Photo: Water ApS

Free Radicals
Hydroxyradicals can be formed by light induced homolysis of ozone in water.
The reaction is very effective and do not need a reaction chamber or filter as the case with oxidation by ozone. Direct photolysis of ozone is primarily used for oxidation of drinking water, where an almost complete oxidation is carried out. Ozone specifically reacts with phenol-groups and benzene rings contained in many of the substances affecting hormonal balances. Therefore it is beneficial to let ozone react with the most reaction willing substances, and then remove the excess ozone by UV light, ensuring an efficient use of the oxidation potential and avoiding emissions of ozone to the recipient.
H2O UV-light at wavelengths under 190 nm are completely absorbet in water under the forming hydroxyradicals and hydrogen. The quantum yield for the forming of radicals are 40%:

The appealing element of this method is that no chemicals have to be dosed. Thereby work labour are saved for handling of chemicals and for the establishment of facilities for dosing and handling the chemicals. 
HO· Advanced Oxidations Processes (AOP) is defined as the forming of hydroxyradicals from different oxidation substances within controlled processes. AOP is more freaquently used for treatment drinking and municipal water. The formed hydroxyradicals quickly unselectively converts organic compounds into reactive radicals.
RH + HO· —› H2O + R· The formed radical of the organic compount is quickly converted again. This convercion could for instance be the oxidation by oxygen.
R· + O2 —› R-O2·
R-O2· + H2O —› R-OH + HO2·
By the reaction a new radical is formed, and can once again react with another organic molecule. Thereby a single HO· can lead to a chainreaction where many chemicals are oxidised. The chain ends when two radicals meets, e.g.
R· + HO· —› R-OH
H2O2 A simple and economical way to form hyroxyradicals is by the homolysis of hydrogen peroxide.
H2O2/UV is the most freaquently used advanced oxidation process. One problem about the process is that hydrogen peroxide has a very low molar absorption coefficient, and therefore a only a small fraction of the available hydrogen peroxide is used. To get a high yield from the process it is important that the used UV-lamp, produces light at wavelengths between 200/250nm.

By: Henrik Andersen  DTU Copenhagen