I n d . Eng. Chem. Res. 1995,34, 2142-2147
2142
Estimation of Cesium Ion Exchange Distribution Coefficients for Concentrated Electrolytic Solutions When Using Crystalline Silicotitanates Zhixin Zheng, Ding Gu, and Rayford G. Anthony* Kinetics, Catalysis and Reaction Engineering Laboratory, Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122
Elmer Klavetter Sandia National Laboratory, Albuquerque, New Mexico 87185
Polzer et al.’s method combined with Bromley’s method for estimating activity coefficients and a Langmuir isotherm for cesium in a simple simulated waste solution containing 5.1 M NaN03 and 0.6 M NaOH was used to estimate distribution coefficients for cesium in a complex simulated waste solution characteristic of the radioactive tank wastes a t Hanford and other U.S. Department of Energy sites. The ion exchange material was a hydrous sodium crystalline silicotitanate, labeled TAM-5, which is being developed by Texas A&M University, Sandia National Laboratories, and UOP Associates. Cesium distribution coefficients collected by Bray et al. on a NCAW simulated waste solution were predicted with deviations of less than 25% for solutions containing 1M, 3 M, and 5 M Na+ and Na:Cs ratios of lo3-lo8. The deviations were less than 5%for the solutions with 1M Na+. Cesium distribution coefficients were also predicted and compared with values measured by Egan et al. for TAM-5 and for a storage tank supernate and a “newly generated waste solution. Excellent results were obtained for the “newly generated” waste simulated solution, which did not contain potassium or rubidium. The predictions for the other simulated waste solution were significantly greater than the measured values, because of the presence of large concentrations of potassium or rubidium. The effect of competitive ion exchange between Cs, Rb, and K was not included in the theory. However, the effect of competitive exchange of Cs, Rb, and K appears to be greater for the Oak Ridge simulated waste solution than for the NCAW waste.
Introduction A new crystalline silicotitanate ion exchanger, labeled TAM-5 and IONSIEVE 910, is being developed by the Department of Chemical Engineering a t Texas A&M University, Sandia National Laboratories, and UOP Associates for removing cesium and other radionuclides from aqueous solutions with high concentrations of sodium. It is an inorganic ion exchanger with a welldefined crystal structure, which has been shown to be very selective for removing cesium from concentrated solutions of sodium in acidic and basic solutions typical of the radioactive defense wastes stored at various U.S. Department of Energy (DOE)sites (Anthony et al., 1993, 1994; Bray et al., 1993; Dosch et al., 1992, 1993; Egan, 1993; Klavetter et al., 1994; Kurath et al., 1993). Designing waste treatment facilities and determining the optimum operating conditions for using TAM-5 to remove radioactive cesium from the aqueous waste requires knowledge of the ion exchange distribution coefficient, which is a function of composition and usually needs to be measured experimentally. Because a wide range of compositions will be encountered in processing the waste, an extensive experimental program will be required. Hence, a method that will allow the prediction of equilibrium performance using a limited set of experimental conditions will significantly reduce the time and effort required for such an experimental program. Therefore, the first objective of this study was to determine the feasibility of using simple simulants, i.e. simulated waste solutions, t o estimate
* Author t o whom correspondence should be addressed. E-mail:
[email protected]. 0888-5885/95/2634-2142$09.00/0
cesium distribution coefficients for TAM-5 when used in complex simulants. The second objective of the study was to determine if a limited set of data collected by using TAM-5 in a complex simulant could successfully be used to estimate the performance as determined by the cesium distribution coefficients for simulants prepared by concentrating, or diluting, the simulant which was used to measure the initial set of cesium distribution coefficients. A third objective was to estimate the levels of potassium and rubidium that begin to impact the selectivity of TAM-5 for cesium. The fourth and final objective of the study was to determine the effect on the predicted cesium distribution coeficient when the complete composition of the solution was unknown. Polzer et al. (1988, 1992) related the parameters in the Langmuir isotherm to “true” thermodynamic constants for interpreting transport of radionuclides in volcanic tuff media. We chose this method, combined with Bromley’s method for estimating activity coefficients and the Langmuir isotherm obtained by using simple simulants to evaluate the first objective. Cesium distribution coefficients were calculated for a neutralized alkaline simulated waste solution, labeled as NCAW, and for a wide range of sodium to cesium, potassium to cesium, and rubidium to cesium ratios for the solution. The same method of prediction was also used to estimate the cesium distribution coefficients for two simulated waste solutions prepared at Oak Ridge National Laboratory (Egan, 1993). We also considered the case of predicting distribution coefficients for an incomplete analysis of a solution. The anions with the highest concentrations were found to have the largest effect on the value of &. Hence, excellent estimates
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995 2143 were obtained by using only a limited amount of information on the solution.
