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The sorption of rare earths on anion exchange resins from nitric acid-mixed solvent systems
J. Inorg. Nucl. Chem., 1964, Vol. 26, pp. 859 to 864. Pergamon Press Ltd. Printed in Northern Ireland
THE SORPTION OF RARE EARTHS ON ANION EXCHANGE RESINS FROM NITRIC ACIDMIXED SOLVENT SYSTEMS L. W. MARPLE Department of Chemistry, Iowa State University, Ames, Iowa (Received 14 October 1963) Abstract--The sorption of lanthanum, dysprosium, and ytterbium on anion exchange resins from nitric acid-isopropyl alcohol-water mixtures was studied in detail. The distribution function of lanthanum shows a maximum that appears to be caused by the invasion of isopropyl alcohol into the resin phase at high mole fractions of alcohol. This is supported by indirect evidence obtained from the effects of polarity of organic solvent, temperature, cross-linkage of resin, and complexing agent on the distribution functions. The size of the maximum decreases as the atomic number increases, and as a result, large differences in distribution coefficients are obtained at high mole fractions of alcohol. The separation of the rare earths by use of Amberlite IRA 400 resin and 0-5 M HNO3-95 per cent isopropyl alcohol eluent is suggested.
PREVIOUS research on the sorption of uranium, thorium, ~1~ and lead ~ from nitric acid solutions on Dowex l-X8 showed that the distribution of these elements could be interpreted in terms of a distribution of a neutral nitrate complex between the resin and solution phases. It was of interest then to find whether or not the distribution of the rare earths between an anion exchange resin and nitric acid-water-alcohol mixtures could be interpreted in the same fashion. Preliminary experiments showed that a linear Log D vs. mole fraction alcohol relationship is observed for the rare earths, but only at mole fractions less than 0.6. Above this, particularly for the light rare earths, an unusual decrease in D occurs as the amount of alcohol is increased. The importance of this behaviour in separations involving rare earths was recognized, and an effort was made to characterize the distribution functions over as wide a range of solvent composition as possible using dioxane, ethyl alcohol and isopropyl alcohol mixed solvent systems. EXPERIMENTAL Data for the distribution coefficients of lanthanum and dysprosium were taken from the work of PiETRZVK~3~ Distribution data for ytterbium in HCl-ethyl alcohol mixtures were obtained by the method used by PmTRZYK.c~ Anion exchange data for the rare earths in HNOs-isopropyl alcohol mixtures were obtained by equilibration of 0"25 mmoles metal ion with 1.00 g resin in a volume of approximately 50 ml. Volume contractions upon mixing measured amounts of solvents were determined and used in the calculation of the distribution coefficients. Analysis of the solution phases of the equilibrated samples was performed by E D T A titration at pH 5.2 using Naphthyl Azoxine S indicator. Both Dowex 1-X8 and Amberlite I R A 400 resins were converted to the nitrate form (when equilibrations with nitric acid were performed) washed, and air dried before use. All solvents were commercial reagent grade chemicals. tll L. W. MARPLE, J. Inorff. Nucl. Chem. 26, 635 (1964). t21 L. W. MARPLE, J. Inorff. NucL Chem. 26, 643 (1964). c3~ D. J. PIETRZY)C,Ph.D. Thesis, Iowa State University (1960). 13
859
860
L.W. MARPLE
RESULTS AND DISCUSSION Lanthanum, dysprosium and ytterbium were chosen as representative rare earths for this investigation, since FARIS and WARTONca) have already shown that the distribution functions of the rare earths in nitric acid-alcohol media form a regular sequence.
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,~oLt r~Acr,o, ALCO,OL FIG. l.--Distributions functions for lanthanum, dysprosium and ytterbium in 0.5 M HNOs-isopropyl alcohol-water mixtures. Dowex l-X8 resin in nitrate form © Lanthanum [] Dysprosium • Ytterbium The results of the distribution functions (distribution coefficients as a function of mole fraction alcohol) for these rare earths in water-isopropyl alcohol media at a constant acidity of 0.5 M HNOz are shown in Fig. 1. One interesting feature is that the distribution function of lanthanum shows a maximum at a mole fraction of 0.6, while ytterbium shows only a slight maximum above a mole fraction of 0.8. This, and the fact that the distribution coefficients decrease with increasing atomic number (below a mole fraction of 0.6), suggests c4, j. p. FARISand J. W. WARTON,,4nalyt. Chem. 33, 1265 (1961).
