed when using alcohols to qualitative and quantitative determine HO^• and SO₄^•- in UV/PDS process.