Electrochemical Oxidation of Environmentally Persistent Perfluorooctane Sulfonate by a Novel Lead Dioxide Anode

Abstract Ti/TiO 2 -NTs/Ag 2 O/PbO 2 anode was prepared by electrodeposition technique and characterized by scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), linear sweep voltammetry (LSV), and accelerated life test. Electrochemical oxidation of persistent organic pollutant wastewater perfluorooctane sulfonate (PFOS) was carried out with the novel PbO 2 anode. Ti/TiO 2 -NTs/Ag 2 O/PbO 2 anode exhibited a pyramid structure, which was the typical PbO 2 electrodes prepared using electrochemical deposition method. XRD spectra indicated that diffraction peaks of PbO 2 coating conformed to that of JCPDS (Joint Committee on Powder Diffraction Standards) card for β-PbO 2 . Ti/TiO 2 -NTs/Ag 2 O/PbO 2 anode showed high oxygen evolution potential, and longer life service, compared with Ti/PbO 2 and Ti/TiO 2 -NTs/PbO 2 anodes. The degradation ratio of PFOS (90 mL of 0.0929 mmol L −1 ) was 74.87%, with a pseudo first-order kinetic constant of 0.0165 min −1 and a half-life of 43.18 min −1 at a constant current density of 30 mA cm −2 after 180 min of electrolysis. PFOS oxidation yielded sulfate, fluoride, and perfluorocarboxyl anions (i.e., C 3 F 7 COO − , C 4 F 9 COO − , C 5 F 11 COO − , C 6 F 13 COO − , and C 7 F 15 COO − ). The electrospray ionization (ESI) mass spectrum confirmed that oxygen in the intermediate products originated from a heavy-oxygen water electrolyte, and the degradation of PFOS was initiated by the dissociation of a sulfonic group. A possible mechanism was revealed; that was, PFOS was desulfated at the anode to form C 8 F 17 · and then transformed into C 8 F 17 OH, followed by intramolecular rearrangement and hydrolysis reactions to form C 7 F 15 COO − . Kolbe decarboxylation occurred in C 7 F 15 COO − at the anode to generate C 7 F 15 ·, which evolved into C 6 F 13 COO − in a similar way and the CF 2 unit fell off from C 7 F 15 COO − . PFOS was gradually degraded into short-chain perfluorocarboxyl anions by repeating the CF 2 unzipping cycle.