Journal of Hazardous Materials B137 (2006) 1056–1064 Study of the removal of dichlorvos and dimethoate in a titanium dioxide mediated photocatalytic process through the examination of intermediates and the reaction mechanism E. Evgenidou a , I. Konstantinou b,∗ , K. Fytianos a , T. Albanis c a Environmental Pollution Control Laboratory, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece b Department of Environmental and Natural Resources Management, University of Ioannina, 30100 Agrinio, Greece c Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece Received 11 February 2006; received in revised form 18 March 2006; accepted 21 March 2006 Available online 2 May 2006 Abstract The photocatalytic oxidation of two selected organophosphorous insecticides (dichlorvos and dimethoate) has been investigated. The aim of the study was the identification of the intermediates that are formed during photocatalytic treatment. Intermediate products from the slurry system were identified by means of solid-phase extraction (SPE) coupled to gas chromatography–mass spectroscopy techniques (GC–MS). Nine possible by-products were identified for dimethoate and three for dichlorvos. A proposed degradation pathway for each insecticide is presented, involving mainly oxidation and dealkylation reactions. The results demonstrated that some of the transient intermediates formed (oxon derivatives, disulfide, chlorinated fragments), were more toxic compared to parent compounds whereas most of them are less toxic than the parent compounds. © 2006 Elsevier B.V. All rights reserved. Keywords: Dichlorvos; Dimethoate; Photocatalysis; Degradation products 1. Introduction Heterogeneous photocatalysis appears to be a very promis- ing technique for the destruction of organic pollutants [1–3]. Among the semiconductors that have been tested, titanium diox- ide demonstrates the most fulfilling results, exhibiting not only an outstanding catalytic efficiency but also low cost and resis- tance to photocorrosion [4]. Moreover, titanium dioxide is envi- ronmentally safe and gives the opportunity to use solar light as an energy source. The latter makes the process quite attractive especially to countries where solar irradiation is highly available. The photocatalytic properties of the semiconductors are based on their electronic structure which is characterized by a filled valence band and an empty conduction band. When they are illuminated with energy greater that their band gap energy E g , excited high-energy states of electron and hole pairs (e - /h + ) are produced. These species can either recombine in the bulk of the semiconductor and dissipate the input energy as heat or they ∗ Corresponding author. Tel.: +30 26410 74120; fax: +30 26410 74179. E-mail address: iokonst@cc.uoi.gr (I. Konstantinou). can migrate to surface of the semiconductor’s particles and react with adsorbed electron donors or electron acceptors. The pho- togenerated holes act as powerful oxidants and the electrons as powerful reductants and initiate a wide range of chemical redox reactions, which can lead to partial or total destruction of a great variety of organic pollutants [5]. In addition to the performance of the treatment process, the study of the formation of by-products is also important. In pho- tocatalytic treatment of toxic pollutants, the monitoring of the disappearance rate of the parent compound is not the most appro- priate parameter for classifying the efficacy of the process [6,7]. Information on the formation and decomposition of the interme- diates or by-products is critical before a clean technology can be established [1]. The formation of intermediates with simi- lar or even higher toxicity than the parent compounds during the photocatalytic degradation of pesticides was often reported [1,8,9]. Organophosphorous pesticides are comprised within the 10 most widely used pesticides all over the world. They have been used as an alternative to organochlorine compounds for pest control. However, they are considered as extremely toxic com- pounds acting on acetylcholinesterase activity [10,11]. Residue 0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2006.03.042