Hydroxyl radical competition

Sulfate radical anion may be converted to the hydroxyl radical in aqueous solution. Evidence for this pathway under polymerization conditions is the formation of a proportion of hydroxy end groups in some polymerizations. However, the hydrolysis of sulfate radical anion at neutral pi I is slow (k— 107 M"1 s 1) compared with the rale of reaction with most monomers (Ar=l08-109 M 1 s 1, Table 3.7)440 under typical reaction conditions. Thus, hydrolysis should only be competitive with addition when the monomer concentration is very low. The formation of hydroxy end groups in polymerizations initiated by sulfate radical anion can also be accounted for by the hydration of an intermediate radical cation or by the hydrolysis of an initially formed sulfate adduct either during the polymerization or subsequently. 

In principle, EPR spectrometry is well suited as a method to monitor kinetic events however, in practice, the time required to tune the spectrometer, and its intrinsically low sensitivity compared to fluorescence or light-absorption spectrometry, affect its competitiveness. Relatively slow reactions on the timescale of minutes, such as the decomposition of the DMPO-superoxide adduct and the subsequent formation of the hydroxyl radical adduct (cf. Pou et al. 1989) are readily followed, either as the first-order disappearance of the DMPO/ OOH signal... 

Of course, superoxide may reduce ferric to ferrous ions and by this again catalyze hydroxyl radical formation. Thus, the oxidation of ferrous ions could be just a futile cycle, leading to the same Fenton reaction. However, the competition between the reduction of ferric ions by superoxide and the oxidation of ferrous ions by dioxygen depends on the one-electron reduction potential of the [Fe3+/Fe2+] pair, which varied from +0.6 to —0.4 V in biological systems [173] and which is difficult to predict.)... 

Thus there is little doubt that the hydroxyl radical, if generated by an unambiguous method such as pulse radiolysis, can be trapped by PBN or DMPO, even if the former has several deficiencies, among them low trapping efficiency and short half-life of HO-PBN. The problem in hydroxyl radical trapping thus rests with the possible competition from the nucleophilic addition-oxidation mechanism, as exemplified in reaction (69) for DMPO and Ox-Red as a general one-electron redox system, or the inverted spin trapping mechanism (70). The treatment to follow will mostly be limited to DMPO. 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

At pH >11, where reaction (30) becomes increasingly important, G( OH) can increase by up to 0.6 molecule (100 eV) (i.e., g(H ), whereas at pH < 3, G( OH) becomes smaller because reaction (3) competes with reaction (27). One should not forget that the product of reaction (27) is 0 rather than OH [88,89]. Thus, if a solute that reacts with O in competition with its protonation in reaction (28) is present, then the reaction products may not be the same in N20-saturated solutions containing hydroxyl radical scavengers in high and low concentrations because O can react differently from OH (see Section 4.3). 

Transfer of radiation-induced electrons and holes (H20 ) from the hydration layer of DNA has been of considerable recent interest. Results from ESR experiments at low temperatures suggest that ionization of hydration water (reaction 4) results in hole transfer to the DNA (reaction 5) [4, 24-28]. Since the proton transfer reaction (reaction 6) to form the hydroxyl radical likely occurs on the timescale of a few molecular vibrations [29], it is competitive with and limits hole transfer to DNA [27]. 

Rate constants for the reaction of hydroxyl radicals with different compounds were determined by Haag and Yao (1992) and Chramosta et al. (1993). In the study of Haag and Yao (1992) all hydroxyl radical rate constants were determined using competition kinetics. The measured rate constants demonstrate that OH0 is a relatively nonselective radical towards C-H bonds, but is least reactive with aliphatic polyhalogenated compounds. Olefins and aromatics react with nearly diffusion-controlled rates. Table 4-3 gives some examples comparing direct (kD) and indirect (kR) reaction rate constants of important micropollutants in drinking water. 

Knowing the reaction rate constants of the direct and indirect reactions and the concentrations, the total reaction rate can be calculated. Unfortunately some data continue to be generated that fail to distinguish between the direct ozone reaction and hydroxyl radical chain reaction. Knowledge of independent rate constants for each pathway is useful to predict competition effects. In drinking water the direct oxidation kinetic is often negligible compared with the indirect, in waste water there is often no clear preference and both pathways can develop simultaneously. This was found for example in the ozonation of 4-nitroaniline at pH = 2, 7 and 11 (T = 20 °C) (Saupe, 1997 Saupe and Wiesmann, 1998). 

As Haber and Weiss (1934) suggested, at lower H202 concentrations and fixed Fe2+ the oxidation reaction approaches second order however, when the ratio of H202 Fe2+ increases, the reaction kinetic approaches zero order and the reaction process depends on the competition between hydroxyl radicals and superoxide radicals. If an excess of hydrogen peroxide is present, then the reactions as shown in Equation (6.123) and Equation (6.124) for 2,4-dinitrotoluene are dominant. The amount of H202 was used up quickly in this study, indicating the importance of Equation (6.123). At concentrations of Fe2+ greater than 600 mg/L, the DRE of BTX reached a maximum value at approximately 82% for benzene and toluene and 73% for xylene. 

