Sorption reactions radionuclides

The chapters of this volume are organized into sections that cover the chemical aspects that are important to understanding the behavior of disposed radioactive wastes. These aspects include radionuclide sorption and desorption, solubility of radionuclide compounds, chemical species of radionuclides in natural waters, hydrothermal geochemical reactions, measurements of radionuclide migration, solid state chemistry of wastes, and waste-form leaching behavior. The information in each of these sections is necessary to predict the transport of radionuclides from wastes via natural waters and thus to predict the safety of the disposed waste. 

Radionuclide transport in natural waters is strongly dependent on sorption, desorption, dissolution, and precipitation processes. The first two sections discuss laboratory investigations of these processes. Descriptions of sorption and desorption behavior of important radionuclides under a wide range of environmental conditions are presented in the first section. Among the sorbents studied are basalt interbed solids, granites, clays, sediments, hydrous oxides, and pure minerals. Effects of redox conditions, groundwater composition and pH on sorption reactions are described. 

Radionuclide Sorption and Desorption Reactions with Interbed Materials from the Columbia River Basalt Formation... 

In order to assess the feasibility of any nuclear waste disposal concept, mathematical models of radionuclide sorption processes are required. In a later section kinetic descriptions of the three common sorption isotherms (3) are compared with experimental data from the mixing-cell tests. For a radionuclide of concentration C in the groundwater and concentration S on the surface of the granite, the net rate of sorption, by a first-order reversible reaction, is given by... 

Chemical components in the waste solutions potentially could affect radioelement solubility and sorption reactions, and thus enhance or reduce radionuclide transport. The effects of 12 chemical components on the solubility and sorption of cobalt, strontium, neptunium, plutonium, and americium were studied to... 

Note that no sorption reactions have been assumed, so all retardation factors are equal to imity. A contaminant source containing radionuclides is released instantaneously at the origin of the fracture at time t = 0 (a first-type Dirac delta inlet condition). To get a more detailed description of the physical system see Sudicky and Frind (1984) or Cormenzana (2000). 

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. 

The reliable long-term safety assessment of a nuclear waste repository requires the quantification of all processes that may affect the isolation of the nuclear waste from the biosphere. The colloid-mediated radionuclide migration is discussed as a possible pathway for radionuclide release. As soon as groundwater has access to the nuclear waste, a complicated interactive network of physical and chemical reactions is initiated, and may lead to (1) radionuclide mobilization (2) radionuclide retardation by surface sorption and co-precipitation reactions and (3) radionuclide immobilization by mineralization reactions, that is, the inclusion of radionuclides into thermodynamically or kinetically stabilized solid host matrices. 

In previous work (l.> > ) it was found that the kinetics of sorption was an important parameter affecting the migration of nuclides in geologic media. For example, in experiments designed to measure the kinetics of reaction for radionuclides in solution with tablets of rock, it was found that periods from several minutes to several hours were required for the radionuclides to reach steady state concentrations on the rock tablets and in the solutions. Figure 1 shows the reaction curves found for the sorption of plutonium and americium from solution by a tablet of granite. The reaction rates for the sorption of plutonium and americium from solution are not the same, and both require a number of hours to reach steady state concentrations. 

The groundwater transport of radionuclides through waterbearing interbed layers in the Columbia River basalt formation will be controlled by reactions of the radionuclides with groundwater and interbed solids. These interactions must be understood to predict possible migration of radionuclides from a proposed radioactive waste repository in basalt. Precipitation and sorption on interbed solids are the principle reactions that retard radionuclide movement in the interbeds. The objective of the work described herein was to determine the sorption and desorption behavior of radionuclides important to safety assessment of a high-level radioactive waste repository in Columbia River basalt. The effects of groundwater composition, redox potential, radionuclide concentration, and temperature on these reactions were determined. 

This equation can be used to describe one-dimensional transport of radionuclides through porous media (e.g. radionuclide elution curves from laboratory columns packed with interbed solids) assuming instantaneous sorption and desorption. Van Genuchten and coworkers have demonstrated the importance of using both sorption and desorption isotherms in this equation when hysteresis is significant. Isotherm data for sorption and desorption reactions of radionuclides with interbed materials are presented in this paper which can be used to predict radionuclide transport. 

As will be shown later, sorption of most radionuclides may be a function of two or more mechanisms. The combination two first-order reactions has been successfully applied to Sr migration over a twenty-year time period in a sandy-aquifer (1) The equations describing two parallel first-order reactions are... 

Investigations of redox processes in natural water systems have emphasized the disequilibrium behavior of many couples (e.g., 37). The degree of coupling of redox reactions with widely varying rates, and its effect on radionuclide transport in an NWRB needs to be considered. Because of the generally slow kinetics of autoxidation reactions, the potential surface catalyzed reduction of a radionuclide at low temperatures in the presence of trace levels of DO may explain certain sorption data (e.g., 38). 

The main components of soils are sand, clay and humus. Whereas interaction between radionuclides and sand is rather weak, as in the case of sediments, sorption by clay minerals and reactions with the organic compounds in humus are most important for the migration behaviour of radionuclides. Cs+ ions are quite strongly bound in clay particles, as already mentioned. Ra + and 2i0pjj2+... 

Fifteen trace elements were determined at ng/g mass fraction levels in high purity Sn by Kolotov et al. (1996) using extraction chromatography on Teflon-supported tributylphosphate (TBP). Tin, together with Sb and In radionuclides, which are formed in secondary nuclear reactions of Sn during irradiation, were retained from 6 M HCl on the column. The Na was removed from the eluted trace element fraction by sorption on hydrated antimony pentoxide (HAP). Gas phase impurities of N, C, and O were also analyzed in the same material by radiochemical photon activation analysis via N, C, and detection. 

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