Nonmonotonic ordering of lanthanides in tributyl phosphate-nitric acid extraction systems

J. Inorg. Nuel. Chem., 1957, Vol. 4, pp. 326 to 333. Pergamon Press Ltd., L o n d o n

NONMONOTONIC ORDERING OF LANTHANIDES TRIBUTYL PHOSPHATE-NITRIC ACID EXTRACTION SYSTEMS*

IN

D. F. PEPPARD,W. J. DRISCOLL,R. J. SIRObIEN, and S. MCCARTY Argonne National Laboratory, Lemont, Illinois U.S.A. (Received 15 February 1957) Abstract--The partition of tracer-level lanthanides, yttrium, and americium between various tri-nbutyl phosphate (TBP) phases and aqueous nitric acid phases has been studied radiometrically as a function of solvent concentration, nitric acid concentration, and Z. In undiluted TBP systems, a plot of log K vs. Z approximates two straight lines joining at Z = 64. The line for the high-Z region has a positive slope somewhat less than that of the line for the low-Z region at 18.5 M HNO3. This difference in slope becomes intensified as the concentration of nitric acid is lowered, the high-Z line ultimately acquiring a negative slope. The slope of the low-Z line remains positive throughout the region studied. This general half-filled shell effect is also noted for systems involving diluted TBP. It is postulated that the extracting species is [M(TBP)a(H20)c~-ad(NO3)3, in which a is a function of the nitric acid concentration and of Z. In all of the systems studied, for plotting purposes yttrium may be assigned an "apparent Z " of 65-68. In the dilute nitric acid systems, the americium plot is nearly coincident with that for praseodymium.

IN THE initial investigations of the trim-butyl orthophosphate, (n-C4HaO)3PO , symbolized as TBP, vs. nitric acid extraction system as a means of fractionation of trivalent lanthanides, it was reported by PEPPARD et al. ~1) that in systems involving undiluted TBP a plot of log K vs. Z is essentially a straight line of positive slope in the high-acid range, whereas an inversion of certain members occurs as the concentration of nitric acid is lowered. Whether the monotonic variation holds throughout the entire 57-71 range in Z for high-acid concentration and the nature and extent of the inversion at lower-acid concentrations were not investigated in the previous study, due to lack of pertinent radioactive lanthanide nuclides of the required purity. Following the acquisition of such nuclides, the investigation was extended, and the study presently reported treats these latter problems. EXPERIMENTAL

Sources o f materials. Tributyl phosphate was obtained from Commercial Solvents Corporation and treated as described below before use. Reagent-grade carbon tetrachloride and toluene were used as diluents as received. The beta-active nuclides Srg0-Y 9°, Ba14°-La14°, Cem-Pr m, 2.6 y P m 147, and (13,16) y Eu a52,154 were obtained from the Isotopes Division of the Oak Ridge National Laboratory. The 40.2h La 14° was separated from its Ba 14° parent (and the small accompanying contamination of Sr 9° and y9o) and the 64h ygo from its Sr 9° parent by solvent extraction processes, t2) * Based on work performed under the auspices of the U.S. Atomic Energy Commission. tx} D. F. PEPPARD,J. P. FARIS, P. R. GRAY, and G. W. MASON d. phys. Chem. 57, 294 (1953). tzl D. F. PEPPARD, G . W . MASON, a n d S. W. MOLINE 326

