Hydroxyl formation rate

Figure 2. Rates of photodegradation of the copolymer when exposed to a broadband UV source (shortest wavelength 297 nm) (A) change in absorbance as a function of irradiation time in the wavelength range of 250-450 nm (spectrum measured on films) (B) change in absorbance at 3580 cm 1 measured by FTIR spectroscopy (rate of hydroxyl formation)...

Winer, A.M., Damall, K.R., Atkinson, R. Pitts, Jr., J.N. (1979) Smog chamber study of the correlation of hydroxyl radical rate constants with ozone formation. Environ. Sci. Technol. 7, 622-626. 

Furthermore, the increase in the scavenging rate of photoproduced electrons resulting from the presence of ozone should decrease the recombination rate of electrons and holes, and thereby augment the formation rate of hydroxyl radicals from basic OH surface groups and adsorbed water molecules (Fig. 1). 

Iron solubility is an obvious factor in Fenton oxidation because the rate of hydroxyl radical formation is directly proportional to [Fe2+]. At elevated pH values, iron hydroxides and oxides form and precipitate, causing a dramatic decrease in hydroxyl radical formation rate. Iron chelators can be used to offset this factor. A related issue is the rate of Fe3 + reduction to Fe2+, which, if insufficient, can result in Fe2+ concentrations that are too low to... 

This last point emphasizes a weakness in the Fenton literature pointed out previously. Often, the rate of peroxide decomposition is assumed to be proportional to the hydroxyl radical formation rate. Flowever, nonhydroxyl radical-forming pathways and the formation of radicals that are not accessible to pollutants may significantly decrease the yield of useable hydroxyl radical. 

As previously discussed, the concentration of Fe2+ is an important factor in the rate of hydroxyl radical formation from hydrogen peroxide. Consequently, any process that can speed the reduction of Fe3+ to Fe2+ will increase the formation rate of hydroxyl radical. UV or visible radiation can play this role by photoreducing iron. However, the photo-Fenton process involves three additional mechanisms that can contribute to pollutant degradation (a) direct photolysis of H202 to yield two hydroxyl radicals (Eq. (22)) (b) photolysis of Fe(OH)2+ to form hydroxyl radical (Eq. (23)) and (c) degradation of pollutants by direct photolysis (i.e., absorption of a photon by the pollutant molecule followed by decomposition of the photoexcited pollutant molecule). 

In the presence of organic compounds, peroxynitrous acid induces oxidation and nitration processes [72,74,76,77]. Oxidation reactions are due to the generation of hydroxyl (reaction 28). In the case of phenol, nitration is most likely to be electrophilic as evidenced by the very steep pH trend of nitrophenol initial formation rates ( Rate oc [H+] [57]). 

Despite its prominent role in atmospheric chemistry [53,54], the hydroxyl radical is considered a minor oxidant in the great majority of surface waters owing to the low formation rates [9] and efficient scavenging by DOM and the bicarbonate/carbonate anions. DOM is at the same time both sensitizer and scavenger of the hydroxyl radical, but CDOM is responsible only for minor formation pathways, which can be summarized as ... 

Studies of the doublet formation rate in polymer stabilized systems are far less numerous than those on electrostatically stabilized systems. Chang [68] has studied the doublet formation rate for 0.57 fim in diameter Si02 particles as a function of the amount of hydroxyl propyl cellulose (HPC) adsorbed onto their surface. Figure 10.32 is a plot of the colloid stability ratio as a function of the amount of HPC added to... 

Despite the complications of layer formation on many silicates, dissolution is generally assumed to be interface-limited and to be accelerated by the presence of protons or hydroxyl ions. Rates are thus often described by the following empirical equation ... 

Figure 5. Effect of processing time on the rate of photooxidation (hydroxyl formation) of PVC (numbers on curves are processing times...

Using the rate constant of Tully et al. (16), the temperature profile of Figure 2, and the measured benzene and hydroxyl mole tions, the predicted rate of benzene destruction by OH is insignificant compared to the measured net benzene formation rate until after the latter passes through its maximum value at 8.5 mm. At... 

The concept that free radicals are important intermediates in photochemical and other redox interactions of oxygen, organic compounds and heavy metals in natural waters has received considerable support recently ((1-3) and references therein this volume). Some of the major primary radicals expected are hydroxyl (OH), superoxide (02 ). and various organic moieties (R, RO, ROO). Of these, OH is of interest because of its extremely high reactivity, significant formation rate from a known source (nitrite photolysis, among others) and the analogy of its known key role in tropospheric chemistry. 

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. 

The metabolism of DMN by a-hydroxylation is outlined in Fig. 12. a-Hydroxylation yields the unstable intermediate, a-hydroxydimethylnitro-samine, which spontaneously decomposes to monomethylnitrosamine and formaldehyde. The former gives rise to methanol via an electrophilic methylating agent. A deuterium isotope effect of 1.8-2.8 has been determined for the N-demethylation of DMN-t/g, indicating that the initial hydroxylation is rate-limiting (91, 276). The rates of formation of formaldehyde and methanol in rat hepatic 10,000 g supernatant have been measured the rates accounted for the metabolic fates of both methyl groups of DMN (280). Equimolar amounts of formaldehyde and methanol were also formed in the solvolysis of... 

The hydroxyl radical is able to cause injury in biological systems, e.g. biomembranes can be deteriorated. Titanium is able to bind H2O2 in a Ti H202 complex. This complex can trap the superoxide radical which is formed during the H2O2 decomposition. By spectrophotometric spintrapping measurements and electron spin resonance measurements no hydroxyl radical formation rate in Ti-H202 could be detected. A similar result was observed with Zr, Au and Al [18]. 

These cathodic leactions affect only the amount of current used for the H2 evolution reaction and not the hydroxyl ion formation rate. Hence, the magnitude of these BCLs can be quantified from the moles of H2 evolved, which is equal to 7 —1 2) Il 2-F. Since,... 

---