590
L. L. BURGER
Vol. 62
URANIUM AND PLUTONIUM EXTRACTION BY ORGANOPHOSPHORUS COMPOUNDS BY L. L. BURGER Hanford Laboratories Operation, General Electric Company, Richland, Washington Received November SO, 1967
Organic phosphates, phosphonates, phosphinates and phosphine oxides are compared as solvents for uranium and plutonium nitrates. Distribution coefficients for U(V1) and Pu(1V) and (VI) indicate a lar e difference in solvent strength among these groups of compounds. The introduction of an electronegative group greatfy reduces the solvent strength. The P-0 stretching frequency can be correlated with solvent strength for these compounds. Some solid complexes of the organophosphorus compounds with uranyl nitrate have been isolated.
Introduction The development of solvent extraction both as an analytical and as an industrial technique has emphasized that the formation of organic-soluble complexes by the solvation of an ion pair by a polar organic molecule is a rather common reaction. One of the more interesting solvents of this type is tributyl phosphate, TBP. As an extractant it was first employed in the analyses of organic acids.l Warf2 demonstrated its ability to extract uranium and the rare earths and pointed to its excellent chemical stability. These early studies were rapidly expanded with its application t o uranium processing as evidenced by the papers presented at the Geneva C~nference.~ The solvent action of TBP is in many respects typical of a large array of organic compounds including alcohols, ethers, ketones and esters. The extraction of uranyl nitrate, for example, can be presented stepwise. The anion replaces water in the coordination sphere of the cation and the organic solvent and water compete for the remaining positions. I n this particular case the complex formed apparently is UOz(NO3)z(TBP)2 based on the evidence, that (a) reasonably uniform equilibrium constants for extraction can be written involving TBP to the second power, (b) the limiting solubility in TBP approaches the ratio 2 TBP/1 U02(NOa)z and (e) solid complexes of UOz(NO3)2 and esters conforming to the above structure can be isolated. I n contrast to the complexes formed with alcohols, ethers and ketones, water is completely eliminated from the molecule. I n view of the extreme difficulty of removing the last two water molecules from the uranyl nitrate molecule, their ready replacement by TBP molecules is evidence of rather stable complexes. Since the basic oxygen of the phosphoryl group is probably responsible for the complexing ability of TBP, it was of interest in theselaboratories to examine other esters of the acids of phosphorus where a wide variation in the electronegativity of the phosphoryl oxygen was possible. It was also of interest to correlate extraction ability and the electronegativity of the phosphoryl oxygen as measured by an independent means. The stretching frequency of the phosphoryl bond was employed as an index of this electronegativity. The (1) H. A. Pagel, P. E. Toren and F. W. McLafferty, Anal. Chem., 21, 1150 (1949). (2) J. C. Warf, J . Am. Chem. S o c . , 71, 3257 (1949). (3) International Conference on the Peaceful Uses of Atomic Energy, Papers No. 441,539, 540,719.
size and nature of the organic group in the ester was also considered to be important since the solubility of the complex in the solvent or in an inert diluent, and hence the degree of extraction will be governed by these factors. A series of 15 phosphates, 21 phosphonates, 5 phosphinates and 3 phosphine oxides were prepared. These are listed in Table I. Those of desirable physical properties, i.e., low mutual solubility with water and low vapor pressure, were then examined as extractants, using the extraction of uranyl nitrate and plutonium nitrates as a measure. The preparation of these compounds and their physical properties are reported elsewhere. TABLE I COMPOUNDS STUDIED Phosphates (RO)aP(O)
Phosphonates (RO)zRP(O)
Diethyl butyl Diethyl isobutyl Diethyl amyl Diethyl decyl Dibutyl methyl Dibutyl ethyl Tributyl Dibutyl hexyl Dibutyl octyl Dibutyl decyl Butyl octyl phenyl Butyl diphenyl Dibutyl ethoxybutyl Tri-butoxyethyl Tri-0-chloroethyl
Diethyl methyl Diethyl butyl Diethyl hexyl Diethyl octyl Diethyl hexadecyl Diethyl phenyl Dibutyl methyl Dibutyl ethyl Dibutyl butyl Dibutyl hexyl Dibutyl isooctyl Dibutyl decyl Dibutyl hexadecyl Dibutyl phenyl Dioctyl phenyl Diisooctyl phenyl Dibutyl hydrogen Diallyl phenyl Dioctyl styrene Diethyl trichloromethyl Diethyl benzoyl
Phospliinates (RO)RzP(O)
Methyl dibutyl Ethyl dibutyl Butyl dibutyl Ethyl dihexyl Ethyl dichlorophenyl Phosphine oxides
Tributyl Dioctyl hydrogen Trip henyl
Simultaneously with this work Higgins, Baldwin and Ruth at the Oak Ridge National Laboratory studied the extraction properties of several of the organophosphorus compounds described here.6 More recently Blake, Brown and Coleman6 have (4) L. L. Burger, t o be published in Ind. Ene. CAem., Data Series. (5) C. E. Higgins, W. H. Baldwin and J. If. Ruth, "OrganoPhosphorus Compounds for Solvent Extraction," ORNL-1338, August 14, 1952 (secret).
