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Solvent extraction of some diprotic nitroaromatic acids and of their complexes with some actinides—III
.I. inorg, m . l . Chem., 1972, Vol. 34, pp, 1043-1052.
Pergamon Press.
Printed in Great Britain
S O L V E N T E X T R A C T I O N OF SOME D I P R O T I C N I T R O A R O M A T I C A C I D S A N D OF T H E I R COMPLEXES WITH SOME ACTINIDES-III EXTRACTION OF TETRAVALENT PLUTONIUM AND 3-NITRO-p-CRESOL-5-SULPHONATE COMPLEXES TRIBUTYLPHOSPHATE
THORIUM WITH
M. BERAN Department of Chemistry, Nuclear Research Institute of Czechoslovak Academy of Sciences, l~e~. near Prague, Czechoslovakia (Received 6 July 1971 )
Abstract-The extraction of tetravalent plutonium and thorium from 1 M (H, Na)CI aqueous solution containing 3-nitro-p-cresol-5-sulphonic acid (HzA) with a n-dodecane solution of tributyl-phosphate (TBP) at 25 _+0.2°C has been studied. The experimental data were explained in terms of the following extractable complexes: PuA(HA)CI-3TBP and PuC14-2TBP; ThA(HA)C1.2TBP, ThACI.~.2TBP and ThA.,.2TBP. The first nitroaromatic complex, MA 2+, was found in the aqueous phase in addition to the first chloride complex, MCI a+, in both cases. The stability constants of these species have been calculated. T HE ARILITY o f 3 - n i t r o - p - c r e s o l - 5 - s u l p h o n i c a c i d ( H 2 A ) to f o r m c o m p l e x e s e x t r a c t e d b y T B P [ 1 ] , has b e e n m u c h s t u d i e d r e c e n t l y [2, 3]. O n l y n e u t r a l s p e c i e s , e.g. t h e u n d i s s o c i a t e d a c i d [2] o r its n o r m a l u r a n y l salt[3], w e r e e x t r a c t e d b y a s o l v a t i o n m e c h a n i s m w i t h T B P . S u c h c o m p l e x e s m a y be o f i n t e r e s t f r o m the v i e w p o i n t o f t h e e x t r a c t i o n m e c h a n i s m , a n d m a y a l s o be i m p o r t a n t in s e p a r a t i o n p r o c e s s e s ; w e t h e r e f o r e d e c i d e d to i n v e s t i g a t e t h e e x t r a c t i o n o f s o m e o t h e r m e t a l s . T h e r e s u l t s fo r t e t r a v a l e n t p l u t o n i u m a n d t h o r i u m are g i v e n b e l o w . EXPERIMENTAL Reagents and apparatus. Most of the chemicals as well as the apparatus used, were as described previously [2, 3]. Sodium nitrite (Lachema, analytical grade) was used to maintain plutonium in the tetravalent state. Stock solutions o f radioactive isotopes. 85-2 mg of metallic ~:~"Pu, the radiochemical purity of which was verified by o-spectrometry, were dissolved in 7-8 ml of I 1.8 M HCI. The resulting stock solution, which was about 0.05 M in Pu and 10 M in HCI, contained > 98% Pu as Pu(IV). The Pu(IV) content was checked by extraction with 0.5 M TTA in xylene from 1 M HCI[4], and was found to be stable for more than 30 days. Thorium-288, an c~-emitter(half-life 1-91 years), was supplied by R.C.C., Amersham as crystalline thorium nitrate. It was purified from its disintegration products by solvent extraction of thorium with undiluted TBP from 10 M HC1 [5] according to the following procedure: to about 1 mCi of 22STh nitrate 55.2 mg of Th(N O:~)4-4H._,O were added, and after dissolution in 1 ml of 0.1 M HCI thorium hydroxide 1. M. Beran, in Solvent Extraction Research (Edited by A. S. Kertes and Y. Marcus) p. 75. Wileylnterscience, New York (1969). 2. M. Beran, J. inorg, nucl. Chem. 33,839 (1971). 3. M. Beran,J. inorg, nucl. Chem. In press. 4. A. M. Poskanzer and B. M. Foreman, Jr.,d. inorg, nucl. Chem. 16,323 (1961). 5. T. Ishimori, K. Watanabe and E. Nakamura, Bull. Chem. Soc. Japan 33,636 (1960). 1043
1044
M. B E R A N
was precipitated by adding 1 ml of approx. 1 M NH4OH. The precipitate was filtered on a sintered glass filter, washed with distilled water and dissolved in 10 ml of 10 M HC1. Thorium was then extracted with 10 ml of undiluted TBP and its radiochemical purity was checked by c~-spectrometry. It contained < 3% of c~-impurities. The organic phase, which served as the thorium feed solution in experiments, was equilibrated with the aqueous phase before each use to backwash the disintegration products of ~2SThfrom it. Distribution experiments. In the plutonium distribution experiments 10 ml of an aqueous phase containing 0"2-1-0 M HCI in 1 M (H, Na)CI, 0.01 M NaNOz and K H A in the required concentration (up to 0.02 M) were prepared by mixing standard solutions of the components. 1 ml of the aqueous phase was pipetted into a penicillin glass tube with a polyethylene stopper; 10/~1 of the plutonium feed solution (diluted 20-200 × with 10 M HCI) and 1 ml of n-dodecane/TBP solution were added and both phases were equilibrated at 25°C. As the extraction of HC1 is negligible [6] under the experimental conditions used, the equilibrium aqueous concentration of hydrochloric acid was considered to be equal to its initial concentration. Thorium distribution experiments were performed as follows: 8 ml of an aqueous phase containing 0-02-0.08 M HC! in 1 M (H, Na)CI and K H A in the required concentration (up to 0.012 M) were prepared by mixing standard solutions of the single components in a 20 ml glass tube. 4 ml of ndodecane/TBP solution and 10/~1 of the thorium stock solution in pure TBP were added and equilibrium was established by shaking at 25°C. The equilibrium aqueous HC1 concentration was measured with a combined glass-calomel electrode calibrated with standard HC1 solutions in 1 M (H, Na)C1. At equilibrium, which required 20 min shaking, 0.1 ml of each phase was taken to determine the plutonium and thorium distribution coefficients by c~-counting as described earlier [3].
RESULTS
AND
DISCUSSION
The plutonium and thorium distribution coefficients were studied as a function of metal ion, nitroaromatic acid and hydrogen ion concentrations. The first two components may be considered as microcomponents and their activity coefficients will be constant. The equilibrium aqueous concentration of nitroaromatic acid, CA, was calculated from its initial concentration using known distribution data[2]. As the equilibrium aqueous concentration of hydrogen ion varies up to 1 M in 1 M HC1, the hydrogen ion activity, all, scale defined by Harned and Robinson [7] has been introduced to eliminate changes in the hydrogen ion activity coefficient, The last factor to influence the metal distribution coefficient is the concentration of TBP in the organic phase, ?TBP. Single experimental values of the plutonium and thorium distribution coefficients are plotted against the above variables in Figs. 1-3. The equilibrium aqueous concentration of metal ions was varied in the region 10-6-10 -4 M at CTBP = 1"1 M , CA = 8"79. 10 -3 M and aa = 0"389 M for plutonium, and CTBP = 1.1 M, CA = 4"19. 10-3 M and a n -~-0"03 M for thorium. No significant dependence was observed, and thus only mononuclear complexes seem to be present. The general equation used for the uranyl extraction data[3] should thus apply for the metal ion, M ~+, distribution coefficient, viz.
D M = p=0 q~0 r=0 s=l e Q R
(1)
1+ Y Y Z #,,~,(cA'~A)"aI~~ p=0 q=0 r=0
6. A. S. Kertes,J. inorg, nucl. Chem. 14, 104 (1960). 7. H. S. Harned and R. A. Robinson, Multicomponent Electrolyte Solutions, p. 60. Pergamon Press, Oxford (1968).
Solvent extraction
1045
t~ -t
-2
J -4
l
-3
-2
Io~ cA Fig. 1. Pu(IV) distribution coefficientsplotted againstequilibrium aqueousconcentration of nitroaromatic acid at different p a . values (O-0-09, ffl-0.41 and ©-0.81) and ~TB~= 1"1 M. The solid curves representdistribution coefficients calculatedfrom the stability constantsin Table 3 accordingto Equation (2). Here ~g iS the fraction of the wholly deprotonized nitroaromatic acid in the aqueous phase; p, q, r and s denote the number of A 2-, H ÷, C I - and T B P species in the complex respectively. The following notation for the individual complexes will be used for simplicity: ~ ) - o r g a n i c phase complex and (p, q, r)-aqueous phase complex. From a slope analysis of the plutonium extraction data, we can see that the log Dpu vs. log CA plots in Fig. 1 give a value o f p - 2 at the higher nitroaromatic acid concentration, while a plot of log Dpu against p a . gives a value of q -~ 1. This implies that only one of two molecules of nitroaromatic acid is bound as the doubly-charged anion, A 2-, in the complex, the second being present as the
acidic anion, H A - . As only the extraction of neutral species is considered with TBP, one chloride anion will be included in the extracted complex. The solvation number s = 3 follows from Fig. 3, giving the total composition (2, 1, 1,3). The plots of log Dpu vs. log CA (Fig. 1) converge to a limiting log Dpu value at low nitroaromatic acid concentrations, which may correspond to the extraction of the (0, 0, 4, 2) chloride complex, the solvation number of which (s = 2) has been accepted from general considerations [8]. 8. Y. M a r c u s and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes, p. 705.
Wiley-Interscience, London (1969).
1046
M. B E R A N
o
-I
-2
-5
log
cA
Fig. 2. Th(IV) distribution coefficients plotted against equilibrium aqueous concentration of nitroaromatic acid at different pan values (O-1.19, ~-1.52 and (3-1.79) and (TBP = I" 1 M. The solid curves represent distribution coefficients calculated from the stability constants in Table 3 according to Equation (6).
It is more difficult to draw conclusions about the thorium complexes. The slopes of the log D Th/1Og CAplots (Fig. 2), viz. -- 1"3, suggest extraction of at least two nitroaromatic complexes. In agreement with the slope of the log DTh[paH plots (near to unity) one of these would be similar to that of plutonium, differing from it in solvation number only (Fig. 3). Its conposition as may be given as (2, 1, 1, 2). The most probable composition of the second, containing one molecule of nitroaromatic acid only, is (1, 0, 2, 2). Regarding the aqueous species, the formation of the first plutonium and thorium nitroaromatic complexes (1, 0, 0) is suggested by our unpublished spectrophotometric measurements under identical experimental conditions used. Similarly, only the first plutonium [9] and thorium [ 10] chloride complexes (0, 0, 1) would be present in the constant 1 M chloride medium. The following simplified versions of Equation (1) may thus be written for the plutonium and thorium distribution coefficients: D f,u
= [fl2113(¢AO~A)2aHe~'Bp ''~ ~0042C2Bp]/( 1 + /~100CAOQ~-F-/~001)
9. S. W. Rabideau and H. D. C o w a n J . A m . chem. Soc. 77, 6145 (1955). 10. W. C. Waggener and R. W. Stoughton, J. Phys. Chem. 56, 1 (1952).
