Photocatalytic Processes

Another innovative technology for wastewater treatment involves the irradiation by UV light of solid semiconductor photocatalysts such as titanium dioxide, T1O7, small particles of which are suspended in solution. Titanium dioxide is chosen as the semiconductor for such applications since it is nontoxic, is very resistant to photocorrosion, is cheap and plentiful, absorbs light efficiently in the UV-A region, and can be used at room temperature. Irradiation at wavelengths less than 385 nm produces electrons, e^, in the conduction band and holes, h+, in the valence band of the metal oxide. The holes in the semiconductor can react with surface-bound hydroxide ions or with water molecules, thereby producing hydroxyl radicals in both cases:

h+ + H20-ยป OH + H+

The holes can also react directly with adsorbed pollutants, producing radical cations that readily engage in subsequent degradation reactions.

Normally, 02 molecules dissolved in the water react with the electron produced at the semiconductor surface, a process that eventually produces more reactive free radicals but is relatively slow. If hydrogen peroxide is added to the water instead, it will, react with the electron to form the anion radical and generate reactive radicals more quickly.

The cost of the electrical energy required to generate the needed UV light is usually the major expense in operation of AOM systems. On this basis, the titanium dioxide methods are even less cost-effective than those described previously, since considerably more electricity is required per pollutant molecule destroyed. Sunlight could be used to supply the UV light, but only about 3% of its light lies in the appropriate UV-A range and is absorbed by the solid. Another problem with the Ti02 processes is the difficulty in separating the various reactants and products from the Ti02 particle if the metal oxide has been used in the form of a fine powder. However, there are now closed systems in which the titanium dioxide slurry is efficiently separated from the purified water and recycled back to the inlet stream.

Some scientists have experimented with immobilizing TiO, as a thin film (1 fj,m thick) on a solid surface such as glass, tile, or alumina. Indeed, TiOi-coated tiles are now used on walls and floors in some buildings. The low-level UV light from fluorescent lighting in such rooms is sufficient to allow the destruction of gaseous and liquid-phase pollutants that touch the oxide on the tiles! For example, odors that upset people are usually present in air at concentrations of only about 10 ppm; at such levels, the UV from normal fluorescent lighting should be sufficient to destroy them with TIC)? photocatalysts. Bacterial infections such as those that cause many secondary infections in hospitals can also be eliminated by spraying walls and floors (in rooms lit by fluorescent bulbs) to give them a titanium dioxide film. Photo-catalysts are quiet, unobtrusive cleansing materials.

Continue reading here: Other Advanced Oxidation Methods

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