Water Research 36 (2002) 1034–1042 Comparison of different advanced oxidation processes for phenol degradation Santiago Esplugas, Jaime Gim! enez*, Sandra Contreras, Esther Pascual, Miguel Rodrı´guez Departamento de Ingenierı´a Quı´mica y Metalurgia de la Universidad de Barcelona, Martı´i Franqu " es, 1, 08028 Barcelona, Spain Received 7 July 2000; accepted 6 June 2001 Abstract Advanced Oxidation Processes (O 3 ,O 3 /H 2 O 2 , UV, UV/O 3 , UV/H 2 O 2 ,O 3 /UV/H 2 O 2 ,Fe 2+ /H 2 O 2 and photocatalysis) for degradation of phenol in aqueous solution have been studied in earlier works. In this paper, a comparison of these techniques is undertaken: pH influence, kinetic constants, stoichiometric coefficient and optimum oxidant/pollutant ratio. Of the tested processes, Fenton reagent was found to the fastest one for phenol degradation. However, lower costs were obtained with ozonation. In the ozone combinations, the best results were achieved with single ozonation. As for the UV processes, UV/H 2 O 2 showed the highest degradation rate. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Phenol; Ozone; UV radiation; Hydrogen peroxide; Fenton; Photocatalysis 1. Introduction Most organic compounds are resistant to conven- tional chemical and biological treatments. For this reason, other methods are being studied as an alter- native to biological and classical physico-chemical pro- cesses. Of these, Advanced Oxidation Processes (AOPs) will probably constitute the best option in the near future. AOPs have been defined broadly as those aqueous phase oxidation processes which are based primarily on the intermediacy of the hydroxyl radical in the mechan- ism(s) resulting in the destruction of the target pollutant or xenobiotic or contaminant compound [1]. The AOPs studied here are pollutant treatment processes, which use ozone, UV, ozone in combination with UV (O 3 /UV), ozone plus hydrogen peroxide (O 3 /H 2 O 2 ), hydrogen peroxide and ultraviolet light (UV/H 2 O 2 ), Fenton’s reagent and photocatalysis, which uses titanium dioxide (TiO 2 ) in combination with light (UV) and oxygen. The main problem of AOPs lies in the high cost of reagents such as ozone, hydrogen peroxide or energy- light sources like ultraviolet light. However, the use of solar radiation as an energy source can reduce costs. Moreover, should be pointed out that AOPs lead normally to the best yields in pollutant destruction when biological treatments are unfeasible. 1.1. AOPs chemistry AOPs have considerable similarities due to the participation of hydroxyl radicals in most mechanisms that are operative during the reaction. Hydroxyl radicals are extremely unstable and reactive because of their high reactivity. The kinetics seem to be first order with respect to hydroxyl radical concentration and to the pollutant. Kinetic constants are in the range from 10 8 to 10 10 M 1 s 1 , whereas radical concentration, even in steady state, in these processes is between 10 10 and 10 12 M. Therefore, the pseudo first order constant is in the range of 1–10 4 s 1 [2]. Given that the hydroxyl radical is such an unstable and reactive species, it must be generated continuously *Corresponding author. Tel.: +34-93-4021293; fax: +34-93- 402-1291. E-mail address: gimenez@angel.qui.ub.es (J. Gim! enez). 0043-1354/02/-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0043-1354(01)00301-3