(2)
Solvent extraction
1047
c~
-2
t -I
O
log ~rsp Fig. 3. Pu(IV) and T h ( I V ) distribution coefficients plotted against T B P concentration in the organic phase. • - P u ( I V ) , CA = 0"00879 M, pall = 0"41; O - - T h ( I V ) , CA = 0"01 M, pall = 1-52. T h e solid curves represent the distribution coefficients calculated as described in Figs. 1 and 2.
and DTn :
[~2112(CAOlA)2(JH"f-~filo22CAOl.~]C2Bp[(l "~'-]~100CAO¢.~-']'- ]~001).
(3)
Rearrangement gives
l(~,2.3/fioo4.2)(cAc~.O2ane~Be+e~Bvl/Dpu = [(1 +/3oo,)/fioo421+ (~3,oo/~ooJCA~A (4) and [(~'2112/~1022)( C AC~A)2a l'I -f- CAOIA]C2Bp/DTh = [(1
+ rio(,,)//~1022 ] +
(/~100]~1022) C AOLA"
(5) The left-hand sides of these equations are plotted against CAaA at various values of/321~s//3oo42 or fiZllZ//~1o22as shown in Figs. 4 and 5. Straight lines with intercepts [(1 +/3ool)/I]oiw_,] and [(1 +/3OOl)/~o22] and slopes equal to filoo]/3oo42and fllOO//3,,,,~ would be obtained at the appropriate values of f121~3/fioo42and fl2~2//3~o22respectively. Using the stability constants for the first chloride complexes given in [9] and [10], all the stability constants in Equations (2) and (3) may be determined. These values, excepting that of fl~oo for thorium, the value of which cannot be found because of the small slope of the straight line in Fig. 5, are given in Table 3. The stability constant values were further refined by successive approximation using Equations (2) and (3). The theoretical distribution coefficients, and their
1048
M. B E R A N "'1 O
+
++
++ + .~_ 4+
0 x
F
0 0 Z O
zx~x
I,
0
I
5
I0
15
CAaA x IO ~ Fig. 4. Graphical solution of Equation (2) for the extraction of Pu(IV). Y = [(~211~/ -3 P ~- ~2q_Bp]/Dpil° T h e ~2113/~0042 values: --~-- 1 2 . 1 0 TM, 0 - 6 . 1 0 ~0O42)(CAO~A)2aHCTB 2 . 10 TM•
--
TM, × -
+ I0+
++ +
+50
÷o~
ID
0 x
°x~x × x 5
×
+0 +
o
o
o 121
0
x
×
Nx x
x
x
x
0
t
I
5
tO
CAaAXlOs Fig, 5. G r a p h i c a l s o l u t i o n o f E q u a t i o n (3) for t h e e x t r a c t i o n o f T h ( I V ) . Y = [(~2,12/ ~O.,2)(CAOtA)2aHJr CAOtA]C~Bp/DTh.T h e ~2112/~1022 ValUeS," "1---15. 10 s, 0 - - 7 . l0 s, × - 1 . l 0 s.
deviations from the experimental values, were then calculated to evaluate how well Equations (2) and (3) represent the experimental data. The refined stability constants for plutonium are summarized in Table 3, and the theoretical distribution coefficients calculated from them using Equation (2) are given in Table 1. If
Solvent extraction
1049
Table 1. Solvent extraction of Pu(IV) 3-nitro-p-cresol-5-sulphonate complexes from 1 M (H, Na)C1 with T B P in n-dodecane; t = 25 -+ 0"2°C CA
CA0~A :g
1
Dputexo)
X 103
aH
X 10~°
CTBP
Dpuwalc)t
0.139 0-0417 0.0175 0.00931 0.00452 0.576 0-150 0.0594 0.0293 0.0102 0.00434 2.00 0.829 0-177 0.0534 0.0148 0.485 0.319 0-281 0'125 0'0314
7.27 3.64 2.18 1.45 0.727 8.79 4.48 2.69 1.79 0-897 0-448 14.0 6.99 2.80 1.40 0.699 8.79
0.808
11.1 5.53 3-31 2.20 1.11 32-5 16.6 9.95 6.62 3.32 1.66 146 72.7 29.1 14.6 7.27 32-5
1.10
0-154 0-0427 0.0174 0.00927 0.00442 0.507 0.156 0.0618 0.0299 0.00975 0.00452 1.98 0.732 0-167 0.0502 0.0154 0.507 0.371 0.260 0.110 0.0327
0.389
0.153
0.389
1.10
1-10
1.10 0.99 0.88 0-66 0.44
lOg
Dpu(cale)
/-)Pu(exp)
+0.045 +0.010 -0.002 - 0.001 -0.009 -0.055 + 0.017 +0.017 + 0.009 -0.019 +0.018 -0.004 - 0-054 - 0.025 -0.026 +0.018 + 0.020 +0.066 -0-033 -0-055 +0-018
o-ex, = + 0.032; trealc-exp = +---0"031 *~A values are calculated by using Equation (2) from [3] and dissociation constants of HzA, K~ H = 2.06 and Ke n = 1.71 . 10-r. tEquation (2) and stability constants given in Table 3.
we compare the average root mean square deviation between the experimental and calculated distribution coefficients (O-ca~c-exo=---0"031) to the average RMS deviation of the experiment as determined from reproducibility data (Ocxo= :-+0.032) we see that the fit is very good. However, this was not so for the thorium data treated in the same way using Equation (3). The comparison between tJrcalc_exo ___0.057 and tYexo ___0.033 indicates the necessity of introducing a correction to this equation. As the greatest negative deviations were observed at the highest values of c~ and pall, the normal nitroaromatic complex (2, 0, 0, 2) was considered to be present: =
--
=
C
o
--
D T h = [/~2112(AaA)-aH"~[~lo22CAaA-l-'~2002(CAOtA)e]C2Bp/(1-}-t~H}oCAOlAff'~OO,).
