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The sorption of certain transplutonium ions on amorphous zirconium phosphate
J. inorg, nucl. Chem., 1966, Vol. 28, pp. 1469 to 1478. Pergamon Press Ltd. Printed in Northern Ireland
THE SORPTION OF CERTAIN TRANSPLUTONIUM IONS ON A M O R P H O U S Z I R C O N I U M PHOSPHATE* E. P. HORWlTZ Argonne National Laboratory, Argonne, Illinois (Received 1 September 1965; in revisedform 2 November 1965)
Abstract--The sorption of tracer concentrations of Am(III), Cm(III), Bk(IV) and Cf(III) on amorphous zirconium phosphate with a POdZr mole ratio of 1'34 was investigated. Tracer Ce(III), Eu(III), and U(VI) were also included in the study. The distribution coefficientswere investigated at 75°C as a function of time and nitric acid concentration. Small separation factors were found for ions with the same charge; large separation factors were found for Bk(IV)/Cm(III), and U(IV)/Am(III). The phosphate hydrolysis of amorphous zirconium phosphates with varying POdZr mole ratios was studied between 1 M and 10-s M nitric acid concentrations at 75°C. Spectrophotometric and neutron activation analysis techniques were used to detect small concentrations of phosphate ion. DURING the last decade numerous basic salts and hydrous oxides have been investigated as ion exchange materials. Most of these studies involved zirconium phosphate gels of various compositions because of their superior stability in acid solutions and high ion exchange capacity. ~1-3~ The sorption of cations on zirconium phosphate of various compositions have been largely concerned with the alkali metals and with those metal ions encountered in nuclear fuel processing; namely, cesium, strontium, yttrium, zirconium, ruthenium, uranium and plutonium31'4'5~ These studies have shown that good separations are sometimes possible a m o n g ions with the same oxidation state; e.g. Cs + and Rb + and among ions with different oxidation states; e.g. y3+ and UO22+, employing zirconium phosphate compounds as an ion exchange medium. The only study reported on the sorption of transplutonium ions on zirconium phosphate was performed from highly salted LiC1 solutions on a zirconium phosphate compound of unknown composition and hydrolysis. ~6~ The objects of this study were the following: (1) to investigate the hydrolysis of amorphous zirconium phosphate compounds as a function of PO4/Zr mole ratio in order to obtain a compound for sorption studies with a workable capacity but a minimum interference from hydrolytic release of phosphate ions, (2) to investigate the selectivity of such a compound for the transplutonium ions, amercium(III), curium (III), berkelium(IV) and californium (III), and * Based on work performed under the auspices of the U.S. Atomic Energy Commission. ~1~C. B. AMPHLETT, Inorganic Ion Exchangers, Chap. 5. Elsevier, Amsterdam (1964). ~) G. NANCOLLASand V. PEK~,REK,J. inorg, nucL Chem. 6, 220 (1958). ~8~ S. AHRLAND, J. ALBERTSSON, L. JOHANSSON, B. NIHLGARD and L. NILSSON, Acta chem. scand. 18, 707 (1964). li~ S. AHRLAND, J. ALBERTSSON, t . JOHANSSON, B. NII-ILGk,RD a n d L. NILSSON, Acta chem. scand. 18,
1357 (1964). ~5~S. AHRLANOand J. ALBERTSSON,Acta chem. scand. 18, 1861 (1964). c6~ ORNL-TM-181, Chem. Tech. Div., Progress Report, p. 64, Jan-Mar (1962). 1469
1470
E.P. HoRwrrz
(3) to investigate the relative affinities of the compound for tripositives transplutonium ions and tripositive light lanthanide ions. EXPERIMENTAL
Preparation of amorphous zirconium phosphate Amorphous zirconium phosphates of varying POdZr mole ratio were prepared by the method described by AMPrn-Ea'r.<~ All compounds were dried at 40-50°C instead of room temperature as in AMeHLETr's procedure. The dried samples were ground and separated into 100-200 mesh and 200325 mesh sizes by means of N.B.S. sieves. The granules of zirconium phosphate were given a final water wash while in the sieves in order to remove small quantities of adherent zirconium phosphate dust. The products were analysed by the method described by CLEARFIELDand S~CNES.ts~
Hydrolysis studies The hydrolysis studies were carried out in standard nitric acid solution on the hydrogen form of the zircomiun phosphate compounds, 100-200 mesh size. All equilibrations were performed at 75°C since this was the temperature chosen for the distribution coefficient measurements. The equilibration of the sample and nitric acid solution was carried out in a small centrifuge tube. Mixing was achieved by means of a magnetic stirrer and stirring bar, 12 mm in length and 3 ram in width. Each tube was stoppered with a tight-fitting teflon plug to prevent escape of water vapour and acid. All tubes were placed in a thermostatted jacket at 75°C. After 5 hr equilibration the supernatant liquid was removed and analysed for phosphate. (Preliminary hydrolysis experiments showed that 5 hr mixing at 75°C was sufficient time for all the compounds of varying PO4/Zr mole ratio to attain equilibrium.) The phosphate analysis was performed spectrophotometrically using the molybdovanadophosphoric acid method ~9~ and radiometrically using activation analysis techniques. For the activation analysis, 1002 of supernatant liquid was evaporated on high purity aluminium plates. A standard plate containing known quantities of zirconium and phosphate was also prepared. The sample and standard plates were irradiated with neutrons for one week at a flux of 2 × 10TM neutron cm -~ sec-1. The concentration of phosphate in the unknown was calculated from the measurement of the resultant beta activity from 14"3 day a2p in the standard and unknown. The phosphate concentrations measured by activation analysis and spectrophotometry were averaged although the two measurements gave concentrations within 4-10 per cent of each other. An attempt was made to analyse zirconium in the supernatant liquid by activation analysis; however, the 9sZr activity was insignificant compared to the background activity resulting from impurities in the aluminium plates. Zirconium was measured by emission spectroscopy; however, the concentrations reported are only accurate to within 4-50 per cent.
Capacity determinations The capacity of the hydrogen form of zirconium phosphate was measured at pH 2-00 for neodymium (III). One gramme of a zirconium phosphate sample was eluted in a coltamn at 75°C until the pH of the effluent was the same as that of the influent solution. The influent solution consisted of 0.050 M neodymium nitrate adjusted to a pH 2.00 with nitric acid. After washing the column with water, the adsorbed neodymium was eluted with 2 M I-INO3. The titration was performed at pH 5 using xylenol orange indicator.
Tracer solutions The alpha active nuclides used in this study were 2a2U (74 years), mAre (458 years), m C m (17"5 years), and 85~Cf(2"2 years). These nuclides were obtained from ANL stocks and purified by ion exchange techniques. The radiochemicai purity of these tracers were established by alpha energy t~ C. B. AMPHLETT,L. McDONALD and M. REDMAN,J. inorg, nucL Chem. 6, 220 (1958). (a~ A. CLEARFIELDand J. S'rCNES,J. inorg, nucL Chem. 26, 117 (1964). ~9~O. B. MICHELSON,Analyt. Chem. 29, 60 (1957).
