Aqueous-phase oxidation

Aqueous Phase Oxidation of Sludge using the Vertical Reaction Vessel System," report to U.S. Environmental Protection Agency, Washington, D.C.,Mar. 1987. 

The total antioxidant activity of teas and tea polyphenols in aqueous phase oxidation reactions has been deterrnined using an assay based on oxidation of 2,2 -azinobis-(3-ethylbenzothiazoline-sulfonate) (ABTS) by peroxyl radicals (114—117). Black and green tea extracts (2500 ppm) were found to be 8—12 times more effective antioxidants than a 1-mAf solution of the water-soluble form of vitamin E, Trolox. The most potent antioxidants of the tea flavonoids were found to be epicatechin gallate and epigallocatechin gallate. A 1-mAf solution of these flavanols were found respectively to be 4.9 and 4.8 times more potent than a 1-mAf solution of Trolox in scavenging an ABT radical cation. 

Vinyl chloride can be completely oxidized to CO2 and HCl using potassium permanganate [7722-64-7] in an aqueous solution at pH 10. This reaction can be used for wastewater purification, as can ozonolysis, peroxide oxidation, and uv irradiation (42). The aqueous phase oxidation of vinyl chloride with chlorine yields chloroacetaldehyde (43). 

The selectivity issue has been related to multi-phase processing [31]. Nitrations include both organic and aqueous phases. Oxidation to phenol as one side reaction takes places in the organic phase, whereas all other reactions occur in the aqueous phase and are limited by organic solubility. For this reason, enhancing mass transfer by large specific interfaces is a key to affecting product selectivity. 

Both models apply the same chemical scheme of mercury transformations. It is assumed that mercury occurs in the atmosphere in two gaseous forms—gaseous elemental HgO, gaseous oxidized Hg(II) particulate oxidized Hgpart, and four aqueous forms—elemental dissolved HgO dis, mercury ion Hg2+, sulphite complex Hg(S03)2, and aggregate chloride complexes HgnClm. Physical and chemical transformations include dissolution of HgO in cloud droplets, gas-phase and aqueous-phase oxidation by ozone and chlorine, aqueous-phase formation of chloride complexes, reactions of Hg2+ reduction through the decomposition of sulphite complex, and adsorption by soot particles in droplet water. 

Figure 7.14 shows the calculated ratio of S(IV) oxidation with the uptake and reaction of N03 to that without the NO, contribution as a function of the chloride concentration in particles (Rudich et al., 1998). For reference, the saturation concentration of Cl- in sea salt particles (i.e., at the deliquescence point) is 6 M at room temperature. Under the assumptions of these particular calculations, the rate of aqueous-phase oxidation of S(IV) is estimated to increase by as much as 25% when N03 chemistry is taken into account. This uptake and reaction of NO, also decrease its gas-phase concentrations. 

There are two major factors to be considered in assessing the contribution of potential oxidants for S(IV) to the net aqueous-phase oxidation. The first is the aqueous-phase concentration of the species, and the second is the reaction kinetics, that is, the rate constant and its pH and temperature dependencies. As a first approximation to the aqueous-phase concentrations, Henry s law constants (Table 8.1) can be applied. It must be noted, however, that as discussed earlier for S(IV) this approach may lead to low estimates if complex formation occurs in solution. On the other hand, high estimates may result if equilibrium between the gas and liquid phases is not established, for example, if an organic film inhibits the gas-to-liquid transfer (see Section 9.C.2). 

As discussed in detail in Sections C.3.d and C.3.e, the fastest atmospheric reactions of S02 are believed to be with H202 and perhaps with Os at higher pH values. Under extreme conditions of large fog droplets (—10 yu,m) and very high oxidant concentrations, the chemical reaction times may approach those of diffusion, particularly in the aqueous phase. In this case, mass transport may become limiting. However, it is believed that under most conditions typical of the troposphere, this will not be the case and the chemical reaction rate will be rate determining in the S(IV) aqueous-phase oxidation. 

Clouds, fogs, and rain, however, have much greater liquid water contents and thus have the potential for contributing more to atmospheric aqueous-phase oxidations. Clouds typically have liquid water contents of the order of 1 g m-3, with droplet diameters of the order of 5-50 yxm the number concentration and size distribution depend on the type of cloud. Fogs, on the... 

