Air-water exchange

Sediment from a 10-cm depth in the bottom sediment of a lake has a 210Pb activity of 2.5 disintegrations per minute (DPM). Sediment collected at the sediment-water interface has an activity of 4 DPM per gram. Assuming constant 210Pb and sediment deposition rates, no sediment compression as the sediment ages, and no mixing or losses in the sediments, how rapidly does sediment accumulate in this lake  

Equation [2-26], the basic equation for radioactive decay, can be used  

Thus far, the discussion of chemical removal from the water column has focused on incorporation of chemicals sorbed to particles into bottom sediment. However, chemicals dissolved in surface waters may also leave the water column and enter the atmosphere as gases or vapors. Conversely, chemicals present in the atmosphere may dissolve into a lake, river, or estuary. For volatile chemicals, which include most common industrial solvents and liquid fuels, the process of water-to-air exchange can be the most important mechanism of chemical removal from a surface water. 

The concentration of a dissolved gas or vapor in a surface water at equilibrium with the atmosphere (Cequil) is determined by Q, the concentration in air, and the Henry s law constant (H) of the chemical  

If the concentration in water (Q) is higher than Cequil, the chemical will volatilize from the water body into the atmosphere. The flux density is proportional to the product of the difference between the actual (Q) and the equilibrium (Cequil) concentrations in the water  

Illustrated Example 20.1 Evaluating the Direction of Air-Water Exchange 

Transfer Velocities in Air Deduced from Evaporation of Water Illustrative Example 20.2 Estimating Evaporation Rates of Pure Organic Liquids 

Transfer Velocities in the Water Phase Deduced from Compounds with Large Henry s Law Constants 

Box 20.1 Influence of Wind Speed Variability on the Mean Air-Water Exchange Velocity of Volatile Compounds 

Surface Renewal Model Boundary Layer Model 

The units for and are expressed as moles per liter and Henry s law constant is dimensionless. 

The preceding equation is also expressed in terms of partial pressure of gas (atmospheres at a given temperature). 

The Henry s constants expressed in Equations 14.22 and 14.23 can be related to each other as follows  

Both Henry s constants will provide similar results, as long as the reader pays careful attention to how they are defined and how the appropriate equations are used. The reader should refer to standard chemistry textbooks for additional details. Excellent discussion on the use of Henry s constants in a range of environmental conditions is provided by Schwarzenbach et al. (1993). 

If the concentration of gas in water (Q ) is higher than CJK, then the movement of gas from water to atmosphere occurs. The flux of gas can be described using a first-order mass transfer relationship as follows  

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Where D.. can be calculated from mass transfer coefficients or an uptake Aalf-time. For example, for air-water exchange is given by... 

This calculation includes an estimation of intermedia transport. Examination of the magnitude of the intermedia D values given in the fate diagrams suggest that water-sediment and air-soil transport are most important, with soil-water, and air-water exchange being slower. This chemical tends to be fairly immobile in terms of intermedia transport. 

Mackay, D., Shiu, W.Y. (1975) The aqueous solubility and air-water exchange characteristics of hydrocarbons under environmental conditions. In Chemistry and Physics of Aqueous Gas Solutions. Adams, W.A., Greer, G., Desnoyers, J.E., Atkinson, G., Kell, K.B., Oldham, K.B., Walkey, J., Eds., pp. 93-110, Electrochem. Soc., Princeton, NJ. 

The potential production of sulfide depends on the biofilm thickness. If the flow velocity in a pressure main is over 0.8-1 ms-1, the corresponding biofilm is rather thin, typically 100-300 pm. However, high velocities also reduce the thickness of the diffusional boundary layer and the resistance against transport of substrates and products across the biofilm/water interphase. Totally, a high flow velocity will normally reduce the potential for sulfide formation. Furthermore, the flow conditions affect the air-water exchange processes, e.g., the emission of hydrogen sulfide (cf. Chapter 4). 

Pilot sewer studies are often carried out in systems operating with recirculation. Specific care must be taken in systems where water-gas exchange processes form a part of the mass balance. Critical points are pumps and bends that may change the flow regime, air-water exchange processes, biofilm and particle structure. Figure 7.2 is a sketch of a pilot sewer used for sewer process studies (Tanaka and Hvitved-Jacobsen, 2000). 

Mean NP concentrations in the water phase were 0.048 xg L-1, and the air—water exchange was quantified by calculating the NP fugacities in both phases. The ratio of the fugacities (/water//gas = Cwater xH/ Cgas X... 

To assess the relative importance of the volatilisation removal process of APs from estuarine water, Van Ry et al. constructed a box model to estimate the input and removal fluxes for the Hudson estuary. Inputs of NPs to the bay are advection by the Hudson river and air-water exchange (atmospheric deposition, absorption). Removal processes are advection out, volatilisation, sedimentation and biodegradation. Most of these processes could be estimated only the biodegradation rate was obtained indirectly by closing the mass balance. The calculations reveal that volatilisation is the most important removal process from the estuary, accounting for 37% of the removal. Degradation and advection out of the estuary account for 24 and 29% of the total removal. However, the actual importance of degradation is quite uncertain, as no real environmental data were used to quantify this process. The residence time of NP in the Hudson estuary, as calculated from the box model, is 9 days, while the residence time of the water in the estuary is 35 days [16]. 

Mackay, D. Shiu, W-Y. "The Aqueous Solubility and Air-Water Exchange Characteristics of Hydrocarbons under Environmental Conditions" in "Chemistry and Physics of Aqueous Gas Solutions" Adams, W. A., Ed., Electrochemical Soc., Inc., Princeton, N.J., 1975. 

