Exchange flux

Depolarization of the cell membrane, for example, favors such events. It should also be noted that any mechanism which affects the action of the Na K pump and the Na transmembrane gradient will also affect the Na Ca exchanger flux. 

Climate Change and Air-Sea Exchange of CO2. The air-sea exchange flux of CO2 is governed by the gas exchange rate and by the difference between... 

The cycles of carbon and the other main plant nutrients are coupled in a fundamental way by the involvement of these elements in photosynthetic assimilation and plant growth. Redfield (1934) and several others have shown that there are approximately constant proportions of C, N, S, and P in marine plankton and land plants ("Redfield ratios") see Chapter 10. This implies that the exchange flux of one of these elements between the biota reservoir and the atmosphere - or ocean - must be strongly influenced by the flux of the others. 

Kindermann, J., Wiirth, G., Kohlmaier, G. H. and Badeck, F.-W. (1996). Interannual variations of carbon exchange fluxes in terrestrial ecosystems. Global Biogeochem. Cycles 10, 737-755. 

The conjugate driving force is the pressure gradient Ap multiplied by — 1. Further, the relative flux of the dissolved substance compared to the flux of the solvent, i.e. the exchange flux /D, is defined by the relationship... 

Zonally integrated giobai sea-air exchange fluxes for CO2 (positive fluxes denote outgassing from the ocean), (a) Effects of the gas exchange pump and the biological pumps (preindustrial). 

Diffusion coefficients of vanadium ions in CMS (Neosepta made by Tokuyama Soda), CMV (Selemion made by Asahi Glass), and CMX (Neosepta made by Tokuyama Soda) cation exchange membranes were determined by measuring the ion-exchange fluxes of the vanadium ions with H3O+ ions using a dialysis cell. The lowest diffusion coefficients were observed in the CMS membrane for all vanadium ions. CMS membranes were found to be most suitable for V—RFB, as it was expected to prevent cross contamination of vanadium ions. ° ... 

Figure 19.5 In multibox models, the exchange between two fairly homogeneous regions is expressed as the exchange flux of fluid (water, air etc.), Q . Normalization by the contact area, A, yields the exchange velocity vex = QJA. This quotient can be interpreted as a bottleneck exchange velocity vex =...

The vertical volumetric exchange flux of water between epilimnion and hypolimnion is ... 

As long as there is no phase change involved, the influence of the transition zone on mass transfer is negligible. The position of the boundary layer is slightly shifted, but the exchange flux is scarcely affected. This is no longer true if the boundary separates two different media, for instance, the water of a lake from the sediments. In this case the drop of diffusivity D(x) and the increase of the partition ratio KAIB (Eq. 19-27) do not coincide (Fig. 19.10). Let us first develop the necessary mathematical tools to describe this new situation and then discuss an example for which the influence of the boundary layer may be relevant. 

Note In Chapter 23 we will further elaborate on the sediment-water exchange flux, especially in Box 23.2. and Table 23.6. 

As shown in Illustrative Example 20.2, another prominent example of a gas-phase-controlled exchange flux is the evaporation of pure organic liquids. 

Whatever the detailed physicochemical model of the interface, most models of the air-water exchange flux are written as the product of two factors, one describing the physics, the other the chemistry. What are these factors ... 

Explain how a change in water temperature could reverse the direction of the net air-water exchange flux even if all other parameters remain unchanged. 

The above equation demonstrates that air-water exchange as well as other boundary exchange processes can be interpreted as a combination of an input flux (7ia/w = oVia/wQa/ 7fia/w) and an output flux (0,a/w = A0v,a/wC, surface). It is a matter of personal taste whether one prefers to keep both terms separated or add them to get the familiar form of a net exchange flux. 

The mass balance equations for the epilimnion and hypolimnion look like Eq. 21-38, except for the air-water exchange fluxes which are replaced by the vertical fluxes across the thermocline, 7) EH and 7) HE. According to the general form of mass transfer models (Eq.18-4), we can express these fluxes as ... 

