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Ion exchange in fused salts
JOURNAL OF CHROMATOGRAPRY
564
ION
EXCHANGE
IN FUSED
SALTS
I. CHROMATOGRAPHIC BEHAVIOUR OF VARIOUS METAL CATIONS FUSED SALTS ON y-ALUMINA AND SYNTHETIC INORGANIC ION EXCHANGE
IN
MATERIALS
G. ALBERTI, A. CONTE AND S. ALLULLI Laboralovio di Cltimica dt~llc Radiazioni e Chinzica Nucleave Genevale cd lnovganica debt?’U&vevsilb, Rome (Italy) (Received. October Iq.th, 1964)
del C.N.E.N.
and Islitacio di CZIilnica
INTRODUCTION
The selective adsorption of metal ions dissolved in fused salts on various inorganic materials has been recently reported by several authors++ zirconium phosphate and zirconium oxidesg0, zeolites’**, So far, y-A1,0,+4, silica geP, glass powder10 and glass-fiber paper 11 have been tested as adsorption materials and their selectivity has been used to separate mixtures of inorganic ions dissolved in fused alkali chlorides and nitrates. ,. There is still, however, little information on the mechanism of selective adsorption. According to MARCUS798Linde Molecular Sieve 4A would retain its ion exchange properties in ionic melts as well as in aqueous solutions. it might be expected, therefore, that ions dissolved in fused salts are also adsorbed by an ion exchange mechanism on other inorganic materials. It might be of interest, both from a theoretical and a practical point of view, to gain further insight into the ion eschange properties of inorganic materials in fused salts, since it is possible to predict, on the basis of the present knowledge of ion exchange mechanism, that these properties will differ considerably from those in aqueous solutions, The main differences to be expected are as follows: (a) Marked changes of selectivity should be observed in’fused salts when compared with aqueous solutions. In aqueous solutions, the ion exchanger tends to show a preference for ions with a larger crystal ra.dius (i.e. a smaller hydrated radius) when the charges of the ions are equal. The opposite should take place in fused salts. (b) The extent of swelling in aqueous solutions and in ionic melts should differ considerably. (c) The non-hydrated hydrogen ion is very small relative to other ions. The selectivity of the exchanger material for H+ ions, in perfectly anhydrous conditions, should therefore be greatly increased, (d) The efficiency of electrolyte esclusion from ion exchangers decreases as the concentration of the solution increases. This provides strong support for the view that invasion of the ionic solvent takes place in fused salts, J, Chromatog.,
18 (1965) 564-571
ION
EXCHANGE
IN
FUSED
SALTS.
565
I.
(e) When a cationic exchanger is in contact with a molten salt, all cations present in the melt will compete for the exchangeable sites. A metal ion dissolved at low concentrations in the fused salt will therefore be adsorbed only when its affinity for the ion exchanger is sufficiently high. Furthermore, the metal ion adsorption could be strongly affected by complex formation. High distribution coefficients for a given metal ion dissolved in fused salts will thus be obtained when the ionic melt has a low complexing power and the metal ion has a high affinity for the ion exchanger. The present paper reports a systematic investigation of the adsorption properties of various inorganic materials in fused salts. Attempts were made to interpret tile mechanism of metal ion adsorption and to confirm the expected differences between the ion exchange properties in molten salts and in aqueous solutions. It seemed convenient to undertake the study of adsorption in molten salts by a chromatographic technique employing glass-fiber paper impregnated with the ion dxchangers to be studied, because of the speed and simplicity compared with column chromatography. EXPERIMENTAL
Finely powdered potassium nitrate (Erba RP) and lithium nitrate (Erba RP) were oven dried at 94.” and mixed in molar proportions of LiNO,: KNO, = 43 : 57. This mixture was dried again at g4”, fused in the chromatography vessel and allowed to stand for I h at the operating temperature (160”) before starting the experiments. The chromatographic apparatus and technique have been described elsewherea. Solutions (cu. X0- 2 M) of the ions to be studied were prepared by dissolving the metal nitrates (Erba R.P) in the LiNO,-KNO, eutectic. Europium nitrate was prepared by dissolving Eu,O, (Johnson-Matthey, Co., London) in hot cont. HNO, and heating to dryness. Fe(NO,), was dissolved in the molten LiNO,-KNO, eutectic containing cont. HNO, in order to avoid Fe,O, precipitation. Metal ions were detected by spraying suitable reagents on the glass-fiber strips. UO,(II) and Eu(III) were detected by fluorescence spot tests lss13. Solutions of Na(I), Rb(I), and Cs(1) nitrates were labelled with 22Na, *ORb and 137Cs, respectively. Rv$aration of glass-fibev slr@s imf-wegnated with zirconizcvn $hos$hate, zircooziacnt oxide and y-AI,O, The glass-fiber paper employed (Whatman GB, 0.15 mm thick, weight 150 mg/cms) was purified by dipping it in HN03 (I : I) and rinsing with distilled water. Strips of this paper (12 cm x 3 cm) impregnated with zirconium phosphate were prepared according to the technique reported elsewherel” and dried at 160~.
