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International Conference on HAZARDOUS WASTE: Sources, Effects and Management 12-16 December 1998

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International Conference on HAZARDOUS WASTE: Sources, Effects and Management 12-16 December 1998 International Conference on HAZARDOUS WASTE: Sources, Effects and Management 12-16 December 1998. Cairo-Esypt TR-5 Thermodynamics of Radio Strontium Exchange with Certain Zeolites A.M.I.Ali,A.M.El-Kamash,M.R.EI-Sourougy and H.F.Aly Atomic Energy Authority, Hot Lab. Centre Cairo, Egypt ABSTRACT EG0000186 Efficient design of ion-exchange columns, Using zeolites, for the decontamination of radioactive wastewater that contains MSr and "'Cs require a detailed study of the thermodynamic of ion-exchange equilibria. Ion-exchange experiments were conducted at 20 °C between certain natural zeolites (chabazite, clinoptilolite, and mordenite) and aqueous solutions of varying ratios of Na+and Sr2* and total concentration of 0.05 and 0.2N. The experiments were designed to investigate the effects of changes in total solution concentration and in relative concentrations of exchangeable cations on the following ion-exchange equilibrium: Sr2+ +2Na = ~S?* +2Na+ Using the isotherms data obtained, a thermodynamic model for the ion-exchange reaction was derived using Gaines and Thomas approach for activity coefficients of zeolite components and Glueckauf extension approach for activity coefficients of aqueous ions. A computer programme to carry out the thermodynamic treatment was given in FORTRAN-77 and modified for best fit results. The results of the forward experiments showed that the ion-exchange isotherm strongly depends on the total solution concentration. Additional experiments demonstrated that the above ion-exchange reaction is reversible for all zeolites under investigation. The derived equilibrium constant, K,, and Gibbs free energy per equivalent of ion-exchange, &G", are obtained for all studied zeolites indicated that all these zeolites have overall selectivity towards Sr2* ions than Na+ ions. INTRODUCTION The potential release of fission products such as "'Sr (t Vi = 29.1 y) and'" Cs (t Vi =30.3 y) into the environment from nuclear power plants and reprocessing plants or by leakage of nuclear waste containers posses a serious of health risk due to their high fission yield and potential mobility in aqueous environments. However, their interaction with the geochemical system, such as, by sorption reactions with the zeolite-rich rocks and soils, may help attenuate this hazard. Natural zeolites (particulary chabazite, clinoptilolite, and mordenite) are known for their favourable selectivity for Sr and Cs through an ion-exchange mechanism(1). Thus, the favourable selectivity of zeolites for alkali and alkaline earth radionuclides makes it effective for cleaning up radioactive liquid wastes arising from the different applications of nuclear industry. The earliest recorded use of zeolites in nuclear waste treatment was in 1960 by Ames TO who used these minerals to selectivity removal of137 Cs and * Sr from the low level radioactive wastes. Mercer and Ames<3)have provided a detailed description of past and present uses of natural zeolites especially in the decontamination of low and medium level wastes and fixation of fission products into zeolites prior to long-term storage. 827 Because ion-exchange processes are affected by aqueous solution chemistry and zeolite composition, a quantitative understanding of zeolite-radionuclide interaction requires chemical models that property account for these effects. The aim of this paper, experiments on ion- exchange between certain natural zeolites and aqueous solutions of Sr^/Na* were conducted. The aim of this paper is to investigate the effects on the exchange equilibrium with changes in total solution concentration and in the relative concentration of exchangeable cations in solution. In addition, the data were used to test an empirical thermodynamic model for zeolit solid solutions to describe ion-exchange equilibria. EXPERIMENTAL The ion-exchange experiments were conducted by equilibrating weighed amounts of Na- zeolites ( chabazite, clinoptilolite, and'mordenite) powder (150 -355 urn grain size) with a series of solution containing Na+ and Sr2+ at different equivalent concentration ratios, but at a constant total normality equal to 0.05 and 0.2 N. The cation exchange capacities of the three different types of zeolites under investigation were 2.24 meq/g for chabazite, 1.83meq/gfor clinoptilolite and 2.43 meq/g for mordenite . Aqueous mixtures of Na +/ST2+ at 0.2 N and with equivalent mole fractions of Sr2* at 0.1,0.2, 1.0 were prepared by mass from reagent - grade NaNO3 and Sr (NO3)2. Solutions at 0.05 were prepared by dilution of 0.2 N mixtures. The initial total Sr concentration of each solution were calculated from the mass of Sr(NO3)2 reagent used and to allow analysis of Sr24' in aqueous samples by liquid scintillation counting, each of the starting solutions was spiked with ^Sr (50pci per gram of solution). The amount of zeolites used in the experiments was 0.5g, and the solution volume ranged from 10 to 250ml. The zeolites and solution