Hydroxyl radical overview

Due to its electrophilic properties the OH° reacts at the position with the highest electron density of the target molecule. Detailed information can be found in von Sonntag (1996), which gives a good overview of the degradation mechanism of aromatics by OH° in water. Buxton et al. (1988) showed that the reaction rate constants for hydroxyl radicals and aromatic compounds are close to the diffusion limit. 

This comparison is only theoretical. In reality a high production of OH° can lead to a low reaction rate because the radicals recombine and are not useful for the oxidation process. Also not considerd are the effects of different inorganic and/or organic compounds in the water. Various models to calculate the actual OH-radical concentration can be found in the literature, some are described in Chapter B 5, Further information concerning the parameters which influence the concentration of hydroxyl radicals is given in Section B 4.4, as well as a short overview about the application of ozone in AOPs in Section B 6.2. 

From the overview presented above it can be found that there are numerous methods of preparation of polymeric photocatalytic membranes. However, it should be stressed that in case of polymer membranes there is always a danger of destruction of the membrane structure by UV light or hydroxyl radicals. This risk is associated with the reactor configuration. Application of a photocatalytic membrane requires irradiation of the membrane itself in order to perform the photodecomposition of pollutants. The lowest UV resistance is exhibited by membranes prepared from polyether-sulfone (PES) and polysulfone (PSU) (Chin et al, 2006 Molinari et al, 2000). This can be attributed to the fact that PES and PSU contain sulfone groups which are highly sensitive to UV light. Other membranes exhibiting low UV resistance are those made of polypropylene (PP), polyacrylonitrile... 

Atmospheric particles very often contain water when they occur as deliquesced aerosol particles, haze, fog, cloud droplets, or even rain droplets (hydrometeors). It has been suggested before that the atmospheric aqueous bulk phase in these systems might also host a lively and important photochemistry which, up to now, has mostly been described insofar as hydroxyl (OH) radicals are generated by the photolysis of nitrate, nitrite, hydrogen peroxide [191-198], and iron-hydroxyl complexes [199]. These processes have been treated in recent overviews such as [200, 201]. 

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