Retardation factor, adsorption

Behavior of Ra in groundwater. The general behavior of Ra has been examined under laboratory conditions and in various environments (see Osmond and Cowart 1992). A major goal of field studies of Ra isotopes have aimed at obtaining bulk, in situ values of adsorption rates and so the retardation factors. Note that Ba serves as a very close chemical analogue to Ra but is typically 10 times more abundant, and so its behavior is related to that of Ra. 

There are various parameters and assumptions defining radionuclide behavior that are frequently part of model descriptions that require constraints. While these must generally be determined for each particular site, laboratory experiments must also be conducted to further define the range of possibilities and the operation of particular mechanisms. These include the reversibility of adsorption, the relative rates of radionuclide leaching, the rates of irreversible incorporation of sorbed nuclides, and the rates of precipitation when concentrations are above Th or U mineral solubility limits. A key issue is whether the recoil rates of radionuclides can be clearly related to the release rates of Rn the models are most useful for providing precise values for parameters such as retardation factors, and many values rely on a reliable value for the recoil fluxes, and this is always obtained from Rn groundwater activities. These values are only as well constrained as this assumption, which therefore must be bolstered by clearer evidence. 

Krishnaswami S, Graustein WC, Turekian KK, Dowd F (1982) Radium, thorium, and radioactive lead isotopes in groundwaters application to the in-situ determination of adsorption-desorption rate constants and retardation factors. Water Resour Res 6 1663-1675 Krishnaswami S, Bhushan R, Baskaran M (1991) Radium isotopes and Rn in shallow brines, Kharaghoda (India). Chem Geol (Isot Geosci) 87 125-136 Kronfeld J, Vogel JC, Talma AS (1994) A new explanation for extreme " U/ U disequilibria in a dolomitic aquifer. Earth Planet Sci Lett 123 81-93... 

The values of q are plotted as a function of the equilibrium concentration. For constituents at low or moderate concentrations, the relationship between q and C can be generated. If n = 1, the (q-C) relationship will be linear (Eq. 9), and the slope of the line (i.e.,ITd) defines the adsorption distribution of the pollutant. Kd is generally identified as the distribution or partition coefficient, and is used to describe pollutant partitioning between liquid and solids only if the reactions that cause the partitioning are fast and reversible, and if the isotherm is linear. For cases where the partitioning of the pollutants can be adequately described by the distribution coefficient (i. e.,fast and reversible adsorption, with linear isotherm), the retardation factor (R) of the subsurface environment can be used as follows ... 

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-... 

Adsorption has a significant impact on the movement of allelo-chemical substances in soil. Such movement in soil by water is important from the standpoint of mechanism of phytotoxin activity in the receiving species at a site remote from the donor plant. Adsorption reduces the solute concentration in the soil solution and consequently minimizes redistribution in the environment. Solute transport has been described by Pick s second law of diffusion and the kinetic models for adsorption and degradation of reactive solutes (, 44). The contribution of adsorption is measured and expressed as the retardation factor, R. 

Geochemical models of sorption and desorption must be developed from this work and incorporated into transport models that predict radionuclide migration. A frequently used, simple sorption (or desorption) model is the empirical distribution coefficient, Kj. This quantity is simply the equilibrium concentration of sorbed radionuclide divided by the equilibrium concentration of radionuclide in solution. Values of Kd can be used to calculate a retardation factor, R, which is used in solute transport equations to predict radionuclide migration in groundwater. The calculations assume instantaneous sorption, a linear sorption isotherm, and single-valued adsorption-desorption isotherms. These assumptions have been shown to be erroneous for solute sorption in several groundwater-soil systems (1-2). A more accurate description of radionuclide sorption is an isothermal equation such as the Freundlich equation ... 

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. 

This retardation is caused by adsorption on clays and organic materials. The adsorption capacity is in turn a function of the surface area and availability of these materials to the adsorbing species in question. In real soils, and especially those of sedimentary origin, the adsorbant components are to be found as discrete lenses of low permeability rather than as an evenly distributed phase as is essentially assumed in the laboratory methodologies for determining the retardation factors. In these lenses penetration to the bulk of the adsorption sites is restricted to diffusion and the small residual convection fluxes. 