(6)
Stability constants refined by successive approximation using Equation (6) are summarized in Table 3, and thorium distribution coefficients calculated from them are given in Table 2. The value O-ca~c-ex,.= - 0"045 shows that Equation (6) fits the experimental data satisfactorily. The solid curves in Figs. 1-3 represent plutonium and thorium distribution coefficients calculated from the stability constants given in Table 3 (successive approximation column) using Equations (2) and (6) respectively.
M. B E R A N
1050
Table 2. Solvent extraction o f T h ( I V ) 3-nitro-p-cresol-5-sulphonate complexes from 1 M (H, Na)CI with TBP in n-dodecane; t = 25 - 0.2°C CA
art
DTh(exp)
X 10a
X 102
0"980 0"641 0"307 0" 116 0"0615 2"09 1"63 0"752 0"264 0" 120 3'98 2"60 1"16 0'540 0"217 3'37 1"59 0-429 0'0914 0"0180
10"9 7"63 4"36 2" 18 1"09 10"5 7"36 4" 19 2" 10 1"08 10"2 7'21 4'09 2"08 1"04 10"1 10" 1 10"0 10'0 10"0
6"43
3"00
1'61
3'00
CA~A X 10 8
2"81 1"97 1"12 0'562 0'281 5'90 4"14 2"35 1" 18 0"607 10"7 7"57 4'29 2" 18 1.09 5'68 5"68 5"62 5"62 5"62
CTBP
DThtealc)t
log ~Th(exp)
l" l0
l' 15 0"686 0"316 0' 131 0"0585 2"29 1"41 0'658 0"277 0" 128 3"95 2"51 1'20 0'519 0"231 4"00 1"76 0"444 0'0710 0"0177
+ 0"069 + 0"030 +0"013 +0"053 --0"021 + 0'040 --0"063 -- 0"058 + 0'021 + 0"028 --0"003 --0"015 +0"015 --0"017 + 0"027 +0"075 + 0"044 +0"015 --0"109 --0"007
1' 10
1"10
1"49 0"99 0"50 0"20 0'10
trexp = ± 0"033; O'caLc-exp= --+0"045 tCalculated from Equation (6) and stability constants given in Table 3.
Table 3. Stability constants o f Pu(IV) and Th(IV) 3-nitro-p-cresol-5-sulphonate and chloride complexes. A q u e o u s phase: H2A in 1 M (H, Na)CI. Organic phase: T B P in n-dodecane, t = 25 ---0.2°C Stability constant
Complex
Formula
(2, 1, 1,3) (0,0,4,2) (1,0,0) (0, 0, 1) (2, 1, 1, 2) (l, 0, 2, 2) (2, 0, 0, 2) (1, 0, 0) (0, 0, 1)
PuA(HA)CI.3TBP PuC14"2TBP PuA 2+ PuC13÷ ThA(HA)C1-2TBP ThAClz'2TBP ThA_~.2TBP T h A z+ ThC! a+
a. Ref. [9]. b. Ref. [ 10]. c. Equation (2). d. Equation (6).
Graphical solution 2.09x 3.49x 1.95× 0.56 a 2'63 × 3"75 x
1.51 b
10lr 10-: l0 s 10TM l07
Successive approximation c(2.05_+0.13) x (3.56-+0.32) x (2.01 _+0.52)x 0.57 "+0.06 a(2.87 --+0-68) × (3.84 -+0"63) x (3"87 -+ 0"50) × (1.58--+0"48)× 1"55--+0"17
1017 10 -2 108 1016 107 l014 107
Solventextraction
1051
It is obvious that all nitroaromatic organic complexes are based on MA C~-2~ species, considerable concentrations of which are present in the aqueous phase in all cases. The normal nitroaromatic complex, UO.,A.2TBP, prevails in the extraction of uranyl ion[3]. This is accompanied by the chloride complex UO._,CI2-2TBP only, the fraction of which does not exceed 6% under the experimental conditions used. Similarly, PuA(HA)CI.3TBP is the main organic complex in plutonium extraction. The fundamental species, PuA 2+, is converted to the neutral form with one acidic nitroaromatic anion and one chloride anion. A similar complex, ThA(HA)C1-2TBP, prevails in thorium extraction. However, considerable concentrations of two other complexes, ThACI2.2TBP and ThA2"2TBP, are present in the organic phase in this case. This follows more clearly from Fig. 6, 0.6
O
4
~
==O3
o
I
0
J
1'2
I
1"4
I
1"6
~'8
2-0
POH
Fig. 6. Fractions of the individualthoriumcomplexes in the organicphase. CA= 0"01M, ?r~v = I M. I00 / 80
~
/I//t~
//
/1
l/J l/
~
j
6O
°
:E
W
40
20
O
I
2
3
Po H Fig. 7. Percentage plutonium, thorium and uranyl ion extracted. CA = 0"01 M , ~BP: 1M
(solidcurves)and2 M (brokencurves).
1052
M. BERAN
where the fractions of individual organic thorium complexes, calculated from the denominator of the right-hand side of Equation (6) by using the stability constants given in Table 3 against pall, are plotted. Finally, the proportion of plutonium, thorium and uranyl extracted has been calculated according to the established equations and stability constants. The appropriate equation and stability constants for uranyl were taken from [3]. It is obvious from Fig. 7 that the extraction of nitroaromatic complexes with TBP offers interesting possibilities of metal separation.
