Photochemical self-cleaning in the environment

Photochemical and photocatalytic processes are responsible for various selfcleaning behaviours in atmosphere, soils, or water basins. 

The main atmospheric cleaner is ozone, photodissociation of which  

hydroxyl, and hydroperoxyl radicals are active in oxidation of hydrocarbons, aldehydes and ketones, whereas hydrogen peroxide is a source of hydroxyl radicals in the Fenton or photo-Fenton reactions (see section in 9.3.3 in Chapter 9 and Chapter 21). 

Photocatalysis and photosensitization processes are common in natural waters and are of special importance because they can stimulate a transformation of molecules that resist direct photolysis, such as transparent species or chromophores, the reactive states of which are inefficiently populated by sunlight absorption. 

Photocatalysis by transition metals shows an effective self-cleaning ability of soils and waters by the degradation and complete mineralization of the dissolved organic matter. Moreover, the oxidation processes are driven by sunlight and atmospheric oxygen and do not pollute the environment by risky side-products (see section 9.3, Chapter 9). 

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