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

Park, Saerom, Lee, Linda S., Medina, Victor F., Zull, Aaron, Waisner, Scott
Chemosphere 2016 v.145 pp. 376-383
benzene, carboxylic acids, electrodes, ethylbenzene, fluorides, free radicals, fuels, groundwater, half life, high performance liquid chromatography, models, oxidation, perfluorocarbons, perfluorooctane sulfonic acid, perfluorooctanoic acid, remediation, slurries, soil, sulfates, sulfonic acid, tandem mass spectrometry, temperature, toluene, xylene
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.