Effective retardation factor

RF,eS effective retardation factor (including the effects of transport on colloids)... 

In order to avoid this apparent impass, an alternative approach is propose.i. This is based around the defining of a formal effective retardation factor tliii couples both the total adsorption capacity of the soil to the structure. 

In real situations it is the initial part of the breakthrough curve that is of importance. It is possible to define an effective retardation factor (Re) by ... 

As can be seen, the effective retardation factor for the saturated case falls from unity in the case of an infinitely thin lens to 0.3% for thicker lenses. Similar behaviour is observed in the unsaturated zone. In the numerical case considered this result implies breakthrough after 30 rather than 6,300 years This concept of defining an effective retardation factor as a function of soil structure has proven to be very fruitful. 

Thirty-six undisturbed soil columns were taken on a 6 x 6 sampling grid immediately adjacent to the field core locations, and were brought to the laboratory. The columns were leached at 2 cm/d until steady state was reached, at which time a pulse of KC1 and napropamide was added to the inlet end. Affluent breakthrough curves for each chemical were fitted to the convection-dispersion equation by the method of moments (11). The effective retardation factor R, which may be calculated from the ratio of the chloride and napropamide vnap velocity parameters obtained by fitting the convection-dispersion equation, is equal to... 

Fig. 7-5. Effective retardation factors, i p,eff> calculated by Contardi et al. (2001) for groundwaters from the Yucca Mountain area, as a function of (L kg ) and colloid concentration, C.F is defined by Eq. [5].

On 100% acrylic materials the quaternary ammonium retarders are used almost exclusively. Other types have been evaluated, however. For example, saturated alkylamines (RNH2 R = C10, C12, C14 and C16 hydrophobes) were found to be just as effective as the quaternary types although other factors, such as aqueous solubility at the optimal dyebath pH and resistance to subsequent discoloration, favour the quaternary compounds [34]- On the other hand, bis (hydroxyethyl) coco amine (12.10) had relatively little effect and the amphoteric carboxymethyldimethylcocoamine (12.11) none at all, although dimethyl-cocoamine oxide (12.12) was quite an effective retarder [34]- Other cationic compounds used [43,44] have included alkylpyridinium salts, imidazoles and imidazolinium salts, alkyldiamines, alkylpolyamines, as well as sulphonium and phosphonium derivatives. 

Diffusion and mass transfer effects cause the dimensions of the separated spots to increase in all directions as elution proceeds, in much the same way as concentration profiles become Gaussian in column separations (p. 86). Multiple path, molecular diffusion and mass transfer effects all contribute to spreading along the direction of flow but only the first two cause lateral spreading. Consequently, the initially circular spots become progressively elliptical in the direction of flow. Efficiency and resolution are thus impaired. Elution must be halted before the solvent front reaches the opposite edge of the plate as the distance it has moved must be measured in order to calculate the retardation factors (Rf values) of separated components (p. 86). 

Table 4.6 gives a few representative values for Kow and Kp for non-polar organic substances on typical soil material and Table 4.7 gives estimates on typical retardation factors estimated for an aquifer. The data show that many non-polar organic substances, with the possible exception of very lipophilic substances such as hexachlorobenzene, are not markedly retarded in aquifers that contain little organic material (foe = 0.001 - 0.005). On the other hand, such substances are effectively retained in soils rich in organic carbon. 

Girvin et al. [358] evaluated the release of PCBs from electrical substation soils contaminated with transformer fluids. They observed that there are two phases to the uptake and release of PCBs with these soils. The initial phase is a rapid, labile phase that is followed by a slower, nonlabile phase. The labile phase occurs at a scale of hours to days while the nonlabile phase releases over weeks and months. Girvin et al. [422] also reviewed the effects of adsorption on the mobility of PCBs and their transport. In an example presented for a hexa-chlorobiphenyl, these authors noted that the PCB isomer would have a retardation factor Rf of 1400 for the particular case given. This means that the ground-... 

The replacement of Portland cement by fly ash class F (ASTM C 618) has been found to reduce the rate of slump loss in a prolonged mixed concrete, and the extent of the reduction is greater with increased cement replacement (Fig. 7.37). Fly ash also was found to be beneficial in reducing slump loss in concretes with conventional water-reducing and retarding admixtures [95], The effect of fly ash on reducing slump loss can be attributed to chemical and physical factors. It was found that the surface of fly ash particles may be partly covered with a vapor-deposited alkali sulfate that is readily soluble [103, 104], Thus the early hydration process of Portland cement is effected because sulfate ions have a retarding effect on the formation of the aluminates. Indeed, fly ash was found to be a more effective retarder than an... 

Hinshelwood (77) calculated methyl retardation factors which characterized the oxidation rates of paraffins. The extent of retarding effect on reaction rate was dependent on the distance of each methyl group in the molecule from the site of attack. 

Livingston (113) made a comprehensive correlation of knock with structure. A number of functional groups were believed to exhibit retarding effects. By a method similar to that of Hinshelwood, Livingston calculated structural retardation factors which were found to vary progressively as the standard compression ratios for knock changed. 

In this discussion equation (8) is used to model the transport of TCE during the test. The parameters needed for the model include the effective dispersion/diffusion coefficients, retardation factor, and the pore fluid velocity. 

These reactions are of the Sn2 type and are sensitive to steric effects of substituents in the azole ring. However, these steric effects are significantly less than, for example, in the analogous pyridine derivatives because the angle subtended by the nitrogen lone pair and an -substituent is about 70 in an azole as opposed to 60 in pyridine. Thus the rate constant for methylation of 2-/-butylthiazole by methyl iodide is only 40 times less than that for the corresponding 2-methyl compound. By comparison, in the pyridine series the retardation factor is over 2000. 

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