Fenton’s reaction

El-Gohary FA, Badawy MI, El-Khateeb MA, El-Kalliny AS (2009) Integrated treatment of olive mill wastewater (OMW) by the combination of Fenton s reaction and anaerobic treatment. J Hazard Mat 162 1536-1541... 

It was also known from much earlier studies that the addition of transition-metal salts (e.g. iron or copper salts) to H2O2 solutions will also result in the formation of OH via Fenton s reaction ... 

Figure 6.9 Abstraction of a phenolic hydroxyl proton by a hydroxyl radical generated by Fenton s reaction with hydrogen peroxide.

Groundwater may be a significant source of water in a lake. This source of water may result in the decomposition of H202 in water through reactions of reduced metals with H202, such as Fenton s reactions (95) ... 

Goldschmidt and Pauncz (1933) suggested that Fenton s reaction is a chain reaction involving the same reactive intermediates occurring during catalytic decomposition of H202 rather than via formation of peroxides of iron ... 

Ferrous iron produced is then oxidized by H202 in Fenton s reaction ... 

Molecular structure of herbicides in the Fenton s reaction system. 

Kuo (1992) suggested that dye decolorization can result from both oxidation and coagulation processes. During the oxidation process, hydroxyl radicals may attack an organic substrate such as an unsaturated dye molecule. Thus, the chromophore of the dye molecule would be destroyed and decolorized. Ferric ions generated from Fenton s reaction might form fer-ric-hydroxo complexes with hydroxide ions, as shown in Equation (6.145) and Equation (6.146) ... 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

The octanol/water partition coefficient (log P) indicates the hydrophobiaty of a substrate. Fligher hydrophobicity of a chemical compound will decrease its aqueous solubility. Because hydroxyl radicals have reaction rates that are most likely diffusion controlled, the reaction rates of hydrophobic compounds in aqueous systems, such as Fenton s reaction, will be negatively affected. Figure 6.22 shows the effect of the octanol/partition coefficient on the oxidation rates of 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophe-nol, and pentachlorophenol. Pentachlorophenol has the lowest aqueous solubility and correspondingly its reaction rates are significantly affected. 

Hydrogen peroxide can serve as an initiator. The optimal concentrations for H2Oz are those with a molar ratio of H202 to CHC13 between 30 and 50. Addition of Fe++ will increase the reaction efficiency due to Fenton s reaction ... 

Decomposition rates for selected chlorophenols in the Fenton s reaction system. (Data from Benitez, F.J. et al., Chemosphere, 41,1271-1277, 2000.)... 

Moreover, metallic ions like Fe2+ are pro-oxidants as they catalyze Fenton s reaction, inducing formation of strongly reactive hydroxyl radical OH- [13,14] ... 

Fenton s reaction in presence of transitions metals of metalloproteins (Fe, Cu, Zn) r or reaction of Harber-Wiss in presence of 02 and metalloproteins OH ... 

LOOH undergoes the Fenton s reaction, which produces either an alkoxy radical (LO ) or an LOO, both of which can oxidize the closest L and the reaction propagates. 

Watts RJ, Bottenberg BC, Hess TF, Jensen MD, Teel AL. Role of reductants in the enhanced desorption and transformation of chloroaliphatic compounds by modified Fenton s reactions. Environ Sci Technol 1999 33 3432-3437. 

Kim Y-S, Kong S-H, Bae S-Y, Hwang G-C. The mechanisms of oxidation and reduction under various pH regimes in Fenton s reaction. Kongop Hwahak 2001 12 755-760. 

Joseph JM, Destaillats H, Hung H-M, Hoffmann MR. The sonochemical degradation of azobenzene and related azo dyes rate enhancements via Fenton s reaction. J Phys Chem A 2000 104 301-307. 

Reactions (19.13)—(19.16) propagate Fenton s reaction (19.12) with the continuous production of OH giving rise to the fast destruction of organic pollutants in the bulk solution. However, a part of this radical is lost by the oxidation of Fe2+ from reaction (19.17) and by decomposition of H2O2 from reaction (19.18) ... 

The rate of Fenton s reaction (19.12) can also decay by oxidation of Fe2+ in the bulk with HO2 by reaction (19.19) and at the anode by reaction (19.20) when an undivided cell is used ... 

In an undivided electrolytic cell, the electro-Fenton process leads to the destruction of organics contained in wastewaters by simultaneous oxidation with OH formed at the anode surface from reaction (19.9) and in the medium from Fenton s reaction (19.12). Parallel slower reaction of pollutants with weaker oxidants such as H202, H02, S20g2-, and O3 formed from reactions (19.1), (19.4), (19.10), and (19.11), respectively, is also possible. In addition, final carboxylic acids can form complexes with iron ions that are difficult to be oxidized by OH. We will see the notable influence of the anode material (Pt or BDD) on the degradation of these compounds in further sections. 

In the photoelectro-Fenton process the mineralization of organic pollutants can be enhanced by the production of more OH from reaction (19.24) and the additional acceleration of Fenton s reaction (19.12). However, we will see that the main action of UVA irradiation is the photodecomposition of complexes between Fe3+ and some final aliphatic acids, as for example oxalic acid (Zuo and Hoigne 1992). 

This section is devoted to discuss the main results obtained for the treatment of organics in wastewaters by electro-Fenton using a divided cell usually with a cationic membrane (e.g., of Nation) to separate the anolyte and catholyte. Although the anode reactions do not affect the degradation of pollutants under these conditions, it is interesting to know the characteristics of their homogeneous oxidation with OH when this radical is uniquely formed in the medium from Fenton s reaction (19.12). 

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