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

Abstract

[EN] 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 × 10¿6 cm2 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 C6F13COO¿. The latter species decarboxylated and finally combined with hydroxyl radicals to allow defluorination to form shorter-chain perfluorocarboxylic acids.