Illustrative Example 6.2 Evaluating the Direction of Air-Water Gas Exchange at Different Temperatures Problem What is the direction (into water or out of water ) of the air-water exchange of benzene for a well-mixed shallow pond located in the center of a big city in each of the following seasons (a) a typical summer situation (T = 25°C), and (b) a typical winter situation (T= 5°C) In both cases, the concentrations detected in air and water are C,a = 0.05 mg-m"3 and C,w = 0.4 mg m 3. Assume that the temperature of the water and of the air is the same. 

Figure 9.1 Illustration of some processes in which sorbed species behave differently from dissolved molecules of the same substance. (a) Dissolved species may participate directly in air-water exchange while sorbed species may settle with solids. (b) Dissolved species may react at different rates as compared with their sorbed counterparts due to differential access of other dissolved and solid-phase reactants. ...

More generally, this is the fraction of BP that is available for processes that only act on the dissolved species (e.g., air-water exchange, see Chapter 20). Hence, for a strongly sorbing compound such as BP, sorption to DOM may significantly influence this PAH s environmental behavior. In such cases, one obviously needs to get a more accurate KjDOC value for the system considered. 

For example, for formaldehyde (R = H) at neutral pH, the pseudo-first-order rate constant for the hydration reaction (forward reaction), k =k [H20], is about 10 s 1 and the first-order rate constant for dehydration, k2, is about 5x 10-3 s-1. In Chapter 20 we will use this example to show that the reactivity of compounds can influence the kinetics of air/water exchange if both processes (reaction and exchange) occur on similar time scales. 

The approach pursued in this and the next chapter is focused on the common mathematical characteristics of boundary processes. Most of the necessary mathematics has been developed in Chapter 18. Yet, from a physical point of view, many different driving forces are responsible for the transfer of mass. For instance, air-water exchange (Chapter 20), described as either bottleneck or diffusive boundary, is controlled by the turbulent energy flux produced by wind and water currents. The nature of these and other phenomena will be discussed once the mathematical structure of the models has been developed. 

At this point, we have to decide which system (A or B) is selected as the reference phase. Our choice determines the actual form of the overall transfer law and explains the asymmetry between the two phases which we meet, for instance, in the equations expressing air-water exchange (see Chapter 20, Eq. 20-3). Here we choose A as the reference system. Then ... 

The above results will be useful for the two-film model of air-water exchange (Chapter 20). A very different bottleneck boundary, that is, the unsaturated zone of a soil, is discussed in Illustrative Example 19.2. 

Note In Chapter 20 we will discuss the air-water exchange and find that indepen-... 

In Chapter 20, the diffusive boundary scheme serves as one of several models to describe air-water exchange. 

Diffusive boundaries also exist between different phases. The best known example is the so-called surface renewal (or surface replacement) model of air-water exchange, an alternative to the stagnant two-film model. It will be discussed in Chapter 20.3. 

Box 20.2 Temperature Dependence of Air-Water Exchange Velocity v(w of Volatile Compounds Calculated with Different Models Overall Air-Water Exchange Velocities... 

Illustrative Example 20.4 Air-Water Exchange of Benzene in Rivers... 

Influence of Surface Films and Chemical Reactions on Air-Water Exchange Advanced Topic)... 

Reaction and Diffusion of Similar Magnitude Illustrative Example 20.5 Air-Water Exchange Enhancement for Formaldehyde and Acetaldehyde... 

All the necessary tools to develop kinetic models for air-water exchange have been derived already in Chapters 18 and 19. However, we don t yet understand in detail the physical processes which act at the water surface and which are relevant for the exchange of chemicals between air and water. In fact, we are not even able to clearly classify the air-water interface either as a bottleneck boundary, a diffusive boundary, or even something else. Therefore, for a quantitative description of mass fluxes at this interface, we have to make use of a mixture of theoretical concepts and empirical knowledge. Fortunately, the former provide us with information which is independent of the exact nature of the exchange process. As it turned out, the different flux equations which we have derived so far (see Eqs. 19-3, 19-12, 19-57) are all of the form ... 

Independent of the model that is used to describe air-water exchange at the sea surface, the flux Ft is proportional to (Eqs. 20-1,20-2, where subscript w is replaced by sw for seawater) ... 

All the physics is hidden in the coefficient via/w which, because it has the dimension of a velocity (LT-1), is called the (overall) air-water exchange velocity. Air-water exchange occurs due to random motion of molecules. Equation 20-1 is a particular version of Eq. 18-4 in which the air-water exchange velocity adopts the role of the mass transfer velocity, vA/B. 

At first sight, there seems to be a basic difference between the two regimes with respect to the influence of Kia/Vl. In the water-phase-controlled regime, the overall exchange velocity, via/w, is independent of Kia/v/, whereas in the air-phase controlled regime v(a/w is linearly related to Ga/w. Yet, this asymmetry is just a consequence of our decision to relate all concentrations to the water phase. In fact, for substances with small Kia/v/ values, the aqueous phase is not the ideal reference system to describe air-water exchange. This can be best demonstrated for the case of exchange of water itself (Kia/V1 = 2.3 x 10 5 at 25°C), that is, for the evaporation of water. 

Figure 20.1 Schematic view of the overall air-water exchange velo-city, via/w, as a function of the air-water partition coefficient, Ku/w, calculated from Eq. 20-3 with typical single-phase transfer velocities v,a = 1 cm s"1, vM = 10 3 cm s1. The broken line shows the exchange velocity v a/w (air chosen as the reference system). The upper scale gives the Henry s Law coefficient at 25°C, Km = 24.7 (Lbar mol"1) x Ku/W.

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