Elution of lanthanides from dowex-50 by acetate solutions

J. lnorg. Nucl. Chem., 1965, Vol. 27, pp. 459 to 462. Pergamon Press Ltd. Printed in Northern Ireland

ELUTION OF L A N T H A N I D E S FROM DOWEX-50 BY ACETATE SOLUTIONS E. H. WARD a n d G . R. CHOPPIN Department of Chemistry, Florida State University, Tallahassee, Florida (Received 2 June 1964)

Abstract--The separation factors have been determined by column elution for Ce, Pm, Eu, Tb, Tm and Y using Dowex-50 and ammonium acetate solutions. Agreement between the experimental separation factors and those predicted from the stability constants of lanthanide acetate complexes show that for elution with 1.1 M NH4Ac solution the principle solution phase species is MAcs and the principal resin phase species is MAc ~. The separations are not as large as with elution using 7-hydroxycarboxylate solutions. THE simpler ~ - h y d r o x y c a r b o x y l i c acids such as glycolic, lactic a n d 7 - h y d r o x y isobutyric are used in s e p a r a t i o n o f the trivalent l a n t h a n i d e s a n d actinides b y c a t i o n exchange. T w o investigations have been r e p o r t e d on the n a t u r e o f the species involved in such systems. (l'z> I n the s e c o n d r e p o r t , a c o m b i n a t i o n o f c a t i o n exchange a n d a n i o n exchange resin studies, it was e s t i m a t e d that at 0.25 M i s o b u t y r a t e ion concent r a t i o n s the c a t i o n resin p h a s e consisted o f nearly equal mixtures o f M B z+ a n d MB2 +, whereas the solution p h a s e consisted p r e d o m i n a n t l y o f M B 4- ( M = l a n t h a n i d e o r actinide, B -+ isobutyrate). Partly as a c o n t i n u a t i o n o f these studies, it seemed interesting to investigate t h e ' e l u t i o n o f l a n t h a n i d e ions f r o m Dowex-50 cation exchange resin b y solutions o f a m m o n i u m acetate. T h e o r d e r o f the stability c o n s t a n t s for l a n t h a n i d e ions a n d acetate (3> is not as simple as t h a t for the ~ - h y d r o x y c a r b o x y l a t e ligands. (4,51 Therefore, the elution sequence c o u l d m o r e clearly indicate the i m p o r tance o f v a r i o u s c o m p l e x species. Secondly, if the resin p h a s e species is the unc o m p l e x e d cation, the v a r i a t i o n o f stability c o n s t a n t s with a t o m i c n u m b e r w o u l d result in a " n o r m a l " e l u t i o n sequence from L a to Eu (i.e., inverse to a t o m i c n u m b e r order) b u t an inverted o r d e r for the heavier cations. F o r these t r a n s e u r o p i u m ions, such an elution sequence c o u l d be o f use for e x a m p l e in the s e p a r a t i o n o f electronc a p t u r e d a u g h t e r s f r o m their parents. EXPERIMENTAL Chemicals. The acetate solutions were prepared from reagent grade glacial acetic acid and were

adjusted to a pH of 4.6 with concentrated ammonium hydroxide. Dowex-50 (4% DVB) of 200-400 mesh was prepared in the NH++ form. Tracers. 1~4Ce, 147pm and 152,t~+Eu were obtained from Oak Ridge National Laboratory. ~'"Y was separated from the 9°Sr parent which had also been obtained from ORNL. l+°Tb and ~°'lna were prepared by neutron irradiation of Tb304 and Tm~O3 in the University of Florida reactor. Equilibrium studies. The distribution coefficient, KD, of ~'Z,l~Eu was measured at 250 : 0.1 C as a function of acetate concentration using standard procedures. ~6' t tl S. BJORNHOLM, M. JORGENSENand B. KLINKEN, Abstracts +!f the X V I I International Con~,ress o/ Pure and Applied Chemistr},, A347, Munich (1959). ~-'~L. W. HOLM, G. R. CHOPPIN and D. MoY, J. lnorA,. Nuc/. Chem. 19, 251 (1961). <:~ A. SON~SSON,Acta Chem. Stand. 12, 165, 1937 (1958). G. R. CtlOPPIN and J. A. CHOPOORIAN,J. lnot~G~+Nucl. Chem. 22, 97 (1961). 459

460

E.H. WARD and G. R. CHOPPtN

Column elutions. Ion exchange resin columns with bed dimensions of 3 mm × 5 cm were prepared and elution using 1"1 M ammonium acetate as eluant conducted at room temperature in a manner reported previously,m y-scintillation spectroscopy using a NaI(TI) well crystal and a TMC CN-1024 analyzer was used to resolve overlapping elution peaks.