The sorption of rare earths on anion exchange resins from nitric acid-mixed solvent systems 861
that the controlling factor in the distribution is the extent of hydration of the complex species that is adsorbed. Specifically, as water in the resin phase is replaced by the organic solvent, those complexes that are weakly hydrated begin to be excluded from the resin phase. Unfortunately, it is very difficult to obtain experimental evidence to directly support the above argument. However, certain indirect evidence has been obtained by variation of certain experimental variables. These results are discussed below.
Effect of polarity of the non-aqueous solvent For solvents with polarity roughly equivalent to that of isopropyl alcohol, one would expect to find the maximum of the distribution function at about the same mole fraction of solvent as was found for isopropyl alcohol. On this basis the aliphatic alcohols should show similar distribution functions compared to isopropyl alcohol. In solvents with low polarity, the distribution functions should be markedly different, particularly in the region of low mole fraction alcohol. The reason for this is that the lower the polarity, the less the free energy change of the solvated species with change in mole fraction solvent. The distribution function maximum should also be displaced toward a higher mole fraction of solvent, since solvents of low polarity are less preferred to those of high polarity by the ion exchange resin. A comparison of the distribution functions in ethyl alcohol-water and dioxanewater mixtures with isopropyl alcohol-water mixtures at an overall acidity of 0.5 M nitric acid is shown in Fig. 2. As was expected, the distribution function in ethyl alcohol-water mixtures was very similar to that in isopropyl alcohol-water mixtures. For dioxane-water mixtures, not only is a change of slope observed in the 0.25 to 0.6 mole fraction region, but the maximum in the distribution function is shifted as well. Evidently, the dielectric constant of the solvent mixtures has little influence on the sorption. On the basis of the extent of association of lanthanum ion with nitrate ion, one would expect greater sorption from dioxane solutions than from isopropyl alcohol solutions, and the reverse is actually observed.
Effect of temperature As pointed out by HELFFERICHC5), the effect of temperature on sorption equilibria is complex. In the case of the rare earths, there is no reason to expect a large temperature dependence. However, if an association reaction of the type La(NOa)3-~- HNOa = HLa(NOa) 4 was responsible for the decrease in the distribution coefficients at high mole fractions, then increasing the temperature might be expected to lessen the decrease. The data for the sorption of lanthanum from 0"5 M nitric acid isopropyl alcohol-water mixtures are shown in Fig. 2, and do not indicate that the maximum decreases with increasing temperature. The slight effect of temperature that was observed also shows that the maximum in the distribution function is not due to a change in the degree of dissociation of nitric acid as the alcohol percentage becomes large. This same conclusion can also be inferred from the fact that under similar conditions, the distribution function of 15~ F. HELFFERICH,Ion Exchange, p. 132. McGraw-Hill, New York (1962).
862
L.W. MARPLE
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FIG. 2.--Distribution functions for lanthanum A. (3 0"5 M HNOs-isopropyl alcohol-water solvent, Dowex l-X8 resin. B. [] 0"5 M HNO3-ethyl alcohol-water solvent, Dowex l-X8 resin. C. zx 0"5 M HNOs-dioxane-water solvent, Dowex l-X8 resin. D. • 0-5 M HNOa-isopropyl alcohol-water solvent, Dowex l-X8 resin, T-42°C E. • 0"5 M HNOa-isopropyl alcohol-water solvent, Amberlite-IRA 400 resin. lanthanum shows a large m a x i m u m while that of ytterbium does not. A change in the degree of dissociation would lead to simultaneous changes in the distribution functions of both elements. This should not be interpreted to say that no change in the dissociation of nitric acid occurs at high mole fractions alcohol. Conductance measurements indicate that the dissociation of nitric acid changes significantly at high mole fractions of alcohol although the extent of dissociation is still very high.
Effect of cross linkage of resin I f a displacement of a solute was occurring upon the invasion of a sufficient amount of organic solvent into the resin phase, then the use of a resin of lower crosslinkage than Dowex l-X8 should enhance the displacement of the solute. The basis of this argument is that resins of low crosslinkage show less preferential sorption of water than resins of high crosslinkage.