Pergamon Press.
Printed in Great Britain
S O L V E N T E X T R A C T I O N OF SOME D I P R O T I C N I T R O A R O M A T I C A C I D S A N D OF T H E I R COMPLEXES WITH SOME ACTINIDES-III EXTRACTION OF TETRAVALENT PLUTONIUM AND 3-NITRO-p-CRESOL-5-SULPHONATE COMPLEXES TRIBUTYLPHOSPHATE
THORIUM WITH
M. BERAN Department of Chemistry, Nuclear Research Institute of Czechoslovak Academy of Sciences, l~e~. near Prague, Czechoslovakia (Received 6 July 1971 )
Abstract-The extraction of tetravalent plutonium and thorium from 1 M (H, Na)CI aqueous solution containing 3-nitro-p-cresol-5-sulphonic acid (HzA) with a n-dodecane solution of tributyl-phosphate (TBP) at 25 _+0.2°C has been studied. The experimental data were explained in terms of the following extractable complexes: PuA(HA)CI-3TBP and PuC14-2TBP; ThA(HA)C1.2TBP, ThACI.~.2TBP and ThA.,.2TBP. The first nitroaromatic complex, MA 2+, was found in the aqueous phase in addition to the first chloride complex, MCI a+, in both cases. The stability constants of these species have been calculated. T HE ARILITY o f 3 - n i t r o - p - c r e s o l - 5 - s u l p h o n i c a c i d ( H 2 A ) to f o r m c o m p l e x e s e x t r a c t e d b y T B P [ 1 ] , has b e e n m u c h s t u d i e d r e c e n t l y [2, 3]. O n l y n e u t r a l s p e c i e s , e.g. t h e u n d i s s o c i a t e d a c i d [2] o r its n o r m a l u r a n y l salt[3], w e r e e x t r a c t e d b y a s o l v a t i o n m e c h a n i s m w i t h T B P . S u c h c o m p l e x e s m a y be o f i n t e r e s t f r o m the v i e w p o i n t o f t h e e x t r a c t i o n m e c h a n i s m , a n d m a y a l s o be i m p o r t a n t in s e p a r a t i o n p r o c e s s e s ; w e t h e r e f o r e d e c i d e d to i n v e s t i g a t e t h e e x t r a c t i o n o f s o m e o t h e r m e t a l s . T h e r e s u l t s fo r t e t r a v a l e n t p l u t o n i u m a n d t h o r i u m are g i v e n b e l o w . EXPERIMENTAL Reagents and apparatus. Most of the chemicals as well as the apparatus used, were as described previously [2, 3]. Sodium nitrite (Lachema, analytical grade) was used to maintain plutonium in the tetravalent state. Stock solutions o f radioactive isotopes. 85-2 mg of metallic ~:~"Pu, the radiochemical purity of which was verified by o-spectrometry, were dissolved in 7-8 ml of I 1.8 M HCI. The resulting stock solution, which was about 0.05 M in Pu and 10 M in HCI, contained > 98% Pu as Pu(IV). The Pu(IV) content was checked by extraction with 0.5 M TTA in xylene from 1 M HCI[4], and was found to be stable for more than 30 days. Thorium-288, an c~-emitter(half-life 1-91 years), was supplied by R.C.C., Amersham as crystalline thorium nitrate. It was purified from its disintegration products by solvent extraction of thorium with undiluted TBP from 10 M HC1 [5] according to the following procedure: to about 1 mCi of 22STh nitrate 55.2 mg of Th(N O:~)4-4H._,O were added, and after dissolution in 1 ml of 0.1 M HCI thorium hydroxide 1. M. Beran, in Solvent Extraction Research (Edited by A. S. Kertes and Y. Marcus) p. 75. Wileylnterscience, New York (1969). 2. M. Beran, J. inorg, nucl. Chem. 33,839 (1971). 3. M. Beran,J. inorg, nucl. Chem. In press. 4. A. M. Poskanzer and B. M. Foreman, Jr.,d. inorg, nucl. Chem. 16,323 (1961). 5. T. Ishimori, K. Watanabe and E. Nakamura, Bull. Chem. Soc. Japan 33,636 (1960). 1043
1044
M. B E R A N
was precipitated by adding 1 ml of approx. 1 M NH4OH. The precipitate was filtered on a sintered glass filter, washed with distilled water and dissolved in 10 ml of 10 M HC1. Thorium was then extracted with 10 ml of undiluted TBP and its radiochemical purity was checked by c~-spectrometry. It contained < 3% of c~-impurities. The organic phase, which served as the thorium feed solution in experiments, was equilibrated with the aqueous phase before each use to backwash the disintegration products of ~2SThfrom it. Distribution experiments. In the plutonium distribution experiments 10 ml of an aqueous phase containing 0"2-1-0 M HCI in 1 M (H, Na)CI, 0.01 M NaNOz and K H A in the required concentration (up to 0.02 M) were prepared by mixing standard solutions of the components. 1 ml of the aqueous phase was pipetted into a penicillin glass tube with a polyethylene stopper; 10/~1 of the plutonium feed solution (diluted 20-200 × with 10 M HCI) and 1 ml of n-dodecane/TBP solution were added and both phases were equilibrated at 25°C. As the extraction of HC1 is negligible [6] under the experimental conditions used, the equilibrium aqueous concentration of hydrochloric acid was considered to be equal to its initial concentration. Thorium distribution experiments were performed as follows: 8 ml of an aqueous phase containing 0-02-0.08 M HC! in 1 M (H, Na)CI and K H A in the required concentration (up to 0.012 M) were prepared by mixing standard solutions of the single components in a 20 ml glass tube. 4 ml of ndodecane/TBP solution and 10/~1 of the thorium stock solution in pure TBP were added and equilibrium was established by shaking at 25°C. The equilibrium aqueous HC1 concentration was measured with a combined glass-calomel electrode calibrated with standard HC1 solutions in 1 M (H, Na)C1. At equilibrium, which required 20 min shaking, 0.1 ml of each phase was taken to determine the plutonium and thorium distribution coefficients by c~-counting as described earlier [3].