mixtures were contained in capped polyethylene bottles and were agitated and thermostatted at 20 ±1°C in shaker water bath. After at least two weaks, samples of starting and experimental solutions were taken for Na+ and Sr1* analysis. Reversibility of the exchange reaction was verified in a second set of experiments. After samples had been taken for the forward experiments, several isotherm points were reversed by adding to the remaining solutions known volume of an 0.2 N NaN03 solution. The new mixtures were allowed to reequilibrate for at least two weaks, then samples were taken for Sr2+ and Na+ analysis. Thermodynamic Model For an exchange reaction involving Sr2+ and Na+, the equilibrium reaction may be written as: Sr2+ +2 Na+ = Sr* + 2Na+ (1) where bars indicate the zeolite phase. Ion-exchange equilibria are often measured by the selectivity coefficient (a) which can be defined as: Sr2+ 2 V =(2Tc)(Srz/Srs)(Nas/Naz) (2) This selectivity coefficient may alter with solution composition (at given zeolite composition) or with exchanger composition (at given solution composition). Then the corrected selectivity coefficient is given by 828 Kc = a y2Na/y Sr (3) Kc is theoretically independent of solution composition, at given zeolite composition, provided that electrolyte imbibition from the external solution and changes in water activity are negligible. The thermodynamic equilibrium constant, Ka, corresponding to the above reaction, is given by: K.-K. fs,/(f*.)2 (4) The activity coefficients of the ions in the solid phase, f, are not independent by measureable, but can be determined indirectly from the measurement of Kc. So, Ka is then obtained from the following equation; r, (5) and the soiid phase activity coefficients are given by : ! lnfsr=(Srz-l)(lnkc+l) + JlnKcdSrz (6) Sr z and = Srz(lnKc+l)-oJlnKcdSrz (7) The standard Gibbs free energy per equivelent of the present exchange reaction is AG° = -RT/2 In Ka (8) The above thermodynamic formulation is valid if imbibtion of neutral electrolyte is negligible, which for zeolites is at solution concentrations approximately < 1.0 M (4). An additional condition is that the effect of water activity change in the zeolite is insignificant. Barrer and Klinowski (5) demonstrated that water activity terms are not significant for most cases of ion- exchange equilibria. However, for ion-exchange involving concentarated electrolyte solutions, terms can be included for the effects of sorbed or imbided solvent and of imbided salts(6). Computional Analysis for the Mode!: A program to carry out the thermodynamic model was written in FORTRAN-77 and modified for best fit results. The principles of this compuational analysis areas follow: 1 .a best fit polynomial is calculated from the input data of the ion-exchange isotherm, 2.the Debye-Huckel constants for Sr2* and Na+ are used to calulate the mean molar salts activity coefficient for each of the equilibration data points, thus the corrected values of these activity coefficient are calculated by using the Glueckaf extention approach^, 3. the correctrd selectivity coefficient, Kc, is calculated for each of the equilibration data points and a best fit polynomial is produced for the plot of In Kc versus Srz, 4. numerical integration is applied for the calculation of the zeolite component activity coefficients, f ».*, and the equilibrium thermodynamic constant, Ka, and 5. The standard free energy per equivalent, AG°, of the binary exchange reaction is calculated. RESULTS AND DISCUSSION Results of binary ion -exchange experiments can be presented conventiently by ion- exchange isotherms, which are usually plotted in terms of the equivalent fraction of the cations in solution against that in the solid in equilibrium with the solution. The isotherm data for the 829 Na7Sr2\ mixtures at 0.05 and 0.2 in each type of zeolite under investigation are plotted in Figs. 1-3. Details of isotherm calculation from experimental data are discussed in(S). The results shown in Figs. 1 -3. indicated a strong dependence of isotherm shape on the total solution concentration for the system Sr2+/Na\ which involves ion-exchange between a monovalent and a divalent cations . These results illustrate that with increasing dilution; chabazite, clinoptilalite and mordenite exhibit increasing preference (selectivity) for Sr2+ relative to Na+. This behaviour, commonly refered to as "concentration valency effect", which gives rise to isotherms that became more rectangular and selective to the ion of higher charge with increasing dilution. Figs.4-5 compare the results of the forward and reverse isotherm experiments at the two different total concentrations (0.05 and 0.2 N) for each type of zeolite. The good agrament between the sets of data indicated that ion-exchange reactions between all types of zeolites and aqueous solutions of Sr2+/Na+are reversible. The quantitative thermodynamic treatment is applied to the previous experimental isotherms data to give a sequential picture of the selectivity of each type of exchangers under investigation. Table 1. Gives the polynomial equations for log K* vs. Sr2^ for different types of zeolite at two different total solution normality.
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