Preferential Transport

Therefore, Eq. 10.5 is limited in its applicability, as are variations of this equation such as the mobile-immobile one (see Sect. 10.2). We discuss non-Fickian transport in detail in Sect. 10.3. 

As noted in Sect. 10.1, heterogeneities play a dominant role in the migration of contaminants in the subsurface. Nonuniform, preferential patterns of flow and transport are ubiquitous. It is important to recognize that, at the field scale, contaminant movement generally is very difficult to anticipate. In natural soils and aquifer materials, macropores, soil cracks and aggregates, fissures, solution channels, root paths, and wormholes, as well as variable mineral composition (e.g., clay aggregates 

To quantify such transport, the advection-dispersion equation, which requires a narrow pore-size distribution, often is used in a modified framework. Van Genuchten and Wierenga (1976) discuss a conceptualization of preferential solute transport throngh mobile and immobile regions. In this framework, contaminants advance mostly through macropores containing mobile water and diffuse into and out of relatively immobile water resident in micropores. The mobile-immobile model involves two coupled equations (in one-dimensional form)  

We discnss network models fnrther in Sect. 11.4, in the context of immiscible displacements. 

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The interaction of all these factors makes it difficult to predict an overall effect of conservation tillage on the potential leaching of a pesticide compared to that in a conventionally tilled field. However, it was found that a prolonged rain immediately after appHcation resulted in short-term levels of pesticide in groundwater to be greater under no-tiU than under conventional till plots, which suggested that preferential transport in no-tiU had occurred... 

One of the first decisions to be made when designing an experiment is the method of detection to be used with a particular solute. If radiolabeled material is available, a simple method of analysis is to count the radiolabel appearing in the receiver compartment as a function of time. While convenient, this can be a dangerous practice. Depending upon the type of radioisotope, its position in the molecule, and its specific activity, radiolabeled compounds can be subject to a variety of chemical and solution-catalyzed degradation pathways. If the stock solution contains a significant amount of radioactive impurities or generates them as a result of solution instability, then the possibility for preferential transport of... 

The presence at the BBB of members of the multidrug resistance-associated protein (MRPs) family, whose members preferentially transport anionic compounds, is still controversial. The seven members of the MRP family belong, like P-gp, to the ATP-binding cassette (ABC) protein superfamily. Mrpl has been found at the BBB in isolated rat brain capillaries, primary cultures of brain capillary endothelial cells and in immortalized capillary endothelial cells, but not in human brain capillaries [59]. Another member, MRP2 has been found at the luminal membrane of the brain endothelial cells [60]. However, further studies are required to show that there are MRP transporters at the BBB (Figure 15.5). As for P-gp, a functional Mrpl was found in primary cultured rat astrocytes [56] and it has been shown to take part in the release of glutathione disulfide from brain astrocytes under oxidative stress [61]. 

We also applied this method for the preparation of our membranes, in order to allow preferential transport nano-paths for molecules. In the first step, the AAMs... 

Results of analogous permeation studies for the hydrophobic toluene molecule are shown in Fig. 20. Now the opposite selectivity pattern is observed i.e., toluene is preferentially transported in the R = -CieHjj membranes. This can be illustrated by defining the alternative selectivity coefficient aci6/0H (Table 3). As was the case for aoH/cia. the acie/on values increase with decreasing tubule diameter. In addition to toluene, acie/on values were determined for p-xylene and naphthalene in the i.d. = 1.9 nm membranes. The following acu/on values were obtained 2.8 for toluene, 6.2 for j9-xylene, and 16 for naphthalene. 

Figure 3. Enrichment of trivalent actinides over Pu(IV) across biological membranes. The assimilation of Am-241 and Cm-244 from the rat GI tract is greater than for plutonium. When plutonium nitrate is inhaled by dogs, daughter Am-241 is preferentially transported to the liver, resulting in depletion in lung and lymph nodes.

A number of drug substances are known to act directly upon the uterus, including uterine relaxants (e.g., (3-agonists) and stimulants (e.g., prostanoids, oxytocin). The administration of drugs to the uterus is achieved by the application of a formulated product to the vagina or the cervix. However, it has been demonstrated that the mechanism by which the drug is transported from the cervicovagina to the uterus is not limited to passive diffusion, but is facilitated by a preferential transport mechanism termed as the first uterine pass effect. 

FIGURE 3.6 A neutral (TRITC-Arg A and D), a singly charged anion (TRITC-Gly B and E), or a doubly charged anion (TRITC-Asp C and F) is continuously electrophoresed. Preferential transport of anions into the field-free channel (ff) at a tee intersection. The electro-osmotic velocity in the side or ground channel (g) has been reduced relative to that in the separation channel (sep) by selectively coating the ground channel with a viscous polymer [387]. Reprinted with permission from the American Chemical Society. 

V, di SimpUcio P, Mandl J, Benedetti A. Preferential transport of glutathione versus glutathione disulfide in rat liver nucrosomal vesicles. J. Biol. Chem. 1999 274 12213-12216. 39. 

The anionic ionophores are highly selective for particular metal cations, and both kinetic and thermodynamic terms determine which cation is selected. Note that the thermodynamic stability of the metal-ionophore complex does not always determine the transport rate. For example, nigericin forms a much more stable complex with potassium but transports sodium much more quickly. In the presence of both sodium and potassium, however, the extra stability of the nigericin-potassium complex results in the preferential transport of potassium over sodium (4). 

In this case the permeation is proportional to the average pore radius r and inversely proportional to but is independent of the mean pressure P that is an important difference from viscous flow. The separation selectivity between two gases will be proportional to M2/M-[f. In the case of vapor transport in mesopores, another mechanism that may occur is the capillary condensation leading to selective filling of pores by a molecule and preferential transport of this molecule. 

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