Studies on the adsorption and kinetics of photodegradation of a model compound for heterogeneous photocatalysis onto TiO 2 Jose ´ Pedro S. Valente * , Pedro M. Padilha, Ariovaldo O. Florentino Department of Chemistry and Biochemistry—IB/UNESP, C. Postal 510, 18618-000 Botucatu, SP, Brazil Received 14 June 2005; received in revised form 21 November 2005; accepted 23 November 2005 Available online 6 January 2006 Abstract An investigation was made on the adsorption and kinetics of photodegradation of potassium hydrogenphthalate in an aqueous sus- pension of TiO 2 . Two models, Langmuir and Freundlich, were used to describe the adsorption process and the model proposed by Lang- muir–Hinshelwood (L–H) was employed to describe the kinetics of the photodecomposition reactions of hydrogenphthalate. The results of the adsorptions were fitted to the models proposed by Langmuir and Freundlich. Adsorption was found to be a function of the tem- perature, with adsorption capacity increasing from 2.4 to 4.5 mg/g when the temperature rose from 20 to 30 °C. The kinetic model indi- cates that the rate constant, k, of the first order reaction, is high in the 10.0 to 100 mg/l interval, which is coherent with the low value of the adsorption constant, K. The results fitted to the L–H model led to an equation that, within the range of concentrations studied here, theoretically allows one to evaluate the photodegradation rate. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Potassium hydrogenphthalate; TiO 2 ; Adsorption-kinetics; Langmuir; Freundlich; Langmuir–Hinshelwood 1. Introduction Heterogeneous photocatalysis has received a great deal of attention as an advanced oxidation process (AOP) for degrading persistent organic compounds, reducing chemi- cal oxygen demand (COD) and eliminating microorganisms in water bodies (Rodriguez et al., 1996; Li and Zhao, 1999; Robert and Malato, 2002; Malato et al., 2003). The advan- tage of this process is that it leads to the total mineralization of organic contaminants, whether they are simple or com- plex molecules. To date, the most efficient procedure for the treatment of contaminated waters by heterogeneous photocatalysis consists of UV light irradiation of suspen- sions of TiO 2 catalyst in solutions containing the organic substrate to be degraded. The mechanism of photocatalysis is based on the activa- tion of a semiconductor by light. The interaction of light with the semiconductor excites electrons from the valence band to the conduction band, producing holes (h+) in the valence band that will act as oxidizing sites, while the photogenerated electrons (e  ) that combine with the dis- solved oxygen hinder the recombination of the electron– hole pair (Linsebigler and Guangquan, 1995; Nogueira and Jardim, 1998; Ziolli and Jardim, 1998; Dionysiou et al., 2000). In this mechanism, the water (molecular or dissociated) adsorbed on the semiconductor acts in the cat- alytic process, reacting with the photogenerated h+ gaps to form highly oxidizing hydroxyl radicals (E 0 = 2.8 V) (Lins- ebigler and Guangquan, 1995; Nogueira and Jardim, 1998). Since photocatalysis is a surface phenomenon, a critical step in intervening in the effectiveness of the photo- de- gradation of a given pollutant is to understand the adsorption process of this pollutant on the catalyzing sur- face. Studying the adsorbability of the organic substrate allows one to predict the mechanism and kinetics that pro- mote the pollutant’s photooxidation. The model chemical species chosen for this study was potassium hydrogenphthalate. This compound has 0045-6535/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.11.050 * Corresponding author. Tel./fax: +55 14 3811 6255. E-mail address: jpedro@ibb.unesp.br (J.P.S. Valente). www.elsevier.com/locate/chemosphere Chemosphere 64 (2006) 1128–1133