Efficient electrochemical decomposition of perfluorocarboxylic acids by the use of a boron-doped diamond electrode Tsuyoshi Ochiai a , Yuichi Iizuka a,b , Kazuya Nakata a , Taketoshi Murakami a , Donald A. Tryk c , Akira Fujishima a,d, ⁎, Yoshihiro Koide b , Yuko Morito e a Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa, 213-0012, Japan b Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1, Rokkakubashi, Yokohama, Kanagawa, 221-8686, Japan c Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae-cho, Kofu, Yamanashi 400-0021, Japan d Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan e U-VIX CORPORATION, 2-14-8 Midorigaoka, Meguro-ku, Tokyo 152-0034, Japan abstract article info Article history: Received 26 August 2010 Received in revised form 19 November 2010 Accepted 4 December 2010 Available online 13 December 2010 Keywords: Diamond film Oxidation Electrical property characterization Surface characterization Electrodes Electrochemical 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 7 F 15 and COOH in PFOA and generates a C 7 F 15 radical and CO 2 . The C 7 F 15 radical forms the thermally unstable alcohol C 7 F 15 OH, which undergoes F - elimination to form C 6 F 13 COF. This acid fluoride undergoes hydrolysis to yield another F - and the perfluorocarboxylic acid with one less CF 2 unit, C 6 F 13 COOH. 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. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Perfluorinated acids have been widely used in industry as surfactants, surface treatment agents, and flame retardants. As the use of perfluorinated acids has increased, some, typically, perfluor- ooctanoic acid (C 7 F 15 COOH; PFOA), have been detected in the environment [1,2]. Analytical studies have revealed their toxicological properties and high stability [3]. Thus, techniques for decomposing them to harmless species under mild conditions are desirable. Until now, the decomposition of perfluorinated acids has been studied by the use of photocatalysis [4–6], sonochemical decomposition [7,8], and nanofiltration [9], etc. However, these methods have required the use of demanding procedures, for example, the use of specialized vessels under harsh treatment conditions. Recently, the electrolysis of organic pollutants by use of boron- doped diamond (BDD) electrodes has received growing attention [10–13]. The wide potential window of BDD electrodes makes it possible to generate various highly active oxidants such as persulfate, which can oxidize organics efficiently. Our previous research also clearly shows the great advantage of BDD electrodes for water purification [14]. Carter et al. reported the electrochemical decompo- sition of perfluorooctane sulfonate (PFOS) using BDD electrodes [15]. A clear decrease in PFOS concentration, together with the rate- limiting mechanism for PFOS oxidation, were shown in this study. However, the amount of F - generation was significantly less than expected, and there was no evidence of CO 2 generation. Therefore, it is still unclear whether or not the total decomposition of PFOS was achieved; the reaction pathway also remains unclear. Herein we report the efficient electrochemical decomposition of PFOA by use of a BDD electrode and propose a decomposition pathway. Moreover, the evaluation and treatment with a medium pressure ultraviolet (MPUV) lamp irradiation of the BDD electrode surface were also investigated for continuous practical use. Diamond & Related Materials 20 (2011) 64–67 Abbreviations: PFOA, perfluorooctanoic acid; PFOS, perfluorooctane sulfonate; BDD, boron-doped diamond; MPUV, medium-pressure ultraviolet lamp; UV, ultraviolet; HPLC, high performance liquid chromatography; XPS, X-ray photoelectron spectrosco- py; LC–MS, Liquid chromatography–mass spectrometry. ⁎ Corresponding author. Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa, 213-0012, Japan. Tel.: + 81 44 819 2040; fax: +81 44 819 2070. E-mail address: fujishima@newkast.or.jp (A. Fujishima). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.12.008 Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond