Chemical Engineering Journal 136 (2008) 236–241 Kinetics of the oxidation of formaldehyde in a flow electrochemical reactor with TiO 2 /RuO 2 anode Mara Terumi Fukunaga a , Jos´ e Roberto Guimar˜ aes a , Rodnei Bertazzoli b,∗ a Departamento de Saneamento e Ambiente, Faculdade de Engenharia Civil, Arquitetura e Urbanismo, Universidade Estadual de Campinas, C.P. 6021, 13083-852 Campinas, SP, Brazil b Departamento de Engenharia de Materiais, Faculdade de Engenharia Mecˆ anica, Universidade Estadual de Campinas, C.P. 6122, 13083-970 Campinas, SP, Brazil Received 23 February 2007; received in revised form 30 March 2007; accepted 1 April 2007 Abstract This paper reports the electrochemical degradation of solutions containing formaldehyde by means of an electrochemical tubular flow reactor with a titanium anode coated with metal oxides (Ti/Ru 0.3 Ti 0.7 O 2 ). Due to the simplicity and low molecular weight of the compound it was possible to achieve high mineralization rates; the oxidation reaction of formaldehyde as well as TOC and COD removal were controlled by mass transfer. For solutions with 0.4 g L -1 of formaldehyde, electrodegradation followed a pseudo first-order kinetics, and the mass transport coefficients were calculated. After the experiments, a 97% reduction of TOC was observed, and the final formaldehyde and COD concentrations were below the detection limit threshold. For solutions with 12 g L -1 of formaldehyde processed at 100 mA cm -2 , a transition from a zero-order kinetics to a first-order kinetics started at the concentration for which the current density corresponded to the value of the limiting current. © 2007 Elsevier B.V. All rights reserved. Keywords: Formaldehyde electrooxidation; Oxide anodes; Electrochemical reactor 1. Introduction Electrochemical technology has been considered as a tool for controlling formaldehyde (FA) concentration in aqueous media [1–4]. The performance of DSA type oxide anodes has been tested for the electrooxidation of organic pollutants in aqueous media, markedly in the potential region in which oxygen evolu- tion occurs as a simultaneous process [5,6]. TiO 2 based RuO 2 or IrO 2 electrodes have been used for the electrooxidation of FA and a (TiO 2 ) 0.7 (RuO 2 ) 0.3 anode composition has exhibited higher FA and formic acid oxidation rates when compared to an IrO 2 containing electrode [1,2]. Low electrochemical activ- ity of IrO 2 electrodes for FA oxidation has also been observed during cyclic voltammetry and electrochemical impedance spec- troscopy experiments [3]. As it is known, IrO 2 containing oxide anodes present lower overpotential for the oxygen evolution, which is a preferential reaction over organics oxidation [5,6]. In an attempt to increase the overpotential for oxygen evolution, and to improve the electrode performance towards eletrooxi- ∗ Corresponding author. E-mail address: rbertazzoli@fem.unicamp.br (R. Bertazzoli). dation of organic molecules, doping of oxide surfaces with Sn and Pb may be an efficient strategy. SnO 2 supported on Ti has been used as electrode for the oxidation of FA with a significant electro-catalytic activity [4]. By adding PbO 2 to TiO 2 /RuO 2 oxide anodes, combustion reaction of organics is favored [1]. Combustion reaction of organics is more likely to occur on SnO 2 and PbO 2 , while on RuO 2 or IrO 2 based anodes a selective oxidation process takes place [5,6]. After the anodic discharge of water, the following step is the formation of hydroxyl rad- icals, which either oxidize the organics or else react to form molecular oxygen. During this process, hydroxyl radicals (OH • ) remain physically or chemically adsorbed on the anode surface, depending on the anode material. Physically adsorbed OH • is responsible for the oxidation of organics directly to carbon diox- ide in a reaction known as combustion. On TiO 2 based RuO 2 or IrO 2 electrodes, OH • is chemically adsorbed in the oxy- gen vacancies of the oxide as MO x+1 , where MO x represents a metallic oxide coating. Following that, it promotes a selec- tive oxidation resulting in lower molecular weight species that undergo further oxidation [5]. As it will be reported in this paper, it is possible to operate an electrochemical oxidation of small organic molecules under full mass transport control in which combustion reaction follows a pseudo-first-order kinetics. 1385-8947/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2007.04.006