May, 1958
URANIUMAND PLUTONIUM EXTRACTION BY ORGANOPHOSPHORUS COMPOUNDS 591
reported on the use of trialkylphosphille oxides for the extraction of uranium and vanadium. Experimental For extraction studies the addition of a diluent to the organic phase was necessary to shift the density away from that of the aqueous solutions and to lower the viscosity. Both saturated hydrocarbons and carbon tetrachloride have been used. In this study the latter was chosen because of a much lesser tendency for the system to split into three phases. Solutions 0.5 molar in ester were used throughout. Equal volumes of aqueous and organic phases were shaken together for 10 to 30 minutes and the phases separated and sampled. Direct light was excluded and the samples for analysis were stored in dark bottles.? Uranium was determined by X-ray absorption or, a t low concentrations, by the fluorimetric method. A few measurements were made with plutonium in both the IV and VI valence states. An aliquot of the plutonium stock solution previously adjusted to the IV or VI state was added to the aqueous phase just before equilibration. I n the case of plutonium(VI), 0.01 molar sodium dichromate was used as a holding oxidant. Plutonium was determined by counting a small aliquot evaporated to a thin film. Data are reported as distribution coefficients, E:, or the ratio of concentrations in t,he organic and the aqueous phases a t equilibrium at 25 . Although the analytical methods are capable of much higher precision, the values reported for E: are probably uncertain to & l o % . Infrared absorption curves from 2 to 14 p were obtained for the pure compounds using a Perkin-Elmer Model 21 instrument with sodium chloride optics. A “Nujol” mull was used for the solid compounds.
1
I
1
I
02
0.3 Y I
m e $0.5
:,i
PHOSPHINE OXIDES
u PHOSPHONATES
Results and Discussion Fig. ].-Infrared I n Table I1 are listed uranyl nitrate distribution coefficients using alkyl phosphate and phosphonate solvents. A large increase in extraction is observed in changing drom the phosphate structure to the phosphonate structure. I n Table I11 data for several of these compounds are compared with data for the phosphinates. Tributylphosphine *IOC oxide is also included, The increase in extraction .wU with increase of the number of phosphorus t o car- -k bon bonds is evident, The distribution coefficients z w uranyl nitrate vary over a range of 100. Similar 0 :W: I O trends were reported by Wiggins.&
P-
-
P
spectra in the P-O and P-O-C vibration region.
I
I PHOSPHINE OXlDl 2- PHOSPHINATES 3- PHOSPHONATES 4- PHOSP,HATES I-
.E 2\
0
TABLE I1 DISTRIBUTION COEFFICIENTS O F URANYL NITRATEAT 25’“ Organic phase 0.5 A4 ester in CCla r -
Phosphates----
Compound Diethyl butyl Diethyl amyl Diethyl decyl Dibutyl methyl Dibutyl ethyl Tributyl Dibutyl hexyl Dibutyl octyl Dibutyl decyl a
Aq. U concn. 0.05 0.2 ilf &I 0.17 0.019 .17 .015 .ll ,017 .14 .013 .15 .015 ,19 .020 .21 ,020 .19 ,018 .17 ,021
-Phosphonates----Compound Diethyl hexyl Diethyl decyl Dibutyl methyl Dibutyl ethyl Dibutyl butyl Dibutyl hexyl Dibutylisooctyl Dibutyl decyl Dibutyl hexadecyl
Aq. U concn. 0.05 0.2M M 1.02 0.44 0.97 .35 1.09 .29 0.96 .32 1.03 .34 1.05 .42 0.98 .34 1.10 .35 0.93
Aqueous concentrations are those before equilibration.
