ROLE OF HUMIC ACIDS IN THE TiO 2 -PHOTOCATALYZED DEGRADATION OF TETRACHLOROETHENE IN WATER ELENA SELLI 1 , DANIELA BAGLIO 2 , LUCA MONTANARELLA 2 and GIOVANNI BIDOGLIO 2 * 1 Dipartimento di Chimica Fisica ed Elettrochimica, UniversitaÁ di Milano, Via Golgi 19, 20133 Milan, Italy and 2 European Commission, Joint Research Centre, Environment Institute, 21020 Ispra (Va), Italy (First received December 1997; accepted in revised form August 1998) AbstractÐThe eect of humic acids on the TiO 2 -mediated photocatalytic degradation of tetrachlor- oethene (PCE) was kinetically investigated at dierent pH and initial substrate concentrations. The pro- cess occurs through two parallel paths: a major oxidative route leading to mineralization and a reductive route leading to the formation of dichloroacetic acid (DCAA), also undergoing photodegra- dation. The rate of PCE decomposition was found to decrease in the presence of humic acids adsorbed on the semiconductor surface, while the concentration of the intermediate dichloroacetic acid increased. This is a consequence of the scavenging action of humic acids toward photoproduced surface oxidant species, which makes conduction band electrons more easily available for interface reactions. Kinetic studies on the eect of humic acids in the TiO 2 -mediated photodegradation of dichloroacetic acid showed that the progressively greater accumulation of this highly toxic intermediate, observed with increasing humic acids content, was a consequence of both an increase in the rate of its production from PCE and a decrease in the rate of its oxidative photodegradation. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐTiO 2 -mediated photodegradation, humic acids, tetrachloroethene, dichloroacetic acid INTRODUCTION Heterogeneous photocatalysis, involving photoin- duced redox reactions at the surface of semiconduc- tor minerals, is a promising technique for the treatment of waters contaminated by organic mol- ecules (Ollis and Al-Ekabi, 1993; Pelizzetti et al., 1994; Homann et al., 1995). Among semiconduc- tors, titanium dioxide is the most widely used in photocatalysis, due to its outstanding stability (Frank and Bard, 1977). Under irradiation with light of suitable wavelength, charge carriers (i.e. electrons and holes) produced at the TiO 2 surface can oxidise and/or reduce many organic and inor- ganic substances (Serpone and Pelizzetti, 1989; Linsebigler et al., 1995). In particular, halogenated compounds (alkanes, alkenes and aromatic com- pounds) were reported to undergo complete miner- alization in water suspensions of TiO 2 , with the formation of CO 2 and mineral acids. Earlier studies focused on the complete photodegradation of pollu- tants (Pruden and Ollis, 1983; Ollis et al., 1984), without investigating the possible formation of in- termediates, which might have a toxicity even higher than that of the starting substrates. Moreover, organic substances, such as fulvic and humic acids, which are widespread in natural waters, may modify the photodegradation paths in the photocatalytic puri®cation treatments of waste and drinking waters. Such humic substances may also play an important role in redox reactions photoinduced by sunlight in aquatic systems at the surface of minerals with semiconducting properties, such as iron oxides. Indeed, natural organic acids have been demonstrated to be capable of sensitising photoredox processes on semiconductors (Vinodgopal and Kamat, 1992; Kamat and Vinodgopal, 1994; Selli et al., 1996). In the present paper, the eects induced by humic acids (HA) in the TiO 2 -mediated photode- gradation of tetrachloroethene (PCE) were investi- gated, to obtain insight into their possible role in the formation and disappearance of byproducts. Tetrachloroethene, a largely inert non-¯ammable solvent, practically omnipresent in trace amounts in groundwaters of industrialized countries, was reported to give some byproducts during both its homogeneous UV-light induced degradation (Mertens and von Sonntag, 1995; Hirvonen et al., 1996) and its TiO 2 -mediated photodegradation in water (Glaze et al., 1993; Kenneke et al., 1993; Gianturco et al., 1996). In particular, Glaze et al. (1993) proposed that the latter process proceeds through two parallel pathways. The usually Wat. Res. Vol. 33, No. 8, pp. 1827±1836, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter PII: S0043-1354(98)00368-6 *Author to whom all correspondence should be addressed. [Tel.: +39-332-78-9383; fax: +39-332-78-9328; e-mail: giovanni.bidoglio@jrc.it]. 1827