Clarification of the role of singlet oxygen for pollutant abatement during persulfate-based advanced oxidation processes: Co3O4@CNTs activated peroxymonosulfate as an example

Singlet oxygen ( 1 O 2 ) has often been identified by the popularly used quenching method as a more important reactive species (RS) than sulfate radicals (SO 4 •− ) 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 1 O 2 in PS-AOPs, this study developed a probe compound-based experimental and kinetic model to quantify the concentrations and exposures of 1 O 2 , SO 4 •− , and • OH, as well as their relative contributions to pollutant abatement during a cobalt oxide incorporated carbon nanotubes activated peroxymonosulfate (Co 3 O 4 @CNTs/PMS) process. Results show that during the Co 3 O 4 @CNTs/PMS process, the exposures and transient concentrations of 1 O 2 were about 19.6 and 41.3 times higher than those of SO 4 •− and • OH, respectively. However, the relative contribution of 1 O 2 to the abatement of most pollutants tested in this study (e.g., sulfisoxazole, sulfamethoxyprazine, trimethoprim, and metoprolol) is generally negligible ( f 1 O 2 ${\mathrm{f}}_{1\mathrm{O}2}$ ≤ 8%) compared to that of SO 4 •− and • OH ( f SO 4 • − ${\mathrm{f}}_{{\text{SO}}_4^{•-}}$  = 15%−98% and f • OH ${\mathrm{f}}_{•\text{OH}}$  = 2%−78%) because of the significantly lower reactivity of 1 O 2 with these compounds than that of SO 4 •− and • OH. Reasons for misidentifying 1 O 2 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 1 O 2 can be generated in significant amounts and be present at higher concentrations than SO 4 •− and • OH in PS-AOP systems, 1 O 2 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.