Glass-fiber paper strips impregnated with y-AlsO, were prepared by immersing first in a 2 M AlCl, solution, then in z M NH,OH. After 2 h the strips were rinsed
with distilled water, air dried, and then oven dried at 500~ for 5 h. X-ray analysis (CuK,) confirmed the formation of y-Al,O,. Glass-fiber paper strips impregnated with zirconium oxide were prepared by immersing first in a 4 M HCl-0.15 & ZrOCl, solution, then in 32 o/0 NH,OH. After 2 h the strips were rinsed with distilled water, air dried and then oven dried at 160’. J. Chomalog.,
18
(1965) 564-571
.
G. ALBERTI, A. CONTE,
566
J. Chvomatog.,
18 (w%)
5’34-57~
S. ALLULLx
l
At the starting point At the starting point with comet At the starting point with comet
At the starting point under all conditions
RF = 0.6 At liquiti front
At front with comet At liquid front
15 % NH&I At frontwithcomet
LiiW-#NO, LW-KNO, IOy0 N&NO, LiNOa-KNO, 15 y0 KC1 LiNO,-KNO, 15 y0 NH&I
LINO,KNO,
LiN03-KNO, LiNO&NO, IOy0 N&NO, LiNOa-KNO, 15 % KC1
.___ Zirconiumphosphate. ‘* Zirconiumoxide.
Th(IV)
Fe(U1)
En(iI1)
UO,(U)
At liquid front 10 y0 NH,NO, LSO,-KNO, 15 y0 KC1 At liquid front LjNO&NO, 15 % NH&l At liquid front
LiNo+No,
At the starting point At the starting point with comet At the starting point
At the starting point At the st&ing point
RF =
At the starting At the starting comet At the starting At the starting comet
comet
point point with
point point with
At thestartingpointwith
At the starting point At liquid front At the starting point At front with comet
At the starting point At front with comet
At liquid front
At the starting point At the starting point.with comet At the starting point
At liquid front At liquidfront
At the starting point At front with comet
At front with comet
At liquidfront
0.5
At liquid front
Xt liquid front At front with comet
front wrth comet
At
RF = 0.66
G. ALBERTI,
568 RESULTS
AND
A. CONTE,
S. ALJ.ULLI
DISCUSSION
Table I presents results obtained by chromatography, 0x1 glass-fiber paper and glass-fiber paper impregnated with zirconium phosphate, zirconium oxide and y-Al,O,, of various metal ions dissolved in the LiNOs-KNO, eutectic. These results clearly indicate that the adsorption of 0(I), Rb(I), Na(I), n(I), and Pb(II), on the ion exchangers tested, is very low. Ni(II), Co(II), WOs(II) and Eu(lII) are moderately adsorbed, whereas Fe(II1) and Th(IV) are strongly adsorbed. Comparison of our results with the scale of the approximate solubilities of oxides in both the LX]KC1r”‘7 eutectic and fused alkali fluorides 1e shows a marked analogy between a& sorption and solubility of a given metal ion. This analogy is illustrated in Table 11, TABLE
II
COMPARISON
BETWEEN
Scale of chavgej ionic
Tl(i)
radius
vatio
’
0.60 0.68 0.68 1.03 1.67
NaU-1 Pb(II)
Zn(II) Co(I1)
Ni(kI) WUI) Eu(III)
2.70 2.78 2.90
3.03 3.oG
ADSORPTION,
AND
A ppvoximatc scale c)f the adsov$tion (hcvcasz’ng dorvnwavd) i?z LiNO,-ICNO,
Apfivoximate scale of oxide solzcbility (decreasing dozer++ zvavd) in IXE-ICCP
Cs(I)
cs,o
lib(I) %1(I) NatI) Pb(Il) Co(H) Ni(II) GO&I) Etl(II1)
-
-_
i&III)
&II) -
Sn(IV) Al(III) Ti(IV) Si(IV)
OXIDJZ. SOLUBILITY
-
* Scarcely aclsorbcd. * * Moclerately adsorbccl. * * * Strongly adsorbed.
