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Heterogeneous photocatalysis is a promising new alternative method for the removal of organic pollutants in water (Carey, 1976). The degradation of organic pollutants in water, using irradiated dispersions of titanium dioxide, is a growing area of both fundamental and applied research.

The development of water and air treatment systems based on heterogeneous photocatalysis is an area of major technical importance (Blanco and Malato, 1993; Matthews, 1993; Ollis et al., 1989; Pelizzetti et al., 1992). Harada et al., (1999) have stated, “... the design of highly efficient photocatalytic systems is of vital interest and one of the most desirable yet challenging goals in the research of environmentally friendly catalysts”. There is general agreement that an important obstacle in the development of highly efficient photocatalytic reactors is the establishment of effective reactor designs for intermediate and large-scale use, as demanded by industrial and commercial applications. To achieve a successful commercial implementation, several reactor design parameters must be optimized, such as the photoreactor geometry, the type of photocatalyst and the utilization of radiated energy. A fundamental issue regarding the successful implementation of photocatalytic reactors is the transmission of irradiation in a highly scattering and absorbing medium composed of water and fine TiO particles.

A successful implementation of photocatalysis requires very efficient catalysts, illumination sources and reactors. In addition, auxiliary equipment for photocatalytic reactors is of major importance to assess the effectiveness of the reactor and of the kinetic reactor modeling. This requires proper characterization of the near-UV lamps used, in the case of artificially powered photocatalytic reactors, and the characterization of the photons absorbed in the photocatalytic reactor unit.

The evaluation of absorption photon rates in slurry reactors is a rather challenging task since light can experience a combination of reflection, scattering and absorption in the TiO particle suspension.

Photocatalytic oxidation of phenol has been studied at a laboratory scale by several researchers (Al-Ekabi and Serpone, 1988; Matthews and McEvoy, 1992; Okamoto et al., 1985b; Tseng and Huang, 1990; Wei and Wan, 1992; Winterbottom et al., 1997). Phenol is a chemical species difficult to convert in conventional bio-treatment processes. Phenol is also a very useful model contaminant in photocatalytic research for ranking reactor performance.

Heterogeneous photocatalysis on metal oxide semi-conductors has been shown to be effective in degrading organic pollutants in gaseous and aqueous streams (Fox and Dulay, 1993; Hoffmann, et al., 1995). In photocatalysis, the definition of energy yield parameters describing the light utilization efficiency is very critical (Fox, 1988).

Modeling photocatalytic reaction processes requires careful consideration of reaction and adsorption phenomena. In order to establish the importance of these matters, experiments can be developed using model pollutants such as methylene blue, phenol, 2-chlorophenol, 2,4-dichlorophenol, catechol (or 1,2 benzenediol), and pyrogallol (or 1,2,3 benzenetriol), each having quite different behaviours of adsorption and reaction.

Heterogeneous photocatalytic oxidation of organic air contaminants is a promising technology that offers distinct advantages. These advantages include potential lower operating costs, the elimination of treatment reagents or electron acceptors, the possible recovery, regeneration and reuse of the photocatalyst and finally its widespread applicability for the complete mineralization of organic compounds (Miller and Fox, 1993; Suri ., 1993). Cabrera ., (1994) indicated that almost any organic pollutant, and many inorganic ones, could be completely mineralized or separated by means of heterogeneous photocatalysis. Additionally, photocatalytic technology can be used in conjunction with solar radiation (Suri ., 1993) at close to ambient temperature (Cassano ., 1995; Falconer and Magrini-Bair, 1998; Miller and Fox, 1993). Photocatalysis also shows important prospects for certain air treatment applications, given that the observed apparent quantum efficiencies can be in excess of 100% (Ibrahim and de Lasa, 2003).

Photocatalysis is a new advanced oxidation process based on the irradiation of semiconductor materials, normally TiO, with UV light having a wavelength smaller than 390 nm. Heterogeneous photocatalysis has emerged as a promising technology that could be instrumental in eliminating pollutants from air and water streams.