Adsorption of radionuclides

Adsorption of Radionuclides. Other appHcations that depend on physical adsorption include the control of krypton and xenon radionuchdes from nuclear power plants (92). The gases are not captured entirely, but their passage is delayed long enough to allow radioactive decay of the short-hved species. Highly rnicroporous coconut-based activated carbon is used for this service. 

The thermodynamic properties of U-Th series nuclides in solution are important parameters to take into account when explaining the U-Th-Ra mobility in surface environments. They are, however, not the only ones controlling radionuclide fractionations in surface waters and weathering profiles. These fractionations and the resulting radioactive disequilibria are also influenced by the adsorption of radionuclides onto mineral surfaces and their reactions with organic matter, micro-organisms and colloids. 

Figure 2. Adsorption of radionuclides on rock minerals, (a) (Topj Photomacrograph of a polished granite surface after exposure to a solution of radio-cesium Dark areas are exposed mica crystallites, (b) (Bottom) Autoradiograph of same surface showing uptake of cesium by mica.

Seme R. J. and Muller A. B. (1987) A perspective on adsorption of radionuclides onto geologic media. In The Geological Disposal of High Level Radioactive Wastes (ed. D. G. Brookins). Theophrastus Publications, pp. 407 -443. 

Wang P., Andrzej A., and Turner D. R. (2001a) Thermodynamic modeling of the adsorption of radionuclides on selected minerals I. Cations. Indust. Eng. Chem. Res. 40, 4428-4443. 

The adsorption of radionuclides to sediments is strongly affected by particle size and mineralogy (Aston, Assinder Kelly, 1985 Ramsay Raw, 1987 Livens Baxter, 1988a). In particular, fine grained material accumulates higher concentrations of all radionuclides and the presence of clay minerals also enhances adsorption. 

As for their stable analogs, the adsorption of radionuclides may depend on experimental conditions. Any variation results from the chemistry of the element, not form its radioactive nature. For example, the adsorption of the alkali metal cesium is largely independent of pH, whereas pH dependence is to be expected for oxyanions (Chapter 5, this volume) such as selenite and selenate and for transition metals such as nickel, silver, and palladium. 

Flocculation. Flocculation is the least costly procedure to concentrate non-hlgh-level waste. The principles are unspecific adsorption of radionuclides on a carrier, such as Fej 03(07) or calcium phosphate, or cocrystallization with a suitable crystalline precipitate, such as strontium with CaCOa. The sludge has to be collected by settling or filtering and is handled as the radioactive waste concentrate. This technique, because of its rather poor decontamination effect, is suitable only for LLW. Usually, the concentrate has a high water content. 

Of special importance is the adsorption of radionuclides onto soil components. The adsorption of Np(V) onto goethite has been studied by Combes et al. (Combes et al. 1992), finding that sorbed Np(V) is present as a mononuclear species similar to that of dissolved NpO J in water. The results do not rule out the possibility that the surface... 

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. 

Kent, D.B., Tripathi, V.S., Ball, N.B., Leckie, J.O., and Siegel, M.D., Surface-Complexation Modeling of Radionuclide Adsorption in Subsurface Environments, U.S. Nuclear Regulatory Commission Report NUREG/CR-4807, 1988, p. 113. 

Bors J, Dultz S, Riebe B (2000) Organophilic bentonites as adsorbents for radionuclides I. Adsorption of ionic fission products. Appl Clay Sci 16 1-13... 

ADTECHS Corporation (ADTECHS) has developed the radionuclides separation (RASEP) process for the removal and stabilization of radionuclides from liquid waste streams. The process uses filtration, selective adsorption, and electrodeposition fixation followed by cement sohdifi-cation. According to the vendor, the technology is commercially available. 

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 ... 

Adsorption of the radionuclide ions Am3+ (241Am) and Eu3+ (152Eu) by malonamide-functionalized MCM-41 materials.129... 

Elements 108 - 116 are homologues of Os through Po and are expected to be partially very noble metals. Thus it is obvious that their electrochemical deposition could be an attractive method for their separation from aqueous solutions. It is known that the potential associated with the electrochemical deposition of radionuclides in metallic form from solutions of extremely small concentration is strongly influenced by the electrode material. This is reproduced in a macroscopic model [70], in which the interaction between the microcomponent A and the electrode material B is described by the partial molar adsorption enthalpy and adsorption entropy. By combination with the thermodynamic description of the electrode process, a potential is calculated that characterizes the process at 50% deposition ... 

In laboratory experiments with radionuclides, knowledge of the mass of the radioactive substances is very important. For example, the mass of 1 MBq of ( i/2 = 14.3 d) is only about 10 ° g, and that of 1 MBq of Tc (ti/2 = 6.0 h) is only about 5 10 g. If there is no carrier present in the form of a large excess of inactive atoms of the same element in the same chemical state, these small amounts of radionuclides may easily be lost, for instance by adsorption on the walls. Whereas in the case of radioisotopes of stable elements the condition of the presence of carriers is often fulfilled due to the ubiquity of most stable elements, it is not fulfilled in case of short-lived isotopes of radioelements, and extraordinary behaviour may be observed (section 13.3). 

The surfaces of plastic materials, such as polyethylene, polypropylene or perspex, do not exhibit ion exchange, but adsorption may be pronounced, in partieular adsorption of organic compounds including organic complexes of radionuclides. 

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