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.