Fatma Beduk Mehmet Emin Aydin Senar Ozcan Department of Environmental Engineering, Selcuk University, Konya, Turkey Research Article Degradation of Malathion and Parathion by Ozonation, Photolytic Ozonation, and Heterogeneous Catalytic Ozonation Processes The oxidation of organophosphorus pesticides (OPPs), such as malathion and parathion, in aqueous solution was studied using conventional ozonation (O 3 ), photolytic ozona- tion (O 3 /UV, O 3 /UV/H 2 O 2 ), and heterogeneous catalytic ozonation (O 3 /TiO 2 /UV) pro- cesses. Experiments were performed in batch mode at laboratory scale and processes were compared in terms of disappearance kinetics. The best results of pesticide mineralization were obtained when TiO 2 particles in combination with ozone (O 3 ) and UV photolysis (l ¼ 254 nm) were applied. Decomposition of 99% of parent compounds were achieved in 10 min and oxon derivatives were completely removed in 30 min. The initial reaction rate increases linearly with increasing catalyst amount. Toxicity measurements of the treated solutions were carried out in order to evaluate the efficiency of the treatment methods. No detoxification was achieved for O 3 and O 3 /UV applications. Heterogeneous photocatalytic ozonation was shown to be feasible for achieving complete decomposition of OPPs and their oxon intermediates. Keywords: Catalysis; Malathion; Oxidation; Ozone, Parathion Received: February 6, 2011; revised: May 3, 2011; accepted: May 7, 2011 DOI: 10.1002/clen.201100063 1 Introduction Organophosphorus pesticides (OPPs) were introduced because of their lower persistency in the environment relative to organo- chlorine pesticides. However, they were found in environment with enough frequency to constitute an ecotoxicological risk [1–3]. Exposure to them could lead to inhibition of acetylcholinesterase (AChE) in target tissues which leads to accumulation of acetylcholine [4, 5]. In the environment, the fate of OPPs is associated with both abiotic and biotic processes, including hydrolysis, volatilization, photo oxidation, chemical oxidation, and microbial transformation. Their degradation in natural environment by various processes sometimes results in the formation of oxygen analogues [6]. This process involves the substitution of the sulfur atom in the P — —S bond of the organophosphate pesticide with an oxygen atom resulting in formation of oxon derivatives. A promising way to perform the mineralization of these substan- ces is the application of advanced oxidation processes (AOP). Photolysis [7, 8], photocatalysis [9–11], the Fenton and photo- Fenton processes [12, 13], and ozone based oxidation [14, 15] for OPPs degradation were investigated in recent years. Among the different AOP technologies, ozone is capable of oxidizing a variety of organic and inorganic compounds in aqueous solution, either by direct reaction of molecular ozone, or through a radical mechanism involving the hydroxyl radical [16]. Radicals are unstable and reac- tive, because one of their electrons is unpaired. On the other hand, ozone alone is insufficient to achieve a complete mineralization of OPPs [14]. Besides, disappearance of parent compounds does not always indicate the successful treatment, because the degraded products may be more toxic than the parent compounds. The majority of OPPs give rise to only slight inhibition of AChE by themselves, unless they undergo oxidative activation resulting with oxon formation [17, 18]. Because of that reason, complete degradation of oxon derivatives of OPPs is critical for application of oxidation processes. Photolytic ozonation is another way of enhancing ozone reactiv- ity. The mechanism of ozone photolysis was studied by Peyton and Glaze [19] who confirmed the formation of hydrogen peroxide as the first reaction intermediate of ozone photolysis. It was also proposed that the secondary reactions lead to the production of hydroxyl radical (OH  ). This process was successfully applied for removal of nitrobenzene [20], herbicides such as linuron [21], synthetic organic compounds, e.g., oxalic and acetic acids [22]. Photolytic ozonation was also used to oxidize phenols [23] and natural organic matter [24]. Ozone was used in combination with hydrogen peroxide to generate OH radicals. Researchers have demonstrated that the addition of hydrogen peroxide can enhance the oxidation of organic compounds [8]. Heterogeneous catalytic ozonation (HeCO) processes with metal ions or metal oxides are also acting as catalysts in order to enhance the reactivity of ozone [25]. The use of ozone and catalysts dates back to the 1970s. In the initial studies, attention focused on mainly metals salts such as Fe(II), Mn(II), Ni(II), or Co(II) use. Homogenous (HoCO) and HeCO processes depend on the water solubility of the catalyst [16]. Photocatalytic oxidation of organic compounds by Correspondence: Dr. F. Beduk, Department of Environmental Engineering, Selcuk University, Campus, 42031 Konya, Turkey E-mail: fatmabeduk@selcuk.edu.tr Abbreviations: HeCO, heterogeneous catalytic ozonation; OPPs, organophosphorus pesticides Clean – Soil, Air, Water 2012, 40 (2), 179–187 179 ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com