Oxidation of carboxylic acids at boron-doped diamond electrodes for wastewater treatment   D. GANDINI 1 , E. MAHE ´ 1;3 , P.A. MICHAUD 1 , W. HAENNI 2 , A. PERRET 2 and Ch. COMNINELLIS 1 * 1 Institute of Chemical Engineering, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland 2 CSEM Centre Suisse d’Electronique et de Microtechnique S.A, Rue Jacquet-Droz 1, CH-2007 Neucha ˆtel, Switzerland 3 On leave from UMR 7612 LI2C-CNRS-UPMC, 4 Place Jussieu, 75252 Paris Cedex 05, France (*author for correspondence, e-mail: christos.comninellis@epfl.ch.) Received 23 July 1999; accepted in revised form 19 October 1999 Key words: boron-doped diamond electrodes, electrochemical oxidation, organics, wastewater treatment Abstract Thin boron-doped diamond films have been prepared by HF CVD (hot filament chemical vapour deposition technique) on conductive p-Si substrate (Si/Diamond). The morphology of these Si/diamond electrodes has been investigated by SEM and Raman spectroscopy. The electrochemical behaviour of the Si/diamond electrodes in 1 M H 2 SO 4 and in 1 M H 2 SO 4 + carboxylic acids has been investigated by cyclic voltammetry. Finally, the electrochemical oxidation of some simple carboxylic acids (acetic, formic, oxalic) has been investigated by bulk electrolysis. These acids can be oxidized at Si/diamond anodes to CO 2 , in the potential region of water and/or the supporting electrolyte decomposition, with high current efficiency. 1. Introduction Oxidative electrochemical processes promising versatil- ity, environmental compatibility and cost eectiveness have a continuously growing importance both in selec- tive organic synthesis and in degradation of organic pollutants. For the oxidative degradation of organics in wastewater treatment, the aim is the complete oxidation of organics to CO 2 [1–4] or the conversion of the toxic organics to biocompatible compounds [5, 6]. The electrode material is clearly an important param- eter when optimizing such processes since the mecha- nism and the products of several anodic reactions are known to depend on the anode material. For example, the anodic oxidation of phenol yields hydroquinone and benzoquinone at Ti/IrO 2 anodes and mainly carbon dioxide at Ti/SnO 2 –Sb 2 O 5 anodes [7]. In previous work a generalized mechanism for the oxidation of organics in the potential region of O 2 evolution was proposed [8–10]. This mechanism explains the complete oxidation of organics to CO 2 by electro- generated hydroxyl radicals on ‘non-active’ electrodes and the selective oxidation on ‘active’ electrodes. Ac- cording to this mechanism, boron doped diamond, a ‘non-active’ electrode, is an ideal anode for the complete oxidation of organics to CO 2 for wastewater treatment. The electrochemistry of synthetic boron-doped dia- mond films has received great attention recently [11–15]. Few electrochemical studies have been made on syn- thetic boron-doped diamond films with the goal of developing application in the electrochemical oxidation of organics for waste water treatment [16–18]. The results have shown that only reactions involving simple electron transfer are active on diamond elec- trodes in the potential region of water stability [17, 18]. For oxidation reactions with more complex mecha- nisms, complex oxidation reactions can take place on diamond electrodes only in the potential region of water discharge [17, 18]. Diamond electrodes have also been successfully used for the anodic oxidation of cyanide and the cathodic recovery of heavy metals [19]. Synthetic boron doped diamond films have also been successfully deposited on titanium base metal (Ti/ diamond) [20]. The configuration of this electrode is similar to DSA (dimensionally stable anodes) electrodes and opens up new possibilities in industrial electro- chemistry. It has been reported that the main reason for the successful preparation of the Ti/Diamond electrodes is the formation of a TiC interlayer between the Ti base metal and the diamond coating [20]. In this work the anodic oxidation of simple carboxylic acids (acetic, formic, oxalic) have been investigated on synthetic diamond film electrodes in acidic medium. These compounds are very resistant to oxidation. In fact acetic acid and formic acid are the final oxidation   Dedicated to the memory of Daniel Simonsson Journal of Applied Electrochemistry 30: 1345–1350, 2000. 1345 Ó 2000 Kluwer Academic Publishers. Printed in the Netherlands.