RESULTS
AND
DISCUSSION
The plutonium and thorium distribution coefficients were studied as a function of metal ion, nitroaromatic acid and hydrogen ion concentrations. The first two components may be considered as microcomponents and their activity coefficients will be constant. The equilibrium aqueous concentration of nitroaromatic acid, CA, was calculated from its initial concentration using known distribution data[2]. As the equilibrium aqueous concentration of hydrogen ion varies up to 1 M in 1 M HC1, the hydrogen ion activity, all, scale defined by Harned and Robinson [7] has been introduced to eliminate changes in the hydrogen ion activity coefficient, The last factor to influence the metal distribution coefficient is the concentration of TBP in the organic phase, ?TBP. Single experimental values of the plutonium and thorium distribution coefficients are plotted against the above variables in Figs. 1-3. The equilibrium aqueous concentration of metal ions was varied in the region 10-6-10 -4 M at CTBP = 1"1 M , CA = 8"79. 10 -3 M and aa = 0"389 M for plutonium, and CTBP = 1.1 M, CA = 4"19. 10-3 M and a n -~-0"03 M for thorium. No significant dependence was observed, and thus only mononuclear complexes seem to be present. The general equation used for the uranyl extraction data[3] should thus apply for the metal ion, M ~+, distribution coefficient, viz.
D M = p=0 q~0 r=0 s=l e Q R
(1)
1+ Y Y Z #,,~,(cA'~A)"aI~~ p=0 q=0 r=0
6. A. S. Kertes,J. inorg, nucl. Chem. 14, 104 (1960). 7. H. S. Harned and R. A. Robinson, Multicomponent Electrolyte Solutions, p. 60. Pergamon Press, Oxford (1968).
Solvent extraction
1045
t~ -t
-2
J -4
l
-3
-2
Io~ cA Fig. 1. Pu(IV) distribution coefficientsplotted againstequilibrium aqueousconcentration of nitroaromatic acid at different p a . values (O-0-09, ffl-0.41 and ©-0.81) and ~TB~= 1"1 M. The solid curves representdistribution coefficients calculatedfrom the stability constantsin Table 3 accordingto Equation (2). Here ~g iS the fraction of the wholly deprotonized nitroaromatic acid in the aqueous phase; p, q, r and s denote the number of A 2-, H ÷, C I - and T B P species in the complex respectively. The following notation for the individual complexes will be used for simplicity: ~ ) - o r g a n i c phase complex and (p, q, r)-aqueous phase complex. From a slope analysis of the plutonium extraction data, we can see that the log Dpu vs. log CA plots in Fig. 1 give a value o f p - 2 at the higher nitroaromatic acid concentration, while a plot of log Dpu against p a . gives a value of q -~ 1. This implies that only one of two molecules of nitroaromatic acid is bound as the doubly-charged anion, A 2-, in the complex, the second being present as the
acidic anion, H A - . As only the extraction of neutral species is considered with TBP, one chloride anion will be included in the extracted complex. The solvation number s = 3 follows from Fig. 3, giving the total composition (2, 1, 1,3). The plots of log Dpu vs. log CA (Fig. 1) converge to a limiting log Dpu value at low nitroaromatic acid concentrations, which may correspond to the extraction of the (0, 0, 4, 2) chloride complex, the solvation number of which (s = 2) has been accepted from general considerations [8]. 8. Y. M a r c u s and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes, p. 705.
Wiley-Interscience, London (1969).
1046
M. B E R A N
o
-I
-2
-5
log
cA
Fig. 2. Th(IV) distribution coefficients plotted against equilibrium aqueous concentration of nitroaromatic acid at different pan values (O-1.19, ~-1.52 and (3-1.79) and (TBP = I" 1 M. The solid curves represent distribution coefficients calculated from the stability constants in Table 3 according to Equation (6).
It is more difficult to draw conclusions about the thorium complexes. The slopes of the log D Th/1Og CAplots (Fig. 2), viz. -- 1"3, suggest extraction of at least two nitroaromatic complexes. In agreement with the slope of the log DTh[paH plots (near to unity) one of these would be similar to that of plutonium, differing from it in solvation number only (Fig. 3). Its conposition as may be given as (2, 1, 1, 2). The most probable composition of the second, containing one molecule of nitroaromatic acid only, is (1, 0, 2, 2). Regarding the aqueous species, the formation of the first plutonium and thorium nitroaromatic complexes (1, 0, 0) is suggested by our unpublished spectrophotometric measurements under identical experimental conditions used. Similarly, only the first plutonium [9] and thorium [ 10] chloride complexes (0, 0, 1) would be present in the constant 1 M chloride medium. The following simplified versions of Equation (1) may thus be written for the plutonium and thorium distribution coefficients: D f,u
= [fl2113(¢AO~A)2aHe~'Bp ''~ ~0042C2Bp]/( 1 + /~100CAOQ~-F-/~001)
9. S. W. Rabideau and H. D. C o w a n J . A m . chem. Soc. 77, 6145 (1955). 10. W. C. Waggener and R. W. Stoughton, J. Phys. Chem. 56, 1 (1952).
