Water-air transfer

Basically, a concept for microbial transformations in sewer networks should cover soluble and particulate components and relevant processes in the water phase, in the biofilm and in the sewer sediments. In addition, mass transfer between these phases and an air-water transfer of oxygen should be taken into account (Figures 1.3 and 5.2). Although only the aerobic microbial activity will be focused on in the concept presented in this chapter, anoxic and anaerobic processes should be considered possible extensions (cf. Chapter 6). 

Another example is the air-water transfer of a compound, illustrated in Figure 1.5. This example will be used to explain volatile and nonvolatile compounds. There is resistance to transport on both sides of the interface, regardless of whether the compound is classified as volatile or nonvolatile. The resistance to transport in the liquid phase is given as Rl = 1/Kl. If we are describing chemical transfer through an equation like (1.3), the resistance to transfer in the gas phase is given as Ro = 1/(HKg). The equilibrium constant is in the Rg equation because we are using the equivalent water side concentrations to represent the concentration difference from... 

Figure 1.5. Air-water transfer analogy to two resistors in a series.

There are many transport conditions where experiments are needed to determine coefficients to be used in the solution. Examples are an air-water transfer coefficient, a sediment-water transfer coefficient, and an eddy diffusion coefficient. These coefficients are usually specific to the type of boundary conditions and are determined from empirical characterization relations. These relations, in turn, are based on experimental data. 

Since condition 2 cannot be valid for an air-water transfer, we will look into the possibility that condition 1 may be assumed. 

Carbon dioxide is one of the maj or global warming gases. The ocean acts as a reservoir for carbon dioxide and therefore will slow the effects of this gas on global warming. How is this air-water transfer rate dependent on the rate of reaction of carbon dioxide with liquid water to form H2CO3 ... 

The evaporation of water is generally used to determine the gas film coefficient. A loss of heat in the water body can also be related to the gas film coefficient because the process of evaporation requires a significant amount of heat, and heat transfer across the water surface is analogous to evaporation if other sources and sinks of heat are taken into account. Although the techniques of Section 8.D can be used to determine the gas film coefficient over water bodies, they are still iterative, location specific, and dependent on fetch or wind duration. For that reason, investigators have developed empirical relationships to characterize gas film coefficient from field measurements of evaporation or temperature. Then, the air-water transfer of a nonvolatile compound is given as... 

Illustrative Example 20.3 Estimating the Overall Air-Water Transfer Velocity from Wind Speed for Different Water Temperatures... 

Table 20.2 Empirical Relationships Between Wind Velocity 10 and Water-Phase Air-Water Transfer Velocity viw a...

For compounds with Kii/Vl larger than about 10 2 the overall air-water transfer velocity is approximately equal to the water-phase exchange velocity viw The latter is related to wind speed uw by a nonlinear relation (Table 20.2, Eq. 20-16). The annual mean of viw calculated from Eq. 20-16 with the annual mean wind speed ul0 would underestimate the real mean air-water exchange velocity. Thus, we need information not only on the average wind speed, but also on the wind-speed probability distribution. 

Deacon derived his model for volatile compounds whose air-water transfer velocities solely depend on the conditions in the water phase. In its original form, which is valid for a smooth and rigid water surface and for Schmidt Numbers larger than 100, it has the form ... 

Table 20.5a Calculation of Overall Air-Water Transfer Velocities for Different Organic Compounds at 25°C Substance-Specific Properties...

Figure 20.7 Overall air-water transfer velocity vla/w as a function of Henry s Law coefficient for two very different wind conditions, 10 = 1 m s l (calm overland condition) and Kl0 = 20 m s 1 (rough ocean conditions). The solid lines are calculated for average compound properties Diz = 0.1 cm2 s 1 and Sc,w = 600. The dashed line indicates the boundary between air-phase- and water-phase-controlled transfer velocities. See Table 20.5 for definitions of parameters and substances.

Calculate the total air-water transfer velocity, v/aAv, of 1,1,1-trichloroethane (methyl chloroform, MCF) and tribromomethane (bromoform, BF) at the surface of the ocean for a wind speed of 15 m s 1 measured 3 m above the water surface at seawater temperatures of 25°C and 0°C, respectively. 

From their experiments, Moog and Jirka (1999a) deduced the following air-water transfer velocity for the small-eddy regime ... 

Note that the inverse of -Kha /ha is identical with aa which was introduced in Eq. 8-21. Here we choose the Annotation to indicate that the ratio is like a partition coefficient which appears in the flux (Eq. 20-1) if different phases or different chemical species are involved (see section 19.2 and Eq. 19-20). In order to show how the combination of both partitioning relationships, one between air and water (Eq. 20-42), the other between neutral and total concentration (Eq. 20-43), affect the air-water exchange of [HA], we choose the simplest air-water transfer model, the film or bottleneck model. Figure 20.11 helps to understand the following derivation. 

This conclusion does not hold in the following example in which tr and tw are of similar size (case 3 in the above list). In Chapter 12 we discussed the hydration/ dehydration of formaldehyde as a pseudo-first-order two-way reaction (Eqs. 12-15 to 12-24). The reaction time of hydration is of the order of 10 s and thus similar to the air-water transfer time. 

Mean wind speed Air-water transfer velocity for 10 5 m s 1... 

NH3 and to a lesser extent mono-, di-, and trimethylamines are the only significant gaseous bases in the atmosphere, and there has been considerable interest in whether the oceans are a source or sink of these gases. Early attempt to assess the air-sea flux from concentration measurements are probably suspect because of the ease with which sample contamination can occur during laboratory processing and analysis. It should be noted here that due to its high solubihty (low value of Henry s law constant), the air-water transfer of NH3 (and the methylamines for the same reason) is under gas phase control (see Section 6.03.2.1.1). The first reliable measurements were probably from the North and South Pacific and indicated that the flux of NH3 from sea to air is of a size similar to that for emission of DMS (Quinn et al., 1990, 1988). Indeed, the authors showed that this similarity was mirrored in the molar ratio of (non-sea-salt) sulfate to ammonium (1.3 0.7) in atmospheric aerosol particles collected on the cruise, indicating that for clean marine air remote from terrestrial sources, the emission of DMS and NH3 from the sea appears to control the composition of the aerosol. 

Brumley BH, Jirka GH (1988) Air-water transfer of slightly soluble gases turbulence, interfacial processes and conceptual models. PhysicoChem Hydrodyn 10 295-319... 

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