If K and qo are known, Kdi can be readily calculated for any equilibrium concentration as follows:
Ion Exchange Equilibrium Ion exchange is a stoichiometric process, where an ion removed from the solution is replaced by another ion from the solid. This is normally represented as follows:
where A and B are ions in the liquid phase, .& and B are ions in the solid phase, and Y A and VB are stoichiometric coefficients and equal to the valence of the counterion. The activities of the cations are related at equilibrium as follows:
Polzer et al. (1992) developed the following equation that relates the Langmuir isotherm parameters to the thermodynamic equilibrium constant, by using Gaines and Thomas’s theory, which satisfies the Gibbs-Duhem equation and implicitly includes the activity coefficients for the solid phase:
In Keq= -vA
where Keqis the thermodynamic equilibrium constant, BA and sig are the activities in the solid phase of
counterions A and B, and a A and CLB are the activities in the liquid phase of A and B. For ion exchange between group I metals, VA and Y B are unity, and eq 2 can be written as the following:
(3) where YA and YB are the activity Coefficients in the solid phase of counterions A and B, YA and YB are the activity coefficients in the liquid phase of A and B, CB and q B are the concentrations of B in the liquid phase and solid phase, and K u is the distribution coefficient of A which is defined as
(4) where QA and CAare the concentrations of A in the solid and liquid phases. The Langmuir isotherm, an alternate method of representing the equilibrium of a cation on the solid with the concentration of the cation in the solution, was utilized successfully by Polzer et al. (1988) to interpret transport of radionuclides in volcanic tuff media. A modified Langmuir isotherm was used in Polzer’s work to include the heterogeneity of the exchanger. Polzer et al. (1992) subsequently related the modified Langmuir isotherm parameters to the “true” thermodynamic constant. The modified Langmuir isotherm has a parameter, p, which describes the heterogeneity of the sorption site (Polzer et al., 1988, 1992) while the Langmuir isotherm assumes uniformly energetic adsorption sites (Holland and Anthony, 1989). Because TAM-5 has a well-defined crystal structure and the Langmuir isotherm was found to be sufficient for summarizing the experimental data, the ion exchange sites for cesium appear to be homogeneous. The Langmuir isotherm is expressed as
where C Bis~ the initial concentration of B in the liquid phase, W N is the solid t o liquid ratio in the system, EB is the initial coverage of the solid, ZA and ZB are the valences of A and B, respectively, and YA and YB are the activity coefficients of A and B in the liquid phase. By using Polzer’s equation, eq 7, and the parameters for the Langmuir isotherm for a given set of conditions, the thermodynamic equilibrium constant for eq 2 can be calculated. Since the thermodynamic equilibrium constant is a function of only temperature, i.e., it does not change with the composition of the liquid phase or the initial coverage of the solid, the Langmuir constant, KL, can be calculated for any set of equilibrium solution compositions, solid t o liquid ratios, and initial coverage by using eq 7. Since the activity coefficients in the liquid phase in eq 7 are functions of composition, errors in the estimates for the liquid phase activity coefficients will cause the thermodynamic constant t o be a function of composition. Bromley’s model for estimating activity coefficients in multicomponent electrolytic solutions was chosen (Bromley, 19731, because of its simplicity and prior application to a wide range of solution compositions. Bromley’s model uses a correction to the Debye-Huckel theory and is stated as follows:
F, = zBiZij2mj j J
,
where q i and Ci are the concentrations of the cation, i, in the solid and liquid phases, respectively, Ki is the Langmuir isotherm constant, and go is the ion exchange capacity.
B.. II =
(0.06
+ 0.6Bg) Izizjl
(11)
(1 + %I)2
where the subscripts i and j denote cations and anions,
2144 Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995
respectively, yi is the activity coefficient of cation i in the liquid phase, A is the Debye-Hiickel constant, which is 0.511 at 25 "C; I is the ionic strength of the liquid phase in molality, zi and zj are the absolute values of the valences of cation i and anion j , respectively, mj is the molality of anionj; and Bo is the Bromley's model parameter. Equations 1-7 are obviously for solutions of two cations ion exchanging with each other. As additional cations are added to the solution the number of equations would increase to include all of the competing cations which will ion exchange with the cation, B, initially on the solid. Because of the high selectivity of TAM-5 for Cs+, we postulated that cesium distribution coefficients could be estimated by assuming that a complex solution could be treated as a pseudo binary solution and by using Bromley's method to calculate the liquid phase activity coefficients. To evaluate this hypothesis, an equilibrium isotherm was developed for cesium cation exchange with sodium in a basic solution with a high concentration of sodium.
Measurement of the Cesium Isotherm for TAM-5 The equilibrium isotherm for ion exchange of cesium in a basic solution with sodium on the solid was determined by using a solution of 5.1 M NaN03, 0.6 M NaOH, and a range of Cs+ concentrations. The equilibration experiments were conducted at 25 "C with 0.1 g of TAM-5 solid in 10 mL of solution. The solution was placed in polypropylene vials to prevent Cs+ from being adsorbed by the containers. Blank tests by adding liquid into the vials without solid were also run to make sure that the vials did not adsorb cesium. The vials containing the mixture of TAM-5 and liquid were shaken (equilibrated) for 24 h. Afterwards, the samples were settled for 12 h and filtered with 0.2 ,um syringe filter and the liquids were analyzed by atomic absorption spectroscopy for the remaining Cs+. Blank tests were also also run to be sure that the filters did not adsorb cesium. The amount of Cs+ ion exchanged onto the solid was calculated by difference by using the following equation:
4 = (C, - C)V/W
(12)
The Langmuir parameters were determined by linear and nonlinear least-mean-squares. The apparent capacity, 40,and the Langmuir isotherm constant,KL,were Umg Cs. The 74 & 4 mglg and (1.38 f 0.02) x calculated Langmuir isotherm and data and are presented in Figure 1.
Prediction of the Cesium Distribution Coefficient for Complex Mixtures Equation 7, activities calculated by using Bromley's method (Bromley, 19731, the equilibrium data, and the Langmuir parameters obtained for the binary solution were used t o calculate a thermodynamic equilibrium constant. The value for Keq in the basic solution was found to be 4.44 x lo4. By utilizing this thermodynamic constant and Bromley's model for the activity coefficients, the Langmuir constant, KL,was calculated for other solutions and experimental conditions by using eq 7, and Kd was calculated by eq 6. The ion exchange capacity, q o , is assumed to be a constant for all solutions. The cesium distribution coefficients were calculated for four types of simulated waste
p
5 .--s a
80% 70
I-
2
60
50
m 40
o E
0 u)
0
.-f 4.-
-
... s LU
u 30 qo = 74+4 mg C s l g
20
K,= (1.38+0.02)~10'~ mLlpgCs
IO
o 0
100 200 300
400
500 600 700 800 900
Equilibrium Cs Concentration in the liquid, C ( vg CslmL )
Figure 1. Langmuir isotherm for a standard solution composed of 5.7 M Na +,5.1 M NOa-, and 0.6 M OH-. Solid line is calculated. Points are the experimental data.
solutions by using the Langmuir equation and compared with experimental values measured by Bray et al. (1993) and Egan (1993). NCAW Simulated Waste Solutions. Three solutions with Na+ concentrations of 1M, 3 M, and 5 M were used as simulated waste solutions for Hanford tank wastes by Bray et al. (1993). The composition of the solution with 5 M Na+ was 5 M Na+, 0.12 M K+, 5.0 x M Rb+, 0.43 M Al(OHI4-, 0.15 M S042-, 1.3 M OH-, 0.23 M C032-, 0.089 M F-, and 0.43 M NOz- and Nos- to balance the charge. The 3 M Na+ and 1M Na+ are simply dilutions of the 5 M Na+ solution. Cs+ concentrations were varied over a wide range to obtain Na:Cs molar ratios of 103-108. Since the variation of the Na:Cs ratio was obtained by varying the cesium concentrations, the KCs and Rb:Cs ratios were also changing over a wide range even though the molar ratio of Na:KRb remained constant at 41.7 for Na:K and lo5 for Na:Rb. In the prediction, we assumed that ion exchange occurs only between Na+ and Cs+ and the effect of competitive exchange with other cations in the liquid was negligible. Of course, this assumption has to fail at some level of K and Rb concentrations. For components NOz- and Al(OH)4- Bromley parameters were unavailable, therefore Bg for these anions were set equal to zero. However, since Bromley's parameters were available for the major components in the simulated waste solutions, the error introduced by setting Bo = 0 for NOz- and Al(0Hk- should be small. A comparison of experimentally determined cesium distribution coefficients with predicted distribution coefficients for the NCAW simulant is shown in Figure 2. For the 1 M NCAW simulant, the calculated values are within 5 % of the experimental values. At the low levels of cesium concentrations, i.e. high NdCs ratio, the calculated value is slightly greater than the experiment values. This difference in the calculated and experimental data increases a t the higher sodium concentrations with the predicted values being about 25% higher than the experimental values at NdCs ratios greater than 60 000. The concentrations of cesium for the 5 and 3 M Na NCAW simulants for NdCs ratios greater than 60 000 are less than 8.33 x M (11.1mg/L) and 5 x lop5 (6.66 mg/L). However, for NdCs molar ratios less than 60 000 and Cs concentrations greater than 10 mg/L the predicted and experimental values are almost equal. The increase of the cesium distribution coefficient with the decrease of the sodium concentration observed experimentally was also predicted.
Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995 2145 1E+5 +,
S
.-Q
2
1E+4
Q
1E+5
I
I
I
C
.-QC
..
1E+4
3M Na'
3M Na*
.- ., .. ..- . . .
u)
b'
0"
1E+2
E
l
1E+1 I 1E+3
,
'
'
.~
1E+4
,
'-, 1E+5
I
~
~
1E+6
~
' ~, ' .
',
1E+7
'
'-. ,
~
1E+8
J
1E+9
Equilibrium NalCs Mole Ratio Figure 2. Prediction of distribution coefficients for NCAW simulated waste solutions by using Bromley's model for estimation of activity coefficients in the liquid phase. Points are experimental data (Bray et al., 1993). Lines are theoretical predictions performed by using the Langmuir isotherm obtained for a solution with 5.1 M NaN03 and 0.6 M NaOH, Figure 1. Solid lines assume Bromley's model parameter, B,, to be zero for N(OH)4- and Nos-. Dashed lines assume the anions in the liquid are only OH- and Nos-.
u"
1E+2
t1
1Et1 1E+3
,,,
'
1
' '
1E+4
1E+5
1E+6
1E+7
1Et9
Equilibrium NalCs Mole Ratio Figure 3. Prediction of distribution coefficients for NCAW simulated waste solutions by using Bromley's model for estimation of activity coefficients in the liquid phase. Points are experimental data (Bray et al., 1993).Lines are theoretical predictions performed by using the Langmuir isotherm data obtained from 5 M Na+ NCAW simulated waste solution. Table 1. Activity Coefficients of Cs+ and Na+ in NCAW Solution Based on Bromley's model" yt for
The first objective to determine if satisfactory predictions of cesium distribution coefficients could be obtained by using data from simple simulants was achieved. Furthermore, even though the predicted values for the high Na:Cs and low Cs concentrations were high for sodium concentrations of 5 and 3 M, the error was less than 25%, and the correct order of magnitude was predicted. To evaluate the effect of the assumption of B, = 0 for A(OH)4- and NOa-, Bromley's coefficients for these anions were assumed t o be equal to the concentration of NOa-. Such a substitution is equivalent to assuming that the A(OH)4- and NO2- anions are not in the solution. The calculated Kd'S were 2%, 4%, and 8% lower than those obtained when assuming By's to be zero for the 1 M, 3 M, and 5 M Na+ NCAW solutions respectively. Hence, the assumption that B, for NOzand Al(OH)4- is small is a valid assumption for NCAW solutions, since the predicted values are reasonably close to the experimental data. Finally, t o determine the effect of the other anions in the solution on prediction of the cesium distribution coefficient, the simulants were assumed t o contain only the anions Nos- and OH-. This assumption is equivalent to assuming the total composition of the solution is unknown, but the group 1 cation concentrations and pH of the solution are known. As illustrated in Figure 2, excellent results were obtained. Obviously, the above results suggest that concentration dependence is built into the thermodynamic constant determined from the simple simulant. Therefore, Bray et al.'s data for the 5 M sodium isotherm were used to calculate a new thermodynamic constant. A value of 2.87 x lo4 was obtained, which is 35% less than the value obtained from 5.1 M NaN03 and 0.6 M NaOH solution. With this constant, estimates were made of the cesium distribution coefficients for the 1 and 3 M simulants, and these results are presented in Figure 3. Of course the data for the 5 M Na fits well because of the curve fitting t o determine Kl, and as expected, the predictions of the distribution coefficients for the 3 M and 1 M solutions are within 3% and 10% of the experimental data.
1E+8
method of calculation assumingB,'s for N(OH)4and NOz- to be zero assumingB,'s for Al(OH)4and Nos- to be that Of NO3assuming the anions in the liquid are 1.3 M OHand Nos- to balance the charge
y Lfor 3M Na+
yi for 1M Na+
0.390 0.493 0.360 0.481
0.528 0.571 0.514 0.566
Na+
5M Na+ 0.314 0.470 0.271 0.451
Cs+ Na+
0.183 0.288 0.476 0.432 0.469 0.562
component, i Cs+ Na+
Cs+
a NCAW simulants: 5 M Na', 0.12 M K+, 5.0 x M Rb+, 0.43 M N(OH)4-, 0.15 M SO&, 1.3 M OH-, 0.23 M co32-,0.089 M F-, 0.43 M NOz- and Nos- to balance the charge. 1 M and 3 M Na were obtained by diluting the 5 M simulant.
The activity coefficients for Cs+ and Na+ calculated by using Bromley's model for the NCAW solutions with A(OH)4-, Sch2-, OH-, c0a2-,F-, NO2- and NOS- are presented in Table 1. These values, as well as their ratios, are obviously significantly different from 1, hence, activity coefficients are required to obtain accurate predictions of the performance of TAM-5 in strong electrolytic solutions. Table 1 also shows that the activity coefficient ratio of Cs+ and Na+ decreases if more anions are considered as Nos-. As expected, the activity coefficient ratio of Cs+ and Na+ approaches unity as the solution is diluted. When the ratio of activity coefficients of sodium to cesium increases, the cesium distribution coefficient decreases. The fact that a Bromley's parameter is not available for either NO2- or A(OHl4- anions apparently results in a low estimate of this ratio which causes a high prediction for the cesium distribution coefficient, Bromley et al. (1973) determined their model parameters for systems with ionic strengths less than 6 molal (m). The ionic strength of the solution used in the isothermal experiments was 6.9 m, and the total ionic strengths of the solutions were greater than 6 m in some cases. Nevertheless, the predictions of the distribution coefficients for 1 M Na+ are excellent over the entire experimental range and the predictions for 3 M Na+ and 5 M Na+ are also quite good even though the predicted distribution coefficients are greater than the experi-
2146 Ind. Eng. Chem. Res., Vol. 34,No. 6,1995 Table 2. Cs+ Ion Exchange Distribution Coefficients in the Solution with Increasing Amounts of Kfa K+ (M)
0
0.0022
0.022
0.22
0.40
K+:Cs+(molarratio) Kd ( m u g ) Kd ( m u g ) predicted
0 1861 2233
115 1775 2233
1146 1545 2215
11458 1033 2356
20833 990 2461
Solution consisted of 5.05 M Na+, 0.25 M A13+,1.95 M NOa-, 2.55 M OH-, 0.05 M SO&, 1.1M NOz-, 0.105 M co32-. K+ was added as KNo3 in the second and third samples, and as KOH in the forth and fifth samples.
