Photocatalytic Degradation of Organic Pollutants: Mechanisms and Kinetics

A wide variety of organic pollutants are introduced into the water system from various sources such as industrial effluents, agricultural runoff and chemical spills (Muszkat et al., 1994; Cohen et al., 1986). Their toxicity, stability to natural decomposition and persistence in the environment has been the cause of much concern to the societies and regulation authorities around the world (Dowd et al., 1998). Development of appropriate methods for the degradation of contaminated drinking, ground, surface waters, wastewaters containing toxic or nonbiodegradable compounds is necessary. Among many processes proposed and/or being developed for the destruction of the organic contaminants, biodegradation has received the greatest attention. However, many organic chemicals, especially which are toxic or refractory, are not amendable to microbial degradation. Researcher showed their interest and started the intensive studies on heterogeneous photocatalysis, after the discovery of the photo-induced splitting of water on TiO2 electrodes (Fujishima and Honda, 1972). Semiconductor particles have been found to act as heterogeneous photocatalysts in a number of environmentally important reactions (Blake, 2001; Pirkanniemi, & Sillanpää, 2002; Gaya & Abdullah, 2008). Materials such as colloidal TiO2 and CdS have been found to be efficient in laboratory-scale pollution abatement systems (Barni et al., 1995; Bellobono et al., 1994; Legrini et al., 1993; Mills & Hunte, 1997; Halmann, 1996), reducing both organic [e.g. halogenocarbons (Gupta & Tanaka, 1995; Martin et al., 1994; Read et al., 1996;), benzene derivatives (Blanco et al., 1996; Mao et al., 1996) detergents (Rao & Dube, 1996), PCB’s (Huang et al., 1996), pesticides (Gianturco et al., 1997; Minero et al., 1996; Lobedank et al., 1997; Haque & Muneer 2003; Muneer & Bahnemann, 2002), explosives (Schmelling et al., 1996), dyes (Vinodgopal et al., 1996), cyanobacterial toxins (Liu et al., 2002)] and inorganic [e.g. N2 (Ranjit et al., 1996), NO3and NO2(Mills et al., 1994; Ranjit et al., 1995; Kosanic & Topalov, 1990; Pollema et al., 1992), cyanides (Mihaylov et al., 1993; Frank & Bard 1977), thiocyanates (Draper & Fox, 1990), cyanates (Bravo et al., 1994), bromates (Mills et al., 1996) etc.] pollutants/impurities to harmless species. Semiconductor photocatalysts have been shown to be useful as carbon dioxide (Irvine et al., 1990) and nitrogen (Khan & Rao, 1991) fixatives and for the decomposition of O3 (Ohtani et al., 1992), destruction of microorganisms such as bacteria (Matsunaga & Okochi, 1995; Zhang et al., 1994; Dunlop et al.,