*
IONIC
POTENTIALS
Appvoximais scale of oxide solacbility (dcwveasing dowgzzvard) in ma2 ten jhorides’~
Rb,O T1,O Na,O
PbO
I
**
***
ZnO
coo
NiO W@ UQ3 Ea1,0,
i% -
0 SKl8,”
QA
TiO, SiO, * Soluble. ++ Sparingly
-I+* Insoluble,
soluble.
which shows not only the approximate scales of, the solubilities and adsorptions but also, for comparison, a scale of the ionic potentials (charge/ionic radius) of the cations under investigation. The correspondence between these scales indicates that as the value of the ionic potential increases, the chromatographic adsorption of a given cation on the tested materials increases, whereas its solubility decreases. At present the nature of these observations is only qualitative. A knowledge of the distribution coefficients and solubility products for a large number of metal ions dissolved in the same ionic melt is required to arrive at more quantitative conclusions. J. Chvomatog., 18 (1965) 564-571
ION
EXCHANGE
In this way
IN FUSED
more
SALTS. I,
569
exact
scales, based on numerical values, would be obtained, and also soMe anomalies due to the different complexing power of the solvent employed, could be avoided. The relation between the above-mentioned scales could be improved by replacing tlhe values of ionic radii by other values involving the actual distances between the cations and the fixed ionic groups of the ion exchangers, e.g. taking into account the charge of the ions and also their polarizability. Table I gives chromatographic results obtained employing as solvent a solution of NH&l in the LiNQ,-IWO, eutectic. These results clearly show the high eluting power of NH&l, already mentioned by GRUEW~. This author employed the same solvent to elute UO,(II), Co(II), Cu(II) and Ni(II) previously adsorbed on pAlSO,. According to GRUEN the high eluting power of WH,Cl is due to the formation of chlorocomplexes. The metal ions were eluted in the same order as mentioned above and confirm the results of spectrophotometry for the relative stabilities of the cltlorocomplexes. In order to derive more exact information concerning the elution mechanism of NH&l, it was necessary to test the eluting power of NH,* and Cl- ions respectively. For this purpose, NH4N0, and KC1 were added separately to the LiNO,-KNO, eutectic. As shown in Table I, all the ions eluted by NH&l are also eluted by NH,,NO, whereas only UO,(II) and Co(I1) are displaced to any extent when KC1 is added to the LiNOa-KNO, eutectic. Thus the eluting power of NH,&1 is due to both NH,+ and Cl- ions. It follows that the results of chromatographic experiments employing solutions of NH&l in the LiNO,-KNO, eutectic cannot be used as evidence of chlorocomplex formation. Although elutions performed with cutectics containing KC1 confirm the formation of chlorocomplexes of UO,(II) and Co(II) already reported by GRUEN~, our results for Ni(II) shqw that the tendency to form complexes of the or type NiC1,2- is very low, since Ni(II) is eluted by eutectics containing NH&l NH,,NOa, but it is not displaced when KC1 is dissolved in the molten nitrates. Furthermore we observed that Ni(I1) does not form an anionic chlorocomplex in noticeable quantities even in the LiCl-KC1 eutectic at 450”, since it behaves as a cation vrhen under the same electrophoresis is carried out in this solvent 11. Co(I1) and UO,(II) conditions behave as anions. The chromatographic technique employing impregnated glass-fiber paper was also used to investigate the disagreement between results obtained by different authors, For example, GRUEN observed that Pr(III) and Nd(III), dissolved in molten alkali nitrates, were not absorbed on y-A120, 1. On the contrary LINDNER ANDJOHNSONS reported that, under the same experimental conditions, rare earths were strongly adsorbed, Table I shows that Eu(II1) is moderately adsorbed on y-Al,O,, in agreement with both the value of the ionic potential and the low solubility of Eu3O3 in molten alkali nitrates. Since the difference between the ionic crystal radii of the rare earths is larger than the difference between their hydrated ionic radii, separations of the trivalent rare earth ions should be achieved in molten salts. BE:NARLE~~ reported that Cu(II), Ni(II), Co(I1) and Mn(I1) dissolved in the LiCl-KC1 eutectic, were adsorbed on glass powder. The adsorption decreased in the same order as mentioned above, with the exception of Cu(II), which can be partially reduced to &(I) in molten alkali chlorides in the presence of 02- ions. It must be mentioned that the order of adsorption is in agreement with the charge of the ion/ ionic radjus ratios, which are respectively: Ni(I1) 2.94, Co(II) 2.77 and Mn(II) 2*5o. J.Clwomato~., 18 (x905)564-571
$70
G.
ALBERTL, A. CONTE, Se ALLULLL
GRUEN~, on the tither hand, reported that Co(II), Ni(II), Cu(II), dissolved in the LiCI-KC1 eutcctic, were not adsorbed on y-A1203. This could be due to different experimental conditions, since GRUEN dehydrated the LiCl-KC1 eutectic carefully with gaseous HCI, whereas BENARLE does not mention any special care in preparing the melt during the experiments. In a previous paperll we reported that the forma,tion of insoluble osides was observed when chromatography was carried out in non-dehydrated molten alkali chlorides, e,g. Ni(I1) and Mn(I1) adsorbed on glassfiber paper in the presence of air did not move when a non-dehydrated eutectic was used. When pyridinium chloride at 130°, or the LiCl-ICC1 eutectic dehydrated with gaseous WC1 were used as eluents, these ions moved with the front of the solvent, ~t’ettability of glass-fiber pa$er As we reported elsewhere 11 the wettabilitv
of glass-fiber paper by fused salts depends upon the presence of traces of water or” 02- ions in the melt, e.g. glass-fiber paper is wetted by a non-dehydrated LiCl-KC1 eutectic, whereas the wettability is greatly lowered when alkali chlorides are dehydrated by gaseous HCI. Furthermore when an atmosphere of gaseous HCI is maintained over the melt, the wettability decreases to such an extent that chromatographic esperiments are no longer possible. In general, glass-fiber paper is wetted with difficulty by the LiNO,-KNO, eutectic, so that chromatographic experiments are sometimes unsuccessful. By adding x-2 oh KOH to the molten nitrates, the wettability is greatly increased. Similar results are obtained with glass-fiber paper impregnated with zirconium phosphate in the hydrogen form. On the basis of these results and assuming that the affinity of the H* ion for the ion eschangers is high (since the non-hydrated H+ ion ‘is very small relative to other cations) the wetting mechanism could be explained as follows: glass-fiber paper is purified by dipping into an aqueous solution of HNO,. The strips are therefore supel-licially converted into the hydrogen form. In perfectly anhydrous melts or at low concentrations of 02’ ions, the estent of exchange between the cations present in the solvent and the H+ ions of the exchanger material is very low and a poor wettability will therefore result. At high concentrations of O+ ions in the melt, the H+ ion can be exchanged since it reacts to form H,O. In non-dehydrated melts, also, the H-Cion could be displaced by other cations, as the hydrated H+ ion is markedly larger than the anhydrous proton, In such cases a wettability will be obtained. We tried also to increase the wettability of glass-fiber paper by replacing the H+ ion with a cation present in the solvent. For this purpose the strips, after purification with HNO,, were dipped in concentrated solutions of KNO,. These strips, dried at 160°, were perfectly wetted by the LiNO,-KNO, eutectic. Further investigations clearly demonstrate the importance of traces of water or OS- ions on the proton exchange in molten salts. A quarititative study of the affinity of the H+ ion, in the presence of different amounts of water, seems to be of interest for the chemistry of the proton in molten salts. CONCLUSIONS
It was shown that some problems connected with the adsorption of metal ions dissolved in fused sdts can be investigated by a simple chromatogrdphic technique employing glass-fiber paper impregnated with various inorganic ion exchangers. J.Chrornatog., 18 (1965)564-573
ION
EXCHANGE
IN
FUSED
SALTS.
1.
57r
The present results support the theory that metal ions a.re mainly adsorbed by ion exchange, although other mechanisms are possible. For example the water of hydration held by several ion cScha.ngers, even at relatively high temperatures, could give rise to aside precipitation. Several metal ions forming insoluble osidcs could therefore be fixed in this way by the adsorption material, The precipitation of oxides is in, agreement with both the analogy between chromatographic adsorption and oxide solubility, and the cluting po\vcr of NW,,* ions, It is well known that the NW4+ ion, behaving as an 02’ ion acceptor, dissolves oxides. Furthermore, in some cases, specific interactions with the fiscd ionic groups of the ion exchanger may take place, e.g. metal ions forming insoluble phosphates can be selectively adsorbed by zirconium phosphate. It will be of interest to obtain more quantitative data (e.g. values of distribution coefficients and of cschangc capacities, percentage of solvent invasion in the ion e&anger, etc.) to improve the knowledge of ion adsorption in fused salts and also to confirm the differences predicted between the properties of ion eschangers in aqueous solutions and in ionic melts. An investigation of anion exchange in fused salts should also prove interesting.
The chromatographic adsorption on y-alumina and synthetic inorga.nic ion exchange materials of various metal cations dissolved in a fused lithium nitratepotassium nitrate eutectic was studied. The results obtained seem to confirm that metal ions are adsorbed by ion eschange, although other mechanisms, e.g. aside or formation of insoluble phosphates, are possible. A relationship precipitation, between the adsorption of the metal ions testecl, the solubility of their oxides and the ionic potentials was found. Some conflicting results of other authors are discussed. REFERENCES M. GRUEN, PYOC, IwCerw. Corzf. Peaceful Uses .-It. Gncr~_v, Geneva, I~.#, \‘ol. 28, p. I 12. M. GRUEN, N&we, 175 (1950) 1181. LINDNER AND S. JOHNSON, %. Elcci?ochem., 04 (1960) roG3. 0. LILJENZIN, I-I. REINMARDT, I-1.WINII~S AND 13.LINDNEA:, Radiochin% Acta, 3 (1963) 161. NRU~~ANN AND G. TSCNIRNE, I
D. D. R. J. D.
J. Ch’omatog.,
18 (1965) 564-571
564
ION
EXCHANGE
IN FUSED
SALTS
I. CHROMATOGRAPHIC BEHAVIOUR OF VARIOUS METAL CATIONS FUSED SALTS ON y-ALUMINA AND SYNTHETIC INORGANIC ION EXCHANGE
IN
MATERIALS
G. ALBERTI, A. CONTE AND S. ALLULLI Laboralovio di Cltimica dt~llc Radiazioni e Chinzica Nucleave Genevale cd lnovganica debt?’U&vevsilb, Rome (Italy) (Received. October Iq.th, 1964)
del C.N.E.N.
