Destruction of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) by ball milling.

Abstract

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) have received high concerns due to their extreme persistence, and very few technologies have been reported for their complete destruction. For sound PFCs wastes disposal, mechanochemical method was employed using a planetary ball mill. Potassium hydroxide (KOH) was identified as the best comilling reagent and nearly complete destruction of both PFOS and PFOA was realized. The measured water-soluble fluoride accounted for most of the organic fluorine. The final products of PFOS after treatment were shown to be KF and K2SO4 by XRD analysis. The mass ratio between PFOS and KOH significantly affected the fluoride recovery but not for PFOS destruction and the sulfate recovery. The gradual formation of sulfate and fluoride reveals that the degradation of PFOS is initiated with the dissociation of the sulfonate group. FTIR spectra further showed the disappearance of the -CF3 and -CF2- groups with the generation of sulfate. The cleavage of C-F bonds in PFOS and the formation of fluoride ion were also identified by XPS spectra. On the basis of these results, possible reaction pathways were proposed. The approach was also successfully applied for the destruction of PFOS and PFOA homologues with different chain lengths.