Sonophotocatalytic/H 2 O 2 degradation of phenolic compounds in agro-industrial effluents Adria ´n M.T. Silva a, * , Ekaterini Nouli b ,A ˆ ngela C. Carmo-Apolina ´rio a , Nikolaos P. Xekoukoulotakis b , Dionissios Mantzavinos b a Department of Chemical Engineering, Faculty of Sciences and Technology, University of Coimbra, Polo II—Pinhal de Marrocos, 3030-290 Coimbra, Portugal b Department of Environmental Engineering, Technical University of Crete, Polytechneioupolis, GR-73100 Chania, Greece Available online 18 May 2007 Abstract The treatment of a model solution containing 13 compounds typically found in olive mill wastewaters (OMW), at a concentration of 50 mg/L each, by means of sonophotocatalysis over 0.75 g/L Degussa TiO 2 suspensions was studied. Experiments were conducted at an ultrasound frequency and intensity of 80 kHz and 120 W, respectively, ultraviolet power of 9, 250 and 400 W with or without the addition of 0.118 mol/L H 2 O 2 . Treatment efficiency was assessed following changes in total phenols (TPh) concentration, individual species concentration, chemical oxygen demand (COD), total organic carbon (TOC) and ecotoxicity. In general, photocatalytic degradation increased with increasing UVA power, while sonolysis alone failed to cause any degradation. Process coupling and addition of extra oxidant resulted in substantial levels of degradation. For instance, sonophotocatalytic treatment at 400 W UVA power with H 2 O 2 for 120 min resulted in complete mineralization followed by significant toxicity reduction. TiO 2 characterization before and after use showed that the catalyst suffered no composition or morphology changes during treatment. However, a substantial surface area increase was noted and this was attributed to the ultrasound de-aggregating catalyst particles. Preliminary tests with an actual OMW showed that the sonophotocatalytic/H 2 O 2 treatment is a promising technology for this type of effluents. # 2007 Elsevier B.V. All rights reserved. Keywords: Agro-effluents; Catalyst characterization; Phenols; Sonophotocatalysis; TiO 2 1. Introduction Olive mill waste waters (OMW) constitute a major environ- mental problem in the Mediterranean region. The classical discontinuous olive oil process has been progressively replaced by continuous centrifugation using three-phase systems and, more recently, two-phase systems because the latter consume lower quantities of water and generate 50% less OMW per unit mass of olive. Nonetheless, in countries like Portugal and Greece most of the mills still operate with the classical and the three- phase olive extraction processes. Discharge of these effluents to water bodies damages seriously the self-capabilities of these environments. Moreover, spreading of OMW onto the fields can reach and contaminate the waterbed due to the associated biorecalcitrant characteristics [1]. Biological treatments are highly affected by the large amounts of phenolic compounds typically found in this kind of effluents and, therefore, other technologies may be needed for the partial or complete treatment of OMW. Photocatalysis is an active area of research in environmental protection, while the development and application of ultrasound- based processes begin to play an important role in this domain. The first treatment is based on the photonic activation of the catalyst by light irradiation and the second on the ultrasonically induced acoustic cavitation [2]. Both processes predominantly involve the formation of free radicals and other reactive moieties which consequently react with and destroy the pollutant species. The mechanisms via which these processes mainly occur have been comprehensively presented in the literature [2,3]. Concerning photocatalysis with titanium dioxide as the catalyst, electrons in conduction band (e cb  ) and holes in the valence band www.elsevier.com/locate/cattod Catalysis Today 124 (2007) 232–239 * Corresponding author. Present address: Laboratory of Catalysis and Mate- rials, Chemical Engineering Department, Faculty of Engineering – University of Porto (FEUP), Rua Dr. Roberto Frias, 4200-465 Porto – Portugal. Tel.: +351 964291576/225081998; fax: +351 225081449. E-mail addresses: adrian@fe.up.pt, adrian@eq.uc.pt (A.M.T. Silva), mantzavi@mred.tuc.gr (D. Mantzavinos). 0920-5861/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2007.03.057