Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 81:352–358 (2006) DOI: 10.1002/jctb.1402 Detoxification of synthetic industrial waste- waters using electrochemical oxidation with boron-doped diamond anodes Pablo Ca ˜ nizares, Cristina S ´ aez, Justo Lobato and Manuel A Rodrigo ∗ Department of Chemical Engineering, Facultad de Ciencias Qu´ ımicas, Universidad de Castilla La Mancha, Campus Universitario s/n 13071 Ciudad Real, Spain Abstract: The electrochemical oxidation on boron-doped diamond (BDD) anodes of synthetic industrial wastewaters polluted with different aromatic compounds has been studied. This technology reduces significantly the organic load and the toxicity of treated aqueous wastes. The shape of the toxicity vs time curve depends strongly on the nature of the phenolic compounds, and especially on the substituent groups contained in the phenol molecule. For the oxidation of hydroxybenzenes a complex shape is obtained, with a plateau zone in the first stages of the electrolyses. This plateau zone is transformed into a maximum in the electrolyses of chlorophenols. In the case of nitrophenols, the toxicity index decreases at the same time as the chemical oxygen demand (COD) concentration, although the decrease in the toxicity is less significant than the decrease in the COD. All these toxicity profiles can be justified in terms of the nature of the intermediates generated during the electrolyses, but they cannot be directly related to the COD variation, which shows an exponential decrease.  2005 Society of Chemical Industry Keywords: electrochemical; oxidation; boron-doped diamond; toxicity; industrial wastes INTRODUCTION Many industrial processes generate wastes con- taining small concentrations of refractory organic compounds. 1–4 Most of these compounds are known by their high toxicity level and their persistence, and thus their recovery or their elimination is required prior to the discharge or the reuse of the waste flow. In appropriate circumstances, the organic com- pounds contained in these wastes can be economi- cally recovered, but usually the best method to treat these wastes is their destruction by different oxida- tion techniques such as Advanced Oxidation Processes (AOP), 5–9 Catalytic Wet Air Oxidation (CWAO) 10 – 13 or electrochemical oxidation. 14 – 17 These technologies can completely mineralise the pollutants and, conse- quently, they can reduce the organic load and toxicity of the waste. Of these technologies, electrochemical oxidation on boron-doped diamond (BDD) anodes appears to be one of the most promising. In recent years, it has been applied successfully to the treatment of aqueous wastes containing non-biodegradable organics such as phenol, 17 – 20 chlorophenols, 21 – 26 nitrophenols 27,28 or aniline. 29,30 Recently, it has been demonstrated that hydroxyl radicals are formed during the electrolysis on BDD anodes of aqueous electrolytes 31 and this has allowed this technology to be classified as an Advanced Oxidation Process. The combination of both the direct oxidation onto the electrode surface and also the hydroxyl-radicals mediated oxidation has been shown to increase the current efficiency of this technique, as compared with other electrochemical technologies. Nevertheless, in spite of these high current efficiencies, the energy requirements in the oxidation of industrial wastewaters are still very high due to the large number of electrons required to completely oxidise the organic matter to carbon dioxide. To decrease the energy requirements, an alternative treatment could be the partial oxidation of pollutants to obtain wastes that can be biologically treated (in a combined process). This option requires less energy, but it is important to consider that some intermediates could also be toxic and such materials must be completely removed. 32 Thus, in the search for less expensive applications it is important to have a good mechanistic understanding of the process and to correlate this mechanism with the toxicity (or the biodegradability) variation. Unfortunately, only a few papers have raised the detoxification of these ∗ Correspondence to: Manuel A Rodrigo, Department of Chemical Engineering, Facultad de Ciencias Qu´ ımicas, Universidad de Castilla La Mancha, Campus Universitario s/n 13071 Ciudad Real, Spain E-mail: Manuel.Rodrigo@uclm.es Contract/grant sponsor: MCT (Ministerio de Ciencia y Tecnolog´ ıa, Spain) Contract/grant sponsor: EU (European Union); contract/grant number: REN2001-0560 (Received 26 January 2004; revised version received 17 May 2005; accepted 8 August 2005) Published online 24 October 2005  2005 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2005/$30.00 352