Principles of UV photooxidation

Photooxidation is an oxidation reaction triggered by light. UV light consists of electromagnetic waves whose energy content depends on their wavelength (λ). The shorter the wavelength, the higher the energy of the radiation.

A typical source of UV radiation emits a range of wavelengths. If a pollutant molecule is to be broken down directly by a radical chain reaction triggered by UV radiation, the binding energy of that molecule must ideally match the energy of the photons emitted by the radiation source. This reaction process is known as photolysis.

These dissociation energies are known for almost all compounds and functional groups; they are determined by the types of chemical bonds present in the molecule.

In addition to photolysis, other effects can be utilised for the degradation of pollutants.

When radiation (hʋ) in the 185 nm range is emitted by the UV lamp, these photons can dissociate oxygen. The atomic oxygen formed can react with further oxygen molecules from the ambient air to form ozone (O3) or directly oxidise a pollutant molecule (R-R), thereby intensifying the photolysis process. Existing water (H₂O) is also split by photons into OH radicals (OH●), which also participate in the oxidation reaction with the pollutants.

If the irradiation time is sufficient, the reactions triggered in this process will then proceed until complete oxidation has taken place.