© by PSP Volume 22 – No 8a. 2013 Fresenius Environmental Bulletin 2442 PHOTOCATALYTIC DEGRADATION OF CHLORPYRIFOS IN WATER USING TITANIUM DIOXIDE AND ZINC OXIDE Abdolmajid Fadaei 1, * and Mahdi Kargar 2 1 Department of Environmental Health Engineering, School of Health, Shahrekord University of Medical Sciences, Shahrekord, Iran 2 Department of Environmental Health Engineering, School of Health, Golstan University of Medical Sciences, Gorgan, Iran ABSTRACT In the present work, degradation of chlorpyrifos in water was investigated using semiconductor oxide cata- lysts, i.e. zinc oxide (ZnO) and titanium dioxide (TiO 2 ). The influence of various parameters, such as type of the catalyst, irradiation time, catalyst concentrations, pH, and sodium bicarbonate salt was also studied. Results indi- cated that the optimal concentration of the catalyst was 0.15 g/L. It was also found that TiO 2 is a better catalyst than ZnO under the same photocatalytic reaction condi- tions. The highest removal efficiency was achieved at pH 9. Results from the present study suggested that the photode- gradation efficiency of pesticides increases with the in- crease of the illumination time. The photodegradation effi- ciency of chlorpyrifos was found to be 80% and 90% for UV/ZnO and UV/TiO 2 , respectively. Photodegradation in the presence of sodium bicarbonate was slower in com- parison to that without the salt. In addition, the efficiency of photocatalytic degradation in distilled water was higher than in natural water. KEYWORDS: photocatalytic degradation, chlorpyrifos, titanium dioxide, zinc oxide. 1 INTRODUCTION Pesticides are extensively used in agriculture, garden- ing, and a wide variety of other household applications. Chlorpyrifos is an organophosphorous insecticide which was used for controlling insect pests in soil, ornamental plants, fruits, and vegetables worldwide [1]. In the photo- catalytic oxidation process, organic pollutants are destroyed in the presence of semiconductor photocatalysts (e.g., TiO 2 and ZnO), an energetic light source, and an oxidizing agent, such as oxygen or air [2]. The positive hole either oxidizes the pollutant directly or it oxidizes water to pro- duce hydroxyl radicals ( ° OH), whereas the electron in the conduction band reduces the oxygen adsorbed on the photo- * Corresponding author catalyst (TiO 2 or ZnO) [3]. In the degradation of organic pollutants, the hydroxyl radical is generated from the oxidation of adsorbed water, where it is adsorbed as OH - , the primary oxidant, and the presence of oxygen can pre- vent the recombination of an electron hole pair. The °OH attacks organic compounds, such as pesticides, chlorin- ated aromatics, and nitro phenols. TiO 2 and ZnO, espe- cially in the recent years, are used as effective, inexpen- sive, nontoxic semiconductor photocatalysts for the deg- radation of a broad range of organic chemicals [4]. Due to its stability and non-toxicity, titanium dioxide (TiO 2 ) has proved to be an excellent catalyst, and its behavior is very well-documented in literature[5]. However, the photocata- lytic effect of other semiconductors like zinc oxide (ZnO) is not so well understood. ZnO is a very interesting, wide- band-gap semiconductor material, mainly because of its direct band gap, large excitation binding energy, and piezo- electric properties. The band gap of this semiconductor material is ca. 3.2 eV, which corresponds to a radiation wavelength of around 390 nm. Therefore, an UV light with a wavelength shorter than 380 nm is needed to excite the electrons in valence band to conduction band [6-8]. Few studies on photocatalytic oxidation process of pesticides exist [9-12]. The main objective of the present work was to study the photocatalytic degradation of chlorpyrifos in water using TiO 2 and ZnO under UV light. The influence of the catalyst concentration, type of catalyst, pH, and Na- HCO 3 concentration was studied. 2 MATERIALS AND METHODS The ZnO catalyst was purchased from FLUKA. The di- ameter, specific surface area, and band-gap energy of ZnO were 14 nm, 10 m 2 .g -1 , and 2.92 eV, respectively. Titanium dioxide (TiO 2 Degussa P25) had a surface area of 55 m 2 /g. Chlorpyrifos was purchased from Supelco, while NaOH and HNO3 were obtained from Merck Co., Germany. The reaction solution (i.e. sodium bicarbonate, NaHCO 3 ) was also purchased from Merck Co., Germany. The concentra- tion of chlorpyrifos in the samples was 6 mg.L -1 (we also used 30% insecticide). The samples were adjusted in the reactor at 5 different retention times of 5, 10, 15, 20, and 25 min. The pH of the samples was in the range of 5-9