Retardation Coefficient

As an example, we can compare the solubility of lindane and 2,4-D pesticides with respect to their major loss pathways. The Ku values of these pesticides are 1.33 x 1CT4 and 5.5 x 10 9 mol L 1, respectively. In addition, the retardation coefficient or degree of sorption of lindane is much higher than that of 2,4-D. Therefore, 2,4-D is more likely to be leached, whereas lindane is more likely to remain near the soil surface from whichit can vaporize. Volatilization is thus the major pathway of lindane, and degradation and leaching are the major loss pathways of 2,4-D in the Asian environments. 

The retardation of the SDS-protein complexes in capillary gel electrophoresis is a function of the separation polymer concentration (P) and the retardation coefficient (KR) ... 

According to these equations, for a given separation system, the main parameters involved in the separation of SDS-protein complexes are the electric force, the frictional force, and the retardation coefficient. These parameters are in turn affected by the strength of the electric field, molecular charge, analyte shape and size, polymer concentration, and temperature. 

Kim, C.G., Clarke, W.P., and Eockington, D. Determination of retardation coefficients of sulfolane and thiolane on soils by Ko -Kocand solubility parameters, batch and colnmn experiments, iSnvzron. Geoi, 39(7) 741-749, 2000a. 

Mansour, M., Thaller, S., andKorte, F. Action of sunlight on parathion. Bull. Environ. Contam. Toxicol, 30(3) 358-364,1983. Manzurola, E. and Apelblat, A. Solubilities of L-glutamic acid, 3-nitrobenzoic acid, p-toluic acid, calcium-L-lactate, calcium gluconate, magnesium-DL-aspartate, and magnesium-L-lactate in water, J. Chem. Thermodyn., 34 (7) 1127-1136, 2002. Maraqa, M.A., Zhao, X., Wallace, R.B., and Voice, T.C. Retardation coefficients of nonionic organic compounds determined by batch and column techniques. Soil Sci. Soc. Am. J., 62(1) 142-152, 1998. 

The analysis was limited in part by the scarcity of measurements, and clear discrepancies between measured and calculated values may be observed. As discussed in Chapter 10, tailing effects often are due to non-Fickian transport behavior, which was not accounted for in this model. Interestingly, the field-scale retardation coefficient values of the reactive contaminants were smaller by an order of magnitude than their laboratory values, obtained in an accompanying experiment. 

The first attempt to account for surface contamination in creeping flow of bubbles and drops was made by Frumkin and Levich (FI, L3) who assumed that the contaminant was soluble in the continuous phase and distributed over the interface. The form of the concentration distribution was controlled by one of three rate limiting steps (a) adsorption-desorption kinetics, (b) diffusion in the continuous phase, (c) surface diffusion in the interface. In all cases the terminal velocity was given by an equation identical to Eq. (3-20) where C, now called the retardation coefficient , is different for the three cases. The analysis has been extended by others (D6, D7, N2). 

If we divide equation (2.31) by the term (1 + KiPb/e), we can see that all convective and diffusive transport is retarded by equilibrium adsorption and desorption. Thus, a retardation coefficient is defined ... 

Now, because the water-borne radioactive element is predominantly associated with the colloids, we no longer have a need for the distribution coefficient. There will still be a partitioning because the major portion of the radioactive elements will still be adsorbed to the sediment. This is a separate equilibrium partitioning coefficient, requiring a new experiment on the clay sediments and the colloids present. The partitioning colloid-clay ratio would most likely be dependent on the surface areas of each present in the sediments. A separate size distribution analysis has resulted in a sediment-colloid surface area ratio of 99 1 for the sediment. This results in a colloid retardation coefficient oiRc = 100 rather than Ri = 4.2 x 10 or i 2 = 6 x 10. ... 

We will now use an example column experiment to determine the retardation coefficient for a given soil. 

As part of a forensic investigation of a continuous Malathion spill, you need to determine the retardation coefficient of the soil at the site for Malathion. You have decided to do so in a column experiment with the soil (illustrated in Figure E6.11.1). Also given in the figure are the results of a pulse test with the nonsorptive tracer, chloride, and the results of a pulse test with Malathion. What is the retardation coefficient, R1... 

This test can result in a retardation coefficient from a comparison of both residence times and the variance. 

Figure E6.11.1. Illustration of the column test for retardation coefficient and results of the tracer tests.

Our two means of determining the retardation coefficient in the column gave R = 20 and R = 22. We can also check whether the organic carbon content of the soil fits what is generally known from the literature. First, there is the relation for R ... 

Even with a fairly sorptive organic compound, the retardation coefficient in rock is not much different from 1.0. Equation (E6.12.1) gives a maximum at y = 0, z = 0,... 

The conflicting results discussed above suggest that further studies are required for understanding mechanisms of convection. More specifically, it is important to investigate hydraulic conductivity, interstitial pressure, and retardation coefficient in different tumor tissues, and how these factors are coupled with infusion-induced tissue deformation. 

Benker, E., G.B. Davis, and D.A. Barry. 1998. Estimating the retardation coefficient of trichloroethene for a sand aquifer low in sediment organic carbon—a comparison of methods. J. Contaminant Hydrol. 30 157-178. 

Maraqua, M.A., X. Zhao, and T.C. Voice. 1998. Retardation coefficients of nonionic organic compounds determined by batch and column techniques. Soil Sci. of Am. J. 

Fig. 4.3. The relative magnitudes of the retardation coefficients for monofunctional compounds. (Data from ref. [7].)...

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