(2)
Solvent extraction
1047
c~
-2
t -I
O
log ~rsp Fig. 3. Pu(IV) and T h ( I V ) distribution coefficients plotted against T B P concentration in the organic phase. • - P u ( I V ) , CA = 0"00879 M, pall = 0"41; O - - T h ( I V ) , CA = 0"01 M, pall = 1-52. T h e solid curves represent the distribution coefficients calculated as described in Figs. 1 and 2.
and DTn :
[~2112(CAOlA)2(JH"f-~filo22CAOl.~]C2Bp[(l "~'-]~100CAO¢.~-']'- ]~001).
(3)
Rearrangement gives
l(~,2.3/fioo4.2)(cAc~.O2ane~Be+e~Bvl/Dpu = [(1 +/3oo,)/fioo421+ (~3,oo/~ooJCA~A (4) and [(~'2112/~1022)( C AC~A)2a l'I -f- CAOIA]C2Bp/DTh = [(1
+ rio(,,)//~1022 ] +
(/~100]~1022) C AOLA"
(5) The left-hand sides of these equations are plotted against CAaA at various values of/321~s//3oo42 or fiZllZ//~1o22as shown in Figs. 4 and 5. Straight lines with intercepts [(1 +/3ool)/I]oiw_,] and [(1 +/3OOl)/~o22] and slopes equal to filoo]/3oo42and fllOO//3,,,,~ would be obtained at the appropriate values of f121~3/fioo42and fl2~2//3~o22respectively. Using the stability constants for the first chloride complexes given in [9] and [10], all the stability constants in Equations (2) and (3) may be determined. These values, excepting that of fl~oo for thorium, the value of which cannot be found because of the small slope of the straight line in Fig. 5, are given in Table 3. The stability constant values were further refined by successive approximation using Equations (2) and (3). The theoretical distribution coefficients, and their
1048
M. B E R A N "'1 O
+
++
++ + .~_ 4+
0 x
F
0 0 Z O
zx~x
I,
0
I
5
I0
15
CAaA x IO ~ Fig. 4. Graphical solution of Equation (2) for the extraction of Pu(IV). Y = [(~211~/ -3 P ~- ~2q_Bp]/Dpil° T h e ~2113/~0042 values: --~-- 1 2 . 1 0 TM, 0 - 6 . 1 0 ~0O42)(CAO~A)2aHCTB 2 . 10 TM•
--
TM, × -
+ I0+
++ +
+50
÷o~
ID
0 x
°x~x × x 5
×
+0 +
o
o
o 121
0
x
×
Nx x
x
x
x
0
t
I
5
tO
CAaAXlOs Fig, 5. G r a p h i c a l s o l u t i o n o f E q u a t i o n (3) for t h e e x t r a c t i o n o f T h ( I V ) . Y = [(~2,12/ ~O.,2)(CAOtA)2aHJr CAOtA]C~Bp/DTh.T h e ~2112/~1022 ValUeS," "1---15. 10 s, 0 - - 7 . l0 s, × - 1 . l 0 s.
deviations from the experimental values, were then calculated to evaluate how well Equations (2) and (3) represent the experimental data. The refined stability constants for plutonium are summarized in Table 3, and the theoretical distribution coefficients calculated from them using Equation (2) are given in Table 1. If
Solvent extraction
1049
Table 1. Solvent extraction of Pu(IV) 3-nitro-p-cresol-5-sulphonate complexes from 1 M (H, Na)C1 with T B P in n-dodecane; t = 25 -+ 0"2°C CA
CA0~A :g
1
Dputexo)
X 103
aH
X 10~°
CTBP
Dpuwalc)t
0.139 0-0417 0.0175 0.00931 0.00452 0.576 0-150 0.0594 0.0293 0.0102 0.00434 2.00 0.829 0-177 0.0534 0.0148 0.485 0.319 0-281 0'125 0'0314
7.27 3.64 2.18 1.45 0.727 8.79 4.48 2.69 1.79 0-897 0-448 14.0 6.99 2.80 1.40 0.699 8.79
0.808
11.1 5.53 3-31 2.20 1.11 32-5 16.6 9.95 6.62 3.32 1.66 146 72.7 29.1 14.6 7.27 32-5
1.10
0-154 0-0427 0.0174 0.00927 0.00442 0.507 0.156 0.0618 0.0299 0.00975 0.00452 1.98 0.732 0-167 0.0502 0.0154 0.507 0.371 0.260 0.110 0.0327
0.389
0.153
0.389
1.10
1-10
1.10 0.99 0.88 0-66 0.44
lOg
Dpu(cale)
/-)Pu(exp)
+0.045 +0.010 -0.002 - 0.001 -0.009 -0.055 + 0.017 +0.017 + 0.009 -0.019 +0.018 -0.004 - 0-054 - 0.025 -0.026 +0.018 + 0.020 +0.066 -0-033 -0-055 +0-018
o-ex, = + 0.032; trealc-exp = +---0"031 *~A values are calculated by using Equation (2) from [3] and dissociation constants of HzA, K~ H = 2.06 and Ke n = 1.71 . 10-r. tEquation (2) and stability constants given in Table 3.
we compare the average root mean square deviation between the experimental and calculated distribution coefficients (O-ca~c-exo=---0"031) to the average RMS deviation of the experiment as determined from reproducibility data (Ocxo= :-+0.032) we see that the fit is very good. However, this was not so for the thorium data treated in the same way using Equation (3). The comparison between tJrcalc_exo ___0.057 and tYexo ___0.033 indicates the necessity of introducing a correction to this equation. As the greatest negative deviations were observed at the highest values of c~ and pall, the normal nitroaromatic complex (2, 0, 0, 2) was considered to be present: =
--
=
C
o
--
D T h = [/~2112(AaA)-aH"~[~lo22CAaA-l-'~2002(CAOtA)e]C2Bp/(1-}-t~H}oCAOlAff'~OO,).
