Fenton processes, production

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The fact that reaction (12) is much slower than reaction (8), implies that Fe is faster depleted from the solution. As a result, Fenton process is halted because the redox chain cannot be supported itself. In addition, it is accepted that (Pignatello 1992 Boye et al. 2003) the hydroperoxyl radical (HO2 ) has a much lower oxidant power than OH. In the presence of organics, Fenton chemistry is even more complex because hydroxyl radical, both iron cations and the oxidation products enter into a series of consecutive and parallel reactions. An example of the complexity of these reactions is discussed elsewhere (Gozzo 2001) but a brief description is given here. The initial step for an organic substrate (R-H) oxidation starts with the interaction of itself with OH, according to (Walling and Kato 1971) ... 

The three intermediate products that reached the highest concentrations in the case of quinoline degradation by the photo-Fenton process were in the order ... 

Regarding the main products corresponding to the opening of the pyridine moiety of quinoline, 2-aminobenzaldehyde and, to a lesser extent, its A -formyl derivative were formed by photocatalysis, whereas only traces of this latter product were detected when the photo-Fenton process was employed. Also, (2-formyl)phenyliminoethanol was detected only in the case of the degradation over Ti02. 

Framjoisse P, Gregor KH. Application of a new Fenton process (FSR process) without sludge production for the treatment of non biodegradable wastewater. Chemical Oxidation. Technologies for the Nineties. Vol. 6. Lancaster Technomic Publishing Co., 1997 208-220. 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

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). 

Irmak et al. (2005) also utilized an analogous divided Pt/carbon-felt cell to destroy 300 cm3 of an 02-saturated 0.6 mM 4-chloro-2-methylphenol solution with Fe2+ as catalyst in H2S04 of pH 2.7 by electro-Fenton and photoelectro-Fenton at Ecat = —0.55 V vs. SCE. The solution was irradiated with UVC light of Amax = 254 nm in photoelectro-Fenton. After applying 141 C to a solution with 1.8mMFe2+ for 300 min, the photoelectro-Fenton process led to overall mineralization with total release of Cl-. This process was much faster than the comparable electro-Fenton one, as expected from the additional photolysis of complexes of Fe3+ with pollutants and the production of more OH from reaction (19.24). 

For electro-Fenton and photoelectro-Fenton, an increase in current always caused quicker mineralization rate, as can be seen in Fig. 19.10 for the photoelectro-Fenton process of 2,4,5-T. This is due to the production of greater amount of OH at the anode surface from reaction (19.9) and/or in the medium from Fenton s reaction (19.12) since more H202 is accumulated (see Fig. 19.4). However, the ACE value gradually decreased at higher current because of the greater relative increase in rate of nonoxidizing reactions of OH like its anodic oxidation to 02 as follows ... 

In the presence of PhSeSePh and excess hydrocarbon substrate the Fenton process [Fe(PA)2/HOOH] produces carbon radicals (RO, which are trapped by PhSeSePh to give PhSe-R products (Table 5-2 and Scheme 5-1). The distribution of PhSe-R isomers appears to reflect the isomer abundance for the R- radicals from the Fenton cycle [Eqs. (5-10) and (5-11)]. For n-hexane and 2-Me-butane the R-SePh isomer distribution (Table 5-2) indicates that the relative reaction probabilities of HO- with a C-H bond in — CH3, CH2, and CH groups are 0.074, 0.44, and 1.00 (the respective C-H bond energies are 100, 95, and 93 kcal), which are in accord with the relative values for aqueous HO- (0.10/0.48/1.00).i3 Thus, PhSeSePh provides the means to trap first-formed carbon radicals and thereby give insight to the mechanism of their generation. 

Advanced oxidation processes (AOP) can be classified using their manner of OH radical production. The most known AOP in water treatment is the combination of ozone with H2O2. Other AOP include the UV-induced AOP (UV/H2O2, UV/ozone, UV/Ti02), Fenton and Photo-Fenton processes, sonolytically induced AOP and those AOP which create OH radicals directly from water by homolysis (gamma radiolysis/electron beam injection, VUV-irradiation). In the following chapters, each technology is discussed with respect to MTBE elimination. 

The applicability of the Fenton process in water treatment is limited to special cases, where the conditions might favor such a process. In drinking water production, for example, the pH value of the raw water usually varies between... 

Highly photo-active and soluble ferrioxalate complexes have been successfully applied to the removal of two antibiotics, sulfametaxazole and trimethoprim, from aqueous solutions via a photo-Fenton reaction. The stability of ferrioxalate avoids the formation of Fe-antibiotic complexes. This improves the quantum yield for ferrous ions production, and consequently both the decomposition of H2O2 and the overall efficiency of the photo-Fenton process. 

Can, O.T. (2014) COD removal from fruit juice production wastewater by electrooxidation electrocoagulation and electro-Fenton processes. Desalin. Water Treat., 52, 65-73. 

In 1876, Henry J.H. Fenton publicly announced that the use of a mixture of H2O2 and Fe " (thereafter so-called Fenton s reagent) allowed the destruction of an organic compound, namely, tartaric acid [1], Such discovery triggered an intense research to elucidate the mechanistic fundamentals and propose different variants and applications of the Fenton process. The possible formation of Fe(IV) as an active Fenton intermediate, as well as the modeling of the real structure of the iron aqua complexes, is still the subject of discussion [2, 3]. However, at present, it is quite well established that the classical Fenton s reaction (1) involves the production of highly oxidative hydroxyl radicals ( OH) in the bulk as the main reactive species, and its optimum pH value is 2.8-3.0 [1] ... 

From a conceptual, operative/technical, and environmental standpoint, the electrochemical alternative to the chemical Fenton process seems more appealing because it allows (i) the on-site electrogeneration of H2O2, thus avoiding problems and costs related to externalized production, transportation, handling, and storage, and (ii) a much... 

The main difference between these two methodologies is related to the way in which the iron ions are incorporated into the system. While in the cathodic Electro-Fenton process an Fe " or Fe electrolyte is added to the reaction mixture, in the anodic Electro-Fenton system, an iron anode is employed as the source of the ferrous ions. In both processes, however, there is a continuous production of H2O2 which is formed from the electrochemical reduction of dissolved oxygen at the cathode surface as described by Eq. 7 [2,12] ... 

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