and Islitacio di CZIilnica
INTRODUCTION
The selective adsorption of metal ions dissolved in fused salts on various inorganic materials has been recently reported by several authors++ zirconium phosphate and zirconium oxidesg0, zeolites’**, So far, y-A1,0,+4, silica geP, glass powder10 and glass-fiber paper 11 have been tested as adsorption materials and their selectivity has been used to separate mixtures of inorganic ions dissolved in fused alkali chlorides and nitrates. ,. There is still, however, little information on the mechanism of selective adsorption. According to MARCUS798Linde Molecular Sieve 4A would retain its ion exchange properties in ionic melts as well as in aqueous solutions. it might be expected, therefore, that ions dissolved in fused salts are also adsorbed by an ion exchange mechanism on other inorganic materials. It might be of interest, both from a theoretical and a practical point of view, to gain further insight into the ion eschange properties of inorganic materials in fused salts, since it is possible to predict, on the basis of the present knowledge of ion exchange mechanism, that these properties will differ considerably from those in aqueous solutions, The main differences to be expected are as follows: (a) Marked changes of selectivity should be observed in’fused salts when compared with aqueous solutions. In aqueous solutions, the ion exchanger tends to show a preference for ions with a larger crystal ra.dius (i.e. a smaller hydrated radius) when the charges of the ions are equal. The opposite should take place in fused salts. (b) The extent of swelling in aqueous solutions and in ionic melts should differ considerably. (c) The non-hydrated hydrogen ion is very small relative to other ions. The selectivity of the exchanger material for H+ ions, in perfectly anhydrous conditions, should therefore be greatly increased, (d) The efficiency of electrolyte esclusion from ion exchangers decreases as the concentration of the solution increases. This provides strong support for the view that invasion of the ionic solvent takes place in fused salts, J, Chromatog.,
18 (1965) 564-571
ION
EXCHANGE
IN
FUSED
SALTS.
565
I.
(e) When a cationic exchanger is in contact with a molten salt, all cations present in the melt will compete for the exchangeable sites. A metal ion dissolved at low concentrations in the fused salt will therefore be adsorbed only when its affinity for the ion exchanger is sufficiently high. Furthermore, the metal ion adsorption could be strongly affected by complex formation. High distribution coefficients for a given metal ion dissolved in fused salts will thus be obtained when the ionic melt has a low complexing power and the metal ion has a high affinity for the ion exchanger. The present paper reports a systematic investigation of the adsorption properties of various inorganic materials in fused salts. Attempts were made to interpret tile mechanism of metal ion adsorption and to confirm the expected differences between the ion exchange properties in molten salts and in aqueous solutions. It seemed convenient to undertake the study of adsorption in molten salts by a chromatographic technique employing glass-fiber paper impregnated with the ion dxchangers to be studied, because of the speed and simplicity compared with column chromatography. EXPERIMENTAL
Finely powdered potassium nitrate (Erba RP) and lithium nitrate (Erba RP) were oven dried at 94.” and mixed in molar proportions of LiNO,: KNO, = 43 : 57. This mixture was dried again at g4”, fused in the chromatography vessel and allowed to stand for I h at the operating temperature (160”) before starting the experiments. The chromatographic apparatus and technique have been described elsewherea. Solutions (cu. X0- 2 M) of the ions to be studied were prepared by dissolving the metal nitrates (Erba R.P) in the LiNO,-KNO, eutectic. Europium nitrate was prepared by dissolving Eu,O, (Johnson-Matthey, Co., London) in hot cont. HNO, and heating to dryness. Fe(NO,), was dissolved in the molten LiNO,-KNO, eutectic containing cont. HNO, in order to avoid Fe,O, precipitation. Metal ions were detected by spraying suitable reagents on the glass-fiber strips. UO,(II) and Eu(III) were detected by fluorescence spot tests lss13. Solutions of Na(I), Rb(I), and Cs(1) nitrates were labelled with 22Na, *ORb and 137Cs, respectively. Rv$aration of glass-fibev slr@s imf-wegnated with zirconizcvn $hos$hate, zircooziacnt oxide and y-AI,O, The glass-fiber paper employed (Whatman GB, 0.15 mm thick, weight 150 mg/cms) was purified by dipping it in HN03 (I : I) and rinsing with distilled water. Strips of this paper (12 cm x 3 cm) impregnated with zirconium phosphate were prepared according to the technique reported elsewherel” and dried at 160~.