It might be expected that changing the size of the alkyl group would change the solubility of the complex. However, the data of Table I1 indicate that changing the alkyl group has very little effect. Substitution in the alkyl chain may produce a ( 6 ) C. A. Blake, K. B. Brown and C. F. Coleman, “Solvent Extraction of Uranium (and Vanadium), from Acid Liquors with Trialkylphosphine Oxides, ORNL-1964, November 17. 1955. (7) The compounds studied are exceptionally stable toward hydrolysis; however, when uranyl nitrate is present, they undergo photochemical decomposition.
z 0 c
3
g
1.0
c E
0
0.1
IO
Fig. 2.-Variation
1200 1250 P - 0 FREQUENCY CM,’,
of distribution coefficient with phosphoryl frequency.
considerable change resulting from the induction effect. This is illustrated by the data in Table IV. It is seen that an electronegative group such as chlorine nearly destroys the solvent strength. The ether-oxygen in the ethoxy- and butoxyethyl phosphates has only a small effect. The electron sink properties of the phenyl group lead t o the same result as does the presence of a chlorine atom. Attaching the phenyl group through an oxygen bond
L. L. BURGER
592
Vol. 62
TABLE I11 URANIUM DISTR~BUTION COEFFICIENTS FOR PHOSPHATES, PHOSPHONATES, PHOSPHINATES AND PHOSPHINE OXIDESAT 25' Organic phase 0.5 M ester in CCL; original aqueous phase 0.2 M uranyl nitrate Phos hates Compounl
Methyl dibutyl Ethyl dibutyl Tributyl
...........
Phosphonates Compound
E.0
0.14 .15 .19
.
Dibutyl methyl Dibutyl ethyl Dibutyl butyl
1.09 0.96 1.03
............
..
..
TABLE IV DIsTRrBuTIoN COEFFICIENTS AQUEOUSSOLUTIONS AND
OF URANYL NITRATE BETWEEN VARIOUS sUB~TITUTED ~ ~ 0
PHOSPHONATES AT 25' Organic phase 0.5 M ester in CCld
PHATES A N D
Aqueous phaseo 0.2 M UOz(N0s)i 0.2 M 3.0 M
Compound
Uon(Noa'P
"'I
Dioctyl phenylphosphonate 0.31 5.0 Diethyl phenylphosphonate .30 (3.0) 19 3.4 Tributyl phosphate Dibutyl ethoxybutyl phosphate .14 2.2 Tributoxyethyl phosphate .I4 .. -056 1.6 Dibutyl hydrogen phosphonate Butyl octyl phenyl phosphate *018 O.s5 Diethyl benzoylphosphonate .018 0.23 Tri-(p-chloroethyl) phosphate < .01 0.17 Triphenyl phosphate < .01 .. Diethyl trichloromethylphosphonate . . O.O3 a Aqueous composition is t,hat before equilibration.
.
..
(phosphates) appears to reduce the solvent strength by a larger factor than when a direct carbon phosphorus bond exists (phenylphosphonates). It may be noted here that the compound triphenylphosphine oxide forms an extremely strong complex with uranyl nitrate. This compound will be considered later. The effect of the different structures on the distribution coefficients of plutonium is similar to that observed for uranium as the data of Table V show. It be noted that the (IV) oxidation state shows a slightly higher extraction. TABLE V DISTRIBUTION COEFFICIENTS OF PLUTONIUM(1V)'AND (VI) NITRATES AT 25" Organic phase 0.5 M ester in CCla -Aqueous 0.9 g./l. 0.2 M
pU(rv)
(Nod2 2.0 M "0, 0.27 .33 .39 3.0 3.7 4.4 16 16
p U ( v 1j
(NOdr 2.0 M "0,
2M "0,
uo2-
u02-
Compound Dibutyl methyl phosphate Tributyl phosphate Dibutyl decyl phosphate Dibutyl methylphosphonate Dibutyl butylphosphonate Dibutyl decylphosphonate Butyl dibutylphosphinate Ethyl dihexylphospbinate
phssea 0.9 &I. 0.2 M
2 A!! 0.71
1.57 2.3 29 32 35 170 200
0.23 .28 .33 1.0 1.3 1.5 5.2 12
0.63 1.1
18 16
18 99 140
Aqueous composition is that before equilibration.
