Mass transfer air water

For diffusion, the approach of Jury et al. (1983, 1984a,b,c) is used as described by Mackay and Stiver (1991) and Mackay (1991) in which three diffusive processes are treated. The air boundary layer is characterized by a mass transfer coefficient ks or U7 of 5 m/h, equal to that of the air-water mass transfer coefficient kA used in D12. 

Further details on air-water mass transfer phenomena may be found in Thibodeaux (1996) and Stumm and Morgan (1981). 

General expressions for the air-water mass transfer can be derived by solving Equations (4.13) and (4.14), for xA and respectively, and substituting the results in both Equations (4.15) and (4.16). The following two expressions are obtained ... 

A major problem in the quantification of air-water transport phenomena in terms of the rate expression [Equation (4.18)] is to find appropriate values for Kl. As far as sewer systems are concerned, the most well-established knowledge concerning air-water mass transfer is on reaeration (Section 4.4). 

Jensen and Hvitved-Jacobsen (1991) developed a direct method for the determination of the air-water oxygen transfer coefficient in gravity sewers. This method is based on the use of krypton-85 for the air-water mass transfer and tritium for dispersion followed by a dual counting technique with a liquid scintillation counter (Tsivoglou et al 1965,1968 Tsivoglou andNeal, 1976). A constant ratio between the air-water mass transfer coefficients for dissolved oxygen and krypton-85 makes it possible to determine reaeration by a direct method. Sulfur hexafluoride, SF6, is another example of an inert substance that has been used as a tracer for reaeration measurements in sewers (Huisman et al., 1999). 

Chapter 9 Air-Water Mass Transfer in the Field. The theory of interfacial mass transfer is often difficult to apply in the field, but it provides a basis for some important aspects of empirical equations designed to predict interfacial transport. The application of both air-water mass transfer theory and empirical characterizations to field situations in the environment will be addressed. 

The principles of air-water mass transfer are often difficult to apply in field measurements and thus also in field predictions. The reasons are that the environment is generally large, and the boundary conditions are not well established. In addition, field measurements cannot be controlled as well as laboratory measurements, are much more expensive, and often are not repeatable. 

A summary of toxaphene atmospheric flux estimates to the Great Lakes is given in Table 8. The reported fluxes for a particular lake in Table 8 differ considerably in magnitude and sometimes even in direction. Seasonal changes in flux direction are due to variations in water temperature (influencing the Henry s law constant) and air concentration. Changes in flux magnitude are driven by air concentration and by wind speed, which affects the air-water mass transfer coefficients. 

Asher WE, Pankow JE (1991) The effect of surface films on concentration fluctuations close to a gas/hquid interface. In Wilhelms SE, Gulliver JS (eds) Air-water mass transfer. American Society of Civil Engineers, New York, p 68 Ewing GC, McAUister ED (1960) Science 131 1374 Robertson JE, Watson AJ (1992) Nature 358 738... 

General theory of air-water mass transfer. The reader is also referred to Chapters 2 and 4, which contains fundamental concepts that apply to all interfacial transfer. 

Air-water transfer rate of chemicals is dependent upon the rate coefficient and the equilibrium that the concentrations in each phase are moving towards. In environmental air-water mass transfer, the flow is generally turbulent in both phases. However, there is no turbulence across the interface in the diffusive sublayer, and the problem becomes one of rate of diffusion. Temporal mean turbulence quantities, such as eddy diffusion coefficient, provide a semiquantitative description of the flux across the air-water interface, however the unsteady character of turbulence near the diffusive sublayer is cmcial to understanding and characterizing interfacial transport processes. 

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