Glass-fiber paper strips impregnated with y-AlsO, were prepared by immersing first in a 2 M AlCl, solution, then in z M NH,OH. After 2 h the strips were rinsed
with distilled water, air dried, and then oven dried at 500~ for 5 h. X-ray analysis (CuK,) confirmed the formation of y-Al,O,. Glass-fiber paper strips impregnated with zirconium oxide were prepared by immersing first in a 4 M HCl-0.15 & ZrOCl, solution, then in 32 o/0 NH,OH. After 2 h the strips were rinsed with distilled water, air dried and then oven dried at 160’. J. Chomalog.,
18
(1965) 564-571
.
G. ALBERTI, A. CONTE,
566
J. Chvomatog.,
18 (w%)
5’34-57~
S. ALLULLx
l
At the starting point At the starting point with comet At the starting point with comet
At the starting point under all conditions
RF = 0.6 At liquiti front
At front with comet At liquid front
15 % NH&I At frontwithcomet
LiiW-#NO, LW-KNO, IOy0 N&NO, LiNOa-KNO, 15 y0 KC1 LiNO,-KNO, 15 y0 NH&I
LINO,KNO,
LiN03-KNO, LiNO&NO, IOy0 N&NO, LiNOa-KNO, 15 % KC1
.___ Zirconiumphosphate. ‘* Zirconiumoxide.
Th(IV)
Fe(U1)
En(iI1)
UO,(U)
At liquid front 10 y0 NH,NO, LSO,-KNO, 15 y0 KC1 At liquid front LjNO&NO, 15 % NH&l At liquid front
LiNo+No,
At the starting point At the starting point with comet At the starting point
At the starting point At the st&ing point
RF =
At the starting At the starting comet At the starting At the starting comet
comet
point point with
point point with
At thestartingpointwith
At the starting point At liquid front At the starting point At front with comet
At the starting point At front with comet
At liquid front
At the starting point At the starting point.with comet At the starting point
At liquid front At liquidfront
At the starting point At front with comet
At front with comet
At liquidfront
0.5
At liquid front
Xt liquid front At front with comet
front wrth comet
At
RF = 0.66
G. ALBERTI,
568 RESULTS
AND
A. CONTE,
S. ALJ.ULLI
DISCUSSION
Table I presents results obtained by chromatography, 0x1 glass-fiber paper and glass-fiber paper impregnated with zirconium phosphate, zirconium oxide and y-Al,O,, of various metal ions dissolved in the LiNOs-KNO, eutectic. These results clearly indicate that the adsorption of 0(I), Rb(I), Na(I), n(I), and Pb(II), on the ion exchangers tested, is very low. Ni(II), Co(II), WOs(II) and Eu(lII) are moderately adsorbed, whereas Fe(II1) and Th(IV) are strongly adsorbed. Comparison of our results with the scale of the approximate solubilities of oxides in both the LX]KC1r”‘7 eutectic and fused alkali fluorides 1e shows a marked analogy between a& sorption and solubility of a given metal ion. This analogy is illustrated in Table 11, TABLE
II
COMPARISON
BETWEEN
Scale of chavgej ionic
Tl(i)
radius
vatio
’
0.60 0.68 0.68 1.03 1.67
NaU-1 Pb(II)
Zn(II) Co(I1)
Ni(kI) WUI) Eu(III)
2.70 2.78 2.90
3.03 3.oG
ADSORPTION,
AND
A ppvoximatc scale c)f the adsov$tion (hcvcasz’ng dorvnwavd) i?z LiNO,-ICNO,
Apfivoximate scale of oxide solzcbility (decreasing dozer++ zvavd) in IXE-ICCP
Cs(I)
cs,o
lib(I) %1(I) NatI) Pb(Il) Co(H) Ni(II) GO&I) Etl(II1)
-
-_
i&III)
&II) -
Sn(IV) Al(III) Ti(IV) Si(IV)
OXIDJZ. SOLUBILITY
-
* Scarcely aclsorbcd. * * Moclerately adsorbccl. * * * Strongly adsorbed.