J. inorg, nucl. Chem. In press.

Nonmonotonic ordering of lanthanides in tributyl phosphate-nitric acid extraction systems 327 The 285-d Ce 1~ was oxidized to the tetravalent state and separated by a solvent extraction process ~2~from traces of beta-active contaminants. The Pm and Eu tracers were further purified by a solvent extraction process ~3) in which the conditions were so adjusted that approximately 10% yield, with concomitant high degree of purity, of product resulted. Beta-active 13-8d Pr 143 was prepared by neutron irradiation of highly-purified gross cerium. 12' The beta-active nuclides, 72d Tb 16°, 129d Tm aT°, (32, 4"2)d yb,169, ~75 and 6"8d t u 177 were obtained from D. C. STEWART of A.N.L., who prepared them by neutron irradiation of highly-purified lanthanides. These were all purified further by techniques similar to those used for the Pm and Eu tracers. 13) Alpha-active 470y Am 241 was obtained from A.N.L. stocks and purified further by solvent extraction of Am(V1). (4) Purification of TBP. The TBP, as received, was found to contain small quantities of butyl alcohol, dibutyl phosphoric acid, monobutyl phosphoric acid, and a component which was an extremely efficient extractant compared with pure TBP. This latter component was assumed to be a pyrophosphate ester. The TBP was stirred with an equal volume of 6 M HC1 at 60°C for a 12 hr period to hydrolyse any pyrophosphate components. (This treatment also results in increasing the quantities of dibutyl phosphoric acid and monobutyl phosphoric acid and presumably results in the formation of some butyl alcohol and phosphoric acid.) The separated TBP phase was cooled to room temperature and scrubbed with two equal-volume portions of water (to remove most of the mineral acid), three equal-volume portions of 5',!~i aqueous sodium carbonate (to remove the di-ester and the mono-ester), and three equal-volume portions of water. The resultant TBP phase was then slowly raised to a temperature of 30°C, under reduced pressure, to eliminate butyl alcohol and water. Preparation of tracer solutions. In the early stages of this investigation it was found that, in certain instances, partial hydrolysis of the tracer-level nuclide had occurred, resulting in an error in the distribution ratio, the measured value being lower than the true value by a few per cent in the low-K region and by several hundred per cent in the high-K region. This source of error was eliminated by heating the tracer-nuclide in aqueous 8 M HNO 3 at 60°C for 24 hr, diluting the cooled solution to 2 M HNOa, and storing the tracer in this form until ready for use. Determination of distribution ratios. In the determination of the distribution ratio, K, of a specific nuclide, defined as the concentration of nuclide in the organic divided by the concentration of nuclide in the aqueous of two equilibrated sensiblyimmiscible liquid phases, a portion of aqueous acid of the indicated concentration containing one or more radioactive nuclides was equilibrated against a portion of solvent which had previously been pre-equilibrated with respect to the corresponding barren aqueous acid. For example, a 1 ml portion of 10.1 M HNO 3 containing approximately 105 c/m each of beta-active Pr ~3 and alpha-active Am TM was equilibrated with a 1 ml portion of undiluted TBP which had previously been contacted with five successive equal-volume portions of barren 10.1 M H NO~. (All solvent pre-equilibrations were made immediately prior to use to avoid any complications arising from solvent hydrolysis.) Aliquots of each phase were evaporated on 3 rail platinum discs 15 16 in. in diameter, by means of induction heating, for beta and alpha counting. Beta counting was done by means of an end-window proportional counter, using ~zJ D. F. PEPPARt), G, W. MasoN, J. L. MAIER, and W. J. DRISC~)LL In press. ¢4~ D. F. PI-PPARD, W. J. DRISCOLt,, and G. W. MASON T o be submitted to J. inorg, nucL Chem.

