Hydrogen peroxide photo-Fenton reaction

Transition metals accelerate the photodecomposition of hydrogen peroxide (photo-Fenton reactions cf. section 5.6). 

Zepp, R. G., B. C. Faust, and J. Hoigne, Hydroxyl Radical Formation in Aqueous Reactions (pH 3-8) of Iron(II) with Hydrogen Peroxide The Photo-Fenton Reaction, Environ. Sci. Technoi, 26, 313-319 (1992). 

However, increases in the photocatalytic activity have been reported for Ti02 doped by lanthanides, tin, and iron (III) (55). It may be questioned whether these increases could, in fact, arise from the photo-Fenton reaction between the cations located at the surface and the hydrogen peroxide formed in situ, or even possibly because of a partial dissolution of these cations in the case of aqueous-phase reactions. 

Zepp RG, Faust BC, Hoigne J. Hydroxyl radical formation in aqueous reactions (pH 3-8) of iron(II) with hydrogen peroxide the photo-Fenton reaction. Environ Sci Technol 1992 26 313-319. 

Ozone, 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). 

These reactions do not involve excitation of the aryl halide but are mentioned here because of interest in degrading chlorinated pollutants. Wastewater treatment involving hydroxyl radical production by photolysis of hydrogen peroxide or ozone is a well-known commercial process. Fe +, HjO, and UV have been used in combination (the photo-Fenton reaction) to enhance the degradation of Aroclor 1242, a mixture of PCBs. The conventional Fenton reaction produces hydroxyl radicals from and Fe + ... 

Destruction of chromophoric and nonchromophoric pollutants in pulp and paper effluents may be achieved by advanced oxidation methods such as photocatalysis, photo-oxidation using hydrogen peroxide (H202)/UV or ozone (Osj/UV systems, Fenton-type reactions, wet oxidation, and by employing strong oxidants such as ozone. 

One of the most friendly, attractive and economically justified techniques used in homogeneous systems consists of the photo-Fenton method. The photo-Fenton reagent (Fe3+/Fe27H202) is an efficient and cheap reagent, which in a very simple way produces OH radicals for wastewater treatment, due to the fact that iron is a very abundant and non-toxic element and hydrogen peroxide is easy to handle and environmentally safe. Iron in its ferrous and ferric forms acts as a photocatalyst and requires a working pH <4. At higher pH values, iron precipitates as a hydroxide. The Fenton reaction, which usually operates at or near ambient temperature and pressure ... 

A typical Fenton s treatment installation requires facilities for storing and dosing chemicals (hydrogen peroxide, ferrous catalyst, acid and alkali for pH adjustment), a reaction tank, a settlement tank or centrifugal pump for solids removal and a UV-VIS lamp if photo-Fenton s is to be employed. The use of Fenton s and photo-Fenton s systems are likely to grow in use over the next decade as environmental legislation becomes more widespread, and as the systems become more efficient. 

Photoreactions that involve transition metal ions, complexes or compounds can usually be classified as (photo)redox (simultaneous oxidation and reduction) processes. A representative non-photoassisted catalytic system is Fenton s reagent that produces HO radicals on reaction of ferrous ions (Fe2 +) and hydrogen peroxide (Scheme 6.287a). Its photochemical counterpart is the photo-Fenton process,1527 in which ferric (Fe3 + ) complexes in aqueous solutions (absorbing over 300 nm) are reduced to ferrous ions (Scheme 6.287b). 

Such a reaction regenerates ferrous ion showing that only low concentration of Fe " is needed in the system [86]. Moreover, the photo-Fenton process may proceed using photons of wavelength close to 500 nm in the case of mixtures of ferric ion and hydrogen peroxide. It can also be performed by solar irradiation making it a low cost process [79, 80, 87, 88]. The photocatalytic cycle may be presented as shown in Scheme 6.6. 

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