*
IONIC
POTENTIALS
Appvoximais scale of oxide solacbility (dcwveasing dowgzzvard) in ma2 ten jhorides’~
Rb,O T1,O Na,O
PbO
I
**
***
ZnO
coo
NiO W@ UQ3 Ea1,0,
i% -
0 SKl8,”
QA
TiO, SiO, * Soluble. ++ Sparingly
-I+* Insoluble,
soluble.
which shows not only the approximate scales of, the solubilities and adsorptions but also, for comparison, a scale of the ionic potentials (charge/ionic radius) of the cations under investigation. The correspondence between these scales indicates that as the value of the ionic potential increases, the chromatographic adsorption of a given cation on the tested materials increases, whereas its solubility decreases. At present the nature of these observations is only qualitative. A knowledge of the distribution coefficients and solubility products for a large number of metal ions dissolved in the same ionic melt is required to arrive at more quantitative conclusions. J. Chvomatog., 18 (1965) 564-571
ION
EXCHANGE
In this way
IN FUSED
more
SALTS. I,
569
exact
scales, based on numerical values, would be obtained, and also soMe anomalies due to the different complexing power of the solvent employed, could be avoided. The relation between the above-mentioned scales could be improved by replacing tlhe values of ionic radii by other values involving the actual distances between the cations and the fixed ionic groups of the ion exchangers, e.g. taking into account the charge of the ions and also their polarizability. Table I gives chromatographic results obtained employing as solvent a solution of NH&l in the LiNQ,-IWO, eutectic. These results clearly show the high eluting power of NH&l, already mentioned by GRUEW~. This author employed the same solvent to elute UO,(II), Co(II), Cu(II) and Ni(II) previously adsorbed on pAlSO,. According to GRUEN the high eluting power of WH,Cl is due to the formation of chlorocomplexes. The metal ions were eluted in the same order as mentioned above and confirm the results of spectrophotometry for the relative stabilities of the cltlorocomplexes. In order to derive more exact information concerning the elution mechanism of NH&l, it was necessary to test the eluting power of NH,* and Cl- ions respectively. For this purpose, NH4N0, and KC1 were added separately to the LiNO,-KNO, eutectic. As shown in Table I, all the ions eluted by NH&l are also eluted by NH,,NO, whereas only UO,(II) and Co(I1) are displaced to any extent when KC1 is added to the LiNOa-KNO, eutectic. Thus the eluting power of NH,&1 is due to both NH,+ and Cl- ions. It follows that the results of chromatographic experiments employing solutions of NH&l in the LiNO,-KNO, eutectic cannot be used as evidence of chlorocomplex formation. Although elutions performed with cutectics containing KC1 confirm the formation of chlorocomplexes of UO,(II) and Co(II) already reported by GRUEN~, our results for Ni(II) shqw that the tendency to form complexes of the or type NiC1,2- is very low, since Ni(II) is eluted by eutectics containing NH&l NH,,NOa, but it is not displaced when KC1 is dissolved in the molten nitrates. Furthermore we observed that Ni(I1) does not form an anionic chlorocomplex in noticeable quantities even in the LiCl-KC1 eutectic at 450”, since it behaves as a cation vrhen under the same electrophoresis is carried out in this solvent 11. Co(I1) and UO,(II) conditions behave as anions. The chromatographic technique employing impregnated glass-fiber paper was also used to investigate the disagreement between results obtained by different authors, For example, GRUEN observed that Pr(III) and Nd(III), dissolved in molten alkali nitrates, were not absorbed on y-A120, 1. On the contrary LINDNER ANDJOHNSONS reported that, under the same experimental conditions, rare earths were strongly adsorbed, Table I shows that Eu(II1) is moderately adsorbed on y-Al,O,, in agreement with both the value of the ionic potential and the low solubility of Eu3O3 in molten alkali nitrates. Since the difference between the ionic crystal radii of the rare earths is larger than the difference between their hydrated ionic radii, separations of the trivalent rare earth ions should be achieved in molten salts. BE:NARLE~~ reported that Cu(II), Ni(II), Co(I1) and Mn(I1) dissolved in the LiCl-KC1 eutectic, were adsorbed on glass powder. The adsorption decreased in the same order as mentioned above, with the exception of Cu(II), which can be partially reduced to &(I) in molten alkali chlorides in the presence of 02- ions. It must be mentioned that the order of adsorption is in agreement with the charge of the ion/ ionic radjus ratios, which are respectively: Ni(I1) 2.94, Co(II) 2.77 and Mn(II) 2*5o. J.Clwomato~., 18 (x905)564-571
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ALBERTL, A. CONTE, Se ALLULLL
GRUEN~, on the tither hand, reported that Co(II), Ni(II), Cu(II), dissolved in the LiCI-KC1 eutcctic, were not adsorbed on y-A1203. This could be due to different experimental conditions, since GRUEN dehydrated the LiCl-KC1 eutectic carefully with gaseous HCI, whereas BENARLE does not mention any special care in preparing the melt during the experiments. In a previous paperll we reported that the forma,tion of insoluble osides was observed when chromatography was carried out in non-dehydrated molten alkali chlorides, e,g. Ni(I1) and Mn(I1) adsorbed on glassfiber paper in the presence of air did not move when a non-dehydrated eutectic was used. When pyridinium chloride at 130°, or the LiCl-ICC1 eutectic dehydrated with gaseous WC1 were used as eluents, these ions moved with the front of the solvent, ~t’ettability of glass-fiber pa$er As we reported elsewhere 11 the wettabilitv
