Hydroxyl radical oxidation

It is well known that the catalytic effect of Ti02 is attributed to the generation of a strong oxidant, hydroxyl radicals [44]. Following this theory, the quantum efficiency of the felt material prepared with the titania/silica fiber was calculated from the aforementioned result. In this case, if the number of molecules is significantly larger than the number of photon, acetaldehyde is oxidized to CH3COOH as follows ... 

Here, we focus our attention on the interplay that exists between solvation processes and ultrafast redox reaction in the vicinity of the strong oxidant hydroxyl radical (OH). This diatomic radical represents one of the most efficient oxidant of cellular components (proteins, lipids, DNA), contributes to Haber-Weiss reaction and plays some important role in fundamental radiation or stratospheric chemistry. Presently, we have investigated short-time water caging effect on transient electron delocalization-relocalization in the vicinity of nascent aqueous OH radicals. This specific electronic channel is represented by Eq.(l). 

If this mechanism is important in natural systems, it would also lead to the formation of the strongly oxidizing hydroxyl radical. This process could have a significant effect on the fate and transport of pollutants. 

The electronic structure of a semiconductor is characterized by a filled conduction band and an empty valence band separated by a bandgap of energy (EG). When the catalyst is illuminated with photons whose energy is equal to or greater than this bandgap, the promotion of an electron from the valence band to the conduction band occurs with the creation of electron-hole pairs (e —h ). The valence-band hole can oxidize electron-donor molecules (water or hydroxyl ions) to produce oxidizing hydroxyl radicals, whereas the conduction-band electron can reduce acceptor molecules such as 02 (to yield a superoxide ion) or a metal ion (reduced to its lower valence states) (Figure 15.1). 

Carbon Monoxide Oxidation. Analysis of the carbon monoxide oxidation in the boundary layer of a char particle shows the possibility for the existence of multiple steady states (54-58). The importance of these at AFBC conditions is uncertain. From the theory one can also calculate that CO will bum near the surface of a particle for large particles but will react outside the boundary layer for small particles, in qualitative agreement with experimental observations. Quantitative agreement with theory would not be expected, since the theoretical calculations, are based on the use of global kinetics for CO oxidation. Hydroxyl radicals are the principal oxidant for carbon monoxide and it can be shown (73) that their concentration is lowered by radical recombination on surfaces within a fluidized bed. It is therefore expected that the CO oxidation rates in the dense phase of fluidized beds will be suppressed to levels considerably below those in the bubble phase. This expectation is supported by studies of combustion of propane in fluidized beds, where it was observed that ignition and combustion took place primarily in the bubble phase (74). More attention needs to be given to the effect of bed solids on gas phase reactions occuring in fluidized reactors. 

Superoxide may also univalently reduce hydrogen peroxide to yield the extremely reactive, electrophilic oxidant, hydroxyl radical (Schaich, 1980) ... 

The first-order reaction rate constant for the isomerization of peroxynitrous acid to nitrate is 4.5 s 1 at 37°C therefore, at pH 7.4 and at 37°C the half-life of the peroxynitrite/peroxynitrous acid couple (let both these species be referred to as peroxynitrite for the sake of brevity) is less than 1 s. The reaction mechanism of peroxynitrite decomposition was a subject of controversy. Primarily proposed was that peroxynitrous acid decomposes by homolysis, producing two strong oxidants hydroxyl radical and nitrous dioxide (B15) ... 

There are numerous hypotheses-but few definitive results-as to what physicochemical characteristics of atmospheric particles are responsible for adverse health effects (Samet 2000 Schlesinger 2000). Hypotheses include general properties such as mass, surface area, or size, as well as more specific chemical properties such as acidity or elevated concentrations of transition metals (Dreher et al. 1997 Samet 2000). For example, it has been suggested that particulate iron is toxic due to its ability to generate the strongly oxidizing hydroxyl radical through the Fenton reaction (Ohio et al. 1996 Smith and Aust 1997 Donaldson et al. 1998 van Maanen et al. 1999) ... 