mental values. This deviation between predicted and experimental values may be due to competitive ion exchange of K+ and Rb’. The Na+:Cs+ratio was varied by varying the concentration of Cs+ in the solution for each of the sodium concentrations at constant concentrations of the other components. Therefore, as Na+: Cs+ increases, the ratios of the other cations to cesium also increase, and for Rb:Cs and K C s the molar ratios covered the range from 0.017t o 8560 and 18 to 9 x lo6, respectively. Experiment To Evaluate the Effect of Rubidium and Potassium on Cesium Distribution Coefficients. In independent experiments we have shown that for K+:Cs+ ratios of 0-20833 the competitive exchange with K+ reduces the cesium distribution coefficient, Kd, from 1861 to 990 m u g . The simulated waste solution used for these experiments was prepared by mixing 10.20 g of NaOH, 9.378 g of Al(N03)3*9H20, 0.071g of Na2S04, 10.20g of NaN03, 7.5 g of NaNOz, 1.129g of Na2C03, 0.3 mL of 1000 ,ug/mL Cs, and 100 mL of deionized, distilled water. The potassium content was increased by adding KN03 or KOH. If all of the species ionized then the molar concentrations would be 5.05 M Na+, 0.25 M A13+, 1.95 M NOS-, 2.55 M OH-, 0.05 M so&, 1.1 M NOz-, 0.105 M cos2-, x m 0 3 or xKOH, and 2.26 x M (3pg/mL) of Cs+. However, probably complexes with OH- in a molar ratio of 4:l OH-:Al, which would reduce the free OH- to 1.55 M. Activity coefficients were based on a solution with 1.55 M OH-, and the experimental and calculated results for increasing the K+ concentration and ratios of K C s are presented in Table 2. This experiment showed that when the K+ concentration was increased to 0.022 M, the Kd for cesium decreased 16%. When K+ reached a concentration of about 0.1 M, the Kd of cesium decreased dramatically. Since the model does not include the effect of potassium, the calculated values of the distribution coefficients should not decrease with the addition of potassium to the solution. The predicted value increases because of the change in activity coefficients caused by increasing the concentrations of NO3and OH- ions by the addition of potassium nitrate or potassium hydroxide, which causes slight changes in
concentration. By considering the similarity of this solution with the NCAW, one can explain that the excellent predictions of cesium distribution coefficients for the 1 M Na+ NCAW are due to the concentration of K+ being diluted to 0.024M. The predicted Kd’s are 25% greater than the experimental data in 3 M and 5 M Na+ NCAW. However, if the ratio of activity coefficients of sodium to cesium increases, the cesium distribution coefficient decreases. The fact that a Bromley’s parameter is not available for either Nos- or Al(OH)4- anions could result in a low estimate of this ratio, which results in the high prediction for the cesium distribution coefficient. Independent experiments to determine the effect of competitive exchange with Rb+ were conducted by using our standard simulated solution, i.e. 5.7 M Na+, 0.6 M OH-, and 7.5 x M Cs+ (100 pg/mL), with the addition of rubidium. The cesium distribution coefficient decreased from 855 to 47 m u g for molar ratios of Rb+:Cs+ of 1.56-156. Since the model does not include the effect of Rb+, no calculations were performed for predictions of Kd when Rb+ is in the solution. Even though the range of K.Cs and Rb:Cs ratios is significant for the NCAW solution, the competitive exchange of K and Rb with Cs in the NCAW solution had very little effect on the cesium distribution coefficients. Comparison of the results for the independent experiments and the data on the NCAW experiments shows that the solution composition, absolute as well as relative concentrations, has a significant effect on the measured as well as predicted distribution coefficients. By considering the results from the independent experiments with K and Rb and the results obtained for the NCAW simulant when the (K+ Al(OH)4-):Cs molar ratio is greater than 6000 and the cesium concentration is less than 11 mg/L, the estimates of the distribution coefficients for these solutions using data obtained from simple simulants will be greater than the values obtained experimentally. As stated earlier, one would expect the competitive exchange with Rb and K to have an effect at some ratio or level of Rb:Cs and K C s . The inclusion of the effect of the aluminum tetrahydroxide anion on the prediction is due to the unavailability of Bromley’s parameter for this anion. However, when a thermodynamic constant is evaluated from an isotherm performed with the simulant matrix, excellent estimates for cesium distribution coefficients are obtained as indicated in Figure 3. Oak Ridge Simulated Waste Solutions. Egan (1993)reported cesium distribution coefficients for two simulated waste solutions with the following sets of compositions: (1)a storage tank supernate with 0.24 M NaOH, 0.14 M Na2C03, 3.9 M NaN03, 0.1 M NaC1, 0.24M KNO3, 0.005M Al(N0313, 0.0001M CaC03, 0.001
+
Table 3. Cs’ Ion Exchange Distribution Coefficients Collected by Oak Ridge National Laboratory in Three Simulated Waste Solutions and the Predicted Results
tvpe of solution: conditions of experiment molar ratio K C s molar ratio Rb:Cs measured, Kd, m u g predicted, Kd, m u g (using complete analysis) predicted, Kd, m u g (assuming anions are OH- and Nos-)
storage tank supernate” 5.0 mg solid in 10 mL solution 0 0
0.1 mg solid in 10 mL solution 10
10
500 1479 1253
1200 1487 1259
newly generated wasteb 5.0 mg solid in 10 mL solution 0 0 0 0 1.7 x 104 2.8 x 104 1.61x 104 1.97 x 104 1.14 x 104 1.23 x 104
0.1 mg solid in 10 mL solution
A storage tank supernate with 0.24 M NaOH, 0.14 M Na2C03, 3.9 M NaN03, 0.1 M NaCl, 0.24 M KNo3, 0.005 M Al(NO&, 0.0001 M CaC03, 0.001 M Zn(NOs)z,6.2 x lo-@M Sr2+,and 5.2 x M Cs+. A “newly generated” waste labeled as NGLLLW with 0.335 M NaOH, 0.587 M NazC03, 0.061 M NaN03, 0.034 M NaC1, 0.025 M LiC1, 0.0117 M NaAlOz, 6.4 x M Sr2+,and 8.7 x M Cs+.
