Ion Exchange in High-Level Nuclear Waste Encapsulation: Atomistic Modeling of Equilibrium Coefficients and Isotherms
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funding agencies and the NSF, unprecedented advances in our ability to predict the behavior of complex systems through simulation may be realized. An effort to develop similar capabilities to address critical nuclear waste issues would seem to be timely. As an initial effort, the application of atomistic simulation to the problem of ion exchange of Cs from zeolites is undertaken. The electrometallurgical treatment program at Argonne National Laboratory [7] is developing conditioning processes for injection of waste chloride salts into microporous aluminosilicates, such as zeolite A or sodalite. The waste containing zeolites are then bonded with glass into composite logs. The logs would be placed in steel containers for disposal in a permanent repository. The long-term stability of the zeolite/salt composites is presently not sufficiently well understood to allow extrapolating the results of short-term leach tests to predict long-term performance in a repository. The eventual goal of our work is provide a detailed molecular-level understanding of transport rates in these systems.
ISOTHERMS FOR CESIUM-SODIUM EXCHANGE Ion exchange is assumed to be the process responsible for the removal of Cs from sodalite. Exchange can be expected to occur with Na, Na + Cs t
(I).
Na + Cs
The overbar indicates the metal ion is in the sodalite, i.e. solid phase. Ion exchange is determined at equilibrium and is a function of physical conditions such as temperature, ion concentration, and pH. At a given temperature and pH, one can develop an isotherm describing the ion concentration in each phase by varying the relative Cs concentration. For the exchange between Cs and Na, the isotherm shows the mole fraction of Cs in each phase. The mole fraction of Cs in the solution phase (Cs) is
(2),
+[Na+] CssCs=[Cs+][Cs+] where the brackets denote concentration of the ion in the given phase. For the solid phase, the Cs mole fraction is
Csz
[]
(3)
[Cs] + [Na]
The isotherm is obtained by plotting Cs, against Csz. An isotherm of Cs-Na ion exchange on Linde Sieve 4A is provided in Figure 1. Linde Sieve 4A is Zeolite A. This is used as an example since no data on the exchange of Cs with Na in sodalite is available in the open literature. The example provided in Figure 1 should be suitable since sodalite is one of the main structural units of Zeolite A. As with any physico-chemical process, the equilibrium point for an ion exchange reaction is reached when the free energy is minimized. For the ion exchange reaction in question, the standard free energy can be defined as AG-
RT IK ZCsZNa
(4)
Ka
where ZCS and ZNa are the charges on the cations and Ka is the equilibrium coefficient. For the example of Cs and Na, the charges are 1. Using y, the ion activity coefficient, andf ,the fugacity of the ions in the solid phase, Equation 5 describes the conditions at each equilibrium point for the mole fraction range from 0 to 1, by defining the equilibrium constant.
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1.
,
0.8
0
0 0.6 u,0.4
C. 0
"0.2 LL
,
05 0.0 0.0
0.2
0.4
0.6
0.8
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