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

Abstract

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 ( k 1) of 2.4 × 10 − 2 dm 3 h − 1 . Under the present reaction conditions, k 1 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 C 7F 15 and COOH in PFOA and generates a C 7F 15 radical and CO 2. The C 7F 15 radical forms the thermally unstable alcohol C 7F 15OH, which undergoes F − elimination to form C 6F 13COF. This acid fluoride undergoes hydrolysis to yield another F − and the perfluorocarboxylic acid with one less CF 2 unit, C 6F 13COOH. By repeating these processes, finally, PFOA was able to be totally mineralized to CO 2 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.