Comparative study of the electrocatalytic oxidation and mechanism of nitrophenols at Bi-doped lead dioxide anodes Yuan Liu, Huiling Liu *, Yan Li Department of Environmental Science & Engineering, State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Municipal and Environmental Engineering Harbin Institute of Technology, Harbin 150090, China 1. Introduction Nitrophenols (NPs) represent a class of widely synthesized chemicals particularly involved in the manufactures of pesti- cides, dyes and pharmaceuticals [1–5]. NPs are anthropogenic, toxic, inhibitory and bio-refractory organic compounds and are considered as hazardous substances and priority toxic pollutants by the United States Environmental Protection Agency (USEPA), and the Public Health Service in the USA [6,7]. The USEPA recommends restricting the concentration of these NPs in natural waters in the range of 1–20 ppb [8], as it takes a long time for NPs to break down in groundwater, as well as in deep soil. The degradation of NPs by biological treatment is difficult and requires long incubation time since the presence of nitro-group confers to the aromatic compound a strong chemical stability and resistance to microbial degradation [9–12]. It is of significant importance to develop new treatment technologies for the destruction and mineralization of NPs in wastewater. In recent years, several works have been focused on different technologies such as chemical [13], sonochemical [14], adsorption [15] and advanced oxidation processes (AOPs) including Fenton [16,17], electro-Fenton [18,19], photocatalytic oxidation [20,21], cataly- tic wet air oxidation [22,23] and electrocatalytic oxidation [24–29]. Within these techniques, electrocatalytic oxidation appears as one of the most promising technologies for the treatment of wastewater containing small amounts of aromatic compounds. The main advantages of the electrocatalytic oxidation processes include environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability to automation and cost effective- ness [30]. The use of high performance anodic materials like lead dioxide (PbO 2 ) electrode can achieve high efficiency and lower the operating cost. PbO 2 has been extensively used to decompose organic contaminants owing to its high electrical conductivity, high oxygen overpotential and chemical inertness and low cost comparing with boron-doped diamond (BDD) [24,31–35]. A general scheme for the electrochemical degradation of organic compounds on metal oxide electrodes (MO x ) has been proposed [36].H 2 O is assumed to be discharged on the anode to form adsorbed hydroxyl radicals: MO x þ H 2 O ! MO x ð  OHÞþ H þ þ e  (1) Applied Catalysis B: Environmental 84 (2008) 297–302 ARTICLE INFO Article history: Received 15 January 2008 Received in revised form 8 March 2008 Accepted 5 April 2008 Available online 18 April 2008 Keywords: Nitrophenols Electrocatalytic oxidation Bi-doped lead dioxide anodes Mechanisms Hammett constant ABSTRACT The electrocatalytic oxidation of o-nitrophenol (o-NP), m-nitrophenol (m-NP) and p-nitrophenol (p-NP) has been studied at Bi-doped lead dioxide anodes on acid medium by cyclic voltammetry and bulk electrolysis. The results of voltammetric studies indicated that these nitrophenol isomers were indirectly oxidized by  OH radical in the solutions. Within the present experimental conditions used (50 mg of nitrophenol L 1 , pH 4.3, 30 mA cm 2 , 303 K), the complete decomposition of nitrophenols (NPs) was achieved. The electrocatalytic oxidation of NPs lay in the order: p-NP > m-NP > o-NP. Molecular configuration including the electron character and hydrogen bonds of NPs significantly influenced the electrocatalytic oxidation of these isomers. Hydroquinone, catechol, resorcinol, benzoquinone, aminophenols, glutaconic acid and maleic acid and oxalic acid have been detected as soluble products during the electrolysis of NPs. The possible degradation pathways of these isomers were proposed. The first stage is the release of nitro group from the aromatic rings. As a consequence, hydroquinone, catechol, resorcinol and benzoquinone are formed. These organic compounds are oxidized initially to carboxylic acids (glutaconic acid, maleic acid and oxalic acid) and later to carbon dioxide and water. Simultaneously, the reduction of NPs to aminophenols takes place at the cathode. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +86 451 53625118. E-mail address: hlliu2002@163.com (H. Liu). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.04.011