The sorption of rare earths on anion exchange resins from nitric acid-mixed solvent systems 863 Experiments using Amberlite IRA-400 show the expected decrease in distribution coefficients at high mole fraction alcohol, in comparison to Dowex l-X8. Curve E, Fig. 2, compared to Curve A shows no decrease in distribution coefficients at low mole fractions, a two fold decrease at the maximum of the distribution function, and a fivefold decrease at a mole fraction of 0.87. This trend is in excellent agreement with that expected on the basis of invasion of isopropyl alcohol into the resin. I00
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FIG. 3.--Distribution functions for lanthanum, dysprosium and ytterbium in 0-3 M HCl--ethyl alcohol-water mixtures. (3 Lanthanum
[] Dysprosium A Ytterbium
Effect of complexing agent The complexation of lanthanum with nitrate ion is certainly not extensive. Likewise, the complexation of lanthanum with other weakly complexing ligands, such as chloride, sulphate, etc., could not be expected to be extensive. Consequently, one would expect similar behaviour of the distribution functions of the rare earths in chloride or sulphate media as found for nitrate solutions. Distribution coefficients over a range of 70-95 per cent ethyl alcohol at an overall acidity of 0.3 M HC1 are shown in Fig. 3. Unfortunately, the rare earths show little sorption by the resin in this system. However, a maximum in the distribution function of lanthanum was observed, and there is some indication of a maximum for dysprosium. The occurrence of a maximum in the distribution function of lanthanum shows that the maximum itself is not specific to systems involving nitrate ion as a
864
L.W. MARPLE
complexing agent. The fact that the same relationship exists between the rare earths in both nitrate and chloride media would appear to be good evidence that the invasion of the non-aqueous solvent into the resin phase is taking place at very high mole fractions of non-aqueous soqvent. Presumably, the use of other systems where complexing and sorption would be more extensive (i.e., in HCl-isopropyl alcohol-water media) would lead to similar results, and thus this problem was not pursued further. Out of all the solvents and complexing agents investigated, it is evident that the use of nitric acid-isopropyl alcohol-water mixtures holds the most promise for use irl the separation of the rare earths, in particular the "heavy" rare earths. Whereas FARISand WARTON(4) and more recently KORKISCHe t al. (6) found little or no separation in the distribution functions of the rare earths gadolinium to lutetium in alcoholic media, good separation of the dysprosium and ytterbium functions is observed in 95 per cent isopropyl alcohol-0.5 M nitric acid solutions. Although further work is necessary, it would seem that the use of a nitric acid-isopropyl alcohol-water system and Amberlite IRA 400 or similar low cross linked resin would give the optimum separation of the rare earths. Further work is in progress on this problem. (6~j. KORKISCH,I. HAZANand G. ARRHENIUS,Talanta 10, 865 (1963).
THE SORPTION OF RARE EARTHS ON ANION EXCHANGE RESINS FROM NITRIC ACIDMIXED SOLVENT SYSTEMS L. W. MARPLE Department of Chemistry, Iowa State University, Ames, Iowa (Received 14 October 1963) Abstract--The sorption of lanthanum, dysprosium, and ytterbium on anion exchange resins from nitric acid-isopropyl alcohol-water mixtures was studied in detail. The distribution function of lanthanum shows a maximum that appears to be caused by the invasion of isopropyl alcohol into the resin phase at high mole fractions of alcohol. This is supported by indirect evidence obtained from the effects of polarity of organic solvent, temperature, cross-linkage of resin, and complexing agent on the distribution functions. The size of the maximum decreases as the atomic number increases, and as a result, large differences in distribution coefficients are obtained at high mole fractions of alcohol. The separation of the rare earths by use of Amberlite IRA 400 resin and 0-5 M HNO3-95 per cent isopropyl alcohol eluent is suggested.
PREVIOUS research on the sorption of uranium, thorium, ~1~ and lead ~ from nitric acid solutions on Dowex l-X8 showed that the distribution of these elements could be interpreted in terms of a distribution of a neutral nitrate complex between the resin and solution phases. It was of interest then to find whether or not the distribution of the rare earths between an anion exchange resin and nitric acid-water-alcohol mixtures could be interpreted in the same fashion. Preliminary experiments showed that a linear Log D vs. mole fraction alcohol relationship is observed for the rare earths, but only at mole fractions less than 0.6. Above this, particularly for the light rare earths, an unusual decrease in D occurs as the amount of alcohol is increased. The importance of this behaviour in separations involving rare earths was recognized, and an effort was made to characterize the distribution functions over as wide a range of solvent composition as possible using dioxane, ethyl alcohol and isopropyl alcohol mixed solvent systems. EXPERIMENTAL Data for the distribution coefficients of lanthanum and dysprosium were taken from the work of PiETRZVK~3~ Distribution data for ytterbium in HCl-ethyl alcohol mixtures were obtained by the method used by PmTRZYK.c~ Anion exchange data for the rare earths in HNOs-isopropyl alcohol mixtures were obtained by equilibration of 0"25 mmoles metal ion with 1.00 g resin in a volume of approximately 50 ml. Volume contractions upon mixing measured amounts of solvents were determined and used in the calculation of the distribution coefficients. Analysis of the solution phases of the equilibrated samples was performed by E D T A titration at pH 5.2 using Naphthyl Azoxine S indicator. Both Dowex 1-X8 and Amberlite I R A 400 resins were converted to the nitrate form (when equilibrations with nitric acid were performed) washed, and air dried before use. All solvents were commercial reagent grade chemicals. tll L. W. MARPLE, J. Inorff. Nucl. Chem. 26, 635 (1964). t21 L. W. MARPLE, J. Inorff. NucL Chem. 26, 643 (1964). c3~ D. J. PIETRZY)C,Ph.D. Thesis, Iowa State University (1960). 13
859
860
L.W. MARPLE
RESULTS AND DISCUSSION Lanthanum, dysprosium and ytterbium were chosen as representative rare earths for this investigation, since FARIS and WARTONca) have already shown that the distribution functions of the rare earths in nitric acid-alcohol media form a regular sequence.
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Jc
O
.
.
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.
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.
.~
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,~oLt r~Acr,o, ALCO,OL FIG. l.--Distributions functions for lanthanum, dysprosium and ytterbium in 0.5 M HNOs-isopropyl alcohol-water mixtures. Dowex l-X8 resin in nitrate form © Lanthanum [] Dysprosium • Ytterbium The results of the distribution functions (distribution coefficients as a function of mole fraction alcohol) for these rare earths in water-isopropyl alcohol media at a constant acidity of 0.5 M HNOz are shown in Fig. 1. One interesting feature is that the distribution function of lanthanum shows a maximum at a mole fraction of 0.6, while ytterbium shows only a slight maximum above a mole fraction of 0.8. This, and the fact that the distribution coefficients decrease with increasing atomic number (below a mole fraction of 0.6), suggests c4, j. p. FARISand J. W. WARTON,,4nalyt. Chem. 33, 1265 (1961).
The sorption of rare earths on anion exchange resins from nitric acid-mixed solvent systems 861
that the controlling factor in the distribution is the extent of hydration of the complex species that is adsorbed. Specifically, as water in the resin phase is replaced by the organic solvent, those complexes that are weakly hydrated begin to be excluded from the resin phase. Unfortunately, it is very difficult to obtain experimental evidence to directly support the above argument. However, certain indirect evidence has been obtained by variation of certain experimental variables. These results are discussed below.
Effect of polarity of the non-aqueous solvent For solvents with polarity roughly equivalent to that of isopropyl alcohol, one would expect to find the maximum of the distribution function at about the same mole fraction of solvent as was found for isopropyl alcohol. On this basis the aliphatic alcohols should show similar distribution functions compared to isopropyl alcohol. In solvents with low polarity, the distribution functions should be markedly different, particularly in the region of low mole fraction alcohol. The reason for this is that the lower the polarity, the less the free energy change of the solvated species with change in mole fraction solvent. The distribution function maximum should also be displaced toward a higher mole fraction of solvent, since solvents of low polarity are less preferred to those of high polarity by the ion exchange resin. A comparison of the distribution functions in ethyl alcohol-water and dioxanewater mixtures with isopropyl alcohol-water mixtures at an overall acidity of 0.5 M nitric acid is shown in Fig. 2. As was expected, the distribution function in ethyl alcohol-water mixtures was very similar to that in isopropyl alcohol-water mixtures. For dioxane-water mixtures, not only is a change of slope observed in the 0.25 to 0.6 mole fraction region, but the maximum in the distribution function is shifted as well. Evidently, the dielectric constant of the solvent mixtures has little influence on the sorption. On the basis of the extent of association of lanthanum ion with nitrate ion, one would expect greater sorption from dioxane solutions than from isopropyl alcohol solutions, and the reverse is actually observed.
Effect of temperature As pointed out by HELFFERICHC5), the effect of temperature on sorption equilibria is complex. In the case of the rare earths, there is no reason to expect a large temperature dependence. However, if an association reaction of the type La(NOa)3-~- HNOa = HLa(NOa) 4 was responsible for the decrease in the distribution coefficients at high mole fractions, then increasing the temperature might be expected to lessen the decrease. The data for the sorption of lanthanum from 0"5 M nitric acid isopropyl alcohol-water mixtures are shown in Fig. 2, and do not indicate that the maximum decreases with increasing temperature. The slight effect of temperature that was observed also shows that the maximum in the distribution function is not due to a change in the degree of dissociation of nitric acid as the alcohol percentage becomes large. This same conclusion can also be inferred from the fact that under similar conditions, the distribution function of 15~ F. HELFFERICH,Ion Exchange, p. 132. McGraw-Hill, New York (1962).