The sorption of certain transplutonium ions on amorphous zirconium phosphate
1471
determinations from ~-pulse height analyses. The ~8~Unuclide was used within one week of purification in order to minimize interference from ~STh daughter. The beta active nuclides used in this study were raCe (33 days) and 152'15~Eu(13, 16 years). These nuclides were obtained from the Oak Ridge National Laboratory and were used without further purification. Stock solutions of the following nuclide mixtures in 0"01 M HNO3 were used in this study: roAm-raCe, 2~XAm-~5~'lS~Eu,~44Cm-mCe, ~44Cm-~52'154Eu, 241Am-252Cfand 2~XAm-~5~U. These solutions were standardized radiometrically by ~-counting using an internal proportional counter, by fl-counting using an end-window proportional counter, and by s-pulse height analyses using a solid state silicon detector plus a Nuclear Data 180 analyser. Determination of distribution coeJ~cients
The distribution coefficients were determined by batch experiments on the hydrogen form of zirconium phosphate, 200-325 mesh size, in standard nitric and sulphuric acids at 75°C. (A temperature of 75°C was chosen in order to facilitate the rate of attainment of equilibrium.) The apparatus used was the same as that used for the hydrolysis experiments. A 10,~or 202 aliquot from one of the standard tracer stock solutions was introduced into the supernatant liquid before equilibration. After a given equilibration time, the solid and liquid phases were separated by centrifugation and the supernatant liquid assayed radiometrically. An aliquot of the supernatant liquid was evaporated on a 3 rail platinum disk ~ in. in diameter, by means of an induction heater, for ~ and fl counting or ~-pulse height analysis. The distribution coefficient was calculated from the following equation: Ao -- A s ml As g where A0 is the activity or counts/min of the solution before equilibration,As is the counts/min of the solution after equilibration, ml is the volume of solution in millilitres, and g is the weight of zirconium phosphate in grammes. The reversibility of the sorption process was examined by first sorbing the activity on the zirconium phosphate from a dilute acid solution, e.g. 0.01 M HNO3, then adding an aliquot of concentrated acid and allowing the system to come to equilibrium. The volume of solution and final acidity were corrected accordingly. The final acidity of the solutions was frequently checked by titration with standard NaOH. The distribution coefficient determinations for Am(III) and Crn(III) were measured in the presence of Eu(llI) which served as an internal normalizing nuclide. In the presence of 241Amor ~44Cm,the fl-activity of as"'~5~Eu was counted through a 10 mg aluminium absorber. A correction of 13 counts/min per 105 counts/min of 241Am and 9 counts/min per 105 counts/min of ~44Cm was still required in order to obtain the fl-activity from ~52'~54Eu. The distribution coefficient determination for Cf(llI), U(VI), Ce(III) and Ce(IV) were measured in the presence of Am(liD which served as an internal normalizing nuclide. Assay plates containing 25~Cf-24~Amor 252U-"4XAmwere measured by ct-energyanalysis. Assay plates containing mCe-~XAm were measured using the same technique as in the case of ~ ' ~5~Eu-~Am. Cerium (I22) measurements were made in the presence of ~10 -~ M C1- which served as a holding reductant. Cerium (IV) measurements were made in the presence of 0-01 M SzO5~--105M Ag÷ which served as a holding oxidant. Ka
RESULTS AND DISCUSSION H y d r o l y s i s studies
The v a r i a t i o n of hydrolysis a n d z i r c o n i u m c o n c e n t r a t i o n with PO4/Zr mole ratio of the a m o r p h o u s z i r c o n i u m p h o s p h a t e samples in 0.1 M H N O 3 is shown in Table 1. It c a n be seen that the hydrolysis decreased with decreasing P O J Z r mole ratio. However, there was n o c o m p o s i t i o n of z i r c o n i u m phosphate within the range studied which was stable to hydrolysis. The samples with a PO4/Zr mole ratio of 1.34 a n d 1"13 do show a substantial reduction in hydrolytic release of phosphate. The z i r c o n i u m c o n c e n t r a t i o n decreased with a n increase in the phosphate concentration. The
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E.P. HORWITZ
TABLE 1.--THE HYDROLYSIS OF ZIRCONIUM PHOSPHATE AS A FUNCTION OF POdZr 5 ml of 0.1 M HNOa, Sample weight ~ 0'55 mmole of Zr, 75°C POJZr of ZP*
mmole of PO4 hydrolysed/g ZP
1.88 1.70 1.50 1.34 1.13
2-1 4.6 1.8 2.0 1.0
x x x x x
MOLE RATIO
mmole of Zr dissolved/g ZP
I0 -x 10-3 10-8 10-8 10-3
1 7 1 2 4
x x x x x
10-s 10-s 10-4 10-4 10-4
* ZP = zirconium phosphate p h o s p h a t e c o n c e n t r a t i o n always exceeded t h a t o f the z i r c o n i u m a l t h o u g h the two a p p r o a c h e d e q u a l i t y (in moles) as the PO4/Zr m o l e r a t i o a p p r o a c h e d 1.0. A p r e v i o u s l y h y d r o l y s e d z i r c o n i u m p h o s p h a t e c o m p o u n d with a m o l e r a t i o o f PO4/Zr o f 1.34 was h y d r o l y s e d a second a n d t h i r d time u n d e r identical c o n d i t i o n s as the first hydrolysis. T h e millimoles o f p h o s p h a t e h y d r o l y s e d was identical within e x p e r i m e n t a l e r r o r in all cases. The v a r i a t i o n o f h y d r o l y s i s o f a z i r c o n i u m p h o s p h a t e c o m p o u n d with a given PO4/Zr m o l e r a t i o with acidity a n d t e m p e r a t u r e is s h o w n in T a b l e 2 a n d 3, respectively. TABLE 2 . - - T H E HYDROLYSIS OF ZIRCONIUM PHOSPHATE AS A FUNCTION OF ACID CONCENTRATION
ml sol/g ZP = 34, 75°C Nitric acid conc. (Molarity) 1-0 0.10 0.010 0.0010
mmole of PO4 hydrolysed/g ZP POJZr = 1 . 3 4 PO4/Zr = 1.13 5"0 2"0 1"9 7.5
X x x x
10-a 10-a 10-8 10-3
2"1 9"9 4"8 6.4
X x x x
10-8 10-4 10-4 10-4
TABLE 3 . - - T H E HYDROLYSIS OF ZIRCONIUM PHOSPHATE AS A FUNCTION OF TEMPERATURE
ml sol/g ZP = 34, PO4/Zr = 1'34, 0"10 M NHO3 Temperature (°C)
mmole of P04 hydrolysed/g ZP
75 50 25
2'0 × 10-3 1"6 × 10"a 1'3 x 10-8
