Ozone readily absorbs UV radiation at 254 nm wavelength (the extinction coefficient 254 nm = 3300 M–1 cm–1) producing H2O2 as an intermediate, which then decomposes to .OH:
Although photochemical cleavage of H2O2 is the simplest method for the production of hydroxyl radicals, the exceptionally low molecular absorptivity of H2O2 at 254 nm (254nm = 18.6 M–1 cm–1) limits the .OH yield in the solution.
The absorptivity of H2O2 can be increased by using UV lamps with output at lower wavelengths. In practice, the power requirement for UV lamps in the process of ozone photolysis is in watts range versus kilowatts for hydrogen peroxide photolysis. If water solutions contain organic compounds strongly absorbing UV light, then UV radiation usually does not give any additional effect to ozone because of the screening of ozone from the UV by optically active compounds such as phenol, 5-methylresorcinol, xylenols, etc.
Although phenolic compounds (phenol, p-cresol, 2,3-xylenol, 3,4-xylenol) are easily oxidized by ozone, complete mineralization to CO2 and H2O is uncommon. Using the O3/UV system complete mineralization of organic compounds with a short molecular chain (glyoxal, glyoxylic acid, oxalic acid, formic acid) can be achieved.