Novel non-radical elimination mechanism of phosphorus flame retardant parent compound in Triton X-100 enhanced solar activated peroxydisulfate process: Key role of energy transfer from singlet oxygen to Triton X-100

The elimination mechanism of emerging contaminants by triplet excited state organic compounds generated from non-radical pathway was rarely concerned in advanced oxidation process (AOP). This study proposed Triton X-100 (TX-100, a nonionic surfactant) enhanced solar activated persulfate process (Solar/PDS/TX-100 process) as a novel method to rapidly eliminate 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO, an organic phosphorus flame retardant parent compound), which mainly depended on 3TX-100* as a new reactive intermediate, instead of reactive oxygen species and SO4 center dot- . The results of DOPO degradation and probe compounds experiments, EPR spectra as well as triplet excited state compound scavenge experiments demonstrated that the observed degradation rate constant (kobs-DOPO) was 3.70 x 10-3 s- 1 in Solar/PDS/TX-100 process, 4.86 times higher than that in Solar/PDS process where kobs-DOPO was 7.61 x 10-4 s- 1, and 3TX-100* was the dominant reactive intermediate leading to the enhancement of DOPO degradation where the second order rate constant of 3TX-100* vs. DOPO was 6.98 x 109 M- 1s- 1. Singlet oxygen was the energy precursor to trigger the transformation from TX-100 to 3TX-100*, which promoted the yield of 3TX-100*. The degradation pathways and products toxicity assessment demonstrated that Solar/PDS/TX-100 process had better performance on DOPO detoxification than Solar/PDS process.