It can be seen f r o m T a b l e 2 t h a t the hydrolysis varies with acid c o n c e n t r a t i o n a n d is at a m i n i m u m in the range o f 10-3 M H N O a . T a b l e 3 shows the slight positive t e m p e r a t u r e coefficient o f the hydrolysis reaction. T h e d a t a in T a b l e s 1, 2 a n d 3 indicate t h a t the z i r c o n i u m p h o s p h a t e c o m p o u n d s with P O a / Z r m o l e r a t i o s o f 1"34 a n d 1.13 are r e a s o n a b l y stable t o w a r d hydrolysis b o t h at high t e m p e r a t u r e a n d o v e r a t h o u s a n d - f o l d acid c o n c e n t r a t i o n range. AHRLAND e t al. ~3) have s h o w n t h a t at c o n c e n t r a t i o n s o f acid greater t h a n 1 M , z i r c o n i u m p h o s p h a t e is a p p r e c i a b l y d e c o m p o s e d b y acid. This is p a r t i c u l a r l y the case o f c o m p o u n d s with low P O J Z r m o l e
The sorption of certain transplutonium ions on amorphous zirconium phosphate
1473
ratios such as the ones listed in Table 2. Several authors (1'z'1°) have shown that hydrolysis of zirconium phosphate becomes appreciable also at lower acidities. This extensive decomposition of zirconium phosphate at acidities beyond the concentration limits shown in Table 2 are of little consequence in transplutonium ion sorption studies. One is limited at high acid concentration by the negligible sorption of the transplutonium ions by zirconium phosphate and at low acid concentration by the hydrolysis of the transplutonium ions themselves. Thus, a practical separation involving transplutonium ions (III) or (IV) would employ nitric acid concentrations no greater than 1 M and no less than 10-3 M. In this nitric acid range the amorphous zirconium phosphate compounds with POJZr mole ratios of 1.34 and 1-13 show relatively small hydrolysis and appear sufficiently stable for a tracer scale study of (III) and (IV) valent transplutonium ions. In addition to the factors influencing hydrolysis shown in Tables 1, 2 and 3 there is a variation in hydrolysis as the ratio of the volume of solution to the weight of sample changes. This property was also reported by AHRLANOe t al. ~3) As the ml/g ratio (solution volume to sample weight) increases, hydrolysis of phosphate increases. In Table 1 the ml/g ratio increases from 30 to 36 as the POJZr mole ratio decreased from 1.88 to 1.13. (This variation in the ml/g ratio was a result of arbitrarly fixing the millimoles of zirconium in each compound hydrolysed at 0.55.) The resultant variation in hydrolysis of phosphate was found to be less than the experimental error in the phosphate analysis. When the ml/g ratio was decreased substantially from the value reported in Table 2, a decrease in hydrolysis of phosphate was observed. Such a situation was observed when the phosphate hydrolysis was measured using an ion exchange column containing one gramme of zirconium phosphate with a PO4/Zr mole ratio of 1.34. The column was eluted with 0.1 M HNO 3 at 75°C at a flow rate of 0.10 ml cm -~ see-1. (Preliminary experiments showed that at this flow rate the hydrolysis of the zirconium phosphate attains equilibrium with the solution in the voids.) The hydrolysis of phosphate was measured for each bed volume (1 ml) for over twenty bed volumes. The phosphate concentration per bed volume was constant over the entire twenty beds volumes at 4.0 × 10-5 mmole of PO4/g of ZP. Thus, the quantity of phosphate hydrolysed from the column was considerably less than the quantity reported in Table 2. The increase in hydrolytic release of phosphate at very high and very low acidities may be explained by two different mechanisms of decomposition of the zirconium phosphate lattice. The increase in hydrolysis of phosphate with increasing pH may be explained by the replacement of the H2PO4- groups on the Zr-O-chains by hydroxyl groups resulting in the formation of very insoluble zirconium hydroxide. ~1~ On the other hand the increase in decomposition at high acidities is probably a result of the breaking of the Zr-O-chains by hydrogen ion. Since one mechanism is favoured by high acidity, the other by low acidity, a minimum in hydrolysis at some acid concentration should be observed. Such a minimum in hydrolysis was observed and is shown in Table 2. The capacity of zirconium phosphate with PO4/Zr ratios of 1.34 and 1.13 for neodymium (III) at pH 2.0 and 75°C is 0.60 mequiv/g and 0.35 mequiv/g, respectively. The increase in capacity with PO4/Zr ratio was also reported by LARSENand VISSER(10) tl0~ E. LARSENand D. VISSER,.Z phys. Chem. 64, 1732 (1960).
1474
E.P. HORWlTZ
and BAETSLE and PELSMAEKERS.(11) The neodymium (III) ion was selected as a substitute for americium (III) and curium (III) in the capacity measurements because o f its similarity in ionic radius and complexing power. The capacities of the two zirconium phosphate compounds are smaller than capacities usually reported in the literatureA 1) The reason for the small capacities of the above compounds compared 10 5
I
I
• ® []
I
I
I
I
Am (/11) Cm ( ~ ) Eu ( = )
104 0.01 M H N O 3
i/ °
I0 3
0 . 0 2 N H 2 S O=4
-
Kd
10 2
-_--
=
®
0.075N .....-..o
.@ H2SO 4 i
o
r-I .078M
_
HNO 3 "
/J
I01
t~
I 0
1 I
I 2
I 3 TIME
I 4 IN
I 5
I 6
7
HOURS
Fro. 1.--Change in the distribution coefficientwith time, POJZr = 1-34,Temp = 75°C. to the literature values t1'1°~ is due mainly to the acidity being fixed at p H 2.0. Since tripositive or tetrapositive transplutonium ions hydrolyse at low acidities, a p H of 2"0 was selected as a practical loading p H of a column containing zirconium phosphate. It was decided to use a compound with a P O J Z r mole ratio of 1.34 for all distribution coefficient measurement because of its combination of stability to hydrolysis and reasonable capacity.
Distribution coe~cient studies Figure 1 shows the variation of distribution coefficient as a function of time for Am(III), Cm(III) and Eu(III) in nitric and sulphuric acid media. The data show that equilibrium is not attained rapidly at either nitric acid concentration shown under the cn~ L. BAETSLI~and J. PELSMAEKERS,Jr. inorff, nucl. Chem. 21, 124 (1961).
