Clarification of the role of singlet oxygen for pollutant abatement during persulfate-based advanced oxidation...

Singlet oxygen (¹O₂) has often been identified by the popularly used quenching method as a more important reactive species (RS) than sulfate radicals (SO₄^•-) and hydroxyl radicals (^•OH) for pollutant abatement during persulfate-based advanced oxidation processes (PS-AOPs), especially those activated by carbon-based catalysts. However, latest studies have demonstrated that the quenching method actually can often mislead the interpretations of the role of RS for pollutant abatement during AOPs due to various confounding effects caused by adding high-concentration quenchers in the system. To clarify the role of ¹O₂ in PS-AOPs, this study developed a probe compound-based experimental and kinetic model to quantify the concentrations and exposures of ¹O₂, SO₄^•-, and ^•OH, as well as their relative contributions to pollutant abatement during a cobalt oxide incorporated carbon nanotubes activated peroxymonosulfate (Co₃O₄@CNTs/PMS) process. Results show that during the Co₃O₄@CNTs/PMS process, the exposures and transient concentrations of ¹O₂ were about 19.6 and 41.3 times higher than those of SO₄^•- and ^•OH, respectively. However, the relative contribution of ¹O₂ to the abatement of most pollutants tested in this study (e.g., sulfisoxazole, sulfamethoxyprazine, trimethoprim, and metoprolol) is generally negligible (f_1O2 ≤ 8%) compared to that of SO₄^•- and ^•OH ( [Formula: see text]  = 15%-98% and f_•OH = 2%-78%) because of the significantly lower reactivity of ¹O₂ with these compounds than that of SO₄^•- and ^•OH. Reasons for misidentifying ¹O₂ as the dominant RS for pollutant abatement by the quenching method were then analyzed based on reaction kinetics principles. The results of this study highlight that while ¹O₂ can be generated in significant amounts and be present at higher concentrations than SO₄^•- and ^•OH in PS-AOP systems, ¹O₂ is unlikely to be the dominant RS for the abatement of most pollutants during the PS-AOPs because of its weak and selective oxidation capacity, and caution should be taken when using the quenching method to evaluate the role of RS for pollutant abatement by the PS-AOPs.