Heat-activated persulfate oxidation of PFOA, 6:2 fluorotelomer sulfonate, and PFOS under conditions suitable for in-situ groundwater remediation

PFOA (perfluorooctanoic acid) oxidation (0.121–6.04 μM) by heat-activated persulfate was evaluated at 20–60 °C with 4.2–84 mM S2O82− and in the presence of soluble fuel components to assess feasibility for in-situ remediation of groundwater. 6:2 fluorotelomer sulfonic acid/sulfonate (6:2 FTSA) and PFOS (perfluorooctanesulfonic acid) persulfate oxidation was also evaluated in a subset of conditions given their co-occurrence at many sites. High performance liquid chromatography electron spray tandem mass spectrometry was used for organic analysis and fluoride was measured using a fluoride-specific electrode. PFOA pseudo-1st order transformation rates (k1,PFOA) increased with increasing temperature (half-lives from 0.1 to 7 d for 60 to 30 °C) sequentially removing CF2 groups ('unzipping') to shorter chain perfluoroalkyl carboxylic acids (PFCAs) and F−. At 50 °C, a 5-fold increase in S2O82− led to a 5-fold increase in k1,PFOA after which self-scavenging by sulfate radicals decreased the relative rate of increase with more S2O82−. Benzene, toluene, ethylbenzene and xylene did not affect k1,PFOA even at 40 times higher molar concentrations than PFOA. A modeling approach to explore pathways strongly supported that for 6:2 FTSA, both the ethyl linkage and CF2–CH2 bond of 6:2 FTSA oxidize simultaneously, resulting in a ratio of ∼25/75 PFHpA/PFHxA. The effectiveness of heat-activated S2O82− on PFOA oxidation was reduced in a soil slurry; therefore, repeated persulfate injections are required to efficiently achieve complete oxidation in the field. However, PFOS remained unaltered even at higher activation temperatures, thus limiting the sole use of heat-activated persulfate for perfluoroalkyl substances removal in the field.