While the volume of liquid water present is much larger in clouds and fogs than that in fine particles, the solute concentrations in the latter may be much higher, and this may serve to increase the rate of aqueous-phase oxidations. More importantly, these fine particles are believed to serve as sites for the condensation of water vapor, leading to the formation of fogs and clouds (Chapter 14.C.2). 

Figure 8.12 shows the projected conversion of S02 to sulfate as a function of the volume of water per cubic meter of air available for conversion in the aqueous phase, covering a range typical of haze particles, fogs, and clouds for atmospheric lifetimes which are typical for each (Lamb et al., 1987). As expected from Eq. (M), the conversion increases with the water available in the atmosphere. As we shall see, the aqueous-phase oxidation does indeed predominate in the atmosphere under many circumstances. Equations (G) and (M) apply as long as the partial pressure of SOz in the gas phase, so,, is measured simultaneously with the solution concentration of S(IV). 

With these comments regarding the characteristics of atmospheric aqueous-phase oxidations in mind, we summarize the kinetics of S(IV) oxidation in solution by a series of individual potential atmospheric oxidants. 

Extrapolation of their results to atmospheric conditions led them to suggest that such heterogeneous reactions could be important in the aqueous-phase oxidation of... 

Lind, J. A., and A. L. Lazrus, Aqueous Phase Oxidation of Sulfur IV by Some Organic Peroxides, EOS Trans., 64, 670 (1983). 

In short, growth of sulfate particles at least in the accumulation mode and the presence of two peaks are both believed to be largely controlled by interactions with water in the atmosphere, including the aqueous phase oxidation of SOz to sulfate. 

Sedlak, D.L. and Andren, A.W., Aqueous-phase oxidation of polychlorinated biphenyls by hydroxyl radicals, Environ. Sci. Technol, 25, 1419-1427, 1991b. 

Gray, L.W., Adamson, M.G.A., Hickman, R.G., Farmer, J.C., Chiba, Z., Gregg, D.W., and Wang, F.T., Aqueous phase oxidation techniques as an alternative to incineration, Lawrence Livermore National Laboratory, UCRL-JC-108867, Albuquerque, NM, 1992. 

Isotope ratios have been used with some success in the past to determine the importance of gas phase (Equation 4) verses aqueous phase (Equations 2A,2B,2C) oxidation of SO2. Saltzman et al. (24) compared the S34S values for SO2 and sulfate from samples collected from Hubbard Brook Experimental Forest (HBEF) in the non-urban northeastern US. They found discriminations which were intermediate to those expected for the individual oxidation mechanisms and suggested that both gas and aqueous phase oxidation were important. Newman (50) found that 6 S values for SO2 in the plume of an oil fired power plant decreased with distance (and time) from the stack which they attributed to equilibrium isotope effects. 

The regression curve of the previously determined (3) relationship between 18°S02- an< 180H20 n aqueous-phase oxidation of SO2 by H2O2... 

Another aqueous-phase oxidant is sodium hypochlorite, in presence of sodium bromide and a PTC this system brominates olefins and acetylenes316. 

Reaction of dissolved gases in clouds occurs by the sequence gas-phase diffusion, interfacial mass transport, and concurrent aqueous-phase diffusion and reaction. Information required for evaluation of rates of such reactions includes fundamental data such as equilibrium constants, gas solubilities, kinetic rate laws, including dependence on pH and catalysts or inhibitors, diffusion coefficients, and mass-accommodation coefficients, and situational data such as pH and concentrations of reagents and other species influencing reaction rates, liquid-water content, drop size distribution, insolation, temperature, etc. Rate evaluations indicate that aqueous-phase oxidation of S(IV) by H2O2 and O3 can be important for representative conditions. No important aqueous-phase reactions of nitrogen species have been identified. Examination of microscale mass-transport rates indicates that mass transport only rarely limits the rate of in-cloud reaction for representative conditions. Field measurements and studies of reaction kinetics in authentic precipitation samples are consistent with rate evaluations. 

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