Kochany and Bolton (1992) studied the primary rate constants of the reactions of hydroxyl radicals, benzene, and some of its halo derivatives based on spin trapping using detection by electron paramagnetic resonance (EPR) spectroscopy. The competitive kinetic scheme and the relative initial slopes or signal amplitudes were used to deduce the kinetic model. Based on a previously published rate constant (4.3 x 109 M 1 s ) in the pH range of 6.5 to 10.0 for the reaction of hydroxyl radicals with the spin trap compound 5,5 -d i methy I pyrro I i ne N-oxide (DMPO), rate constants for the reaction of hydroxyl radicals with benzene and its halo derivatives were determined. 

The highest bicarbonate concentration (0.01 M) produced the greatest decrease in the degradation rate due to the competition for hydroxyl radicals by bicarbonate ions however, a 0.001-M concentration of bicarbonate ion barely affected the oxidation rate of fluorene (Beltran et al., 1995). Similar tests were conducted by Beltran et al. (1995) on PHEN and ACEN, and the... 

Bansal KM, Patterson LK, Schuler RH (1972) Production of halide ion in the radiolysis of aqueous solutions of the 5-halouracils. J Phys Chem 76 2386-2392 Barnes JP, Bernhard WA (1994) One-electron-reduced cytosine in acidic glasses conformational states before and after proton transfer. J Phys Chem 98 887-893 Barvian MR, Greenberg MM (1992) Independent generation of the major adduct of hydroxyl radical and thymidine. Examination of intramolecular hydrogen atom transfer in competition with thiol trapping.Tetrahedron Lett 33 6057-6060... 

Barvian MR, Greenberg MM (1992) Independent generation of the major adduct of hydroxyl radical and thymidine. Examination of intramolecular hydrogen atom transfer in competition with thiol trapping. Tetrahedron Lett 33 6057-6060... 

Methanesulfonic acid, dimethyl sulfoxide and dimethyl sulfone are potential intermediates in the gas phase oxidation of dimethylsulfide in the atmosphere. We nave measured the rate of reaction of MSA with OH in aqueous solution using laser flash photolysis of dilute hydrogen peroxide solutions as a source of hydroxyl radicals, and using competition kinetics with thiocyanate as the reference solute. The rate of the reaction k (OH + SCN ) was remeasured to be 9.60 1.12 x 109 M 1 s 1, in reasonable agreement with recent literature determinations. The rates of reaction of the hydroxyl radical with the organosulfur compounds were found to decrease in the order DMSO (k = 5.4 0.3 x 109 M-i s 1) > MSA (k = 4.7 0.9 x 107 M l S 1) > DMS02 (k = 2.7 . 15 x 107 M 1 s ). The implications of the rate constant for the fate of MSA in atmospheric water are discussed. 

Gas phase kinetic studies of the reactions of hydroxyl radical are most conveniently carried out with direct monitoring of the OH radical with time using laser induced fluorescence (111. The low absorption coefficient of the aqueous hydroxyl radical ( 188nm 540 M 1 cm-1, (12)) precluded the direct measurement of this reactant species by its absorbance. Also, the absence of a readily observable product species for the reaction of OH + MSA at the wavelength range (275-575 nm) easily accessible in our experiments, has lead us to monitor the concentration of OH in solution indirectly by competition kinetics (13), measuring the absorption of the thiocyanate radical anion (ejsonm = 7600 M cm 1 (12)). 

The trapping of photoelectrons (17.14) leaves photogenerated holes available for reaction with hydroxyl ions to form hydroxyl radicals. Decreased activity above the optimum metal ions concentration is possibly due to the oxidation of Fe2+ by hydroxyl radicals or holes. The competition of holes between Fe+2 and OH— means that less OH radicals would be generated for the bacteria inactivation. The reactivity... 

The best way to prove the existence of the hydroxyl radical is to perform kinetic competition experiments with hydroxyl-radical scavengers [125]. Using the kinetic criterion, we can also exclude the intermediacy of the ferryl oxidant. However, such experiments in isolated heart models are, in practice, very difficult. 

Onstein P, Stefan MI, Bolton JR (1999) Competition Kinetics Method for the Determination of Rate Constants for the Reaction of Hydroxyl Radicals with Organic Pollutants Using the UV/H2O2 Advanced Oxidation Technology The Rate Constants for the tert-Butyl Formate Ester and 2,4-Di-nitrophenol,/. Adv. Oxid. Technol. 4, No. 2 231-236. 

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