Photocatalytic degradation of organics in water in the presence of iron oxides: Influence of carboxylic acids Eva Rodrı ´guez, Guadalupe Ferna ´ ndez, Beatriz Ledesma, Pedro A ´ lvarez, Fernando J. Beltra ´n * Departamento de Ingenierı´a Quı´mica y Quı´mica Fı´sica, Universidad de Extremadura, 06071 Badajoz, Spain 1. Introduction Nowadays the presence in the water environment of many synthetic organic chemicals able to alter living being endocrine systems is a subject of great concern [1]. Removal of this kind of contaminants from water is compulsory and only tertiary treatment methods such as membrane technologies, adsorption or advanced chemical oxidation fulfill this task. Among these technologies, only advanced oxidation processes (AOPs) destroy the contaminants. AOPs are characterized for the production of hydroxyl radicals, very strong oxidizing species that unselectively react with most of the contaminants in water [2]. AOPs based on the simultaneous use of radiation and semiconductors have attracted the interest of many researchers [3,4]. In particular, the combination of UV-A radiation and titania constitutes one of the most studied advanced photochemical oxidation technologies [5,6]. Another interesting photocatalytic oxidation process is the combination of UV-A radiation and iron, just dissolved or as oxides. The efficiency of this oxidizing system can be improved by the presence of some carboxylic acids that in the presence of iron form carboxylate complexes that absorb UV-A radiation with high quantum yield to trigger radical chain mechanisms of oxidation [7–10]. Literature reports some works on the removal of contaminants with UV-A radiation/iron/carboxylate oxidation systems. Thus, oxalic, citric and tartaric acids have been used with UV-A/Fe(III) to improve the removal of contaminants such as dyes, herbicides, benzene, etc., through the formation and photodecomposition of iron–carboxylate complexes that improve the generation of oxidizing free radicals [11–14]. These processes are of great interest since carboxylic acids, such as oxalic acid, are usually intermediate or end products of the hydroxyl radical oxidation of organic contaminants in water [15]. Also, the use of iron oxides as photocatalysts is recommended due to their abundance on earth. In fact, literature reports some recent works where compounds such as bisphenol A, 2-mercaptobenzothiazol and pentachlorophenol are removed from water with these oxidizing systems [16–19]. However, when using iron oxides and carboxylic acids to photocatalytically remove contaminants from water, studies on iron leaching into water are also needed due to the importance of dissolved iron–carboxylic acids complexes on the process performance. Main factors affecting iron oxide dissolution are the properties of the overall system (temperature, UV light), the composition of the solution phase (pH, redox potential, concentration of acids, reductants and complexing agents) and the properties of the oxide (specific surface area, stoichiometry, crystal chemistry, crystal defects or guest ions). However, only the composition of the solution and the tendency of ions in solution to form complexes were considered important in mechanistic studies [20,21]. Iron oxide dissolution requires the breaking of bonds Applied Catalysis B: Environmental 92 (2009) 240–249 ARTICLE INFO Article history: Received 27 April 2009 Received in revised form 3 July 2009 Accepted 15 July 2009 Available online 22 July 2009 Keywords: Bisphenol A Photocatalytic oxidation Iron oxides Hematite Magnetite Carboxylic acids Titania Hydrogen peroxide ABSTRACT The effects of four carboxylic acids: malic, citric, tartaric and oxalic acids on the leaching of iron from two commercial iron oxides (hematite, a-Fe 2 O 3 , and magnetite, Fe 3 O 4 ) have been investigated. The variables studied were the doses of iron oxides and carboxylic acids used as well as aqueous pH, temperature and the presence of hydrogen peroxide and/or UV-A radiation. On the whole, Fe 3 O 4 led to higher amounts of leached iron than a-Fe 2 O 3 , and oxalic acid was the most effective carboxylic acid used. The importance of iron leaching has been considered to explain the photodegradation of bisphenol A (BPA) by UV-A/iron oxides systems. The influence of the presence of hydrogen peroxide and/or titania on the efficiency of these oxidation systems was also investigated. At the conditions tested, advanced oxidation with the UV- A/iron oxide/oxalic acid/H 2 O 2 /TiO 2 system led to the lowest BPA half life (<15 min) among those processes studied. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +34 924 289 387; fax: +34 924 289 385. E-mail address: fbeltran@unex.es (F.J. Beltra ´ n). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.07.013