862
L.W. MARPLE
o
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10
0
,I
.2
.3 MOLE
.4
.5
.6
.7
.8
.9
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FRAGTION
FIG. 2.--Distribution functions for lanthanum A. (3 0"5 M HNOs-isopropyl alcohol-water solvent, Dowex l-X8 resin. B. [] 0"5 M HNO3-ethyl alcohol-water solvent, Dowex l-X8 resin. C. zx 0"5 M HNOs-dioxane-water solvent, Dowex l-X8 resin. D. • 0-5 M HNOa-isopropyl alcohol-water solvent, Dowex l-X8 resin, T-42°C E. • 0"5 M HNOa-isopropyl alcohol-water solvent, Amberlite-IRA 400 resin. lanthanum shows a large m a x i m u m while that of ytterbium does not. A change in the degree of dissociation would lead to simultaneous changes in the distribution functions of both elements. This should not be interpreted to say that no change in the dissociation of nitric acid occurs at high mole fractions alcohol. Conductance measurements indicate that the dissociation of nitric acid changes significantly at high mole fractions of alcohol although the extent of dissociation is still very high.
Effect of cross linkage of resin I f a displacement of a solute was occurring upon the invasion of a sufficient amount of organic solvent into the resin phase, then the use of a resin of lower crosslinkage than Dowex l-X8 should enhance the displacement of the solute. The basis of this argument is that resins of low crosslinkage show less preferential sorption of water than resins of high crosslinkage.
The sorption of rare earths on anion exchange resins from nitric acid-mixed solvent systems 863 Experiments using Amberlite IRA-400 show the expected decrease in distribution coefficients at high mole fraction alcohol, in comparison to Dowex l-X8. Curve E, Fig. 2, compared to Curve A shows no decrease in distribution coefficients at low mole fractions, a two fold decrease at the maximum of the distribution function, and a fivefold decrease at a mole fraction of 0.87. This trend is in excellent agreement with that expected on the basis of invasion of isopropyl alcohol into the resin. I00
I0 u') <[
I 0
|
I
I
I
.I
.2
.3
.4
MOLE
.5
FRACTION
.6
.7
.8
.9
I.O
ALCOHOL
FIG. 3.--Distribution functions for lanthanum, dysprosium and ytterbium in 0-3 M HCl--ethyl alcohol-water mixtures. (3 Lanthanum
[] Dysprosium A Ytterbium
Effect of complexing agent The complexation of lanthanum with nitrate ion is certainly not extensive. Likewise, the complexation of lanthanum with other weakly complexing ligands, such as chloride, sulphate, etc., could not be expected to be extensive. Consequently, one would expect similar behaviour of the distribution functions of the rare earths in chloride or sulphate media as found for nitrate solutions. Distribution coefficients over a range of 70-95 per cent ethyl alcohol at an overall acidity of 0.3 M HC1 are shown in Fig. 3. Unfortunately, the rare earths show little sorption by the resin in this system. However, a maximum in the distribution function of lanthanum was observed, and there is some indication of a maximum for dysprosium. The occurrence of a maximum in the distribution function of lanthanum shows that the maximum itself is not specific to systems involving nitrate ion as a
864
L.W. MARPLE
complexing agent. The fact that the same relationship exists between the rare earths in both nitrate and chloride media would appear to be good evidence that the invasion of the non-aqueous solvent into the resin phase is taking place at very high mole fractions of non-aqueous soqvent. Presumably, the use of other systems where complexing and sorption would be more extensive (i.e., in HCl-isopropyl alcohol-water media) would lead to similar results, and thus this problem was not pursued further. Out of all the solvents and complexing agents investigated, it is evident that the use of nitric acid-isopropyl alcohol-water mixtures holds the most promise for use irl the separation of the rare earths, in particular the "heavy" rare earths. Whereas FARISand WARTON(4) and more recently KORKISCHe t al. (6) found little or no separation in the distribution functions of the rare earths gadolinium to lutetium in alcoholic media, good separation of the dysprosium and ytterbium functions is observed in 95 per cent isopropyl alcohol-0.5 M nitric acid solutions. Although further work is necessary, it would seem that the use of a nitric acid-isopropyl alcohol-water system and Amberlite IRA 400 or similar low cross linked resin would give the optimum separation of the rare earths. Further work is in progress on this problem. (6~j. KORKISCH,I. HAZANand G. ARRHENIUS,Talanta 10, 865 (1963).