Migration through medium, rate

In a reactive transport model, the domain of interest is divided into nodal blocks, as shown in Figure 2.11. Fluid enters the domain across one boundary, reacts with the medium, and discharges at another boundary. In many cases, reaction occurs along fronts that migrate through the medium until they either traverse it or assume a steady-state position (Lichtner, 1988). As noted by Lichtner (1988), models of this nature predict that reactions occur in the same sequence in space and time as they do in simple reaction path models. The reactive transport models, however, predict how the positions of reaction fronts migrate through time, provided that reliable input is available about flow rates, the permeability and dispersivity of the medium, and reaction rate constants. 

The filter coefficient, X, varies as deposited material changes the morphology of the porous medium and as conditions surrounding the collection sites change. It has been noted that the filtration coefficient increases as fines migrate through a clean filter bed the retained fines increase the specific surface area. This increase in X is short-lived, and the magnitude of the filter coefficient decreases as additional fines are retained. Since Iwasaki published his notes on filtration in 1937, numerous variations of the rate expression have been recorded (72). 

Chromatography is a compound separation method based on the differences in rates at which the individual components of a mixture migrate through a stationary medium under the influence of a moving phase. The use of gas-liquid chromatography with an electron capture detector has been demonstrated in the analysis of organolead compounds [62]. 

If the rate of adsorption, the rate of desorption, and the equilibrium partitioning of a nuclide between a solid medium and solution are known, then the rate of migration of a nuclide through the medium can be predicted with the ARDISC model. 

Safety analysis of nuclear waste repositories requires realistic prediction of the rates of migration of nuclides from the repository through the host geological medium to the accessible environment. These predictions require sorption isotherms rather than single values of distribution coefficients, and there must be substantial confidence that the sorption isotherms... 

The mobility, or rate of migration, of a molecule increases with increased applied voltage and increased net charge on the molecule. Conversely, the mobility of a molecule decreases with increased molecular friction, or resistance to flow through the viscous medium, caused by molecular size and shape. Total actual movement of the molecules increases with increased time, since mobility is defined as the rate of migration. 

Diffusion is the most fundamental mechanism of gas migration in that it requires only a partial pressure (concentration) gradient. The rate of diffusion of a gas is then determined by the medium in which diffusion takes place, its temperature and absolute pressure, and the diffusion coefficient of the gas. The diffusion coeffiecient is a function of molecular weight, the shape of molecules, and their intermolecular attraction. Every gas thus has a different diffusion coefficient. The influence of the medium in which gas diffusion occurs is related to the density of the medium gases diffuse less quickly through a solid than through another gas. The rate at which a gas diffuses in a specified medium is sometimes termed its diffusivity. 

A method of separating charged molecules through their different rates of migration under an electric field. Usually a medium that minimizes diffusion is used, e.g., gel, paper, or capillary electrophoresis. 

In synthetic polymers, the interpretation is necessarily more difficult The form of Equation 4 and Equation 5 requires that the kinetics of formation and decay of complexes are modelled adequately by rate-constants and that they take place in a homogeneous medium. If, as in synthetic polymers, the population of excimer trap sites, may occur through energy migration or rotational diffusion, a rate-constant may not be an adequate representation of the process, some time-dependent parameter being required (see below.) Heterogeneity may also play an important role. Thus in earlier work the fluorescence decay of excimer-forming polymers was modelled adequately by a scheme based upon simple excimer kinetics to which had been added terms to account for the occurrence in co-polymers of monomer sites which, by their isolation, could not form excimers (4-10). For polymers which contain isotactic and syndiotactic sequences, or rather, are made up of meso and racemic triads (14), the kinetics may be similarly a superimposition of simple schemes appropriate for the different sequences. 

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