328

D . F . PEPPARD, W. J. DRISCOLL, R. J. SIRONEN, and S. MCCARTY

suitable absorber where needed. Alpha counting was done by means of an internal proportional counter. In general, each of the entire series of nuclides was studied in the presence of a single specific nuclide considered to be the normalizing element. For example, in the 10.I M HNO3 x undiluted TBP system Amm was used as the normalizing nuclide with all of the data normalized to the average value obtained for Am. A set of data were considered acceptable only if all Am values were within 109/ooof the average value. In instances in which Am ul was used as an internal normalizing nuclide, the beta activity was counted through 10 mg of aluminium absorber. (A correction of 13 c/m per 10z c/m of Am m was still required.) In instances in which Pm x47was used as an internal normalizing standard, a great preponderance of Pm ~47 was used, so that only a small subtraction from the total measured activity was required in order to obtain that due to P m . 147 The activity of the accompanying nuclide was obtained by using 40 mg of aluminium absorber to lower the Pm 147contribution to a sufficiently low value to permit valid correction. Although it was established, in separate experiments with Pr m and Tm, 17° that 30 see mixing periods were sufficient to establish equilibrium in both the high-acid and low-acid studies, all data were obtained using 3 min mixing periods. The phases, in glass-stoppered 5 ml glass cylinders, were mixed by violent shaking, the two phases co-transferred to a centrifuge cone, the mixture centrifuged at full speed for a 1 min period in an International Clinical Centrifuge, Model CL, the lower phase transferred (by pipette) to a new centrifuge cone (leaving a small heel of lower phase to minimize the possibility of inclusion of traces of the upper phase), both phases centrifuged in their separate cones for a 1 min period, and aliquots of each phase then taken for analysis. In several instances, the resulting pregnant organic extract was contacted with fresh barren aqueous acid and a redetermination of K made to establish the attainment of equilibrium and the absence of hydrolysis errors. All data were obtained at 22 -q- 2°C. The temperature effect within this range was shown to be within the error of the determination. RESULTS AND CONCLUSIONS In Fig. 1, the variation of log K with concentration of HNOa in the aqueous phase is shown for Eu(63) and Tb(65) and for Pr(59) and Tm(69). It may be noted that 63 and 65 are symmetrically located with respect to 64 as are 59 and 69 and that the 63 and 65 curves intersect at a point corresponding to approximately 2.9 M HNO 3 as do the 59 and 69 curves. Other examples of the same behaviour, not shown graphically, are the 57 and 71 pair and the 58 and 70 pair. In Fig. 2, the variation of log K with concentration of HNO~ in the aqueous phase is shown for a number of lanthanides, yttrium, and americium. The placement is arbitrary in order to avoid confusion, each of the curves being displaced upward one log unit with respect to the curve immediately below it with the exception of the curve for Am(95), which is placed at the very top for ease of comparison. On an absolute plot, the curve for Am is very nearly coincident with that for Pr in the region to the left of 8 M HNO 3, and at 18.5 M HNO3 the value of the K for Am is only twice that of the K for Pr. It may be noted that on the basis of the shape of the curves yttrium(39) falls properly between Tb(65) and Tm(69).

Nonmonotonic ordering of lanthanides in tributyl phosphate-nitric acid extraction systems 329 Some of the data of Fig. 2 are presented in a different fashion in Figs. 3 and 4 in order to accent the Z effect. In Fig. 3, the variation of log K, at a constant concentration of H N O 3 acid, with Z is shown for several different H N O 3 concentrations. The 18.5 M HNO3 plot 3.07'

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approximates a straight line. However, it is represented better by two straight lines meeting in the neighbourhood of Z ~ 64. (This curve does not contain a point for Ce, since it was found impossible to prevent partial oxidation of tracer-level Ce(III) to Ce(IV) in this acid medium, even in the presence of hydrogen peroxide.) It is to be noted that as the concentration of H N O 3 decreases, the slope of the right-hand

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portion of the curve decreases more rapidly than does that of the left-hand portion until a definite inversion of some of the high-Z elements is demonstrated by the 3.89 M HNO3curve. This inversion is exaggerated at still lower HNO3 concentrations, as shown by the 0.98 M H N O a curve. This effect, ascribed to a behaviour as two subgroups, Z = 57-64 and Z = 64-71, is shown for other concentrations of H N O a in Fig. 4, in which two arbitrary straight lines, meeting at Z ~- 64, are drawn for each set of data. Whether these lines should be drawn with the point of intersection precisely at 64 is difficult to decide, and no trustworthy Gd tracer has been available for resolving the question. However, on the basis of the half-filled shell hypothesis, (5) it seems logical to draw the lines as in Fig. 4. Two points should be noted about Fig. 4. The curve, as drawn, for 2.94 M HNOz is essentially symmetrical about Z--~ 64; and the slope of the plot in the Z = 57-64 range never becomes negative, even at 0.266 M HNO3. From the fact (Figs. 1 and 2) that some of the lanthanides display a definite maximum followed by a definite minimum in the log K vs. M HNOz plot, it was considered likely that the dependence of K upon the TBP concentration was also a function of H N O 3 concentration. It was, therefore, decided to study the TBP dependence at two concentrations of HNOa, one to the left of the maximum and one to the right of the minimum. The results are shown for 1.96 M HNOz in Figs. 5 and 7 and for 15.5 M HNOz in Figs. 6 and 8. The diluent used was carbon tetrachloride. (51 I"1. BOMMER Z. anorg. Chem. 241,273 (1939).

