Perfluorinated alkyl acids, such as perfluorooctanoic acid (PFOA), are extremely persistent water contaminants. Persulfate activation has already been described as a possible strategy for oxidizing PFOA, although the reaction of sulfate radicals with PFOA is challenging in the presence of common water constituents such as inorganic ions and natural organic matter, which strongly compete for sulfate radicals. The present study investigated FeS-assisted heterogeneous activation of persulfate for degradation of PFOA in water matrices. This approach breaks the dominance of the bulk-phase competitors for sulfate radicals, thus PFOA is also degraded in the presence of natural organic matter (up to 100 mg L^–1), Cl^- and NO₃^- (up to 100 mM). The hypothesis of heterogeneous reaction is supported by kinetic isotope effect measurements and a change in product pattern, and therefore a shift in participation of the various reaction pathways compared to homogeneous PFOA degradation. Significant PFOA degradation was achieved at moderate PFOA concentrations (c_0,PFOA = 1 mg L^–1) over a broad range of reaction conditions, although it is most efficient at higher reactant concentrations.

PFOA enrichment at the FeS surface is higher (K_D = 100 – 500 L kg^-1) than expected for a mineral with almost no inner surface; it seems to be the pre-requisite for reactions of radical consumers near the FeS surface and in the water matrix to be partially decoupled from each other.