Sonochemical Degradation of Perfluorooctanesulfonate in Aqueous Film-Forming Foams

Aqueous film-forming foams (AFFFs) are fire extinguishing agents developed by the Navy to quickly and effectively combat fires occurring close to explosive materials and are utilized today at car races, airports, oil refineries, and military locations. Fluorochemical (FC) surfactants represent 1−5% of the AFFF composition, which impart properties such as high spreadability, negligible fuel diffusion, and thermal stability to the foam. FC’s are oxidatively recalcitrant, persistent in the environment, and have been detected in groundwater at AFFF training sites. Ultrasonic irradiation of aqueous FCs has been reported to degrade and subsequently mineralize the FC surfactants perfluorooctanoate (PFOA) and perfluorooctanesulfonate (PFOS). Here we present results of the sonochemical degradation of aqueous dilutions of FC-600, a mixture of hydrocarbon (HC) and fluorochemical components including cosolvents, anionic hydrocarbon surfactants, fluorinated amphiphilic surfactants, anionic fluorinated surfactants, and thickeners such as starch. The primary FC surfactant in FC-600, PFOS, was sonolytically degraded over a range of FC-600 aqueous dilutions, 65 ppb < [PFOS]<sub>i</sub> < 13 100 ppb. Sonochemical PFOS−AFFF decomposition rates, <i>R</i><sub>AFFF</sub><sup>−PFOS</sup>, are similar to PFOS−Milli-Q rates, <i>R</i><sub>MQ</sub><sup>−PFOS</sup>, indicating that the AFFF matrix only had a minor effect on the sonochemical degradation rate, 0.5 < <i>R</i><sub>AFFF</sub><sup>−PFOS</sup>/<i>R</i><sub>MQ</sub><sup>−PFOS</sup> < 2.0, even though the total organic concentration was 50 times the PFOS concentration, [Org]<sub>tot</sub>/[PFOS] ∼ 50, consistent with the superior FC surfactant properties. Sonochemical sulfate production is quantitative, Δ[SO<sub>4</sub><sup>2−</sup>]/Δ[PFOS] ≥ 1, indicating that bubble-water interfacial pyrolytic cleavage of the C−S bond in PFOS is the initial degradation step, in agreement with previous studies done in Milli-Q water. Sonochemical fluoride production is significantly below quantitative expectations, Δ[F<sup>−</sup>]/Δ[PFOS] ∼ 4 vs 17, suggesting that in the AFFF matrix, PFOS’ fluorochemical tail is not completely degraded, whereas Milli-Q studies yielded quantitative F<sup>−</sup> production. Measurements of time-dependent methylene blue active substances and total organic carbon indicate that the other FC-600 components were also sonolytically decomposed.