Sorbing radionuclide retardation

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 ... 

The sorbing radionuclides will be retarded in their diffusion into the matrix by a factor R because they sorb to the micropore surfaces and are constantly withdrawn from the water. They accumulate on the surfaces and can attain high concentrations there. The volumetric sorption coefficient K differs between nuclides. For nonsorbing nuclides K=0 but for sorbing nuclides it can range from less than 1 to hundreds of thousands. 

In current performance assessments, radionuclide retardation in the geosphere is represented as a sorption process. Data acquired within the NSARP are used to support the representation of sorption within assessment models of the longterm performance of the repository. Sorption is generally quantified using a sorption coefficient, which is the ratio of the concentration of radionuclide sorbed on the solid (per unit mass) to the concentration in solution (per unit volume). It is assumed that sorption is linear (i.e. the sorption coefficient is independent of radionuclide concentration) and reversible. The assumption of linearity is considered reasonable for the expected range of concentrations predicted along possible flowpaths from the repository to the... 

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. 

Sorption can significantly diminish the mobility of certain dissolved components in solution, especially those present in minor amounts. Sorption, for example, may retard the spread of radionuclides near a radioactive waste repository or the migration of contaminants away from a polluting landfill (see Chapters 21 and 32). In acid mine drainages, ferric oxide sorbs heavy metals from surface water, helping limit their downstream movement (see Chapter 31). A geochemical model useful in investigating such cases must provide an accurate assessment of the effects of surface reactions. 

To mitigate radioactive contamination, it is important to understand the processes and mechanisms of interactions between radionuclides and the solid material of aquifers. Cationic radionuclides may be sorbed by processes such as ion exchange or surface complex formation, thus retarding their transport by groundwater. 

That colloid-facilitated radionuclide transport should take place is a straightforward deduction from the component parts of the problem (i) highly-retarded radionuclides will strongly sorb to colloidal materials (ii) colloids exist in most groundwater environments in relatively high concentrations and (iii) colloids can be transported through the subsurface over appreciable distances. Why then is the debate over the significance colloid-facilitated radionuclide transport at the field scale still considerable ... 

Sorption also is a potential retardation mechanism for radionuclide transport through fractures. The surfaces of fractures are often lined with minerals that may be capable of sorbing some of the radionuclides however, there is limited characterization of the extent and distribution of fracture-lining minerals along potential flow paths (Carlos et al., 1995). In the conceptual model used for Yucca Mountain, it is conservatively assumed that there is no sorption (i.e., = 0) in fractured tuff... 

---