(6)
Stability constants refined by successive approximation using Equation (6) are summarized in Table 3, and thorium distribution coefficients calculated from them are given in Table 2. The value O-ca~c-ex,.= - 0"045 shows that Equation (6) fits the experimental data satisfactorily. The solid curves in Figs. 1-3 represent plutonium and thorium distribution coefficients calculated from the stability constants given in Table 3 (successive approximation column) using Equations (2) and (6) respectively.
M. B E R A N
1050
Table 2. Solvent extraction o f T h ( I V ) 3-nitro-p-cresol-5-sulphonate complexes from 1 M (H, Na)CI with TBP in n-dodecane; t = 25 - 0.2°C CA
art
DTh(exp)
X 10a
X 102
0"980 0"641 0"307 0" 116 0"0615 2"09 1"63 0"752 0"264 0" 120 3'98 2"60 1"16 0'540 0"217 3'37 1"59 0-429 0'0914 0"0180
10"9 7"63 4"36 2" 18 1"09 10"5 7"36 4" 19 2" 10 1"08 10"2 7'21 4'09 2"08 1"04 10"1 10" 1 10"0 10'0 10"0
6"43
3"00
1'61
3'00
CA~A X 10 8
2"81 1"97 1"12 0'562 0'281 5'90 4"14 2"35 1" 18 0"607 10"7 7"57 4'29 2" 18 1.09 5'68 5"68 5"62 5"62 5"62
CTBP
DThtealc)t
log ~Th(exp)
l" l0
l' 15 0"686 0"316 0' 131 0"0585 2"29 1"41 0'658 0"277 0" 128 3"95 2"51 1'20 0'519 0"231 4"00 1"76 0"444 0'0710 0"0177
+ 0"069 + 0"030 +0"013 +0"053 --0"021 + 0'040 --0"063 -- 0"058 + 0'021 + 0"028 --0"003 --0"015 +0"015 --0"017 + 0"027 +0"075 + 0"044 +0"015 --0"109 --0"007
1' 10
1"10
1"49 0"99 0"50 0"20 0'10
trexp = ± 0"033; O'caLc-exp= --+0"045 tCalculated from Equation (6) and stability constants given in Table 3.
Table 3. Stability constants o f Pu(IV) and Th(IV) 3-nitro-p-cresol-5-sulphonate and chloride complexes. A q u e o u s phase: H2A in 1 M (H, Na)CI. Organic phase: T B P in n-dodecane, t = 25 ---0.2°C Stability constant
Complex
Formula
(2, 1, 1,3) (0,0,4,2) (1,0,0) (0, 0, 1) (2, 1, 1, 2) (l, 0, 2, 2) (2, 0, 0, 2) (1, 0, 0) (0, 0, 1)
PuA(HA)CI.3TBP PuC14"2TBP PuA 2+ PuC13÷ ThA(HA)C1-2TBP ThAClz'2TBP ThA_~.2TBP T h A z+ ThC! a+
a. Ref. [9]. b. Ref. [ 10]. c. Equation (2). d. Equation (6).
Graphical solution 2.09x 3.49x 1.95× 0.56 a 2'63 × 3"75 x
1.51 b
10lr 10-: l0 s 10TM l07
Successive approximation c(2.05_+0.13) x (3.56-+0.32) x (2.01 _+0.52)x 0.57 "+0.06 a(2.87 --+0-68) × (3.84 -+0"63) x (3"87 -+ 0"50) × (1.58--+0"48)× 1"55--+0"17
1017 10 -2 108 1016 107 l014 107
Solventextraction
1051
It is obvious that all nitroaromatic organic complexes are based on MA C~-2~ species, considerable concentrations of which are present in the aqueous phase in all cases. The normal nitroaromatic complex, UO.,A.2TBP, prevails in the extraction of uranyl ion[3]. This is accompanied by the chloride complex UO._,CI2-2TBP only, the fraction of which does not exceed 6% under the experimental conditions used. Similarly, PuA(HA)CI.3TBP is the main organic complex in plutonium extraction. The fundamental species, PuA 2+, is converted to the neutral form with one acidic nitroaromatic anion and one chloride anion. A similar complex, ThA(HA)C1-2TBP, prevails in thorium extraction. However, considerable concentrations of two other complexes, ThACI2.2TBP and ThA2"2TBP, are present in the organic phase in this case. This follows more clearly from Fig. 6, 0.6
O
4
~
==O3
o
I
0
J
1'2
I
1"4
I
1"6
~'8
2-0
POH
Fig. 6. Fractions of the individualthoriumcomplexes in the organicphase. CA= 0"01M, ?r~v = I M. I00 / 80
~
/I//t~
//
/1
l/J l/
~
j
6O
°
:E
W
40
20
O
I
2
3
Po H Fig. 7. Percentage plutonium, thorium and uranyl ion extracted. CA = 0"01 M , ~BP: 1M
(solidcurves)and2 M (brokencurves).
1052
M. BERAN
where the fractions of individual organic thorium complexes, calculated from the denominator of the right-hand side of Equation (6) by using the stability constants given in Table 3 against pall, are plotted. Finally, the proportion of plutonium, thorium and uranyl extracted has been calculated according to the established equations and stability constants. The appropriate equation and stability constants for uranyl were taken from [3]. It is obvious from Fig. 7 that the extraction of nitroaromatic complexes with TBP offers interesting possibilities of metal separation.