If the solvent strength of these compounds is indeed a functioll of the electronegativity of the then it be oxyge*' in the phosphoryl instructive to examine this bond. The two structures P+-o- and P=o represent the limiting
Phosphine oxideEa0 Compound
Phosphinates Compound
E2
Methyl dibutyl Ethyl dibutyl Butyl dibutyl Ethyl dihexyl
Ea0
3.8 3.2 3.9 4.9
...... ......
..
Tributyl
23
..
..
......
cases, and different contributions of each should lead to different infrared stretching frequencies. sThe - absorption band corresponding t o the phosphoryl stretching frequency has been well established as being in the 1100-1300 cm.-l region.*-" I n Table VI are listed the absorption maxima of the phosphoryl bonds for the compounds studied. Daasch and Smithloreport a phosphoryl frequency of 1275 cm.-l for trimethyl phosphate, in reasonTABLE VI PHOSPHORYL-BOND STRETCAINQ FREQUENCIES Phosphates Diethyl butyl Diethyl isobutyl Diethyl amyl Dibutyl methyl Dibutyl ethyl Tributyl Dibutylhexyl Dibutyl ootyl Dibutyl decyl Butyl octyl phenyl Butyl diphenyl Dibutyl ethoxybutyl Tr!-butoxwthyl Tn-p-chlorethyl
7.90 7.90 7.94 7.90 7.86 7.88 7.88 7.92 7.89 7.81 7.80 7.88 7.85 7.82
Cm.-l 1266 1266 1259 1266 1272 1269 1269 1262 1267 1280 1283 1269 1275 1278
Phosphinates Methyl dibutyl
8.41
1189
p
. 4 0 1192 ~ ~ ~ $ ' & ' ~88.40 ~ ;1192 ~
Tzkz2ine
IPhosphonates p Cm.-I Diethyl methyl 8.10 1235 Diethyl butyl 8 . 0 7 1239 Diethyl hexyl 8.05 1242 Diethyl hexadecyl 8.04 1244 Diethyl phenyl 8.06 1241 Dibutyl methyl 8.06 1241 Dibutyl ethyl 8.03 1245 Dibutylbutyl 8.04 1244 Dibutyl hexyl 8.06 1241 Dibutyl isoiictyl 8.03 1245 Dibutyl hexadecyl 8.03 1245 Dibutyl phenyl Dioctyl phenyl Diisooctyl phenyl Dibutyl hydrogen Diallyl phenyl Diethyl trichlormethyl Diethyl benzoyl
8.03 8.03 8.01 8.00 8.03
1245 1245 1248 1250 1245
7 . 8 5 1273 8 . 0 1 1248
Oxides
Dioctyl hydrogen Triphenyl
8.64 1157 8 , 6 4 1157 8 . 3 8 1193
able agreement with the phosphates listed in the table. Data for triphenylphosphine oxide and dibutyl butylphosphonate are also in agreement. However, their values for the phosphoryl band in diethyl pheny~phosphonate, 1257 cm. -1, and in dibutyl hydrogen phosphonate, 1265 cm.-', do not agree well with those reported here. For trimethylphosphine oxide they found the absorption at 1176 cm.-I, some distance removed from the 1157 cm.-1 position found for the alkylphosphine oxides examined in this work. Tributyl phosphate and diethyl butylphosphollate were included in a group of compounds studied byT3ellamy and Beecher." They found the phosphoryl bands a t 1274 and 1242 cm.-1, respectively, for the two compounds, in good agreement with the present work. Earlier discussions of the general vibrational spectra of organophosphorus compounds were published by Gores and by Meyrick and T h o m p ~ o n . ~ The infrared absorption in the regions corrcSOC., 9, 138 ( 1 9 ~ 0 ) . ( 8 ) R. c. Gore, ~ i s c . (9) C. Meyrick and H. W. Thompson, J. Chem. Soc., 225 (1950). (10) L. W. Daasch and D. C. Smith, "Infrared Spectra of Phosphorus Compounds," NRL Report 3657, April, 1950; A n d . Chsm., 23, 853 (1951). (11) L. J. Bellamy and L. Beecher, J . Chem. S O L ,475, 1701 (1952).