The sorption of certain transplutonium ions on amorphous zirconium phosphate
1475
conditions employed. The data in Fig. 1 were obtained by using the hydrogen form of zirconium phosphate, which initially contained no Am(III)-Eu(III) tracer. When the process was reversed by starting with a zirconium phosphate containing the sorbed tracer Am(III)-Eu(III) ions, then adding the nitric acid, equilibrium was obtained in less than one hour. The distribution coefficients obtained by approaching the equilibrium from different directions were identical within the experimental error of ~:5 per cent. Similar results were obtained with Cm(III), Since the rate of exchange equilibria in such compounds has been shown to be controlled by the rate of diffusion of ions into the solid phase, el) one can postulate that the diffusion of hydrogen ion into the zirconium phosphate granule is more rapid than the diffusion of the Am(Ill), Cm(III) and Eu(III) ions. Figure 1 shows that when the media was changed from nitric to sulphuric acid, the rate of attainment of equilibrium for Am(llI), Cm(III) and Eu(III) increased substantially. This effect may be explained by the fact that sulphuric acid more readily decomposes zirconium phosphate than nitric acid tz) and in doing so apparently opens the structure by breaking across linking bonds between polymer chains3 x~) Thus, ions can diffuse into the structure more readily. Figure 1 also shows that although equilibrium is attained slowly, there is a rather rapid initial sorption of Am(Ill), Cm(III) and Eu(III). For example, in the case of the sorption of Am(Ill) and Eu(III) in 0.01 M HNO3, more than 99 per cent of the activity was sorbed on zirconium phosphate in less than 15 min. The distribution coefficients Ce(IV) and U(VI) were also studied as a function of time and gave results similar to those shown in Fig. 1. The shape of the sorption curves in nitric acid may be explained qualitatively by an initial surface adsorption of the tracer concentration of ions by the zirconium phosphate granules followed by diffusion of the ions into the interior of the granule. The variations of the distribution coefficients with nitric acid concentration for a number of ions is shown in Fig. 2. Ce(IV) was used as a substitute for Bk(IV) because the two ions are chemically very similar, tl~) however, the cerium nuclide is more easily detectable radiometrically. The UO22+ ion was used as an approximate substitute for AmO2~+ because of the instability of the latter ion. ~la) It can be seen from Fig. 2 that the experimental points for all the ions except Ce(IV) are very close to straight lines with negative slopes equal to the charge on the ion. BAETSL~and Htrvs ~14) and AHRLAND and ALBERTSSON(5) found similar results for Eu(III) and Ce(III), respectively. The law of mass action is, therefore, obeyed for these ions within the nitric acid concentrations studied31) The Ce(1V) slope only approximates --4. This is probably due to the sulphate ion produced from the oxidation of Ce(III) to (1V) by peroxydisulphate complexing Ce(IV) and thus lowering the charge of the ion. The reversibility of the sorption process was checked for all the ions shown in Fig. 2, with the exception of Ce(IV), by approaching equilibrium from the reverse direction. This was accomplished by sorbing the ion on the solid phase and subsequently ~ls)j. j. KATZand G. T. SEABORO,The Chemistry of the Actinide Elements, Chap. 10, Wiley,New York (1957). t13~j. j. KATZand G. T. SEABORG,The Chemistry of the Actinide Elements Chap. 8, Wiley,New York (1957). tx~ L. BAETSL~and D. HoYs,J. inorg, nucl. Chem. 21, 133 (1961).
1476
E.P. HORW1TZ 105 • ®
o • n a •
Am (]]I) Cm (11r) C f ( rrr )
Ce (rll) Eu (r~) Ce (]3Z) U (vr)
10 4
Kd
10 3
I0 ~
I0 0.01
0.02
0.05 NITRIG
AGID
0.10 GONG.
0.20
0.50
1.0
(Molority)
FIG. 2.--Change in the distribution coefficient with nitric acid conc., POdZr = 1.34, Temp = 75°C., equilibration time -----4-5 hr. removing a fraction of sorbtion by increasing the acid concentration. In every case the distribution coefficients measured fit the straight lines shown in Fig. 2 within the experimental error of :~ 5 per cent. The separation factors (defined as the ratio of the distribution coefficients of two ions) for several pairs of ions are shown in Tables 4 and 5. D a t a from sulphuric acid
The sorption of certain transplutonium ions on amorphous zirconium phosphate
1477
TABLE 4.--THE SEPARATIONFACTORS ON AMORPHOUS ZIRCONIUMPHOSPHATE
PO4/Zr = 1"34,Temp 75°C Cm/Am
Cf/Cm
Eu/Am
Am/Ce
1"20
2-45
1-46
1-86
1'15
2'10
1"36
--
Nitric acid media Sulphuric acid media
TABLE 5.--THE SEPARATIONFACTORS ON AMORPHOUS ZIRCONIUM PHOSPHATE PO4/Zr = 1.34, T e m p 75°C, 0"20 M H N O a
Ce(IV)/Cm(III) 1.75 × 102
Ce(IV)/Cf(III) 7"14 × 10
U(VI)/Cm(III) 5"94 × 10
media at 75°C are included in Table 4 to show the effect of a complexing anion on the separation factor. The separation factors listed in Table 4 show that amorphous zirconium phosphate has very little selectivity for Am(III), Cm(III) and Cf(III). The selectivity for these ions is even less when nitric acid is replaced by sulphuric acid. Although the separation factors for Cm/Am and Cf/Cm show that separations of these ions on columns of zirconium phosphate are theoretically possible, much larger separation factors are obtained employing organic ion exchange resins and chelating agents. The data in Table 4 and Fig. 1 also show that there is no possible group separation between the transplutonium ions (Am-Cm-Cf) and the lanthanide ions (Ce-Eu). Table 5 shows that amorphous zirconium phosphate does have large selectivities for ions with different oxidation states. The separation of Ce(IV), actually Bk(IV), from Cm(III) and Cf(III) using columns of inorganic ion-exchange material appear promising. This separation is especially interesting in view of the fact that zirconium phosphate, unlike organic ion exchange resins, is stable in the presence of strong oxidizing agents. Table 5 also shows a large separation factor for U(VI)/Cm(III). Since the UOz2+ ion is similar in size to the AmO22- ion, ~14)zirconium phosphate may have comparable affinity for the AmO2~+ ion. If AmOz 2+ could be stabilized for a sufficient period of time, a separation of Am(VI) from Cm(III) using an ion exchange column could be affected. Work on both of these separations is currently in progress. The affinity of Dowex-50 for the tripositive lanthanide ions and tripositive americium and curium has been shown to decrease with increasing atomic number. ~15) This trend has been explained qualitatively by showing that the hydrated radii increases, and thus sorption decreases, with an increase in atomic number. The sorption sequence, Cf > Eu > Cm > Am ~> Ce, on amorphous zirconium phosphate is the exact reverse of that on Dowex-50. The sorption increases with increasing atomic number or decreasing crystal radii. (The crystallographic radii of Cf(III) has not been measured; however, by an approximate extension of the actinide contraction, the Cf(III) could be smaller than the Eu(III) ion.) The replaceable hydrogen atoms of the acid phosphate groups in zirconium ~15~ j. p. SURLS, JR. and G. R. CHOPPIN, J. A m . chem. Soc. 79, 855 (1957).
1478
E.P. HORWITZ
phosphate have been shown to play a role similar to that of the sulphate acid groups of organic ion exchange resins such as Dowex-50. tl} The acidity of these phosphoric acid groups is intermediate between a sulphonic acid resin and a carboxylic acid resin. In view of the trend in sorption of the transplutonium and lanthanide ions on zirconium phosphate, one can postulate that the phosphoric acid groups dehydrate the metal ion to some extent in the formation of a complex. It is interesting to note that the sorption behaviour of zirconium phosphate for the (III), (IV) and (VI) ions shown in Fig. 2 parallels the extraction behaviour of various alkyl phosphoric acids for these ions, tl~) although the organic extractants show much greater selectivity. Acknowledgements--The author wishes to thank Mr. K. JENSENfor performing the zirconium phos-
phate analyses and the spectrophotometric phosphate analyses. The author also wishes to thank Mr. M. ESSLINGfor assistance in the neutron activation analysis of phosphorus. (le) D. F. PEPPARD,G. W. MASON,J. L. MAIERand W. J. DRISCOLL,d. inorg, nucL Chem. 4, 334 (1957).
THE SORPTION OF CERTAIN TRANSPLUTONIUM IONS ON A M O R P H O U S Z I R C O N I U M PHOSPHATE* E. P. HORWlTZ Argonne National Laboratory, Argonne, Illinois (Received 1 September 1965; in revisedform 2 November 1965)
Abstract--The sorption of tracer concentrations of Am(III), Cm(III), Bk(IV) and Cf(III) on amorphous zirconium phosphate with a POdZr mole ratio of 1'34 was investigated. Tracer Ce(III), Eu(III), and U(VI) were also included in the study. The distribution coefficientswere investigated at 75°C as a function of time and nitric acid concentration. Small separation factors were found for ions with the same charge; large separation factors were found for Bk(IV)/Cm(III), and U(IV)/Am(III). The phosphate hydrolysis of amorphous zirconium phosphates with varying POdZr mole ratios was studied between 1 M and 10-s M nitric acid concentrations at 75°C. Spectrophotometric and neutron activation analysis techniques were used to detect small concentrations of phosphate ion. DURING the last decade numerous basic salts and hydrous oxides have been investigated as ion exchange materials. Most of these studies involved zirconium phosphate gels of various compositions because of their superior stability in acid solutions and high ion exchange capacity. ~1-3~ The sorption of cations on zirconium phosphate of various compositions have been largely concerned with the alkali metals and with those metal ions encountered in nuclear fuel processing; namely, cesium, strontium, yttrium, zirconium, ruthenium, uranium and plutonium31'4'5~ These studies have shown that good separations are sometimes possible a m o n g ions with the same oxidation state; e.g. Cs + and Rb + and among ions with different oxidation states; e.g. y3+ and UO22+, employing zirconium phosphate compounds as an ion exchange medium. The only study reported on the sorption of transplutonium ions on zirconium phosphate was performed from highly salted LiC1 solutions on a zirconium phosphate compound of unknown composition and hydrolysis. ~6~ The objects of this study were the following: (1) to investigate the hydrolysis of amorphous zirconium phosphate compounds as a function of PO4/Zr mole ratio in order to obtain a compound for sorption studies with a workable capacity but a minimum interference from hydrolytic release of phosphate ions, (2) to investigate the selectivity of such a compound for the transplutonium ions, amercium(III), curium (III), berkelium(IV) and californium (III), and * Based on work performed under the auspices of the U.S. Atomic Energy Commission. ~1~C. B. AMPHLETT, Inorganic Ion Exchangers, Chap. 5. Elsevier, Amsterdam (1964). ~) G. NANCOLLASand V. PEK~,REK,J. inorg, nucL Chem. 6, 220 (1958). ~8~ S. AHRLAND, J. ALBERTSSON, L. JOHANSSON, B. NIHLGARD and L. NILSSON, Acta chem. scand. 18, 707 (1964). li~ S. AHRLAND, J. ALBERTSSON, t . JOHANSSON, B. NII-ILGk,RD a n d L. NILSSON, Acta chem. scand. 18,
1357 (1964). ~5~S. AHRLANOand J. ALBERTSSON,Acta chem. scand. 18, 1861 (1964). c6~ ORNL-TM-181, Chem. Tech. Div., Progress Report, p. 64, Jan-Mar (1962). 1469
1470
E.P. HoRwrrz
(3) to investigate the relative affinities of the compound for tripositives transplutonium ions and tripositive light lanthanide ions. EXPERIMENTAL
Preparation of amorphous zirconium phosphate Amorphous zirconium phosphates of varying POdZr mole ratio were prepared by the method described by AMPrn-Ea'r.<~ All compounds were dried at 40-50°C instead of room temperature as in AMeHLETr's procedure. The dried samples were ground and separated into 100-200 mesh and 200325 mesh sizes by means of N.B.S. sieves. The granules of zirconium phosphate were given a final water wash while in the sieves in order to remove small quantities of adherent zirconium phosphate dust. The products were analysed by the method described by CLEARFIELDand S~CNES.ts~
Hydrolysis studies The hydrolysis studies were carried out in standard nitric acid solution on the hydrogen form of the zircomiun phosphate compounds, 100-200 mesh size. All equilibrations were performed at 75°C since this was the temperature chosen for the distribution coefficient measurements. The equilibration of the sample and nitric acid solution was carried out in a small centrifuge tube. Mixing was achieved by means of a magnetic stirrer and stirring bar, 12 mm in length and 3 ram in width. Each tube was stoppered with a tight-fitting teflon plug to prevent escape of water vapour and acid. All tubes were placed in a thermostatted jacket at 75°C. After 5 hr equilibration the supernatant liquid was removed and analysed for phosphate. (Preliminary hydrolysis experiments showed that 5 hr mixing at 75°C was sufficient time for all the compounds of varying PO4/Zr mole ratio to attain equilibrium.) The phosphate analysis was performed spectrophotometrically using the molybdovanadophosphoric acid method ~9~ and radiometrically using activation analysis techniques. For the activation analysis, 1002 of supernatant liquid was evaporated on high purity aluminium plates. A standard plate containing known quantities of zirconium and phosphate was also prepared. The sample and standard plates were irradiated with neutrons for one week at a flux of 2 × 10TM neutron cm -~ sec-1. The concentration of phosphate in the unknown was calculated from the measurement of the resultant beta activity from 14"3 day a2p in the standard and unknown. The phosphate concentrations measured by activation analysis and spectrophotometry were averaged although the two measurements gave concentrations within 4-10 per cent of each other. An attempt was made to analyse zirconium in the supernatant liquid by activation analysis; however, the 9sZr activity was insignificant compared to the background activity resulting from impurities in the aluminium plates. Zirconium was measured by emission spectroscopy; however, the concentrations reported are only accurate to within 4-50 per cent.
Capacity determinations The capacity of the hydrogen form of zirconium phosphate was measured at pH 2-00 for neodymium (III). One gramme of a zirconium phosphate sample was eluted in a coltamn at 75°C until the pH of the effluent was the same as that of the influent solution. The influent solution consisted of 0.050 M neodymium nitrate adjusted to a pH 2.00 with nitric acid. After washing the column with water, the adsorbed neodymium was eluted with 2 M I-INO3. The titration was performed at pH 5 using xylenol orange indicator.
Tracer solutions The alpha active nuclides used in this study were 2a2U (74 years), mAre (458 years), m C m (17"5 years), and 85~Cf(2"2 years). These nuclides were obtained from ANL stocks and purified by ion exchange techniques. The radiochemicai purity of these tracers were established by alpha energy t~ C. B. AMPHLETT,L. McDONALD and M. REDMAN,J. inorg, nucL Chem. 6, 220 (1958). (a~ A. CLEARFIELDand J. S'rCNES,J. inorg, nucL Chem. 26, 117 (1964). ~9~O. B. MICHELSON,Analyt. Chem. 29, 60 (1957).
