Perfluoroalkyl acids (PFAAs) only represent a small fraction of per- and polyfluoroalkyl substances (PFASs) in commercial products and at contaminated sites. Despite the phase-out of long-chain perfluoroalkyl acids (PFAAs) by major PFAS manufacturers, substantial amount of PFAAs can continuously enter into the environment because of the degradation of the precursor compounds. Large-scale field studies on polluted groundwater have shown that in situ chemical oxidation (ISCO) targeting chlorinated solvents and hydrocarbons affected the composition and distribution of PFAAs and promoted the transformation of precursor compounds. However, there is little knowledge available on the transformation kinetics and mechanisms of precursor compounds in these engineered systems. 6:2 fluorotelomer sulfonate (6:2 FTS) is widely detected in the environment, sometimes exceeding ppm levels in groundwater. Peroxymonosulfate (PMS)-based advanced oxidation process has been explored in recent years for use in wastewater treatment, and it is also promising for ISCO. In this study, we investigated the transformation of 6:2 FTS by Co(II)-activated PMS at conditions representative of ISCO. We found that 6:2 FTS was efficiently transformed in the Co(II)-activated PMS system with a pseudo-first-order rate constant of 1.90 h–1 at 20 °C. 6:2 FTS was transformed to 6:2 fluorotelomer carboxylic acid (6:2 FTCA) rapidly and both 6:2 FTS and 6:2 FTCA was further transformed to shorter-chain perfluoroalkyl carboxylic (PFCAs) simultaneously, with perfluorohexanoic acid (PFHxA) being the predominant product. The transformation of 6:2 FTS was controlled by the steady-state concentration of sulfate radical, resulting in highest reaction rate at circumneutral pH and low background electrolyte concentrations. Further degradation of PFCAs required higher concentrations of PMS, lower pH, as well as elevated temperature. We further applied quantum chemistry simulation by density functional theory calculation, to investigate the reaction mechanism of 6:2 FTS and PFCAs with sulfate radical and to explain the significance of the different reaction conditions. Our results indicate that application of PMS in ISCO would significantly increase the concentrations of shorter-chain PFCAs through the transformation of FTSs.

Transformation of 6:2 fluorotelomer sulfonate by activated peroxymonosulfate at conditions representative of in situ chemical oxidation

2019 Spring National Meeting

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