Sorption of radionuclides

The oldest, most well-established chemical separation technique is precipitation. Because the amount of the radionuclide present may be very small, carriers are frequently used. The carrier is added in macroscopic quantities and ensures the radioactive species will be part of a kinetic and thermodynamic equilibrium system. Recovery of the carrier also serves as a measure of the yield of the separation. It is important that there is an isotopic exchange between the carrier and the radionuclide. There is the related phenomenon of co-precipitation wherein the radionuclide is incorporated into or adsorbed on the surface of a precipitate that does not involve an isotope of the radionuclide or isomorphously replaces one of the elements in the precipitate. Examples of this behavior are the sorption of radionuclides by Fe(OH)3 or the co-precipitation of the actinides with LaF3. Separation by precipitation is largely restricted to laboratory procedures and apart from the bismuth phosphate process used in World War II to purify Pu, has little commercial application. 

Sorption and desorption isotherms were obtained for sorption of radionuclides under oxidizing and reducing conditions. The Freundlich equation accurately describes most of these isotherms. Most radionuclides are apparently irreversibly sorbed on each of the geologic solids since the slopes of sorption and desorption isotherms for a given radionuclide are different. This hysteresis effect is very large and will cause a significant delay in radionuclide transport. It, therefore, should be included in modeling radionuclide transport to accurately assess the isolation capabilities of a repository in basalt. 

One can attempt to overlay the effects of sorption on this scenario. It is apparent that strong sorption of radionuclides will tend to depress the equilibrium concentration of the element in... 

Sorption of radionuclides on particulates in solution is frequently observed. The particles may be coarsely or finely dispersed. Their surface properties (surface layer, charge, ion-exchange and sorption properties) play a major role. In general, they offer a great number of sorption sites on the surfaee, and microamounts of radionuclides may be found on the surface of these particles instead of in solution. Sorption of radionuclides on colloidal particles leads to formation of radioeolloids (carrier colloids, section 13.4). 

Mobility and transport of radionuclides in the geosphere are influenced markedly by their interaction with solids. Migration is retarded, or even stopped, if the interaction is strong, in particular if the radionuclides are incorporated into the solids. Sorption of radionuclides on solids has been investigated extensively for materials in the neighbourhood of planned high-level waste repositories. 

Representation of sorption of radionuclides under natural conditions. Several approaches have been used to represent variability of sorption under natural conditions. These include (i) sampling values from a probability distribution... 

As discussed previously (Section 9.06.3.1.1), plots of pH sorption edges (see Figure 3) are useful in summarizing the sorption of radionuclide by substrates that have amphoteric sites (i.e., SOH, SO , SOHJ). The pH sorption edges of actinides are similar for different aluminosilicates (quartz, a-alumina, clinoptUolite, montmorillo-nite, and kaolinite). For example, Np(V) and U(VI) exhibit similar pH-dependent sorption edges that are independent of specific aluminosilicate identity (Bertetti et al, 1998 Pabalan et al., 1998). Under similar solution conditions, the amount of radionuclide adsorbed is primarily a function of the surface area. This observation has led several workers to propose that the amount of actinide sorption onto natural materials can be predicted from the surface site density and surface area rather the specific molecular structure of the surface (Davis and Kent, 1990 Turner and Pabalan, 1999). 

Two different processes could be important for the initiation of radionuclide transport by carrier colloids (i) reversible sorption of radionuclides from solution onto pre-existing colloids and (ii) detachment of colloids from the host rock with high concentrations of previously sorbed irreversibly bound colloids. In both cases, radionuclides that sorb strongly to the rock matrix and would normally migrate very slowly will travel at a rapid rate while they are bound to colloids. 

Experimental studies. Sorption of radionuclides by colloids is affected by the same solution composition parameters discussed in the previous section on sorption processes. The important parameters include pH, redox conditions, the concentrations of competing cations such as Mg " " and K, and the concentrations of organic ligands and carbonate. The high surface area of colloids leads to relatively high uptake of radionuclides compared to the rock matrix. This means that a substantial fraction of mobile radionuclides could be associated with carrier colloids in some systems. The association of radionuclides with naturally occurring colloids and studies of radionuclide uptake by colloids in laboratory systems give some indication of the potential importance of colloid-facilitated radionuclide transport in the environment as discussed below. 

Huie Z., Zishu Z., and Lanying Z. (1988) Sorption of radionuclides technetium and iodine on minerals. Radiochim. Acta 44/45, 143-145. 

Sorption of radionuclides onto the container walls is a common problem. The problem does not arise for all radionuclides and types of water, but its occurrence must be checked and may be prevented as discussed in Section 4.5. The usual sample preservation technique is mild acidiAcation, but sAonger acid may be necessary to preserve Aansuranium radionuclides. A basic solution must be used for radioiodine. 

Introduction. Colloidal suspensions are defined as suspensions of particles with a mean diameter less than 0.45 xm, or a size range from 1 nm to 1 pm. They represent potentially important transport vectors for highly insoluble or strongly sorbing radionuclides in the environment. Colloids are important in both experimental systems and natural settings. In the former, unrecognized presence of colloids may lead to overestimation of the solubility and underestimation of the sorption of radionuclides if they are included in the estimation of the concentration of radionuclide solution species. In natural systems, they may provide an important transport mechanism for radionuclides not filtered out by the host rock. In fractured rock, local transport of radionuclides by colloids may be important. 

Berry, J. A. 1992. A review of sorption of radionuclides under the near- andfar-field conditions of an underground radioactive waste repository. Parts /-///. UK DOE Report DOE/HMIP/RR/92.061. 

Linklater, C. M. 1991. Sorption of radionuclides on mineral surfaces. In Proceedings of the 3rd International Conference On Nuclear Fuel Reprocessing and Waste Management, Sendai, Japan. 988-993. 

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