Perfluorooctanesulfonic acid and its conjugate base perfluorooctanesulfonate) are chemicals having an eight-carbon fluorocarbon chain and a sulfonic acid or sulfonate functional group. Collectively, the two species are called PFOS. The acid is an example of perfluorosulfonic acid. The acid and the sulfonate salts are perfluoroalkyl substances (PFAS). Once a poplular fluorosurfactant, PFOS is now regarded as a global pollutant. PFOS was the key ingredient in Scotchgard, a fabric protector made by 3M, and related stain repellents. The acronym "PFOS" refers to the parent sulfonic acid and to various salts of perfluorooctanesulfonate. These are all colorless or white, water-soluble solids. Although of low acute toxicity, PFOS has attracted much attention for its pervasiveness and environmental impact. It was added to Annex B of the Stockholm Convention on Persistent Organic Pollutants in May 2009.
History In 1949, 3M began producing PFOS-based compounds by electrochemical fluorination. In 1968, organofluorine compounds were detected in the blood serum of consumers, and in 1976, perfluorooctanoic acid (PFOA) or a related compound such as PFOS were suggested as components. In 1997, 3M detected PFOS in blood from global blood banks, although the company's internal documents indicate knowledge of this decades earlier, dating from the 1970s. In 1999, the U.S. Environmental Protection Agency began investigating perfluorinated compounds after receiving data on the global distribution and toxicity of PFOS, the key ingredient in Scotchgard. For these reasons, and USEPA pressure, the primary American producer of PFOS, 3M, announced, in May 2000, the phaseout of the production of PFOS, PFOA, and PFOS-related products. Most other manufacturers (particularly, those in Europe) phased out the production of PFOS and perfluorooctanoic acid (PFOA) in 2000 and 2006, respectively. A shorter-chain PFOS, perfluorohexanesulfonic acid (PFHxS), was included in Annex A to the Stockholm Convention in 2022. Currently, most of PFOS and PFOS-related chemicals are produced in China.
Production The main method used for the industrial scale production of PFOS is electrochemical fluorination (ECF). ECF is an electrolysis method whereby the precursor of octanesulfonyl fluoride is electrolyzed in a solution of hydrogen fluoride to give perfluorooctanesulfonyl fluoride. ECF produces 70% linear PFOS, 25% branched and 5% terminal; this is not a function of the production process but rather that the precursor also exhibits this isomer ratio. Although 89 constitutional isomers of PFOS are possible, environmental samples usually consist of the linear isomer and 10 branched isomers. Telomerisation involves constructing the PFOS molecule using short chain (often 2-carbon) precursors and adding a sulfonate group as a final step. This production process results in 100% linear PFOS. This production method, whilst cleaner and resulting in a much purer product than ECF, is not known to have been widely used except for the production of reagent grade PFOS and analytical standards.
Indirect routes Perfluorooctylsulfonyl compounds degrade to PFOS. Examples include N-methyl perfluorooctane sulfonamidoethanol (N-MeFOSE), a carpet stain repellent, and N-ethyl perfluorooctane sulfonamidoethanol (N-EtFOSE), a paper treatment. Also perfluorooctanesulfonamide is a precursor. About 50 precursors were named in the 2004 proposed Canadian ban on PFOS.
Chemical properties Perfluorooctanesulfonic acid is a strong acid and as such does not exist in aqueous settings. Instead it is fully ionized. Regardless, the acid and its anion highly resist degradation under environmental conditions. Waste water treatment plants also fail to degrade PFOS. The C8F17 subunit of PFOS is hydrophobic and lipophobic, like other fluorocarbons, while the sulfonic acid/sulfonate group confers polarity. PFOS is an exceptionally stable compound in industrial applications and in the environment because of the aggregate effect of many carbon–fluorine bonds. PFOS is a fluorosurfactant that lowers the surface tension of water more than that of hydrocarbon surfactants.
Uses Perfluorooctanesulfonic acid was usually used as the sodium or potassium salts.
PFOS was the key ingredient in Scotchgard, a fabric protector made by 3M, and numerous stain repellents. PFOS, together with PFOA, has also been used to make aqueous film forming foam (AFFF), a component of fire-fighting foams, and alcohol-type concentrate foams. PFOS compounds can also be found in some impregnation agents for textiles, paper, and leather; in wax, polishes, paints, varnishes, and cleaning products for general use; in metal surfaces, and carpets. In the semiconductor industry, PFOS is used in multiple photolithographic chemicals including: photoacid generators (PAGs) and anti-reflective coatings (ARCs). It has been phased out in the European Union semiconductor industry due to health concerns. PFOS is the key ingredient in Skydrol, a fire-resistant hydraulic fluid used in commercial aviation. The most important emission sources of PFOS have been metal plating and fire-fighting foams. Because of concerns about PFOS, F-53B has been used as a replacement for mist suppression in metal plating.
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