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Destructive waste water treatment changes the hazardous chemical compound, removing the environmental hazard. One type of destructive waste water treatment is UV photo-oxidation. UV photo-oxidation uses ultraviolet light to activate a catalyst to physically decompose the pollutant into non-toxic components. Although UV light alone will degrade organic compounds, reaction rates are too slow to be considered for industrial applications.(Hoegger et al, 1997) Several catalysts such as hydrogen peroxide, ozone and titanium dioxide are known to perform this type of reaction on organic compounds. (Yue, 1993) Titanium dioxide is probably the best choice due to its low cost, low toxicity, resistance to photo-corrosion and catalytic efficiency. (Alfano, et al., 1970)
Our focus will be the photo-oxidation of benzoic acid and caffeine over a titanium dioxide immobilized catalyst. To reduce the contaminates concentration to appropriate levels UV light was used to activate the titanium dioxide catalyst. The activated catalyst causes water to dissociate and form unstable hydroxyl radicals. These unstable radicals are formed when the titanium dioxide semiconductor promotes it’s electrons from the valence band to the conduction band where they are shared with water and oxygen molecules to form the hydroxyl radical. The following mechanism describes the event. (Venkatadri and Peters, 1993)
TiO2 + hv -> electron(cb) + h+vb
h+vb + OH- -> OH.
h+vb + H2O -> OH. + H+
electron(cb) + O2 -> O2-
electron (cb) + h+vb -> heat
Where h+vb represents the valance-band holes, electron(cb) represents the conduction band electrons and O2- represents the super-oxide ion. With oxygen acting as an electron acceptor, electron trapping prevents the photo-corrosion of the catalyst titanium dioxide. The results are that the organic contaminant was destroyed and the hydroxyl radical was terminated by the recombining of the hole and electron that created heat as a product.
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