Electrochemical oxidation of 1H,1H,2H,2H-perfluorooctane sulfonic acid (6:2 FTS) on DSA electrode: Operating parameters and mechanism

The 6:2 FTS was the substitute for perfluorooctane sulfonate (PFOS) in the chrome plating industry in Japan. Electrochemical oxidation of 6:2 FTS was investigated in this study. The degradabilities of PFOS and 6:2 FTS were tested on the Ti/SnO₂-Sb₂O₅-Bi₂O₃ anode. The effects of current density, potential, and supporting electrolyte on the degradation of 6:2 FTS were evaluated. Experimental results showed that 6:2 FTS was more easily degraded than PFOS on the Ti/SnO₂-Sb₂O₅-Bi₂O₃ anode. At a low current density of 1.42mA/cm², 6:2 FTS was not degraded on Ti/SnO₂-Sb₂O₅-Bi₂O₃, while the degradation ratio increased when the current density ranged from 4.25 to 6.80mA/cm². The degradation of 6:2 FTS at current density of 6.80mA/cm² followed pseudo first-order kinetics with the rate constant of 0.074hr^-1. The anodic potential played an important role in the degradation of 6:2 FTS, and the pseudo first-order rate constants increased with the potential. The surface of Ti/SnO₂-Sb₂O₅-Bi₂O₃ was contaminated after electrolysis at constant potential of 3V, while the fouling phenomenon was not observed at 5V. The fouled anode could be regenerated by incinerating at 600°C. The intermediates detected by ultra-performance liquid chromatography coupled with a triple-stage quadrupole mass spectrometer (UPLC-MS/MS) were shorter chain perfluorocarboxylic acids. The 6:2 FTS was first attacked by hydroxyl radical, and then formed perfluorinated carboxylates, which decarboxylated and removed CF₂ units to yield shorter-chain perfluorocarboxylic acids.