array of intermediate products, eventually resulting in CO2 and H20. For chlorinated

compounds, HC1 is also produced.


 Cl ci Ci CI
 C Q= + *OH C C -OH
 a) CI CI Cl

 H Cl H Cl
 I I I I
 H-C-C-CI + *OH H-C-C. + H20
 b) H CI ci Cl




Figure 2-2. Example of hydroxyl radical attack a) on tetrachloroethylene by self-addition
 b) on 1,1,1-trichloroethane by hydrogen abstraction (notice chlorine shift in
 the radical) (adapted from Mao et al., 1991 and 1992).

 There are several literature reviews available on the mineralization of particular

classes of organic chemicals (Hoffman et al., 1995; Matthews, 1988; Ollis et al., 1991;

Serpone, 1995). Blake (1995) and Halmann (1996) have compiled extensive surveys of

this research and have also included sections on typical reactor designs and methods for

catalyst improvement. Research has also been focused on the determination of the

intermediate degradation pathways, for Mao et al. (1991 and 1992) has shown that the

conditions of a system (pH, oxygen concentration, competition with other compounds)

can affect the pathway a single compound can take towards complete mineralization.

 2.2 Titanium Dioxide

 Metal oxides are the most popular catalysts used for potable water and air

treatment; of these, TiO2 and ZnO have been the most researched and are considered the

most efficient. Of these two, TiO2 tends to be favored due to its stability in extreme

conditions (low or high pH), insolubility, heat resistance, non-toxicity, and low cost. TiO2