Efficient electrochemical decomposition of perfluorocarboxylic acids by the use of a boron-doped diamond electrode

The electrochemical decomposition of environmentally persistent perfluorooctanoic acid (PFOA) was achieved by the use of a boron-doped diamond (BDD) electrode. The PFOA decomposition follows pseudo-first-order kinetics, with an observed rate constant (k1) of 2.4 × 10− 2 dm3 h− 1. Under the present reaction conditions, k1 increased with increasing current density and saturated at values over 0.60 mA cm− 2. Therefore, the rate-limiting step for the electrochemical decomposition of PFOA was the direct electrochemical oxidation at lower current densities. In the proposed decomposition pathway, direct electrochemical oxidation cleaves the C–C bond between the C7F15 and COOH in PFOA and generates a C7F15 radical and CO2. The C7F15 radical forms the thermally unstable alcohol C7F15OH, which undergoes F− elimination to form C6F13COF. This acid fluoride undergoes hydrolysis to yield another F− and the perfluorocarboxylic acid with one less CF2 unit, C6F13COOH. By repeating these processes, finally, PFOA was able to be totally mineralized to CO2 and F−. Moreover, whereas the BDD surface was easily fluorinated by the electrochemical reaction with the PFOA solution, medium pressure ultraviolet (MPUV) lamp irradiation in water was able to easily remove fluorine from the fluorinated BDD surface.

Research Highlights
► Perfluorooctanoic acid is decomposed by electrolysis on a diamond electrode.
► The diamond electrode surface was fluorinated by the electrolysis.
► Ultraviolet irradiation was able to remove fluorine from the diamond surface.