PROBABLE FATE photolysis gradual photooxidation expected to occur, main route of degradation in an aquatic environment, photoxidation half-life in water 3.2-160 days, photooxidation half-life in air 12.9-129 days, half-life for photooxidation by peroxy radicals 58 days oxidation hydroxyl radicals may displace nitro groups, may react with photochemically pro-... 

Many studies of wet air oxidation have claimed highly efficient conversion of DOC to low molecular weight polar species such as short-chain organic acids and diacids. Not much is known about the mechanisms of the reactions taking place in wet oxidation. Hydroxyl radicals have been suggested as intermediates, but very little information of a mechanistic nature is available to prove or disprove this hypothesis. In a study of wet air oxidation of phenanthrene (Larson et al., 1988), the structures of the partial oxidation products identified suggested a radical mechanism that could have been initiated by -OH (Figure 4.12). 

Another example of the differences in bio degradability of advanced oxidation (hydroxyl-radical mediated) transformation products are for quaternary amines. A study by Adams and Kuzhikannil (2000) showed that transformation products of alkyldimethylbenzyl ammonium chlorides (Bar-quats) were, on average, significantly more biodegradable than the parent compounds. However, the biodegradability of transformation products of dioctyl-dimethyl ammonium chlorides (Bardac LF) was significantly less than that of the parent. 

Although HO% HO2 , and O2 are all oxidants, hydroxyl radicals have the strongest oxidizing ability for a variety of organic compounds. When organic compounds encounter the Fenton s reagents, chain reactions would take place as shown below ... 

Catalytic activity of composites B-N-Fe and Si-N-Fe when applying UV radiation in presence of hydrogen peroxide, oxalic acid, and EDTA is determined by formation of photo-Fenton, ferric-oxalate, and Fe-EDTA systems in the solution which leads to generation of the super-oxidant-hydroxyl radicals. At the same time, solutions practically are not polluted by iron. High catalytic activity of the composites is determined by combination of heterogeneous and homogeneous catalyses. 

This dosimeter relies on the radiolytic reduction of the dichromate ion (Cr207) to chromic ion in aqueous perchloric acid solution. In this aqueous solution the radiolytically produced hydrogen atoms reduce Cr(VI), while the hydroxyl radicals oxidize Cr. Matthews (1981) suggested adding silver ions to the solution, since these scavenge the oxidizing hydroxyl radicals. 

In electrochemical mineralization, oxygen atoms are transferred from water to the organic pollutant R via the strong oxidizing hydroxyl radicals (E° = 2.74 V/SHE) produced by water discharge (Fig. lb) ... 

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] ... 

The oxidations of [(NQgFeOmid)] " and [(NC)6Fe(iV-Me-imid)] by HaOa proceed by dissociation of the corresponding imidazole and a rate-limiting substitution of the oxidant. Hydroxyl radicals are formed in the initial one-electron redox... 

When the reduction of a solute is desired, the oxidizing hydroxyl radicals have to be removed before they are able to attack the solute. This is achieved by adding an alcohol to the solution at a concentration much higher than that of the solute. Alcohols do not react with the hydrated electron, but scavenge the OH radical. Propanol-2, for example, reacts according to ... 

As in the case of phenol oxidation, hydroxyl radicals formed by water discharge on the BDD anode (eq. 20.1) participate in the oxidation of 3-MP to nicotinic acid (eq. 20.4) ... 

The production of numerous active oxidants hydroxyl radicals, hydrogen peroxide, ozone, peroxodisulfate, etc., has been simplified with the use of the BDD anodes. The AOPs use these oxidants to destroy low concentrations of biorefractory organic species. Some of these oxidants are unstable, and thus the innovative BDD anodes allow an easier use of the AOPs in the field of wastewater treatment. 

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