Photocatalytic degradation of carbofuran using semiconductor oxides

Abstract

The photocatalytic degradation of carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate) was investigated in an aqueous solution using Degussa P-25 TiO₂ and ZnO as photocatalysts. The progress of degradation was monitored using TOC analyzer, HPLC, GC–MS and UV–vis spectrophotometer. The effects of various experimental parameters such as initial concentration of carbofuran, pH of the solution, catalyst loading and light intensity were systematically studied in order to achieve maximum degradation efficiency. The complete mineralization of carbofuran was confirmed by TOC analyzer. The degradation with ZnO showed less efficiency than TiO₂. The formation of NO₃⁻ was identified and quantified using HPLC. In addition, four different intermediates formed during the degradation process were also identified and characterized by GC–MS. The mineralization rate was compared with lamps of wavelength 254 and 365 nm under similar conditions. The rate with 254 nm was observed to be very close to that of 365 nm.

Introduction

The presence of pesticides in the effluent of pesticide formulating and manufacturing industries severely affects the eco-system. They are also used in different forms to overcome the loses from pests in the cultivation of plants. Though they contribute to a great extent in the agricultural productivity, they become major pollutants due to their extensive use. Carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methyl carbamate), an insecticide and nameticide, is included in the general group of carbamate derivative pesticides. It is an active acetylcholinesterase inhibitor and is toxic to fish and mammals. Katsumata et al. [1] reported that more than five million pounds of carbofuran were applied in the United States in the year 1995. The use of carbofuran has received intensive concern not only due to its extensive use but also due to its high oral toxicity [2], [3], [4]. Due to its long persistence in water, it is increasingly detected in soil surface and wastewater. It exhibits refractory character to bio-degradation method [5]. Hence an effective and inexpensive technique is badly needed for treating such pollutant. Recently, it has been demonstrated that semi-conducting materials mediated photocatalytic oxidation of organic compounds is a successful, convenient alternative to conventional methods for the removal of organic pollutants from water. Among the various semiconducting materials, TiO₂ and ZnO are considered to be promising photocatalysts, particularly TiO₂ (anatase), due to its: (i) high photocatalytic activity, (ii) resistance to photocorrosion, and (iii) biological immunity. The high photocatalytic activity of anatase form of TiO₂ exhibits good application in the degradation of various organic pollutants [6], [7], [8], [9]. The illumination of these semiconducting particles with light energy produces excited high-energy state of electron and hole pairs (e⁻/h⁺) that can migrate to the surface of the particle and initiate a wide range of redox reactions, which can lead to complete mineralization of organic pollutants. The degradation of these compounds takes place via formation of partially oxidized intermediates that also undergo degradation by fragmentation. The complete mineralization of carbofuran was achieved in 16 h with TiO₂ coated on glass plates [2]. Twenty percent Mineralization was observed with TiO₂ coated on the blades of agitator after 6 h [10]. In the present study TiO₂ was used as powder in batch reactor. The effects of experimental parameters such as concentration of carbofuran, pH of the solution, catalyst loading and light intensity on the degradation efficiency were examined and the results are discussed. The degradation efficiencies of TiO₂ and ZnO were also compared to select the suitable photocatalyst for the degradation of carbofuran. The degradation of carbofuran was also investigated with low-pressure mercury vapour lamps of wavelengths 254 and 365 nm and their efficiencies were compared. Furthermore, the photoproducts of carbofuran produced during the photocatalytic degradation process have been identified, and based on the formation of intermediates possible degradation pathway of carbofuran is proposed.