Ind. Eng. Chem. Res., Vol. 34, No. 6, 1995 2147 M Zn(NO&, 6.2 x M Sr2+,and 5.2 x M Cs+ and (2) a “newly generated” waste labeled as NGLLLW with 0.335 M NaOH, 0.587 M Na2C03,0.061 M NaN03, 0.034 M NaC1, 0.025 M LiC1, 0.0117 M NaA102, 6.4 x lop6M Sr2+,and 8.7 x M Cs+. Experimental and the predicted distribution coefficients are presented in Table 3. The “newly generated” waste simulant does not contain K+ or Rb+, and the prediction of Kd is within the experimental error of the experiments. The concentration of aluminum in this simulant is low compared to cesium, and the competitive ion exchange of SrOH+ is apparently negligible compared to experimental error in measuring cesium distribution coefficients. As expected, when competitive exchange with K exists, such as in the storage tank supernate solution, the predicted Kd is higher than the observed Kd, because the model does not take into account the competitive effect. If only NO3- and OH- are used in the calculation of the activity coefficients, the predicted values of Kd decrease. Further work is required to determine the effect of using only a limited amount of information for the prediction of the distribution coefficients. Future work is directed toward incorporation of the effect of the competitive ion exchange of Rb, K, Na, and Cs.
Conclusions By measuring the Langmuir isotherm in a simple solution, cesium distribution coefficients with a high degree of accuracy can be predicted for complex electrolytic solutions for a variety of solution concentrations. When potassium and aluminum are in the complex simulant then the method fails if (K+ Al(OH)4-):Cs molar ratios are greater than 6000 and the cesium concentration is less than 11mg/L. However, if cesium distribution coefficients are measured in a complex solution, then the method can be used t o estimate a “thermodynamic constant” for that simulant and then use it to successfully calculate distribution Coefficients for more or less concentrated solutions if the ratios of the matrix elements are constant. When this is done the competitive effect of other cations with cesium is implicitly accounted for in the predictions. Activity coefficients for cesium and sodium must be calculated, and Bromley’s model for predicting these coefficients is adequate for predicting good values of the distribution Coefficients, even though in some cases Bromley’s parameters were used beyond the conditions for which they were initially determined. Furthermore, only the parameters for sodium, cesium, and dominant anions, which are nitrate and hydroxide ions in most of the waste, need t o be used in the estimation of the activity coefficients. Polzer et al.’s (1992) method and Bromley’s equations have been extended for estimating with a reasonable degree of accuracy the performance of TAM-5 for complex simulants by using a limited amount of experimental data. Future work is planned for improving the methods for predicting the performance of TAM-5 in multicomponentcomplex electrolytic solutions including the competitive effect of potassium and rubidium.
+
Acknowledgment This work was performed at Texas A&M University and Sandia National Laboratories. The work at Texas A&M was funded by Sandia National Laboratories under Texas A&M Research Foundation contract number RF8350, and Sandia National Laboratories is supported by the U. s. Department of Energy under contract number DE-AC04-94AL85000.
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Received for review April 7 , 1995 Accepted April 12, 1995 @
IE940473C Abstract published in Advance A C S Abstracts, May 15, 1995. @