Nonmonotonic ordering of lanthanides in tributyl phosphate-nitric acid extraction systems 331

From Figs. 5 and 6, it may be seen that the "half-filled shell effect" is operative in both concentrated TBP and dilute TBP systems involving these specific acid concentrations. 103 [

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From Fig. 7, it is seen that the dependence of K upon TBP concentration is approximately second power for the 1.96 M HNO 3 system, the arbitrary lines being drawn for second- and third-power dependencies. The high-Z and low-Z lanthanides appear to have essentially identical TBP dependencies at this acid concentration. However, in Fig. 8, in which the arbitrary lines are drawn for second-, third-, and fourth-power TBP dependencies, it is seen that the K for a low-Z lanthanide approximates a third-power dependency while that for a high-Z lanthanide approximates a fourth-power dependency. From the facts shown by Figs. 5-8, it seemed likely that a given pair of lanthanides for which Z was respectively less than 64 and greater than 64 should show inverted order of extraction at a considerably higher-acid concentration in a system involving diluted TBP than in one involving undiluted TBP. To test this hypothesis, an acid dependency study involving Eu(63) and Tm(69), using Am(95) as internal standard, was made using as solvent a solution made by diluting 25 volumes of pure TBP to 100 volumes with toluene and pre-equilibrating the resulting mixture with respect to the appropriate aqueous acid.

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Nonmonotonic ordering of lanthanides in tributyl phosphate-nitric acid extraction systems 333' The results, along with the 63 and 69 data from Fig. 1, are shown in Fig. 9. It may be seen that the 63 and 69 curves intersect at approximately 7-0 M H N O 3 as opposed to the intersection point at 4.5 M HNO3 in the undiluted TBP system. It may be noted, further, that for the undiluted TBP system, each of the curves has a point of inflection in the 5-6 M HNO3 region and that for the 25 ~ TBP system each has both a maximum and minimum with a point of inflection at approximately 5 M H N O 3. DISCUSSION It is interesting to note that the various plots of log K vs. Z may be considered to approximate a composite of two straight-line portions meeting at Z ~ 64 and that, with decreasing H N O 3 concentration, the slope of the high-Z portion decreases more rapidly than does that of the low-Z portion. It is to be noted, further, that whereas, with decreasing H N O 3 concentration, the slope of the high-Z portion ultimately becomes negative, that of the low-Z portion remains positive throughout the region investigated. When placed on these plots, Y falls at an apparent Z of 66-68, except for the plots in which the high-Z slope is nearly zero, in which case Y can not be placed with greater accuracy than 65-68. It follows, then, that on some plots in which the high-Z portion has a negative slope, Y will be double-valued with respect to apparent Z. For example, in the 2.94 M HNO3 x undiluted TBP system the apparent Z values for Y are 60.6 and 67.6. However, it should be noted that the apparent Z for Y never assumes a value to the left of 64 unless it is double-valued. It will be noted that even at 18-5 M HNOa the two portions of the plot do not form a continuous straight line. At 15.5 M H N O 3 the departure from a single straight line is quite marked. On the latter plot the apparent Z for Yis 67.6. However, if the Y value is plotted at 66.5 and only lanthanides of Z less than 65 are used in the plot, a single straight line results. This half-filled shell effect has been shown by BOMMERtS~ for a plot of ionic radius of lanthanide vs. Z and has been demonstrated by THOMPSON, CUNNINGHAM,and SEA~ORGt61 for the elution of both lanthanides and actinides from a Dowex-50 ionexchange bed using a buffered citrate solution as elutriant. WHEELWRIGHT,SPEDDING, and SCHWARZENBACHtT~ report a half-filled shell effect in their study of the stability of lanthanide complexes with ethylenediaminetetraacetic acid. It is postulated that the extracting species is [M(TBP)a(H20)x_~](NO3) 3 in which x is probably 6, but is conceivably dependent upon Z and smaller in the high-Z than in the low-Z range. The data are consistent with the assumption that the value of a approaches, in the neighbourhood of 2 M H N O 3, 2 for all Z values and, in the neighbourhood of 16 M HNO3, 3 for low-Z nuclides and 4 for high-Z nuclides. Although the exact numbers are in some doubt, it is obvious that a considerable difference in TBP dependencies for Eu and Tm must be assumed in any explanation of the data of Fig. 9. (6) S. G. THOMPSON, B. B. CUNNINGHAM,and G. T. SEABORG J. Amer. chem. Soc. 72, 2798 (1950). (7) E. J. WHEELWRIGHT, F. H. SPEDDING, and G. SCHWARZENBACH ]bid. 75, 4196 (1953).