Ozone in the aqueous phase

Ozone utilization efficiency (RL) will vary with changes in average UV intensity. The value of RL represents a ratio between the amount of decomposed ozone in the aqueous phase to the amount of ozone in the gas phase initially introduced into the treatment chamber. Additionally, RL corresponds to the fraction of ozone actually utilized for the destruction of solutes (Hayashi et al., 1993). RL values are maximized at UV intensities from about just over 10 W/m2 to about 100 W/m2. 

Iverfeldt A, Lindqvist O. 1986. Atmosperic oxidation of elemental mercury by ozone in the aqueous phase. Atoms Environ 20(8)1567-1573. 

Ozone in the aqueous phase was analyzed by the indigo method of Bader and Holgne (28,29). using the dlsulfonate rather than the trisulfonate as originally described by those authors. This method (hereafter called the HBI method) was calibrated in purified water against the lodlmetrlc method of Flamm ( ) (BKI) and checked by UV absorbance using the extinction coefficient of Hart et al. (3D. 

Aqueous Phase. In contrast to photolysis of ozone in moist air, photolysis in the aqueous phase can produce hydrogen peroxide initially because the hydroxyl radicals do not escape the solvent cage in which they are formed (36). 

Ozone in the gas phase can be deterrnined by direct uv spectrometry at 254 nm via its strong absorption. The accuracy of this method depends on the molar absorptivity, which is known to 1% interference by CO, hydrocarbons, NO, or H2O vapor is not significant. The method also can be employed to measure ozone in aqueous solution, but is subject to interference from turbidity as well as dissolved inorganics and organics. To eliminate interferences, ozone sometimes is sparged into the gas phase for measurement. 

The reaction mechanism of ozone with the solute should be known to establish a selective oxidation. Non-targeted compounds contained in the water phase should not have a higher reactivity to ozone. Otherwise ozone might already react to a large extent in the aqueous phase, consuming much of the ozone so that it is not available for the oxidation of the target solute in the solvent phase. The achievable selectivity depends much on the distribution of the solute between the gas, water and solvent phase, which should be checked by partition coefficients from the literature or experimentally determined. 

Volatile organic compounds (VOCs), especially trihalomethanes, are frequently found in drinking water due to the chlorination of humic acids. When UV irradiation is applied to the pre-ozonation of humic acids, the decomposition of VOC precursors increases (Hayashi et al., 1993). The ozonation rates of compounds such as trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane, 1,2-dichloroethane, and 1,2-dichloropropane were found to be dependent on UV intensity and ozone concentration in the aqueous phase by Kusakabe et al. (1991), who reported a linear relationship between the logarithmic value of [C]/[C0] and [03]f for 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene. The other two organochlorines followed the same first-order kinetics with and without UV irradiation (Kusakabe et al., 1991). Thus, the decomposition rate can be expressed as ... 

None of the five compounds analyzed can be destroyed in the gas phase without UV irradiation however, both trichloroethylene and tetrachloroet-hylene were degraded under UV irradiation. In the aqueous phase, UV irradiation destroyed the five compounds tested. The degradation rates increased linearly with UV intensity. Finally, the utilization efficiency of ozone, as well as the corresponding destruction rates of organic compounds, is influenced by UV intensity. The maximum efficiency roughly occurred in light intensity ranging from 10 to 100 W/m2. 

Hashem, T.M., Zirlewagen, M., and Braun, A.M., Simultaneous photochemical generation of ozone in the gas phase and photolysis of aqueous reaction systems using one UV light source, Water Sci. Technol., 35, 41—48, 1997. 

However, the photochemical production of ozone on a small scale represents a potential application of the novel incoherent Xe2 excimer lamps. This was first demonstrated by Eliasson and Kogelschatz (1991), and on laboratory and preparative scales by Laszlo et al. (1998) and by Hashem et al. (1997). The latter research group realized the simultaneous generation of ozone in the gas phase and its transfer to the aqueous phase followed by the VUV irradiation of this water by... 

Simultaneous Photochemical Generation of Ozone in the Gas Phase and Photolysis of Aqueous Reaction Systems Using one VUV Light Source, Wat. Sci. Technol. 35 41-48. 

