Principles of UV photooxidation

Photooxidation is an oxidation reaction triggered by light. UV light consists of electromagnetic waves whose energy content depends on the 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 cleaved directly by a radical chain reaction induced by UV radiation, the bond energy of that molecule must ideally match the energy of the photons emitted by the radiation source. This reaction process is called photolysis.

These dissociation energies, which are determined by the types of chemical bonds present in the molecule, are known for almost all compounds and functional groups. In addition to photolysis, other effects can be utilised for the degradation of pollutants.

If the UV lamp emits radiation (hν) in the 185 nm range, these photons can dissociate oxygen. The atomic oxygen formed can react with other oxygen molecules from the ambient air to form ozone (O₃), or directly oxidise a pollutant molecule (R-R), thereby enhancing the photolysis process. Water (H₂O) present is also split by photons into OH radicals (OH•), which also participate in the oxidation reaction with the pollutants.

Provided the irradiation duration is sufficient, the reactions triggered in this process proceed until complete oxidation has taken place. Reaction scheme: Photolysis R-R + hν → R• + R•; ozone formation O₂ + hν → 2/3 O₃ → 2 O•; OH radical formation H₂O + hν → OH• + H•.