CHAPTER 2
 LITERATURE REVIEW

 2.1 Heterogeneous Photocatalysis

 Advanced oxidation processes (AOPs) are considered to be an alternative water

treatment technology for removing harmful compounds and microbes from public water

supply, comparable with traditional practices such as activated carbon adsorption, air

stripping, and chlorine disinfection. AOPs can not only disinfect the water from virulent

microbes, but can also oxidize toxic heavy metals, organic pesticides and solvents,

chlorinated compounds, and inorganic chemicals from the water without producing a

waste stream unsuitable for disposal or consumption. For example, these processes can be

used for chemical spill cleanup, treatment of industrial effluents, and wastewater

treatment (Turchi and Ollis, 1990).

 AOPs involve the generation of highly reactive oxidative species produced by

several pathways, including reactions with H202 and 03 with or without ultraviolet (UV)

irradiation and heterogeneous photocatalysis. Heterogeneous photocatalysis entails the

use of a solid photocatalyst (usually a semiconductor) in contact with either a liquid or

gas, while homogeneous photocatalysis uses a catalyst of the same phase as the

contaminated media. While UV use or an oxidant alone may yield partial degradation of

a compound, the combined use of UV with an oxidant (with or without a photocatalyst)

has been shown to yield complete mineralization of organic contaminants to carbon

dioxide (Ollis et al., 1991).