The sorption of certain transplutonium ions on amorphous zirconium phosphate
1471
determinations from ~-pulse height analyses. The ~8~Unuclide was used within one week of purification in order to minimize interference from ~STh daughter. The beta active nuclides used in this study were raCe (33 days) and 152'15~Eu(13, 16 years). These nuclides were obtained from the Oak Ridge National Laboratory and were used without further purification. Stock solutions of the following nuclide mixtures in 0"01 M HNO3 were used in this study: roAm-raCe, 2~XAm-~5~'lS~Eu,~44Cm-mCe, ~44Cm-~52'154Eu, 241Am-252Cfand 2~XAm-~5~U. These solutions were standardized radiometrically by ~-counting using an internal proportional counter, by fl-counting using an end-window proportional counter, and by s-pulse height analyses using a solid state silicon detector plus a Nuclear Data 180 analyser. Determination of distribution coeJ~cients
The distribution coefficients were determined by batch experiments on the hydrogen form of zirconium phosphate, 200-325 mesh size, in standard nitric and sulphuric acids at 75°C. (A temperature of 75°C was chosen in order to facilitate the rate of attainment of equilibrium.) The apparatus used was the same as that used for the hydrolysis experiments. A 10,~or 202 aliquot from one of the standard tracer stock solutions was introduced into the supernatant liquid before equilibration. After a given equilibration time, the solid and liquid phases were separated by centrifugation and the supernatant liquid assayed radiometrically. An aliquot of the supernatant liquid was evaporated on a 3 rail platinum disk ~ in. in diameter, by means of an induction heater, for ~ and fl counting or ~-pulse height analysis. The distribution coefficient was calculated from the following equation: Ao -- A s ml As g where A0 is the activity or counts/min of the solution before equilibration,As is the counts/min of the solution after equilibration, ml is the volume of solution in millilitres, and g is the weight of zirconium phosphate in grammes. The reversibility of the sorption process was examined by first sorbing the activity on the zirconium phosphate from a dilute acid solution, e.g. 0.01 M HNO3, then adding an aliquot of concentrated acid and allowing the system to come to equilibrium. The volume of solution and final acidity were corrected accordingly. The final acidity of the solutions was frequently checked by titration with standard NaOH. The distribution coefficient determinations for Am(III) and Crn(III) were measured in the presence of Eu(llI) which served as an internal normalizing nuclide. In the presence of 241Amor ~44Cm,the fl-activity of as"'~5~Eu was counted through a 10 mg aluminium absorber. A correction of 13 counts/min per 105 counts/min of 241Am and 9 counts/min per 105 counts/min of ~44Cm was still required in order to obtain the fl-activity from ~52'~54Eu. The distribution coefficient determination for Cf(llI), U(VI), Ce(III) and Ce(IV) were measured in the presence of Am(liD which served as an internal normalizing nuclide. Assay plates containing 25~Cf-24~Amor 252U-"4XAmwere measured by ct-energyanalysis. Assay plates containing mCe-~XAm were measured using the same technique as in the case of ~ ' ~5~Eu-~Am. Cerium (I22) measurements were made in the presence of ~10 -~ M C1- which served as a holding reductant. Cerium (IV) measurements were made in the presence of 0-01 M SzO5~--105M Ag÷ which served as a holding oxidant. Ka
RESULTS AND DISCUSSION H y d r o l y s i s studies
The v a r i a t i o n of hydrolysis a n d z i r c o n i u m c o n c e n t r a t i o n with PO4/Zr mole ratio of the a m o r p h o u s z i r c o n i u m p h o s p h a t e samples in 0.1 M H N O 3 is shown in Table 1. It c a n be seen that the hydrolysis decreased with decreasing P O J Z r mole ratio. However, there was n o c o m p o s i t i o n of z i r c o n i u m phosphate within the range studied which was stable to hydrolysis. The samples with a PO4/Zr mole ratio of 1.34 a n d 1"13 do show a substantial reduction in hydrolytic release of phosphate. The z i r c o n i u m c o n c e n t r a t i o n decreased with a n increase in the phosphate concentration. The
1472
E.P. HORWITZ
TABLE 1.--THE HYDROLYSIS OF ZIRCONIUM PHOSPHATE AS A FUNCTION OF POdZr 5 ml of 0.1 M HNOa, Sample weight ~ 0'55 mmole of Zr, 75°C POJZr of ZP*
mmole of PO4 hydrolysed/g ZP
1.88 1.70 1.50 1.34 1.13
2-1 4.6 1.8 2.0 1.0
x x x x x
MOLE RATIO
mmole of Zr dissolved/g ZP
I0 -x 10-3 10-8 10-8 10-3
1 7 1 2 4
x x x x x
10-s 10-s 10-4 10-4 10-4
* ZP = zirconium phosphate p h o s p h a t e c o n c e n t r a t i o n always exceeded t h a t o f the z i r c o n i u m a l t h o u g h the two a p p r o a c h e d e q u a l i t y (in moles) as the PO4/Zr m o l e r a t i o a p p r o a c h e d 1.0. A p r e v i o u s l y h y d r o l y s e d z i r c o n i u m p h o s p h a t e c o m p o u n d with a m o l e r a t i o o f PO4/Zr o f 1.34 was h y d r o l y s e d a second a n d t h i r d time u n d e r identical c o n d i t i o n s as the first hydrolysis. T h e millimoles o f p h o s p h a t e h y d r o l y s e d was identical within e x p e r i m e n t a l e r r o r in all cases. The v a r i a t i o n o f h y d r o l y s i s o f a z i r c o n i u m p h o s p h a t e c o m p o u n d with a given PO4/Zr m o l e r a t i o with acidity a n d t e m p e r a t u r e is s h o w n in T a b l e 2 a n d 3, respectively. TABLE 2 . - - T H E HYDROLYSIS OF ZIRCONIUM PHOSPHATE AS A FUNCTION OF ACID CONCENTRATION
ml sol/g ZP = 34, 75°C Nitric acid conc. (Molarity) 1-0 0.10 0.010 0.0010
mmole of PO4 hydrolysed/g ZP POJZr = 1 . 3 4 PO4/Zr = 1.13 5"0 2"0 1"9 7.5
X x x x
10-a 10-a 10-8 10-3
2"1 9"9 4"8 6.4
X x x x
10-8 10-4 10-4 10-4
TABLE 3 . - - T H E HYDROLYSIS OF ZIRCONIUM PHOSPHATE AS A FUNCTION OF TEMPERATURE
ml sol/g ZP = 34, PO4/Zr = 1'34, 0"10 M NHO3 Temperature (°C)
mmole of P04 hydrolysed/g ZP
75 50 25
2'0 × 10-3 1"6 × 10"a 1'3 x 10-8