May, 1958
URANIUM AND PLUTONIUM EXTRACTION BY ORGANOPHOSPHORUS COMPOUNDS 593 TABLEVI1 URANYL NITRATE~RGANOPHOSPHORUS ESTERCOMPLEXES
U0z(NOa)z Calcd. -2(BuOP(0)Bu2) Found - ~ [ ( B U O ) ~ P ( O ) Calcd. phenyl 1 Found -2(BuaPO) Calcd. Found -2( triphenyl PO) Calcd. Found
%C
%H
%N
%U
M*P.,'C*
Soluhility
34.1 32.8 37.2 36.1 34.6 34.8 45.5 45.9
6.4 6.1 5.1 4.9 6.5 6.4 3.2 3.2
3.24 3.24
28.2 28.0
56-57
..
.. ..
Soluble in organic solvents; insoluble in HzO Soluble in organic solvents; insoluble in HzO Soluble in organic solvents; insoluble in
2.94 3.06
25.0 24.1
.. .. ..
sponding to the P=O bond and the P-0-C linkage are plotted in Fig. 1 for typical compounds. The phosphoryl vibration shifts toward longer wave lengths on changing from phosphates to phosphonates to phosphinates to phosphine oxides. I n the case of the phosphinates, the P=O band center is uncertain and the absorption maxima in the vicinity of 8.1 p probably arises from the hydrocarbon groups. The band positions are about 7.87 1 (1270 cm.-l) for phosphates to 8.05 p (1242 cm.-l) for phosphonates to 8.40 p (1192 cm.-I) for phosphinates, and about 8.64 p (1157 cm.-l) for phosphine oxides. The inductive effect of chlorine shows up in a greater contribution of the P=O configuration or an increased frequency. The presence of the phenyl group raised the frequency in the phosphates and phosphine oxides studied. However, with only one phenyl phosphorus bond present, as occurs in the phenylphosphonates, the shift was not observed. The effect of electronegative groups in shifting the phosphoryl frequency to higher values was noted by Daasch and Smith.lo Their study, however, included only one dialkyl alkylphosphonate and no alkyl dialkylphosphinat es. It may be noted that the center of the broad p-0-C band also appears to shift, in this case to a higher frequency, On chal'gillg from a phosphate to a phosphinate structure. Figure 2 shows the correlation between the P=o bond stretching frequency and extraction coefficients. The correlation between the four classes of ComPounds confirms the expected relation between the extraction ability of the ester and the electronegativity of the phosphoryl oxygen. Within a class of compounds the scatter is too great for meaningful comparisons. The presence of strongly electronegative groups such as chlorine atoms Seems to have a greater effect in depressing
..
42-43 51-53
..
HzO 289-293
Insoluble in organic solvents; insoluble in H20; slightly soluble in hot alcohol
the extraction coefficient than the phosphoryl frequency shift would predict. Caution must be used in applying any quantitative significance to the above distribution coefficients, because these coefficients change with a change in solute concentration. A comparison can be made on the basis of constant activity in the aqueous phases a t equilibrium. However, one then finds an extreme range in concentration in the organic phase and a consequent wide variation in effective solvent concentration. Without a knowledge of the activity coefficients, quantitative comparisons are again difficult. I n the absence of sufficient data t o calculate thermodynamic equilibrium constants, the simple comparison used here is probably as good as any. Solid Complexes.-An interesting consequence of the increased complexing ability of phosphone oxides, phosphinates and phosphonates over that of the phosphates is that crystalline compounds of the structure UO2(NO&?(Ester)zcan be separated easily from solution. Pickard and Kenyon12 prepared similar complexes using phosphine oxides and the halides of group I1 elements. Table VI1 lists the properties of four uranyl nitrate complexes. These were prepared by mixing alcohol solutions of both uranyl nitrate hexahydrate and the ester. It may be noted that water is not present in the resultant complex. The triphenylphosphine oxide complex is characterized by extreme thermal stability and can be heated to 290-3100 before any evidence of e\volution of oxides of nitrogen appears. Its high melting point and insolubility suggest a very Kgh lattice energy in contrast to the other complexes studied. It also has the additional unique characteristic of being strongly fluorescent. Acknowledgment.-The author is indebted to Beatrice J. McClanahan for her assistance in carrying out many of the distribution measurements. (12) R. H. Pickard and J. Kenyon, ibia., 89,262 (1906).
