Fenton systems

Fukushima M, K Tatsumi (2001) Degradation pathways of pentachlorophenol by photo-Fenton systems in the presence of iron (III) humic acid and hydrogen peroxide. Environ Sci Technol 35 1771-1778. 

In the presence of metal catalysts, hydrogen peroxide oxidations proceed in improved yields. The most common catalyst is an iron(II) salt which produces the well-known Fenton system or reagent. Dimethyl sulphoxide is oxidized to the sulphone using this system although a range of unwanted side-products such as methanol and methane are produced Diphenyl sulphoxide does not react using this reagent due to its insolubility and in all cases some iron(III) is formed by other side-reactions. 

Bremner DH, Molina R, Martnez F, Melero JA, Segura Y (2009) Degradation of phenolic aqueous solutions by high frequency sono-Fenton systems (US-Fe203/SBA-15-H202). Appl Catal B 90 380-388... 

Garcia-Montano J, Torrades F, Garcfa-Hortal JA et al (2006) Degradation of Procion Red H-E7B reactive dye by coupling a photo-Fenton system with a sequencing batch reactor. J Hazard Mater B 134 220-229... 

Later even more complexity was demonstrated (Makino et al., 1992) in the reaction between DMPO and Fe111 in water. The HO-DMPO" formed was transformed into a hydroxamic acid [24] which is a tautomer of 2-hydroxy-DMPO [25] in a Fenton system transfer of a hydroxyl (cf. p. 133) from the ligand-FeOOH complex to either of these species leads to additional epr-active nitroxyls [26] and [27] in reaction (72). 

Costa, RCC Lelis, MFF Oliveira, LCA Fabris, JD Ardisson, JD Rios, RRVA Silva, CN Lago, RM. Novel active heterogeneous Fenton system based on Fe3-xMx04 (Fe, Co, Mn, Ni) The role of species on the reactivity towards H2O2 reactions. Journal of Hazardous Materials, 2006 129, 171-178. 

Moura, FCC Araujo, MH Costa, RCC Fabris, JD Ardisson, JD Macedo, WAA Lago, RM. Efficient use of Fe metal as an electron transfer agent in a heterogeneous Fenton system based on Fe°/Fc304 composites. 

Radical decomposition of CP by photo-Fenton system. (Data from Benitez, F.J. et al., Chemo-sphere, 41, 1271-1277, 2000.)... 

For setting up a probe system for use in biological systems, it is required that it provides OH but also adequately behaves with respect to competition kinetics. The Fenton system seems to fulfill the first criterion in that it produces the required products in good yields but certainly not the second one. As can be seen from Table 3.7, the measured yields and the calculated ones [based on competition kinetics, Eq. (43), and established rate constants] dramatically disagree. The reason for this is not yet known, but it is evident that this system cannot be used with advantage as a reference for OH production. 

Based on his own experimental results and their analysis, Traube has made the correct (in terms of modem understanding) conclusion that H202 displays high reactivity in the presence of a catalyst in the system. This is the pathway of various substrate oxidations by hydrogen peroxide, proceeding in the Fenton system (iron ion + H202), and some biochemical processes. 

Critical review of oxidation reactions in the Fenton system. 

Thus, development of the investigations in this branch is not an extensive process. Interest in this problem is regularly kept alive by new theoretical ideas. All currently known elementary reactions proceeding in the Fenton system are shown by the overall sketch in Figure 6.1 which presents the whole variety of highly active intermediate particles, synthesized in seemingly simple reactions. However, it should be noted that the fate of free radicals and charged particles in the mechanism is determined by the process kinetics, where the basic factors are the reactor operation mode and physicochemical parameters of the catalyst. 

Figure 6.1 The mechanism of hydrogen peroxide dissociation in the Fenton system. The sketch is based on the data by Kazarnovsky [7], Haber-Weiss [9], Bard [6] and Uri [25].

Let us consider the reaction of benzene oxidation with hydrogen peroxide in the Fenton system as the classical situation [30], In the absence of iron ions benzene does not in practice interact with H202. The addition of bivalent iron salt to the system C6H6-H202-H20 induces benzene oxidation to phenol and diphenyl according to the following mechanism ... 

As noted in the previous chapter, substrate interaction with OH and H02 radicals produces different products, which composition depends on H202 dilution. Hence, OH radicals participate in nonselective oxidation, whereas H02 promotes selective gas-phase oxidation of substrates with hydrogen peroxide [32], This situation is observed for both low H202 concentrations in the reaction mixture and liquid-phase oxidation in the Fenton system, where the OH radical is the key active site. In the Fenton system, connection channels between two reactions are set with the help of a general intermediate, the OH radical, which represents a nonselective active site due to the ability to attack another complex molecule by various... 

As follows from the above, there is no principal possibility of implementing high selectivity oxidation of the substrate in the Fenton system. 

The works devoted to catalytic hydroxylation of aromatic and heterocyclic compounds with hydrogen peroxide under soft conditions should be mentioned [34], The authors showed that the main experiments were performed in the Fenton system and the process can be described by the following generalized mechanism ... 

Photo-Fenton systems, enhanced by Fe(III) complexation by humic and fulvic acids [100-102], could be a very important OH source in surface waters. However,... 

If hydrogen peroxide is available in excess in a Fenton system, or if additional Fe3+ is present, the overall reaction yields greater 02 formation [4] by favoring reaction (3). 

Reaction (13) is typical of aliphatics and alcohols, whereas reaction (14) is common for double bonds, especially in conjugated and aromatic systems. To form the final products, the radicals R and ROH undergo additional reactions. Free radical scavengers are a very important component of Fenton systems, and their importance in pollutant degradation will be discussed further in Secs. Ill and IV. 

In contrast to the results of Bossmann et al., Lindsey and Tarr [31,32] observed equivalent rate constants for polycyclic aromatic hydrocarbon (PAH) degradation with Fenton systems as had been previously observed for PAH reaction with hydroxyl radicals as generated by pulse radiolysis techniques [33], Such kinetic agreement suggests, but does not confirm, equivalent mechanisms. PAHs, unlike 2,4-dimethylaniline, are not expected to directly coordinate iron in aqueous solutions. 

Although there is still debate as to whether hydroxyl radicals or ferryl species are the key oxidants in Fenton systems, most literature reports on the mechanisms of degradation of organic compounds invoke the hydroxyl radical. Based on the reports discussed above, it seems likely that hydroxyl radical is a major oxidant during Fenton degradations. Although ferryl ions or other highly oxidized forms of iron may occur, either to a limited extent or more abundantly under specific conditions, this section will deal with documented reaction pathways and kinetics for hydroxyl radical or species assumed to be hydroxyl radical. The reader should keep in mind that ferryl pathways may need to be considered under certain conditions. 

Attempts have been made to immobilize iron in photo-Fenton systems. Iron immobilized in Nafion was successfully used to degrade 4-chlorophe-nol [82] and Orange II [83]. In both studies, the authors indicated that the Nafion -bound iron was resistant to aging or fouling. 

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