It can be seen f r o m T a b l e 2 t h a t the hydrolysis varies with acid c o n c e n t r a t i o n a n d is at a m i n i m u m in the range o f 10-3 M H N O a . T a b l e 3 shows the slight positive t e m p e r a t u r e coefficient o f the hydrolysis reaction. T h e d a t a in T a b l e s 1, 2 a n d 3 indicate t h a t the z i r c o n i u m p h o s p h a t e c o m p o u n d s with P O a / Z r m o l e r a t i o s o f 1"34 a n d 1.13 are r e a s o n a b l y stable t o w a r d hydrolysis b o t h at high t e m p e r a t u r e a n d o v e r a t h o u s a n d - f o l d acid c o n c e n t r a t i o n range. AHRLAND e t al. ~3) have s h o w n t h a t at c o n c e n t r a t i o n s o f acid greater t h a n 1 M , z i r c o n i u m p h o s p h a t e is a p p r e c i a b l y d e c o m p o s e d b y acid. This is p a r t i c u l a r l y the case o f c o m p o u n d s with low P O J Z r m o l e
The sorption of certain transplutonium ions on amorphous zirconium phosphate
1473
ratios such as the ones listed in Table 2. Several authors (1'z'1°) have shown that hydrolysis of zirconium phosphate becomes appreciable also at lower acidities. This extensive decomposition of zirconium phosphate at acidities beyond the concentration limits shown in Table 2 are of little consequence in transplutonium ion sorption studies. One is limited at high acid concentration by the negligible sorption of the transplutonium ions by zirconium phosphate and at low acid concentration by the hydrolysis of the transplutonium ions themselves. Thus, a practical separation involving transplutonium ions (III) or (IV) would employ nitric acid concentrations no greater than 1 M and no less than 10-3 M. In this nitric acid range the amorphous zirconium phosphate compounds with POJZr mole ratios of 1.34 and 1-13 show relatively small hydrolysis and appear sufficiently stable for a tracer scale study of (III) and (IV) valent transplutonium ions. In addition to the factors influencing hydrolysis shown in Tables 1, 2 and 3 there is a variation in hydrolysis as the ratio of the volume of solution to the weight of sample changes. This property was also reported by AHRLANOe t al. ~3) As the ml/g ratio (solution volume to sample weight) increases, hydrolysis of phosphate increases. In Table 1 the ml/g ratio increases from 30 to 36 as the POJZr mole ratio decreased from 1.88 to 1.13. (This variation in the ml/g ratio was a result of arbitrarly fixing the millimoles of zirconium in each compound hydrolysed at 0.55.) The resultant variation in hydrolysis of phosphate was found to be less than the experimental error in the phosphate analysis. When the ml/g ratio was decreased substantially from the value reported in Table 2, a decrease in hydrolysis of phosphate was observed. Such a situation was observed when the phosphate hydrolysis was measured using an ion exchange column containing one gramme of zirconium phosphate with a PO4/Zr mole ratio of 1.34. The column was eluted with 0.1 M HNO 3 at 75°C at a flow rate of 0.10 ml cm -~ see-1. (Preliminary experiments showed that at this flow rate the hydrolysis of the zirconium phosphate attains equilibrium with the solution in the voids.) The hydrolysis of phosphate was measured for each bed volume (1 ml) for over twenty bed volumes. The phosphate concentration per bed volume was constant over the entire twenty beds volumes at 4.0 × 10-5 mmole of PO4/g of ZP. Thus, the quantity of phosphate hydrolysed from the column was considerably less than the quantity reported in Table 2. The increase in hydrolytic release of phosphate at very high and very low acidities may be explained by two different mechanisms of decomposition of the zirconium phosphate lattice. The increase in hydrolysis of phosphate with increasing pH may be explained by the replacement of the H2PO4- groups on the Zr-O-chains by hydroxyl groups resulting in the formation of very insoluble zirconium hydroxide. ~1~ On the other hand the increase in decomposition at high acidities is probably a result of the breaking of the Zr-O-chains by hydrogen ion. Since one mechanism is favoured by high acidity, the other by low acidity, a minimum in hydrolysis at some acid concentration should be observed. Such a minimum in hydrolysis was observed and is shown in Table 2. The capacity of zirconium phosphate with PO4/Zr ratios of 1.34 and 1.13 for neodymium (III) at pH 2.0 and 75°C is 0.60 mequiv/g and 0.35 mequiv/g, respectively. The increase in capacity with PO4/Zr ratio was also reported by LARSENand VISSER(10) tl0~ E. LARSENand D. VISSER,.Z phys. Chem. 64, 1732 (1960).
1474
E.P. HORWlTZ
and BAETSLE and PELSMAEKERS.(11) The neodymium (III) ion was selected as a substitute for americium (III) and curium (III) in the capacity measurements because o f its similarity in ionic radius and complexing power. The capacities of the two zirconium phosphate compounds are smaller than capacities usually reported in the literatureA 1) The reason for the small capacities of the above compounds compared 10 5
I
I
• ® []
I
I
I
I
Am (/11) Cm ( ~ ) Eu ( = )
104 0.01 M H N O 3
i/ °
I0 3
0 . 0 2 N H 2 S O=4
-
Kd
10 2
-_--
=
®
0.075N .....-..o
.@ H2SO 4 i
o
r-I .078M
_
HNO 3 "
/J
I01
t~
I 0
1 I
I 2
I 3 TIME
I 4 IN
I 5
I 6
7
HOURS
Fro. 1.--Change in the distribution coefficientwith time, POJZr = 1-34,Temp = 75°C. to the literature values t1'1°~ is due mainly to the acidity being fixed at p H 2.0. Since tripositive or tetrapositive transplutonium ions hydrolyse at low acidities, a p H of 2"0 was selected as a practical loading p H of a column containing zirconium phosphate. It was decided to use a compound with a P O J Z r mole ratio of 1.34 for all distribution coefficient measurement because of its combination of stability to hydrolysis and reasonable capacity.
Distribution coe~cient studies Figure 1 shows the variation of distribution coefficient as a function of time for Am(III), Cm(III) and Eu(III) in nitric and sulphuric acid media. The data show that equilibrium is not attained rapidly at either nitric acid concentration shown under the cn~ L. BAETSLI~and J. PELSMAEKERS,Jr. inorff, nucl. Chem. 21, 124 (1961).
