Electrochemical oxidation of 6:2 fluorotelomer sulfonic acid (6:2 FTSA) on BDD: electrode characterization and mechanistic investigation

6:2 Fluorotelomer sulfonic acid (6:2 FTSA) is used as surfactant and foam stabilizer in the formulation of air firefighting foams (AFFFs). 6:2 FTSA is produced as an alternative to persistent and bioaccumulative long-chain perfluoroalkyl compounds. This study investigates the electrochemical degradation of 6:2 FTSA on a boron-doped diamond (BDD) anode. First, the BDD anode was characterized by cyclic voltammetry, revealing that the direct oxidation of 6:2 FTSA occurred at an anodic potential of 2.72 V versus Ag/AgCl (saturated KCl) electrode. Increasing the scan rate resulted in an increased current intensity of the direct oxidation peak, and this relationship was analyzed using the Randles-Sevcik equation to calculate the diffusion coefficient of 6:2 FTSA in aqueous media (D = 4.16 x 10(-6) cm(2) s(-1) at room temperature). This value is in close agreement to the predicted value obtained by the Wilke-Chang correlation. In electrolysis experiments under potentiostatic control, increasing the anode potential over 2.72 V greatly enhanced the 6:2 FTSA removal, and the simultaneous formation of short-chain perfluorocarboxylic acids (perfluorohexanoic acid, perfluorpentanoic acid and perfluorobutanoic acid) and fluoride release were observed. Based on these observations, the 6:2 FTSA degradation pathway was predicted to start by the attack of hydroxyl radicals to the non-fluorinated carbons to form a perfluorocarboxylate, followed by a single electron transfer to the anode to yield a reactive radical C(6)F(13)COOaEuro . The latter species decarboxylated and finally combined with hydroxyl radicals to allow defluorination to form shorter-chain perfluorocarboxylic acids. [GRAPHICS] .