Hydroxyl radical competition kinetics

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. 

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... 

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. 

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). 

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. 

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 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. 

Fig. 8.17 Competition kinetic scheme of the reaction of hydroxyl radicals with a substrate M in the presence of scavengers S/ and the corresponding second order rate expressions.

Barb et al. have also considered the ferric ion catalysis kinetics at low values of R2 where, as stated above, deviations from, the von Bertalan equation (c) occur. They conclude that with decreasing R2 reactions (3) and (1) will become of comparable importance as chain-terminating reactions, since peroxide will no longer be of such a concentration as to eliminate the ferrous reaction in the competition for the hydroxyl radical. At... 

Absolute rate constants have been determined for the reaction of the hydroxyl radical with a variety of aromatic compounds in aqueous solution. The rate constants obtained are significantly higher than values previously reported. Rate constants for the reaction of the hydroxyl radical with methyl alcohol and ethyl alcohol have also been determined by competition kinetics using three of these absolute rate constants as reference values. Comparison of our results with the published values from competition kinetics suggests that the rate constants for the reaction of hydroxyl radicals with iodide ion and thiocyanate ion are significantly higher than reported in earlier work. The ultraviolet absorption bands of the various substituted hydroxycyclohexadienyl radicals formed have been observed. 

Relative reactivities of organic compounds toward hydroxyl radicals have also been determined by competition kinetics using carbonate ions (I), thiocyanate ions (I, 2, 19), and iodide ions (21) as the reference reactant. However, the values reported for carbonate differ by a factor of two (1, 21, 23) and the values for both iodide and thiocyanate have been questioned (7,12) since there is a possibility that diiodide formation rather than iodine atom formation may be rate determining (12) and the analogous possibility has been suggested (7) for thiocyanate. There appears to be a need for further work to establish accurate values for a number of rate constants which may serve as reference values. 

Absolute Rate Constants. Absolute rate constants for the hydroxyl radical reactions, as determined from the formation curves of the hydroxycyclohexadienyl radicals, are summarized in Table I. Detailed data for benzoate ion are shown in Table II. In all cases the rate curves fit closely to a first order rate law. A detailed examination of this case seems warranted not only as an example of the data, but because of the possible use of this reaction as a reference reaction in competition kinetics. 

Table III. Rate Constants for the Reaction of Hydroxyl Radicals with Methanol and Ethyl Alcohol Determined by Competition Kinetics...

It is difficult to control and define the levels of hydroxyl radicals in a reaction system and rate constants are often derived using competition kinetics sometimes referred to as relative rate techniques. In this experimental approach, a compound, U, whose reaction rate is not known is allowed to react with hydroxyl radical in the presence of a reference compound, R, whose reaction rate is known. Assuming the compounds are only reacting with OH, the proportionate loss of the reference... 

Abstract—The influence of the hydroxyl radical (OH) on the photodegradation of the estrogen-like compound, bisphenol A (BPA), was examined in this study. The formation rate of OH, normalized to the vernal equinox solar noon condition of Higashi-Hiroshima (34°N) was in the range 0.70-3.25 X 10 °M s in Kurose river water. The total consumption rate constant of OH in river water ranged from 1.66 to 3.89 X 10 s . Based on the photochemical formation rate and the total consumption rate constant of OH, steady-state OH concentrations on the order of 3.33-8.35 X 10 M were determined. The reaction rate constant for OH with BPA determined by competition kinetics was found to be 1.55 X 10 ° s in water containing nitrate ions that photochemically produced OH. 

Addition of materials capable of reacting with electrons reduces the lifetime of enabling kinetics of its fast reactions to be determined. The powerful oxidant OH (the hydroxyl radical) has only a weak absorption in the ultraviolet, and its reactivity is best measured by a competition method based on its very fast oxidation of thiocyanate ion CNS to yield the intensely absorbing (CNS) ion (Xmax 472 nm). Addition of a second substrate X will provide competition for OH, and the intensity of the absorption of (CNS)2 will be systematically reduced as [X] is increased, enabling a rate constant to be derived. 

By pulse radiolysis experiments Ambroxol and bromhexine were shown to be scavengers of both superoxide and hydroxyl radicals (Felix et al. 1996). The dismutation of superoxide was accelerated 3-fold by bromhexine and 2.5-fold by Ambroxol over the rate of spontaneous dismutation. The reaction constants of hydroxyl radicals with bromhexine and Ambroxol were determined by competition kinetics to be 1.58 0.15x 10 M" s" and 1.04 0.1 X10 JVr s", respectively. 

Danilczuk M, Perkowski AT, Schlick S (2010) Ranking the stability of perfluorinated membranes used in fuel cells to attack by hydroxyl radicals and the effect of Ce(III) a competitive kinetics approach based on spin trapping ESR. Macromolecules 43 3352-3358... 

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