The sorption of certain transplutonium ions on amorphous zirconium phosphate
1475
conditions employed. The data in Fig. 1 were obtained by using the hydrogen form of zirconium phosphate, which initially contained no Am(III)-Eu(III) tracer. When the process was reversed by starting with a zirconium phosphate containing the sorbed tracer Am(III)-Eu(III) ions, then adding the nitric acid, equilibrium was obtained in less than one hour. The distribution coefficients obtained by approaching the equilibrium from different directions were identical within the experimental error of ~:5 per cent. Similar results were obtained with Cm(III), Since the rate of exchange equilibria in such compounds has been shown to be controlled by the rate of diffusion of ions into the solid phase, el) one can postulate that the diffusion of hydrogen ion into the zirconium phosphate granule is more rapid than the diffusion of the Am(Ill), Cm(III) and Eu(III) ions. Figure 1 shows that when the media was changed from nitric to sulphuric acid, the rate of attainment of equilibrium for Am(llI), Cm(III) and Eu(III) increased substantially. This effect may be explained by the fact that sulphuric acid more readily decomposes zirconium phosphate than nitric acid tz) and in doing so apparently opens the structure by breaking across linking bonds between polymer chains3 x~) Thus, ions can diffuse into the structure more readily. Figure 1 also shows that although equilibrium is attained slowly, there is a rather rapid initial sorption of Am(Ill), Cm(III) and Eu(III). For example, in the case of the sorption of Am(Ill) and Eu(III) in 0.01 M HNO3, more than 99 per cent of the activity was sorbed on zirconium phosphate in less than 15 min. The distribution coefficients Ce(IV) and U(VI) were also studied as a function of time and gave results similar to those shown in Fig. 1. The shape of the sorption curves in nitric acid may be explained qualitatively by an initial surface adsorption of the tracer concentration of ions by the zirconium phosphate granules followed by diffusion of the ions into the interior of the granule. The variations of the distribution coefficients with nitric acid concentration for a number of ions is shown in Fig. 2. Ce(IV) was used as a substitute for Bk(IV) because the two ions are chemically very similar, tl~) however, the cerium nuclide is more easily detectable radiometrically. The UO22+ ion was used as an approximate substitute for AmO2~+ because of the instability of the latter ion. ~la) It can be seen from Fig. 2 that the experimental points for all the ions except Ce(IV) are very close to straight lines with negative slopes equal to the charge on the ion. BAETSL~and Htrvs ~14) and AHRLAND and ALBERTSSON(5) found similar results for Eu(III) and Ce(III), respectively. The law of mass action is, therefore, obeyed for these ions within the nitric acid concentrations studied31) The Ce(1V) slope only approximates --4. This is probably due to the sulphate ion produced from the oxidation of Ce(III) to (1V) by peroxydisulphate complexing Ce(IV) and thus lowering the charge of the ion. The reversibility of the sorption process was checked for all the ions shown in Fig. 2, with the exception of Ce(IV), by approaching equilibrium from the reverse direction. This was accomplished by sorbing the ion on the solid phase and subsequently ~ls)j. j. KATZand G. T. SEABORO,The Chemistry of the Actinide Elements, Chap. 10, Wiley,New York (1957). t13~j. j. KATZand G. T. SEABORG,The Chemistry of the Actinide Elements Chap. 8, Wiley,New York (1957). tx~ L. BAETSL~and D. HoYs,J. inorg, nucl. Chem. 21, 133 (1961).
1476
E.P. HORW1TZ 105 • ®
o • n a •
Am (]]I) Cm (11r) C f ( rrr )
Ce (rll) Eu (r~) Ce (]3Z) U (vr)
10 4
Kd
10 3
I0 ~
I0 0.01
0.02
0.05 NITRIG
AGID
0.10 GONG.
0.20
0.50
1.0
(Molority)
FIG. 2.--Change in the distribution coefficient with nitric acid conc., POdZr = 1.34, Temp = 75°C., equilibration time -----4-5 hr. removing a fraction of sorbtion by increasing the acid concentration. In every case the distribution coefficients measured fit the straight lines shown in Fig. 2 within the experimental error of :~ 5 per cent. The separation factors (defined as the ratio of the distribution coefficients of two ions) for several pairs of ions are shown in Tables 4 and 5. D a t a from sulphuric acid
The sorption of certain transplutonium ions on amorphous zirconium phosphate
1477
TABLE 4.--THE SEPARATIONFACTORS ON AMORPHOUS ZIRCONIUMPHOSPHATE
PO4/Zr = 1"34,Temp 75°C Cm/Am
Cf/Cm
Eu/Am
Am/Ce
1"20
2-45
1-46
1-86
1'15
2'10
1"36
--
Nitric acid media Sulphuric acid media
TABLE 5.--THE SEPARATIONFACTORS ON AMORPHOUS ZIRCONIUM PHOSPHATE PO4/Zr = 1.34, T e m p 75°C, 0"20 M H N O a
Ce(IV)/Cm(III) 1.75 × 102
Ce(IV)/Cf(III) 7"14 × 10
U(VI)/Cm(III) 5"94 × 10
media at 75°C are included in Table 4 to show the effect of a complexing anion on the separation factor. The separation factors listed in Table 4 show that amorphous zirconium phosphate has very little selectivity for Am(III), Cm(III) and Cf(III). The selectivity for these ions is even less when nitric acid is replaced by sulphuric acid. Although the separation factors for Cm/Am and Cf/Cm show that separations of these ions on columns of zirconium phosphate are theoretically possible, much larger separation factors are obtained employing organic ion exchange resins and chelating agents. The data in Table 4 and Fig. 1 also show that there is no possible group separation between the transplutonium ions (Am-Cm-Cf) and the lanthanide ions (Ce-Eu). Table 5 shows that amorphous zirconium phosphate does have large selectivities for ions with different oxidation states. The separation of Ce(IV), actually Bk(IV), from Cm(III) and Cf(III) using columns of inorganic ion-exchange material appear promising. This separation is especially interesting in view of the fact that zirconium phosphate, unlike organic ion exchange resins, is stable in the presence of strong oxidizing agents. Table 5 also shows a large separation factor for U(VI)/Cm(III). Since the UOz2+ ion is similar in size to the AmO22- ion, ~14)zirconium phosphate may have comparable affinity for the AmO2~+ ion. If AmOz 2+ could be stabilized for a sufficient period of time, a separation of Am(VI) from Cm(III) using an ion exchange column could be affected. Work on both of these separations is currently in progress. The affinity of Dowex-50 for the tripositive lanthanide ions and tripositive americium and curium has been shown to decrease with increasing atomic number. ~15) This trend has been explained qualitatively by showing that the hydrated radii increases, and thus sorption decreases, with an increase in atomic number. The sorption sequence, Cf > Eu > Cm > Am ~> Ce, on amorphous zirconium phosphate is the exact reverse of that on Dowex-50. The sorption increases with increasing atomic number or decreasing crystal radii. (The crystallographic radii of Cf(III) has not been measured; however, by an approximate extension of the actinide contraction, the Cf(III) could be smaller than the Eu(III) ion.) The replaceable hydrogen atoms of the acid phosphate groups in zirconium ~15~ j. p. SURLS, JR. and G. R. CHOPPIN, J. A m . chem. Soc. 79, 855 (1957).
1478
E.P. HORWITZ
phosphate have been shown to play a role similar to that of the sulphate acid groups of organic ion exchange resins such as Dowex-50. tl} The acidity of these phosphoric acid groups is intermediate between a sulphonic acid resin and a carboxylic acid resin. In view of the trend in sorption of the transplutonium and lanthanide ions on zirconium phosphate, one can postulate that the phosphoric acid groups dehydrate the metal ion to some extent in the formation of a complex. It is interesting to note that the sorption behaviour of zirconium phosphate for the (III), (IV) and (VI) ions shown in Fig. 2 parallels the extraction behaviour of various alkyl phosphoric acids for these ions, tl~) although the organic extractants show much greater selectivity. Acknowledgements--The author wishes to thank Mr. K. JENSENfor performing the zirconium phos-
phate analyses and the spectrophotometric phosphate analyses. The author also wishes to thank Mr. M. ESSLINGfor assistance in the neutron activation analysis of phosphorus. (le) D. F. PEPPARD,G. W. MASON,J. L. MAIERand W. J. DRISCOLL,d. inorg, nucL Chem. 4, 334 (1957).