Hence, the one-box and two-box models yield the same result. There is a simple reason for that. Since the only removal processes of PCE act at the lake surface, at steady-state the surface concentration in both models (C°°for the one-box model, ClE for the two-box model) must attain the same value to compensate for the input /, tot. Furthermore, since the hypolimnion has neither source nor sink, the net exchange flux across the thermocline must be zero, and this requires C(E= C,H. 

The external processes (boundary fluxes) can be combined into four pairs of generalized exchange fluxes that is (a) input/output by streams, rivers, or ground-water, (b) air-water exchange, (c) sediment-water exchange, (d) exchange with adjacent water compartments. If the box represents a pond or lake as a whole, flux (d) does not exist. The fluxes into the system are controlled by external parameters such as the concentration in the inlets, the atmospheric and the sedimentaiy concentrations. These concentrations can be constant or variable with time. 

Figure 23.1 General view of a linear one-box model of a well-mixed pond, a lake, or part of a lake or ocean. See Box 23.1 for definitions. If the box represents the complete water body of a lake, the terms with vex (water exchange with adjacent boxes) do not exist. Similarly, if the box is not in contact with the atmosphere, the air-water exchange flux (va,w) is absent.

Figure 23.3 The processes which contribute to the exchange flux between open water (op) and sediment column (sc) (a) settling of suspended particles (b) exchange flux of dissolved phase across a stagnant bottom boundary layer, (c) particle resuspension followed by equilibration between particle and open water.

Molecular diffusion of the dissolved phase of the chemical between the open water and the pore water accompanied by sorption/desorption with the local particles can be described by an exchange flux (see Fig. 23.3b and Box 23.2) ... 

Note that if the sediment surface were to consist of freshly sedimented particles with concentration Cssc = C°p, then the pore water in equilibrium with these particles would have the aqueous concentration C c = C p, and thus according to Eq. 23-24 the diffusive exchange flux Fsed difr would be zero. However, in most cases the sediment surface is not in equilibrium with the water column, because diagenetic processes change the physicochemical properties of the sediments and thus its solid-water distribution ratio, Kf, relative to. Furthermore, the sediment surface usually reflects a longer history of exposure to the chemical under consideration than the water column. Therefore, water and sediments would approach equilibrium only if the external loading to the lake has changed very slowly in the past. For manmade chemicals this is usually not the case. 

The third exchange process is related to the possible resuspension and resettling of sedimentary particles (Fig. 23.3c). As shown in Box 23.2, processes 2 and 3 can be combined into a single exchange flux with one single specific exchange rate, vsedex, which combines both mechanisms (Box 23.2, Eqs. 5 to 7) ... 

As it turns out, the appropriate coordinate to describe the concentration change along the river due to a boundary exchange flux is the integrated surface area 5(x), measured from an arbitrary cross section, where s(0) = 0, to the coordinate x (Fig. 24.3). The infinitesimal increment ds is related to dx by ... 

To understand this point, we must recall that the sediment surface acts as a wall boundary with respect to diffusion (Chapter 19). According to Eq. 19-33, the exchange flux, Fsed, and thus the corresponding apparent exchange velocity, vsedex, decreases with elapsed time t. From Eq. 19-33, with C p replaced by/wCt, we get ... 

When a dynamic equilibrium prevails at the a/p phase boundary, the exchange fluxes Pi b and Jfb occur across the interface and cancel each other individually. 

We can see that two SE s on each side of the interface are involved in the transfer. Matter transport across the interfaces and, in particular, the dynamic equilibrium exchange fluxes j therefore concern the building elements or components k. At equilibrium,... 

One notes that RA is inversely proportional to the exchange flux (/a) °f the dynamic equilibrium interface. 

Transport of Ag+ across the AgI/Ag2S boundary has been studied experimentally as a function of A g (which was determined with the help of microsensors of the type Ag/AgBr) [H. Schmalzried, et al. (1992)]. From flux vs. driving force curves, the exchange flux j° has been evaluated and found to be ca. 1 A/cm2 at 260 °C. Introducing this high value of j° into Eqn. (10.41) and noting that the boundary resistance is... 

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