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

Abstract

Ti/TiO2-NTs/Ag2O/PbO2 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 PbO2 anode. Ti/TiO2-NTs/Ag2O/PbO2 anode exhibited a pyramid structure, which was the typical PbO2 electrodes prepared using electrochemical deposition method. XRD spectra indicated that diffraction peaks of PbO2 coating conformed to that of JCPDS (Joint Committee on Powder Diffraction Standards) card for β-PbO2. Ti/TiO2-NTs/Ag2O/PbO2 anode showed high oxygen evolution potential, and longer life service, compared with Ti/PbO2 and Ti/TiO2-NTs/PbO2 anodes. The degradation ratio of PFOS (90mL of 0.0929mmolL−1) was 74.87%, with a pseudo first-order kinetic constant of 0.0165min−1 and a half-life of 43.18min−1 at a constant current density of 30mAcm−2 after 180min of electrolysis. PFOS oxidation yielded sulfate, fluoride, and perfluorocarboxyl anions (i.e., C3F7COO−, C4F9COO−, C5F11COO−, C6F13COO−, and C7F15COO−). 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 C8F17· and then transformed into C8F17OH, followed by intramolecular rearrangement and hydrolysis reactions to form C7F15COO−. Kolbe decarboxylation occurred in C7F15COO− at the anode to generate C7F15·, which evolved into C6F13COO− in a similar way and the CF2 unit fell off from C7F15COO−. PFOS was gradually degraded into short-chain perfluorocarboxyl anions by repeating the CF2 unzipping cycle.