RESULTS The results of the batch equilibrium measurements of KD where

KD=

c.p.m}52'4Eu per g resin c.p.m. 15~'4Eu per ml solution

are given in Table 1. A plot of log KD vS log (Ac-) for these data has a linear slope of --3.95. Table 2 gives the composite result of the column elutions using 1.1M ammonium acetate solutions. Twelve elutions were performed with various combinations of two or three ions and the results combined in Table 2. The separation factors relative to Eu(III) were calculated after subtraction of the "free column volume" of 3.5 drops from the elution peak value. DISCUSSION In the earlier paper from this laboratory t~ it was shown that the value of the slope of a log KD vs log (Ac) plot can provide information on the principle species involved. A slope value of --4 indicates a combination of M 3+ in the resin phase and MAc 2+ in the solution phase, or, alternately, MAc 2+ in the resin phase and MAc 3 in the solution phase. However, these conclusions are based on the assumption that there is a single species predominant in each phase. Furthermore, the sensitivity of the slope values is not high. In the acetate systems, the linearity of the slope over a range of acetate concentrations large enough to preclude a single pair of species reflects this tack of sensitivity. The stability constants which have been reported la.a~ would predict that the principal solution phase species at 1M acetate concentration would be MAc 3. Consequently, with some reservations, we may interpret the slope as providing evidence that MAc 2+ is the predominant resin phase species and MA% the predominant solution phase species at the higher acetate concentrations. More definitive proof that MAc ~+ and MAc a are the species involved in this system is obtained from a consideration of the dependency of the separation factors on the stability constants. If we assume, based on the evidence cited in the previous paragraph, that with l . l M acetate solution the resin phase species is MAc ~+ and the solution phase species is MA%, then (MAc)R KD -- (MA%)---~s"

From Equations (6) and (9) of reference (2), it follows that K,,?.(K2Ka) -1

where/(2 and K s are the second and third stability constants. This same dependency on (K~K.~)1 should exist for the separation factors since they are the ratio of K,'s. ':' S. G. TIIOMPSON, B. G. HARVEY, G. R. CHOPPIN a n d G. T. SEABORG, J. Amer. Chem. Soe. 76, 6229 (1954). ¢"~ R. S. KOIAI a n d J. E. POWELI, Inorg. Chem. !, 293 (1962).

Elution of lanthanides from Dowex-50 by acetate solutions

461

TABLE 1.-----DISTRIBUTION COEFFICIENT FOR E U ( I I I ) WITH DOWEX-50 AND

NH4Ac SOLUTION

Cone.

Acetate

(M)

Ka

0-1~ 0.3~ 0.5~ 0-7~ 0.900 1"1~

35,3~ 720 78.9 21.4 8.93 4.23

TABLE 2 . - - E L U T I O N ORDER AND SEPARATION FACTORS FOR YTTRIUM AND SOME LANTHANIDES

USING DOWEX-50 AND 1'I M NH4Ac SOLUTION Observed

Isotope

Atomic No.

Elution Peak drop no.

ct[u

le°Tb 17°Tm 9oy I~4Eu 147Pm 1*aCe

65 69 39 63 61 58

18'0 19-0 20.0 22"0 28:0 50"0

0"78 0"84 0"89 1"00 1"32 2"51

TABLE 3.--COMPARISON OF PREDICTED AND EXPERIMENTAL SEPARATION FACTORS

Predicted ~ u Ion Tb Tm Y Eu Pm Ce

K1 58 42 34 90 94 48

Ks

asu H Expt.

K3

19"7 16"2 13"5 18"2 15'6 10"0

(K~K~) -~

(KIK~Ka) ~

(KzKa) -1

1"4 2'4 3"6 1"0 1"1 3'4

1"2 1"6 2"8 1"0 1"3 4"9

0"72 0"79 1"0 1'0 1'4 2"5

5"0 6"0 5"2 4"0 3"4 2"8

TABLE 4.--SEPARATION FACTORS OF T b ( I I I ) AND T m ( l I I ) AT DIFFERENT ACETATE CONCENTRATIONS

Predicted %bTm Experimental CtTb~m (K, K2) -x

(K~K2Ka) -t

(K2Kz) -~

1.7

1"3

l'1

1.07 (1.1 M) 1"11 (0.9 M) 1"17 (0.7 M)

0"78 0"84 0"89 1"00 1"32

2'51

462

E.H.

WARD and G. R. CHCPPIN

Similarly, if the species are (MZ+)R and (MAc2+)s (R and S signify resin and solution phase, respectively), we would expect the separation factors to be a function of (K1K2)-1. If (MS+)n and (MAca) s are the principal species, the separation factors would show a correlation with (KIK~Kz)-x. In Table 3 the separation factors predicted by these calculations are compared with the experimental values. The stability constants listed in Table 3 are those measured by SONESSONfor an ionic strength of 2-0M. The values of KOLAT and POWELLmeasured at 0.1M are larger but the ratios Kn+I/K~ are very similar for the two ionic strengths. Also, the ratios such as Kn(Eu)/K~(Ce) are similar. Since we use ratios, i.e. (KzKs)eo-1/(K2Ks)~.u-1, use of either set of stability constants predicts similar separation factors and we may assume that these are equally valid at 1.1M. The errors quoted for the stability constants would lead to an estimated uncertainty of approximately 15-20 per cent in the predicted separation factors. From Table 3, it is readily apparent that the agreement in separation factors between experiment .and prediction is good only for the (MAc~+)R-(MAcs)s combination. Moreover, the agreement is sufficiently satisfactory to add considerable weight to the validity of this approach. As the concentration of acetate in the solution phase decreases, it would be expected that species of lower complexation would increase in relative concentration in both resin and solution phases. The results of column elutions of Tb(III) and Tm(III) at two lower concentrations are presented in Table 4. This pair was chosen since their inversion from the normal order would be rather sensitive to the species involved The expected increase in separation is evident. The full width at half maximum value of the elution curves was approximately 20 per cent which is too great with these small separation factors to allow satisfactory separations of electron-capture daughters, etc., for this inverted elution order after Tb(III).

Acknowledgements The authors wish to acknowledgethe assistanceof Dr. W. H. ELLISin the reactor irradiation of Tb and Tm. This research was conducted under AEC Contract AT-(40-1)-1797.