System assumptions that should be valid for such applications include fluid flow in the porous media is isotropic and adsorption is fast, reversible, and linear (cf. Freeze and Cherry 1979). Given these constraints, the comparative transport of a conserved (nonadsorbed) tracer, such as Br , and an adsorbed or retarded species, such as Am, can be described as shown in Fig. 10.29. A comparison of migration distances of the two species after time t, is made at concentrations where C(measured)/Co(initial) = 0.5 for the conserved and adsorbed species. The migration distance X of the conserved species after time r is a measure of the average groundwater velocity (U), or X = vt. Similarly, the migration distance of the adsorbed species (X,) i related to its velocity of movement (v ) by Xf = vj. The retardation factor (/tj for the adsorbed species is then given by... 

Equation 2.66 shows that the velocity of a chemical solute concentration Vq is slower than the solution (water) velocity v by a retardation factor Rq. The retardation is caused by the adsorption of the chemical on the solid. Adsorption can be defined using the Freundlich isotherm or Langmuir isotherm. The general form of the Freundlich isotherm is... 

The magnitude of 1 + (pb/0) / is the retardation factor, estimating the degree to which the cation s movement is reduced. is simply the ratio of moles of Cs" adsorbed to moles in solution, and must be determined by an adsorption experiment using the soil material from the site of interest. The K4 can have a value of 100 or... 

Large retardation factors can mean that the leaching of metal cations from the soil surface into the subsoil is slow, even assuming that metal adsorption is by exchange processes only. As will be explained in Chapter 4, many metals adsorb in addition by strong forces, and this form of metal retention in soils is likely to be practically irreversible, leading to long-term immobilization. 

The adsorption phenomenon of the stationary phase is substantially reduced once a large part of the adsorption sites are occupied. This creates an effect of elongation of the spots. As a result, the Rf (retardation factor) of a compound in the pure state is slightly different from the Rf of the same compound present in a mixture. 

The simplifications and assumptions of the retardation factor approach in treating chemical reactions can be illustrated by comparing the one-site Langmuir isotherm and surface complexation formulation for adsorption of a metal onto iron oxide with one type of surface sites. 

There are two reasons in favour of the theory of the dynamic adsorption layer at weakly and strongly retarded bubble surfaces. At first, the theory has been developed also for large Re, and secondly it was developed for small Re also for the case of surface retardation controlled by surfactant concentration as well as by other retardation factors. 

Equations (3.75) and (3.76) are valid only if the partition coefficient Kr, does not depend on the contents of the sorbate proper and water composition, i.e., corresponds with Henry s adsorption isotherm. This is possible at very low concentrations of an adsorbed component. At high concentrations, when adsorption corresponds with Freundlich, Langmuir isotherms or depends on ion exchange with another component, the retardation factor is expressed much more complexly. For Freundlich isotherm (equation (2.177)), the retardation factor may be estimated from equation... 

A retardation factor was calculated from these data and yielded a distribution coefficient Kf) of 0.0017, which corresponds to an adsorption of 0.16 mg of surfactant per gram of soil, but these values are not significantly different from zero taking the experimental error into account. Regardless, these values are lower than those reported by Rouse et al. [45] for other surfactants on similar soils. We attribute this in part to the very favorable characteristics of dihexyl sulfosuccinate and in part to the use of the IPA cosolvent. High values of surfactant retention... 

The product f-F is usually referred to in chromatography of column flow as the retardation factor, Fr (Giddings, 1965). In a polymer flood, there are therefore two competing effects on the retardation factor adsorption, tending to make Fr> 1, and velocity enhancement, which tends to make Fr< 1. Note that if Fr = 1, then it is probable that there are no adsorption or excluded-volume effects however, it could be that they fortuitously cancel. 

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