The effects of inorganic anions on degradation kinetics and isotope fractionation during the transformation of tris(2-chloroethyl) phosphate (TCEP) by UV/persulfate

Abstract

Tris(2-chloroethyl) phosphate (TCEP), as a typical chlorinated flame retardant, is attracting more attention as a carcinogen. Although persulfate-based oxidation exhibits good performance in removing refractory organic pollutants, the kinetics of persulfate-based remediation are affected by inorganic anions, which causes inaccurate remediation efficiency. This study combines steady-state radical concentration modelling with isotope fractionation to investigate the effects of inorganic anions on TCEP degradation by UV/persulfate (UV/PS). In the absence of anions during UV/PS system, the observed degradation rate was (9.7 ± 0.1) × 10−5 s−1, which was approximately 93 % attributed to sulfate radical (SO4−•) oxidation based on radical modelling. Carbon isotope fractionation, coupled with the identification of transformation products by mass spectrometry, suggests a carbon bond split during TCEP degradation with a carbon isotopic fractionation value (ε) of −1.6 ± 0.2 ‰ (± 95 % confidence intervals). With respect to co-existing anions in UV/PS system, the addition of chloride (Cl−) had a negligible effect on degradation rates, while the addition of hydrogencarbonate (HCO3−) caused them to decrease, and the addition of hydrogenphosphate (HPO42−) caused them to increase. Radical modelling suggested that SO4−• was transformed to chlorine radicals (Cl•/Cl2−•), phosphate radicals (HPO4−•), and carbonate radicals (CO3−•). Furthermore, the overlapping 95 % confidence intervals (C.I.) and the statistical tests (p > 0.05) both agree that Cl− and HPO42− gain identical ε values. Nevertheless, when HCO3− coexisted in the UV/PS system, the ε values were distinct. The addition of HCO3− would result in ε variation of TCEP in the UV activated PS process, which should receive more attention when applying remediation.