In the aqueous phase ozone absorbs at 254 nm with a maximum molar absorptivity coefficient 3300 cm Hydroxyl radicals are produced via the ultraviolet photolysis of ozone to produce electronically excited singlet oxygen atoms ... 

The use of gaseous ozone for in situ chemical oxidation is more effective than aqueous-based systems for the treatment of contaminated soils, because the diffusivity of ozone gas is much greater than that for aqueous species. In addition, the concentration of ozone in the gas phase can be orders of magnitude higher than that obtainable in aqueous solutions, and ozone is more stable in the gas phase than in water. ... 

Pedersen and Sehested (2002) showed that the aqueous-phase reaction of isoprene with ozone was insignificant for the processing of isoprene in the atmosphere. They estimated the overall and individual lifetimes of isoprene due to reactions with ozone and the hydroxyl radical, at 25 "C and typical in-cloud conditions. The results (Table 3) indicate that clouds generally should not contribute much to the processing of isoprene in the atmosphere. Only in the aqueous phase, were the lifetimes of isoprene due to reactions with ozone and with OH radicals comparable. Similar conclusions were drawn for methyl vinyl ketone, while for methacrolein the clouds could reduce the overall atmospheric lifetime by 50 %. 

Rudzinski (2004) compared the rates of the aqueous-phase reaction of isoprene with sulphate radicals against the rates of the gas- and the aqueous-phase reactions of isoprene with OH radicals, NO3 radicals and ozone. The rates were evaluated for 25 C, typical atmospheric concentrations of reactants (Herrmann et at, 2000), and a LWC of 10 and 10. The partitioning of reactants between phases was described using Henry s Law. The results, shown in part in Table 4, indicated that the aqueous-phase reaction of isoprene with sulphate radicals was competitive against other reactions only in the aqueous phase and at very high values of liquid water content (LWC =10 ). 

The reactions controlling the lifetime of ozone at the surface depend on the chemical composition of the aqueous film. If this is not specified, it is therefore not possible to estimate the steady-state concentration for this photooxidant in the aqueous phase. However, the chemical effect of ozone in films or atmospheric droplets can be estimated by assuming that the steady-state concentration of ozone is still in equilibrium with the concentration of the ozone in the atmosphere. An atmospheric concentration of ozone of 1012 molecules per cubic centimeter would result in an aqueous equilibrium concentration of about 1 nanomolar (20°C) as the environmental factor to be considered in the aqueous film. Figure 7 gives examples for the rate constants for different types of compounds and a scale for the corresponding half-lives of these compounds exposed to the estimated concentration of ozone. (For further rate constants see Ncta et al., 1988 or Hoigne and Bader, 1983 and Hoigne et al., 1985). 

While ozone is not produced in the aqueous phase, at least 12 different chemical pathways consuming ozone have been identified. As a result of the relatively small aqueous-phase solubility of ozone, none of these reactions is rapid enough to significantly influence gas-phase ozone concentrations (Pandis and Seinfeld, 1989a). The fastest of these reactions is that with 02 ... 

Chemical reactions occur in the gas phase and in the aqueous phase (cloud droplets) that both oxidize elemental mercury to divalent mercury and reduce the divalent mercury to elemental mercury. The most important gas phase oxidation pathways are the reactions with ozone (Hall 1995) and OH radicals (Sommar etal. 2001). Small amounts of Hg , which are dissolved in liquid water in the atmosphere (fog or clouds), can also be oxidized by ozone (Munthe 1992) or by OH radicals (Garfeldt et al. 2001). The oxidation in the aqueous phase occurs at a significantly higher rate than in the gas phase but, due to the low solubility of Hg in water and the low liquid water content in the atmosphere, the overall rate of oxidation is comparable to the gas phase oxidation rate (Pirrone etal. 2001). Reduction of divalent mercury back to Hg may also occur by sulfite (S03 ) ions or HO2 radicals furthermore, complexation of divalent mercury with soot may occur to form particulate divalent mercury (figure 17.2). 

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