of glass-fiber paper by fused salts depends upon the presence of traces of water or” 02- ions in the melt, e.g. glass-fiber paper is wetted by a non-dehydrated LiCl-KC1 eutectic, whereas the wettability is greatly lowered when alkali chlorides are dehydrated by gaseous HCI. Furthermore when an atmosphere of gaseous HCI is maintained over the melt, the wettability decreases to such an extent that chromatographic esperiments are no longer possible. In general, glass-fiber paper is wetted with difficulty by the LiNO,-KNO, eutectic, so that chromatographic experiments are sometimes unsuccessful. By adding x-2 oh KOH to the molten nitrates, the wettability is greatly increased. Similar results are obtained with glass-fiber paper impregnated with zirconium phosphate in the hydrogen form. On the basis of these results and assuming that the affinity of the H* ion for the ion eschangers is high (since the non-hydrated H+ ion ‘is very small relative to other cations) the wetting mechanism could be explained as follows: glass-fiber paper is purified by dipping into an aqueous solution of HNO,. The strips are therefore supel-licially converted into the hydrogen form. In perfectly anhydrous melts or at low concentrations of 02’ ions, the estent of exchange between the cations present in the solvent and the H+ ions of the exchanger material is very low and a poor wettability will therefore result. At high concentrations of O+ ions in the melt, the H+ ion can be exchanged since it reacts to form H,O. In non-dehydrated melts, also, the H-Cion could be displaced by other cations, as the hydrated H+ ion is markedly larger than the anhydrous proton, In such cases a wettability will be obtained. We tried also to increase the wettability of glass-fiber paper by replacing the H+ ion with a cation present in the solvent. For this purpose the strips, after purification with HNO,, were dipped in concentrated solutions of KNO,. These strips, dried at 160°, were perfectly wetted by the LiNO,-KNO, eutectic. Further investigations clearly demonstrate the importance of traces of water or OS- ions on the proton exchange in molten salts. A quarititative study of the affinity of the H+ ion, in the presence of different amounts of water, seems to be of interest for the chemistry of the proton in molten salts. CONCLUSIONS
It was shown that some problems connected with the adsorption of metal ions dissolved in fused sdts can be investigated by a simple chromatogrdphic technique employing glass-fiber paper impregnated with various inorganic ion exchangers. J.Chrornatog., 18 (1965)564-573
ION
EXCHANGE
IN
FUSED
SALTS.
1.
57r
The present results support the theory that metal ions a.re mainly adsorbed by ion exchange, although other mechanisms are possible. For example the water of hydration held by several ion cScha.ngers, even at relatively high temperatures, could give rise to aside precipitation. Several metal ions forming insoluble osidcs could therefore be fixed in this way by the adsorption material, The precipitation of oxides is in, agreement with both the analogy between chromatographic adsorption and oxide solubility, and the cluting po\vcr of NW,,* ions, It is well known that the NW4+ ion, behaving as an 02’ ion acceptor, dissolves oxides. Furthermore, in some cases, specific interactions with the fiscd ionic groups of the ion exchanger may take place, e.g. metal ions forming insoluble phosphates can be selectively adsorbed by zirconium phosphate. It will be of interest to obtain more quantitative data (e.g. values of distribution coefficients and of cschangc capacities, percentage of solvent invasion in the ion e&anger, etc.) to improve the knowledge of ion adsorption in fused salts and also to confirm the differences predicted between the properties of ion eschangers in aqueous solutions and in ionic melts. An investigation of anion exchange in fused salts should also prove interesting.
The chromatographic adsorption on y-alumina and synthetic inorga.nic ion exchange materials of various metal cations dissolved in a fused lithium nitratepotassium nitrate eutectic was studied. The results obtained seem to confirm that metal ions are adsorbed by ion eschange, although other mechanisms, e.g. aside or formation of insoluble phosphates, are possible. A relationship precipitation, between the adsorption of the metal ions testecl, the solubility of their oxides and the ionic potentials was found. Some conflicting results of other authors are discussed. REFERENCES M. GRUEN, PYOC, IwCerw. Corzf. Peaceful Uses .-It. Gncr~_v, Geneva, I~.#, \‘ol. 28, p. I 12. M. GRUEN, N&we, 175 (1950) 1181. LINDNER AND S. JOHNSON, %. Elcci?ochem., 04 (1960) roG3. 0. LILJENZIN, I-I. REINMARDT, I-1.WINII~S AND 13.LINDNEA:, Radiochin% Acta, 3 (1963) 161. NRU~~ANN AND G. TSCNIRNE, I
D. D. R. J. D.
J. Ch’omatog.,
18 (1965) 564-571