Technical Note Photocatalytic and combined anaerobic–photocatalytic treatment of textile dyes F. Harrelkas a,b , A. Paulo c , M.M. Alves c , L. El Khadir b , O. Zahraa d , M.N. Pons a , F.P. van der Zee c, * a Laboratoire des Sciences du Génie Chimique, CNRS, Nancy University, INPL, 1 Rue Grandville, BP 20451, 54001 Nancy Cedex, France b Laboratoire d’Automatique et d’Etudes des Procédés, Université Cadi Ayyad, Faculté des Sciences Semlalia, Avenue Prince Mly Abdellah, BP 511, 40000 Marrakech, Morocco c IBB – Institute for Biotechnology and Bioengineering, Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal d Département de Chimie-Physique des Réactions, UMR 7630-CNRS, Nancy University, INPL, 1 Rue Grandville, BP 20451, 54001 Nancy Cedex, France article info Article history: Received 1 February 2008 Received in revised form 13 May 2008 Accepted 16 May 2008 Available online 27 June 2008 Keywords: Anthraquinone dyes Autoxidation Azo dyes COD removal Phthalocyanine dyes Titanium dioxide abstract A photocatalytic process based on immobilized titanium dioxide was used to treat crude solutions of azo, anthraquinone and phthalocyanine textile dyes. In addition, the process was applied to the treat autox- idized chemically reduced azo dyes, i.e. representatives of recalcitrant dye residues after biological sequential anaerobic–aerobic treatment. Photocatalysis was able to remove more than 90% color from crude as well as autoxidized chemically reduced dye solutions. UV-absorbance and COD were also removed but to a lower extent (50% in average). The end products of photocatalytic treatment were not toxic toward methanogenic bacteria. The results demonstrate that photocatalysis can be used as a pre- or post-treatment method to biological anaerobic treatment of dye-containing textile wastewater. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The textile-processing industry is putting a severe burden on the environment, through the release of heavily polluted wastewa- ters. Biological treatment methods are favored as they are consid- ered environment-friendly and relatively cheap. The most logical biological strategy is sequential anaerobic–aerobic treatment (Field et al., 1995). The anaerobic phase is important for reductive cleavage (decolorization) of azo dyes, as well as for (partial) decol- orization of other types of dyes such as anthraquinone, phthalocy- anine, and triphenylmethane dyes (Delee et al., 1998). The consecutive aerobic phase would serve to biomineralize aromatic amines from azo dye cleavage (Pinheiro et al., 2004), as well as to remove some of the other types of dyes by adsorption and bio- degradation (Easton, 1995). However, there are some limitations: it is not certain that all aromatic amines can be degraded, and the complete removal of other types of dyes is questionable (Van der Zee and Villaverde, 2005). Due to these limitations, more and more research is focusing on combining biological treatment of dye-containing wastewaters with other techniques, such as coagu- lation-flocculation, adsorption on solids (activated carbon, natural products such as agro wastes), and most importantly advanced oxidation processes (AOPs). AOPs are based on the use of the hydroxyl radical as primary oxidant of organic pollutants. These treatments can lead to com- plete mineralization of organic molecules into CO 2 and water (Legrini et al., 1993), with the hetero-atoms being transformed into chloride (Chen et al., 1995), sulfate (Kato et al., 2005), ammo- nium (Hidaka et al., 1995), etc. Systems such as UV-hydrogen per- oxide (Aleboyeh et al., 2005; Baldrian et al., 2006), ozonation (Farré et al., 2005; Liu et al., 2007) and photo-Fenton (Ruppert et al., 1993) have been extensively described in literature and have demonstrated their efficiency. More recently, techniques such as photocatalysis (Ollis, 2000), sonolysis (Drijvers et al., 1999; Minero et al., 2008) and c-radiolysis (Getoff, 1996; LaVerne et al., 2007) have shown promising prospects. Solar photocatalysis is especially attractive as it is based on a fully renewable and cheap energy source (Malato et al., 2002). Titanium dioxide is a stable and non-toxic semi-conductor, which is widely used in so- lar photocatalysis. However, the UV radiation that can be ab- sorbed by TiO 2 represents only 5% of the solar spectrum. This limits the overall yield of the process. In order to improve this yield, combinations with other AOPs have been suggested (Zhang et al., 2008; Neelavannan and Ahmed Basha, 2008). For the treat- ment of various types of wastewater, including dye-containing textile wastewater, a combination of AOPs with biological pro- cesses as pre-treatment and/or polishing step have been sug- gested (Bousselmi et al., 2002; Bahnemann, 2004). Anaerobic bioprocesses are particularly attractive: their energy input is min- imal as no aeration is required. 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.05.026 * Corresponding author. Tel.: +351 253 604 400x5413; fax: +351 253 678 986. E-mail address: frankvdz@deb.uminho.pt (F.P. van der Zee). Chemosphere 72 (2008) 1816–1822 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere