Synergism in the solvent extraction of di, tri and tetravalent metal ions—III

J .Inorlb Nucl. Chem., 1962, Vol. 24, pp. 1429 to 1448. ~

Prms Ltd. Printed in England

SYNERGISM IN THE SOLVENT EXTRACTION OF DI, TRI AND TETRAVALENT METAL IONS--III ANTISYNERGISM WITH THENOYL TRIFLUORACETONE* T. V. HEALY~,D. F. PEPPARD and G. W. MAsoN Argonne National Laboratory, Argonne, Illinois (Received 26 December 1961; in revised form 19 March 1962) Almract---Greatly enhanced extraction (synergism) of di, tri and tetravalent cations from aqueous solution has previously been reported for the system H20/M n+/HTl"A/S/dilmmt, where S is a neutral organophosphorus ester and HTTA is thenoyl trifluoracetone. Examples are now given where S contains no phosphorus and is an amide, alcohol or ketone. Complete destruction of this greatly enhanced extraction by addition of excess S whether the neutral additive is a phosphorus ester, an amide, alcohol or ketone is demonstrated. These antisynergic effects are shown to be greater than, but the same type as, the effects due to some so called inert diluents. This phenonamon of antisynergism is shown to be connected with the water content of the organic phase and the destruction of the anhydrous synergic species MCITA)~y. The HTTA thermodynamic activity is lowered and water probably enters the complex. Absorptiometric measurements demonstrate three different spectra for HTI'A/TBP in non-aqueoos solution (a) the enolate form in dry TBP, probably hydrogen bonded to the TBP Co) the normal enol form in hexaue solution in the presence or absence of low concentrations of TBP and water and (c) the ketohydrate form in water eqnih'brated TBP. UO2(TTA)2 o r Th(TTA)4 in d r y T B P have t h e H T T A enolate spectra whereas in wet T I ~ the metalfIWA enol spectra obtains, evidence that water helps to destroy the anhydrous synergic species, causing antisynergism to occur. THe enhancement of the extraction of a cation from aqueous solution by a mixture of acidic and neutral substances, the extraction being better than by either constituent alone was first called synergism by BLAKE et al. (1) They used a mixture of an acidic dialkyl phosphate and certain neutral organophosphorus esters in the extraction of uranium. They also noted that above a certain concentration of ester, synergism no longer occurred and, in fact, a decrease in extraction took place. With some phosphorus acids no synergism occurred, there being an immediate decrease in the extraction in comparison with the acid alone. They considered thi~ to be due to interaction between the acid and the ester. Other workers have noted this phenomenon, now called antisynergism, and I~'PARD et al.(2) made use of it in a separation technique. The nature o f the interaction has been examined(a) and it is considered to be due to the formation o f an association product between the acid and the neutral esters, thereby effectively removing these reagents from the solvent extraction system. * Based on work performed under the auspices of the U.S. Atomic Energy Commigsion. t Harwell Exchange Fellow at Argonne National Laboratory. (t) C.A. BrAgs, C. F. BAES,K. B. BROWN,C. F. COLm~ANand J. C. WHrrE, Proceedi~s of the Second International Conference on Peaceful Uses of Atomic Energy, Geneva, 1958, 15/P/ 1550. United Nations (1958). (2) D F. P~p~a3, G. W. MASONand R. J. Snto~reN,J. Inorg. Nucl. Chem. 10, 117 (1959). (3) j. R. FmtRARO, D. F. I~PPAgD, J. Phys. Chem. 65, 539 (1961); H. T. BAKERand C. F. BARS, Report ORNL-2443 (1957); 2486 (1957); 2451 (1957); 2737 (1959). 1429

1430

T.V. I'IEALY,D. F. PEI'I'ARDand G. W. MASON

Part I of this series(4) describes the very great enhancement of extraction (synergism) for di, tri and tetravalent cations for the system H20/Mn+/HTTA/S where H T T A is thenoyl trifluoracetone and S is a neutral organophosphorus ester. The present work shows that large synergistic enhancement o f cation extraction may also be obtained ff S is an amide, an alcohol or a ketone. The species extracted are discussed in the paper. Further addition of S, whether it be phosphorus ester, amide, alcohol or ketone, eventually causes the synergistic system to break down and an antisynergism develops. On examination, this phenomenon does not a p p ~ r to work by the same mechanism as obtains in the phosphorus acid-phosphorus ester antisynergistic system. In this work, antisynergism in the H T T A system is studied both by tracer techniques and by absorption spectrophotometry. EXPERIMENTAL The alpha active nuclides 8 x 104 years 230Th, 1"6 × 105 year 233U and the 470 year 241Am were all obtained from ANL stocks. The ~-active nuclide 2.6 year 147Pm was obtained from Isotopes Division of the Oak Ridge National Laboratory. All these tracers were subjected to further purification cycles using liquid/liquid extraction techniques. The method of determining distribution ratios and the solvent extraction techniques employed have been previously deseribed.(4) Tri-n-octyl phosphine oxide (TOPO) was used without further purification. Tributyl phosphate (TBP) received from Commercial Solvents Corporation and dibutyl butyl phosphonate (DBBP) from the Virginia Carolina Chemical Co., were purified as described previously.(5) HTTA received from the Dow Chemical Co., was recrystallized from benzene with a molecular weight of 222.2. N, n-butyl acetanilide (BAA), obtained from the Matheson Co. Inc., and N, N-dibutyl acetamide from Eastman Organic Chemicals, were used without further purification. The alcohols isobutyl, ethyl hexyl and n-decyl were also obtained pure from Eastman. Absorption spectral measurements were made with a Care), 14 Automatic Recording Spectrophotometer using silica cells of 1 cm or 0.1 cm path length. RESULTS Tracer work on the extraction coefficients of three groups (a) trivalent Am/Pro (b) tetravalent Th and (c) divalent UO2 from aqueous into hexane/HTTA solutions was carried out in the presence of various additives. These added materials varied from very small to very large concentrations and caused both a positive synergism and antisynergism, the extraction coefficients varying over a range as high as l0 s. The additives varied from a number ofneutral phosphorus esters, alcohols, ethers, ketones, to so called "inert" diluents, such as benzene or chloroform. The absorption spectroscopy of mixtures o f H T T A and TBP or TOPO was examined at various concentrations in hexane, both dry and water equilibrated, for evidence of interaction. Uranyl and thorium T T A salts in TBP were also studied spectrophotometrically.

Synergism and antisynergism in Arn/Pm tracer studies It was reported in Part I of this series(4) that addition of a neutral organophosphorus ester increased the extraction of Am/Pm from aqueous solutions into a HTTA/hexane solution by many powers of ten. At a constant acidity and constant H T T A concentration it was shown that the partition coefficient K plotted on log/log paper against the neutral additive gave a line of slope@2. This was true up to a concentration o f (4) T. V. HeALY,,IT.Inorg. Nucl. Chem. 19, 314 (1961). (5) D. F. I~I'PARD, W. J. DRISCOLL,R. J. SmoNeN and S. MCCARTY,J. Inorg. Nuel. Chem. 4, 334 (1957).

Synergism in the solvent extraction of di, tri and tetravalent metal ions--IlI

1431

between 10 -2 and 10-1 molar additive. However, more additive causes the slope of the line to decrease gradually, become horizontal and finally turn down in a negative slope. Fig. 1 shows this behaviour in the system H20/Am/Pm/HTTA/DBBP/hexane. ,o4 HEXANE

O,O50 M HTTA 0.01M. HCI

iO3 --

i02 KAm Pm IOI

,

I00i io-i I 10-3

I 10-2

I i0-1

I i0 0

I0 I

MOLARITY OF DBBP

FIo. 1 . - - S ~ and antisyuergism with trivalent complexes. This starts off with a slope of -t-2 as in the complex Am(TTA)3(DBBP)2 for a second power dependency of DBBP. The line then curves round and eventually has a slope of --4. Also, as reported previously,(e) so called "inert" diluents gave varying K values over a range o f 106, hexane, carbon tetrachloride, benzene and chloroform in that order decreasing the K values obtained in cyclohexane. Fig. 2 shows this effect in io3 -

i0 2

--

K Am

Pm I O -

iO0 CYCLOHEXAN E

0.05 M, TOPO 0.0375 M. H T T A 0 . 0 2 N HCI

i0-I _

1°-2 i0 -i

I

I

i00

I0

MOLARITY OF CHLOROFORM FIG.

2.--Antisyner~'am with "'inert" diluent.

(6) T. V. H e , Y, .I. Inorg. Nuci. Chem. 19, 328 (1961).

1432

T.V. I-Is~Y, D. F. PSPP~D and G. W. MASON

the substitution of chloroform for cyclohexane using the system H20/Am/Pm/HTTA/ TOPO/cyclohexane. A log/log plot of K vs. the molarity of chloroform produces a IO3 0,0,6 M TSp/81enzene

I

/ / ~ t

oooNHc

102

i01

io o

I

Pm KAm i0 -i

10-2

~0-~

/ " /"

i0 - 4

•'" 0.92 M TBP/Benzene

/I

I0 -3

10-2

NTTA MOLARITY

I iO-I

i00

IN BENZENE

F[6. 3.--HTTA dependency in both synergistic and antisynergistic systems.

lOt i IO~ _

i

KAm 10-I

i0-~

I0 -4 10-3

I I0 -2 MOLARITY

[ i0 -I OF HGI

P A% I0 0

1~o. 4.--[1-I +] delxadcncy in boffi s,~¢r~tic and antisyner~stic system.

Synergism in the solvent extraction of di, tri, and tetravalent metal ions--III

1433

slope of --4, the same antisynergisticslope that obtains in excess D B B P (see Fig. I). Similarly, substitutionof benzene for cyclohexane in the system A m / P m / H T r A / T B P / cyclohexane also furnishes a slope of --4. It has been previously reported(4) that, in the synergisticsystem H 2 0 / M 3 + / H T T A / T B P there was a power dependency of -{-3for H T T A and --3 for [H+]. This work was repeated at both low and high T B P concentrations, that is at the middle of the synergistic region and the middle of the antisynergistic region. Figs. 3 and 4 show that, in the two regions, the same power dependency holds both for H T T A and for [H+]. Synergism had not previously been demonstrated by the two donor solvents, ethyl hexyl alcohol (EHA) and methyl isobutyl ketone (MIK), in combination with H T T A . It is now shown on the same basis as for the H T T A / T B P synergism, that both these non-phosphorus compounds exhibit a power dependency of plus two for I

,°2i

I

O.15M H T T A / D i t u e n t 0 . 0 0 2 N HCI

IO t K Ara Prn IO o

iO -I Cyclohexone H T T A in

Benzene to-Z

m

10-3

I

I

iO-2 MOLARITY

I0 o

iO -I OF ADDITIVE

IN

131

DILUENT

Fxo. 5.--Ethyl hexyl alcohol and methyl isobutyl ketone as both synergists and antisyner~ts. trivalent metals (Fig. 5), indicating the species Am(TTA)3(EHA)2 , Pm(TTA)30~HA)2, Am(TTA)3(MIK)2 and Pm(TrA)3(MIK)2. Their synergistic powers do not match those of the HTTA/TBP system but nevertheless they increase the partition coefficient up to 400 times greater than for either HTYA or the neutral reagent alone. Here again, the difference between two so called "inert" diluents cyclohexane and benzene is demonstrated (Fig. 5), a factor of up to 30 in the partition coefficients being observed. The same synergism and antisynergism are observed in this series also but because of the relatively small effects it is not possible to say with certainty, what the antisynergistic slope or dependency is. Other differences from the phosphorus ester systems are (I) that greater concentrations of EHA and MIK are required before the synergistic effect is really noticeable and (2) the antisynergic effect is not really definite until concentrations of greater than 1 molar EHA or MIK are present. Synergism and antisynergism in thorium tracer studies As reported previously,(4) the system H20/Th/HTTA/TBP has a power dependency of plus one for TBP concentration as the species Th(TTA)4(TBP)I is synergistically extracted into the organic phase. Further increase in the TBP concentration above the 10-~-10 - 1 molar region causes the synergistic extraction system

1434

T.V. I-IEALY, D. F. PEPPARDand G. W. MASON

to break down, the power dependency line of slope -t-1 bends down, becomes horizontal andeventually decreases to a constant slope of --2. Fig. 6 demonstrates I03

O.OIMHTTA. . ~ O.OIMTBP

R.USNEU'rRI~L~l?Ot'nvE O.ION HCI

QOI M.HTTA O.ION.HCI

1(~3'

I

IOs

162

I

I

I0' IO° MOLARITY OF NEUTRAL ADDITIVE

tOE

F~o. 6.--A phosphorus ester, an amide and an alcohol as both synergists and antisynergists in Th/HTTA/TBP systems. this graphically and also shows similar behaviour for two other synergistic reagents which are non-phosphorus containing compounds. Ethyl hexyl alcohol in combination with HTTA has been shown in the previous paragraph to exert a synergistic effect on the trivalent rare earths and actinides. This is also true for thorium where Fig. 6 shows both the synergistic effect with the extraction of the species Th(TTA)4(EHA)I and the antisynergistic effect. N, n-butyl acetanilide (BAA) is a donor solvent which one would expect to have some extracting properties for metal salts. It is shown here to have very good synergistic properties in combination with HTTA (Fig. 6), the extracted thorium species being Th(TTA)4(BAA)t. Like TBP it exerts both synergistic and antisynergistic properties with BAA dependencies also of -F 1 and --2 respectively. Also, On this composite curve in Fig. 6 is shown the effect produced by addition of BAA or EHA to the synergistic system H20/Th/HTTA/TBP containing 0-01 molar HTTA and 0.01 molar TBP. It is interesting to see that addition of the alcohol EHA causes an immediate antisynergistic effect and eventually the extraction is the same as that of the antisynergistic part of the EHA curve itself in the system H20/Th/HTTA/ EHA which contains no TBP. In this case the initial synergistic effect of the TBP has completely disappeared in excess EHA. Addition of BAA to the previous H20/Th/ HTTA/TBP system causes a slight initial rise in the thorium partition coefficient, then a gradual fall until the partition line coincides with that of the H20/Th/HTTA/ BAA system which has a slope of --2. The antisynergistic effect of so called "inert" dilu¢nts has been commented on in the work on the trivalent metal series above. This is also true for thorium except that the slope of the power dependency of the diluent is changed from --4 to --2. Fig. 7 illustrates the strong antisynergic effect of chloroform additions. Benzene is not shown but has somewhat less of an effect with the same slope of --2. MIK, mesityl

Synergism in the solvent extraction of di, tri and tetravalent metal ions--Ill

1435

oxide and the dibutoxy ether of ethylene glycol, all have a similar antisynergistic effect of slope --2, but only one, MIK, is shown, because the lines are more or less co-incident with each other. BAA and three alcohols, isobutyl, decyl and ethyl hexyl have somewhat greater antisynergistic effects, all with slopes of --2. All the above effects are illustrated in Fig. 7 as part of a composite whole where the Th/HTTA/TBP io ~

102 L-

,,,,

KTh

F

CHLO O ,

DECYL ALCOHOL

i0-I_

,UTY

ETHYLHEXYLALCOHO~ L

~ , ~ "

iO-2

,6

10"2

I

]

i0-1

iO0

I iOI

MOLARITY OF NEUTRALADDITIVE

FIG. 7.--An~ynergic effects in Th/HTYA/TBP systems produced by amides,

alcohols, ketones and "inert" diluents.

io e

O.OI M HTTA ~Hexone O.OOI M TOPO\ O.I N HGI i0 I KTh

-¢'~./q'~ I0 0

i 0 -I X

,o - t L iO-I MOLARITY

IO 0 OF ALCOHOL

iO I

FIG. 8.--Antisynergic effects in Th/TOPO systems produced by alcohols.

1436

T.V. He,Y, D. F. Pm~.~v and G. W. MASON

synergistic effect is destroyed by different types of solvents and all with a power dependency of --2. T o P e is one of the stronger donor solvents, much stronger than TBP, yet antisynergistic agents like alcohols cause large decreases in the partition coefficients. For example, in the system H20/Th/HTTA/TOPO, both isobutyl alcohol and ethyl hexyl alcohol have inverse second power dependencies, and slopes of --2 are illustrated in Fig. 8. In an attempt to ascertain if the maximum on the synergistic-antisynergistic curve varied with the relative proportion of HTTA to neutral additive, two curves were obtained for the system H20/Th/I-ITTA/EHA, one at 0"01 molar and another at 0-08 molar HTFA.. Fig. 9 indicates that, although the

100

O.08M.HTTA 0.10 N.HCl KT h

o. ~ / ' ~ ~ ~

0.01M.HTTA O. ION. HCI

O.OI /

o.oc

t.OOI

I

eel

_

L

O, I

_

I

I

MOLAR,TYOF NEUTRALADD,T,VEIETHYLHEXANO')'NC¥CLOHEXANE Fro. 9.--Effect of [HTrA] on Th/EHA synergistic and antisynergistic curves. system containing the 0.01 molar HTTA had a larger synergistic effect below 0.01 molar EHA than the other system, the maximum occurred in the same region, in both cases. Probably, if a very large HTI'A concentration had been used (say 0.51 molar), there might have been no visible synergistic effect with EHA at all, only an antisynergistic effect.

Synergism and antisynergism in uranyl (II) tracer studies The uranyl/HTTA/S systems exhibit the same synergistic and antisynergisfic effects as shown by the other elements. The difference is in the amount of antisynergism, that is, there is a shift to the right in the maximum of the syn./antisyn. curves. This is illustrated in the H20/UO2/HTTA/TBP system in Fig. 10. The synergistic line has a slope of + 1, and the small antisynergistic line has a slope of--2 similar to the thorium system. Also shown on this graph is the synergistic effect produced in the uranyl system by the BAA dependency of Jr 1. The antisynergistic effect of CHC13 on the H20]UO2/HTTA/TBP system also causes an initial flattening out at the top part of the curve, and the overall antisynergistic effect of CHC13 is thereby much reduced. The final dependency, however, is still the same, namely --2.

Synefgimlin the mlveat egtracti~ of di, tri and tetravaJeatmetal iota--HI

1437

Absorption spectral evidm~ for water/HTTA/donorsolvent interaction It was initially observvd that the absorption spectra of solutions of HTTA in dry and water saturated organic solvents were somewhat ditfcmnt, the dry solution spectra having a peak in the 350-400 m/t region when compared with the wetted ,o 3 _

x~x~LOPE2 -

102 -

/

O l /i

nK~ /uLNED S IPLACED BYFACTOR OFO lt2 SLOPE+A/

i0° , io"4

I

I

I I1 4o-3

Fro. 1

~C.

~

I

I II I I I 11 I I I tl i 162 Io" 1o0 MOLARITY OF NEUTRAL ADDITIVE m~d ~ with unmyl cemmku.

t

I I io

solution. This peak poX'on moved toward longer wave length with increasing HTTA concentration. The increasing height of the peak, not surprisingly, did not follow Beer's Law. It was also observed that addition of a donor solvent such as TBP to a dry hexane solution of I-ITTA similarly increased this peak height when compared with the ~ HTTA solution. In contrast, addition of TBP to an aqueom saturated bexane solution of HTFA earned initially an increase followed by a decrease in peak heisht. This rise and fall in peak height with addition of TBP showed a s ~ pattern to the synergism followed by antisynergimn exhibited in the extraction of metals by HTTA-TBP mixtures from aqueous solution. The absorption spectral data of HTTA-TBP mixtures were therefore investisated, solutions being made up with three diffcmmt concentrations of HTTA in hcgane, namely 0.1, 5XI0-3 and 5xlO-S M H T r A respectively. In order to be able to ascertain the species present,, the most dilute solution (5 X 10-s M HTTA) was examined first in the spectrophotometer.using bexane as the blank solution. These dilute f l I T A solutions contained various concentratiom of TBP, one series being dry and the other equilibrated overnight with water. Table 1 gives the spectral peaks, huml~ or shoulders of importance in the various solutions. This table shows very clearly that HTTA plus a great excess of TBP in the prosen~ of water gives a spectrum almost identical with that of HTrA hydrate in aqueous soIution(~ whereas inthe absence of water the spectrum corresponds to the aqueous en01ate ion.O) Neithcg of these spectra have been obtained in a non-aquvom solution (,}B.I. z m L C e x ~ T I D ~ 8

(1947).

1438

T.V.I..lz~Y, D. F. P~P, taD and G. W. MASON TABLZ I.---SP~,AL PEAKSoF HTTA SOLUTIONS (5 xlO-SM) CoN'r~'~NO TBP n~ Aqueous equilibrated solutions TBP Molarity (M)

Wavelength (n~)

Dry solutions

Density (1 cm cell)

Wavelength (m,)

Density (I cm c011)

0.000

338 314

0"58 h 0"68

338 315

0"65 h 0"79

0.001

338 315

0"58 h 0.68

338 315

0.64 h 0"79

0-10

338 315 265

0"58 h 0"64

338 250

0.99 0"49

1"1

360-320 286 262

0"20 sh 0.44 0.51

338 250

0"98 0"48

3"6

288 262

0"41 0.53

338 250

0"98 0.48

HTTA hydrate 292 266

pH 1-3 0.40 0.50

Enolate ion 338 262

pH 9 0.89 0.32

Aqueous solutions

0"40 sh

previously, but their correspondence with the two aqueous spectra is perhaps shown more clearly in Table 2, where the extinction coe~cients o f the spectral peaks are given. TAsus 2.---CoMI"AS.tSONO1~AQUEOUSAND TBP SOLUTIONSOF HTTA TBP solutions 5 × 10-SM HTTA in TBP Dry

Water equilibrated

E~s •- 19,600 E2so 9,000

E290 ffi 8~200 E2e2 : 11,000

:

Aqueous solutions Enolate ion EmE26z

ffi

17,700 6,250

HTFA hydrate E292 : 7,940 E2e~ ffi 10,100

This means that in the absence of water H T T A combines with TBP, the basic donor properties of the ester presumably causing hydrogen bonding on to the enolic form of the acid HTTA. In the presence of water this bonding is much weaker than the keto hydrate formation which destroys the enol bond completely. There is also infra-red evidence(s) to support the formation of TBP-hydrate in the water (s)j. R. F m m ~ o and T. V. H~LY, Unpublished work.

Syn~

in the solvent extraction of di, tri and tetravalent metal ions---III

1439

equilibrated TBP solutions of HTTA, the 1260 cm -1 waveband moving over to 1270 cm-1. Both the wetted and dry dilute TBP-I-ITrA solution spectra are similar to those of the pure HTTA in wetted and dry solutions and all have the normal enol structure formed in organic solutions of HTTA. When the addition of TBP to HTTA solution reaches about O.1 molar TBP, the spectral evidence (Table I) shows that the usual enol spectrum is changing. This change is either towards that of the keto hydrate (wet) or enolate (dry) spectra and is almost complete at a concentration of I molar TBP. In the enolate species the H T r A probably forms a complex with TBP, the ester presumably hydrogen bonding on to the enolate form of the HTrA. The three H T r A spectra are illustrated in Fig. 1I, l

I

0.0075 M TTC,/Oy¢lohexolte 0.020 M TBP/ lu $ GHGI

IO0

Ku

\x ope - 2 I0

\

%

\ i

I

I t.O

O,I

I0

MOLARITY OF CHLOROFORM

Fro. 11.~Antisyn~gic effect of chloroform with uranyl complexes. and the probable species are given below. HC---CH

II

HC

C

H

oII

C

oI

CH2

C

\s /

H

HC--CH

II

II

oII

CF3 HC

C

C

I

oI CH

C

CFs

\S /

0 H

ENOL

F~TOItYDRATE

HC--CH

II

fl

(RO)sP--*O----H I O O

HC

12

C

II

CH

\S / ENOLATE

I

C

CF3

1440

I'. V. H e , Y, D. F. Pro,PARDand G. W. MASON

Addition o f TBP to higher concentrations of H T r A in hexane cannot show directly the three spectral species owing to the very high extinction coefficients of HTTA. But these mixtures of T B P - H T T A solutions can be examined spectrally using the H T T A solution alone as blank. In this way the increasing height of a peak can be plotted against an increasing TBP concentration in a constant molarity H T T A solution. The wavelength of the peaks varies with the H T T A concentration and Beer's Law does not apply. This is because the spectrophotometer is really being used beyond its capabilities due to the poor transmission of the more concentrated H T r A solutions. Table 3 illustrates these points by showing the increasing peak wavelength and the small change in peak height (I cm cells) as the H T T A concentration increases.

TABLE 3.--EFt~croFTBP ADDrnON ON PEAK eeiotrrAND WAVELENGTH OF H T T A / H E X A N E Wet solutions

Sample

0"16 ~ D396 * " - - 10-1M H T T A + I ' I 0"12 =~ D387 *-- 5 × 10-3M HTI"A+I'I --0.35 =~ D33s 4-- 5 × 10-5M H T r A +1.1 0"44. •ffi D256 4" 5 xI0-SM HTI"A +1"1

vs. Reference solution

Dry solutions

M. TBP 10-1M HT'rA - - - - - * D 3 9 7 - ~ 3"9 M. TBP 5 x 10-3M H T r A --, D3e5 -- 2-2 M. TBP 5 × 10-SM HTTA ..-, D34o =~ 0-39 M. TBP 5 × 10-SM HTTA ~ D33s "ffi0"98

As shown above, the presence of T B P in dry H T T A solutions increases the peak height at all the H T T A concentrations. Although the peak position has moved toward longer wavelength at increased H T T A molarities, this is undoubtedly a measure of the production of the enolate species. Similarly,the very slightrise and fallof this peak height in the water saturated solutionsplus the changing wavelength must also indicate the competition for the H T T A between T B P and water. These data are given below, the dry and wet solutions meaning dry hexane and water equilibrated hexane solutions respectively. Sample A and reference blank A contain 0.I M H T T A . Similarly B and C contain 5 x I 0 - 3 M and 5 × I 0 - 5 M H T T A respectively.

A similareffectto that of T B P on H T T A solutionscan be shown by most electron donor solvents. For instance,the effectof dry T O P O in dry 0. I M H T T A solution in bexane is shown by the gradual increase in the peak height at a wavelength of 397 m # using 0"I M H T T A in a I cm reference cell. In this case, the optical density of this peak increasesfrom 0.52 at 0-02 M, 0.95 at 0.04 M, 1-35 at 0"06 M, 1.53 at 0.08 M, 1.88 at 0-I M to 5.0 at 0.2 M TOPO. This isa greatereffectthan that produced by the corresponding T B P solutionA in Table 4, and indicatesthat, in dry solution,the HTrA-TOPO interaction is much stronger as would be expected from the very strong electron donor properties of T O P O as compared with TBP. In aqueous equilibratedhexane solutions of H T r A - T O P O , the formation of ketohydrate and T O P O hydrate would also be greater than in the case of T B P for the same reason. The following table gives a few examples of the complexing of H T T A by a donor solvent in the absence of water, and the competition between water and donor solvent for HTI"A complexing in the presence of water. The optical density of the 397 m # peak is used to compare the effect of the donor solvents TOPO, TBP, N-butyl acetanilide (BAA) and dibutyl acetamide (DBA) on 0.1 molar H T T A in hexane.

Synergism in the solvent extraction of di, tri and t~travakmt metal ions--HI

i

i~ ~

0 ~'~

~

O~

O~

•~0

. -0

Z

e~

e

o

o

•

o

i o

o

o

o

I"

t~ o

1441

1442

T.V. Hs~Y, D. F. PsI~a~D and G. W. MAsoN TAm.~5.---Co~A~soN ov HT~A--SOLVENT~ c ' r I o N

PEAKS

Reference blank is 0.1 M HTTA/hexane Dry solutions

Water equilibrated/ solutions

D397

0"1 M HTrA

1"9

O'1 M TOPO

0.80

0.1 M HTTA 0"1 M TBP 0.1 M HTI'A 0.1 M BAA 0"1 M HTrA 0.1 M DBA

0"62 0"30

Ds97 0"19 0.40 0"62 0~1

This peak height gives a measure in the dry solutions of their donor strengths and also of their possible synergic effect on the formation of metal/TTA/S complexes. The peaks in the wet solutions are less reliable but give a measure of the competition between donor solvent and water in the formation of ketohydrate. A more reliable measure would be the production of a series of absorption spectra from dry and aqueous equilibrated hexane solutions of 10-5 molar HTFA with increasing amounts of donor solvents added as illustrated in Table 1. In this way one can assess the amount ofenolate and ketohydrate which is formed from initial 90-100 per cent enol form of HTTA normally found in organic solvents. The diluent hexane has been used in all previous experiments with HTTA-donor solvent. But, by varying the diluent, the HTTA solvent interaction peak also varies both in height and in wavelength. The diluent in which water is least soluble produces the least difference between the HTTA-donor solvent interaction peaks when the dry and the aqueous equilibrated solutions are compared spectrophotometrically. The table below bears this out for the solvents cyclohexaue (CH), bexane (H), benzene (Bz) and butanol (BuOH). For the donors, hexoue (MIK), ethyl hexyl alcohol (EHA) and TBP which bring even more water into the water equilibrated organic phase the differences are even more striking, due to the tendency to euolate formation in dry solutions and ketohydrate in wet solutions. This order in Table 6 is also the order of increasing antisynergism in the metal/TrA/ solvent extraction. TAm.e 6.--EF~'r oF Dn.u'E~rrsON HTrA-TBP xtcr~gAc'rmNpeT,x 0.1 M HTTA +0.1 M TBP.--Dry solution is +ve sample in 1 cm cell Wet solutionis -ve reference in 1 cm cell Peak height Wavelength (m~t)

CH 0.2 396

H 0.25 397

Bz 0.35 404

BuOH 0.75 397

MIK 2 401

EHA 2.5 399

TBP 5 398

Absorption spectralevidencefor water/MO'TA)x/TBPinteraction The absorption spectra of UO2(TTA)2TBP in benzene or hexaue is essentially the same as that of UO2(TTA)2 in these solvents, having two large peaks at 330 and 380 m/t. The only difference is above 400 m# where the well defined peaks of the UO2(TTA)2TBP complex at 426, 440 and 454 m/~ are missing from the UO2CITA)2 spectrum. These spectra are also very similar in the presence of slight excess TBP

Synergism in the solvent extraction of di, tri and tetravalent metal ions---HI

1443

and/or in the presence of an aqueous phase. By dissolving a small quantity of the solid UO2(TTA)2 complex in dry liquid TBP, a new spectrum is formed with peaks at 256 and 336 m/~. This is essentially the spectrum of HTTA dissolved in dry TBP plus a smaller amount of the spectrum of UO2(TTA)2TBP in a ratio of about 2:1 (Fig. 12). On saturating this TBP solution of UO2(TTA)2 with water, the $pe¢truHl is chaul~ed back to that of UO2(TTA)2 with peaks at 330 and 377 m/~ (Fig. 12). The pronounced

1.0

A. 8. C.

5 x tO"4 M HTTA I cm cell Dry HTTA in TBP vs. dry TBP Wet HTTA in TBP vl. wet TBP Wet HTTA in Hexone vs. Helane

>. Fz 0.5 o 0

0 220

260

300 340 380 WAVELENGTH (m/~)

4~0

FIG. 12.--Absorption spectra of HTI'A in organic solution, (A) enolate (B) ketohydrate and (C) enol. peaks above 400 m/z of UO2(TTA)2TBP are absent. The sequence above indicates that addition of a little TBP to UO2(TTA)2 forms the UO2(TTA)2TBP complex. Addition of (1) great excess dry TBP breaks it down to form a HTTA-TBP complex and (2) great excess wet TBP breaks it down to form UO2(TTA)2 which may or may not have a hydrate attached. The following table gives the extinction coefficient obtained in this series. The spectra of Th(TI'A)4 and Th(TTA)4TBP in benzene are practically the same. Both have one large peak at 350 m# with an extinction coefficient of about 85,000. It was expected that solutions of Th(TTA)4 in wet and dry TBP would behave similarly to the uranium salts and give different spectra. Actually they gave somewhat as both gave a large peak of E about 80,000. The wavelengths of the peaks dilfered by about 10 ma. The peak was at 349 for the spectrum of Th(TTA)4 in water saturated TBPindicating it was practically identical with the Th(TTA)4 and the Th(TTA)4TBP spectra. By analogy with the uranium species it was probably a Th(TTA)4 hydrate

1444

T.V. I-IsALY,D. F. PEPPARDand G. W. MASON TABLE 7.mExTINC'rlON COEIqqC~I'S OF UO2(TTA)2 COMPLEX~

UO2(TTA)2 in benzene

--

E33s 26,400

UO2(TTA)2TBP ill benzene

--

E332

E3s4 21,200

--

E380

m

E430

27,500 22,000 1,450 UO2('ITA)2

in water sat'd TBP

UO2(TTA)2 in dry TBP

-E256

Es3o E377 25,000 21,000 E336

--

16,000 33,000 HT]'A in dry TBP

E262 6,250

E338 17,700

.

.

--

- -

E444

E4SS

950

500

--

--

E422

E435

E449

550

400

200

.

.

spectra in hydrated TBP. The Th(TTA)4 spectrum in dry TBP had a peak at 340 m# which is essentially that of HTTA dissolved in dry TBP indicating that the excess dry TBP had broken down the thorium complex liberating the HTTA which then complexed with the dry TBP. Again this behaviour is analogous to that of UO2(TTA)2 when dissolved in dry TBP. DISCUSSION Synergism not confined to HTTA plus phosphorus esters Synergism in metal extraction by HTTA plus phosphorus esters has been demonstrated previously.¢ 4) It is shown in this paper that phosphorus esters can be replaced by ketones, alcohols, and amides, and it should be mentioned that HTTA plus sulphoxides or plus amines also exhibits synergism. Two obvious alternatives to HTTA, namely 8-hydroxy quinolin and cupferron, both exhibited somewhat limited synergism with TBP. Several alcohols plus HTTA exhibit synergism with di, tri and tetravalent metal species. Among the stronger alcohols in this respect are n-dccyl alcohol and ethyl hexyl alcohol (EHA). Species apparent from tracer work (Figs. 5 and 9) are Th(TI'A)4EHA, Am(TTA)3(EHA)2 and PmO'TA)3(EHA)2. As an example of this synergism, addition of about 2 per cent of ethyl hexyl alcohol to a solution of 0.15 molar HTTA in cyclohexane increases the partition coefficient of Am or Pm by a factor of 200 to 300. The fact that these alcohols and also ketones plus HTTA exhibit this phenomenon of synergism shows that it is not confined to strong electron donor solvents. Two of the stronger donor ketones are methyl isobutyl (MIK) and methyl isopropyl ketone, the species extracted by MIK being Am(TTA)3 (MIK)2 and Pm(TTA)3(MIK)2. It has been reported in the literaturec8) that extraction of metals with HTFA is greater if hexone (MIK) rather than benzene is used as the solvent, but no reason was given for this. It is now seen that gradual replacement of benzene by MIK solution causes a rise in the partition coefficient of the metal as the M(TTA)=(MIK)~ species is formed. Eventually, the partition coefficient decreases again by a relatively small amount as benzene is completely substituted by the MIK. N-butyl acetanilide (BAA) plus HTTA together have a large synergistic effect on the extraction of thorium from aqueous solution, increases of greater than 104 being obtained (Fig. 6). The species extracted was found, from this tracer work, to be tv~ j. R. THOMAS,H. W. ~ L L , (1949); T. KmA and S. ~ ,

T. E. HICKS,B. RtmlN and J. S~L.DICK,Report KLX-44 Bull. Chem. $oc. Japan, 31, 1007 (1958).

Synergism in the solvent extraction of di, tri and tetravalent metal ions--HI

1445

Th(TTA)4BAA. Other species found from tracer and macro work were UO2(TTA)2 BAA and Nd(TTA)3(BAA)2. Ultra-voilet and visible absorption spectral data to be reported elsewheretlO) also confirm the formulae for these species. Antbynergbm Antisynergism was first reported as a phenomenon by BLAKEeta/.(x) and later for separation procedures by PEPI'A.RDet al.t2) in the organophosphorus acid-phosphorus ester series. In both casest3) antisynergism, that is the reverse of synergism, was considered to be due to interaction between the synergistic agents--acid plus ester. On adding TBP in excess to a H20/metal/HTTAfrBP synergistic system, the system breaks down, the partition coefficient decreases and the phenomenon of antisynergism occurs. At first, it was thought that this antisynergism was likewise due to interaction between the acid (HTTA) and the ester (TBP) to form an association product. An examination of the power dependencies of the metal extractions on TBP or other s'mdlar phosphorus ester (S) for the systems H20/Th/HTTA/S indicated a positive synergistic power dependency for S of -~ I followed by a negative antisynergistic power dependency for S of --2 due to excess of S. The same ~ 1 and --2 dependencies were found when the amide BAA and the alcohol EHA were used instead of S. It was also remarkable that if either EHA or BAA were gradually added to the feted synergistic system H2Ofrh/0.01 M HTI"A/0.01 M TBP, antisynergism was found to occur (See Fig. 6). These lines, also of slope --2 eventually became coincident with the antisynergistic part of the H20/Th/0"01 M HTrA/BAA or EHA systems. It was also found (Fig. 7) that other alcohols, amides, ketones and ethers all produced the same antisynergistic slope of --2 when added to the system H20/Th/0.01 M H T r A / 0~)1 M TBP/hexane. Furthermore, so called inert diluents such as benzene or chloroform on addition to this system also produced antisynergistic slopes of --2. It appeared from all these data that antisynergism in the case of thorium could hardly be due to direct interaction of the HTTA with either the TBP or the other donor or nondonor solvents. Experiments substituting Am or Pin for thorium in similar systems showed exactly the same results except for the fact that in all cases the synergistic power dependency slope was -t-2 with a corresponding antisynergistic dependency of --4. It should also be noted that, in both the synergistic and antisynergistic regions, the trivalent elements had the usual power dependency of + 3 for the HTTA and --3 for the hydrogen ion dependency. The following table summarizes the synergistic and antisynergistic power dependencies of UO2, Th, Am and Pm systems for both donor and non-donor solvents at constant H T r A and hydrogen ion concentration. TAnLE8.--S~maomu AND~¢m,cseaOv~ Pow~ v ~ E s o ~ due to phosphorus esters, amides, alcohols, ketones and inert A n t ~

Synergism due to phosphorus esters, amides, alcohols and ketones Th Am, Pm UO2

+1 +2 +I

(10) T. V. I-Ill~Y and J. Fean~o, Unpublished work

dfluents

--2 --4

--2

1446

T.V. HEALY,D. F. I~PP~D and G. W. MASON

It appears that the only common constituents in the organic phases of these systems is HTTA and the water content. Different donor solvents give the same positive and negative power dependencies while different so called inert diluents give similar negative power dependencies. Also the HTTA and hydrogen ion dependencies are consistently as expected both in the synergistic and antisynergistic regions. This fact indicates that, throughout the system, the MCITA)x is an integral part of the extracted species, x being the valency of M. One is led therefore to the assumption that water in the organic phase has an appreciable effect on the breakdown of the anhydrous synergistic complex M(TTA)~Sy. Spectrophotometric evidence to support this is shown first in the HTTA-TBP system and then in the M/HTTA/TBP system. The absorption spectra of dilute HTFA solutions in hexane whether in the presence or absence of an aqueous phase is very similar to the same solutions also containing small amounts of TBP. These spectra are of the usual enolic form of HTTA in organic solvents. These are also the concentrations used in obtaining synergistic extraction of metals from aqueous solution by HTTA/TBP solutions. However, if a great excess of TBP is added to HTTA in the presence of an aqueous phase i.e. under conditions where antisynergistic extraction of metals occurs, the spectrum is changed to that of HTTA ketohydrate. There is also infra-red evidence of reformation of TBP hydrate and of a shift in the carbonyl peaks.(S) On addition of TBP in increasing quantities to a fixed concentration of HTFA in the absence of water the carbonyl infrared absorption peak at 1600 cm-1 shifts gradually to 1650 cm-1. The same TBP addition to HTTA in the presence of D20 shows initially a shift of the 1600 cm- 1 peak to 1650 cm- 1 but further TBP addition appears to shift the peak back towards 1600 cm-1~8) This provides further evidence for the initial HTTA-TBP complexing in the presence and absence of water, followed by the breakdown of the complex on further addition of water saturated TBP. Great excess of TBP plus HTTA, in the absence of water, forms a completely different ultra-violet absorption spectrum, namely one similar to that of the enolate ion seen in aqueous alkaline solution. The gradual increase in peak height of the spectrum as more dry TBP is added to HTFA is a measure of formation of the HTTA-TBP complex. However addition of TBP to HTTA in the presence of an aqueous phase shows the peak height (Table 4) rising through a maximum which corresponds approximately to the synergism and antisynergism concentration regions in the metal/HTTAfFBP systems. An examination of the absorption spectra of organic solutions of UO2(TTA)2 and UO2(TTA)2TBP in the presence or absence of an aqueous phase shows them to be essentially the same except for the well defined peaks of the latter complex at wavelength of > 400 m#. Dissolution of either complex in dry TBP furnishes essentially the spectra of dry HTTA in dry TBP, that is the uranyl complex is broken down and the liberated HTTA is hydrogen bonded to the dry TBP. In the presence of an aqueous phase the spectrum is different--it is that of UO2(TTA)2. This tends to show directly the competition of water with TBP. In the presence of an aqueous phase, addition of TBP to UO2(TTA)2 in hexane, produces the UO2(TTA)2TBP complex. Further addition of excess TBP causes this complex to be broken down back to UO2(TTA)2. This would also explain the synergism followed by the antisynergism exhibited in the extraction of uranium in this system. Thoriu~ is shown to behave similarly, forming initiaUy the synergistic Th(TTA)4TBP and then, with antisynergism, returning to

Synergism in the solvent extraction of di, tri and tetravalent metal ions--IlI

1447

Th(TTA)4. Confirmatory evidence for this TBP/water competition in the metal/T]'A complex system is obtained from infra-red studies.(8) It was observed that addition of dry TBP to solid UO2(TTA)2 gives an infra-red spectrum (in the 1600 crn-I region) similar to that obtained for solid UO2(TTA)2TBP. Two equally intense peaks are obtained at 1622 and 1595 cm-1. However, on contacting a dilute solution of UO2(TTA)2 in TBP with D20, the higher frequency peak becomes a small shoulder

0.8

Spectro of 2 x IO"4 M UOt(TTA)t in 0.1cm Cell A in pure dry TSP B in wote¢ soturoted TBP A-I ond B-I ore from 1.0cm cells m

0.2

°2

B

~

350 400 WAVELENGTH (m~¢)

450

1~o. 13.--Effect of water on absorption spectra of UO2(TTA)2 in TBP. relative to the main peak at 1595 cm- 1, indicating a shift to the UO2(TrA)2 spectrum obtained in wet solution. Similsr results were obtained when the Th(TTA)4 was dissolved in excess TBP both dry and wet. Karl Fischer water content measurements have shown(10) that as one adds Th, UO2 or Nd to HTTA/TBP mixtures in hexane the water content of the phase gradually decreases until at the stoichiometric quantity of metal to HTFA and TBP the hexane solution is practically free of water. Addition of excess TBP causes the water content of the hexane phase to rise again. It is interesting to see that the order of bringing about antisynergism in the solvents used is also the same order as the solubility of water in these solvents. For example, cyclohexane, which has the poorest antisynergistic effect and lowest water solubility (0434 g H20 per litre) is followed by hexane (0.07 g/1.), CC14 (0-15 g/1.), benzene (0"6 g/l.), CHCI 3 (1"5 g/l.), dibutyl carbitol (14 g/l.), MIK (19 g/l.), decanol (24 g/l.), 2-ethylhexanol (26 g/1.) and TBP (60 g I-I20 per litre). While the mechanism of formation of the synergistic complexes M(TTA)xSy is relatively weI1understood through the reaction: M ~+ + xTTA.- + y S ~ M(TTA,)xSy = K[M(VrA.)xSt]. and the equilibrium[TTAe-Ix[S], [MX+ ]

1448

T.V. I-IEALY,D. F. PF.PPARD and G. W. MASON

itappears difficultto interpretthe mechanism ofantisynergism as enough is not known about it. Addition of more donor solvent will simply cause an increase in the activity of IS] and so the only way to cause antisynergism must be by decreasing the [TTA d activity. The subscript e for [TTA] means the enol form as distinct from the ketohydrate or enolate forms. It has been shown that the H T T A - T B P interaction in the absence of water gives the H T T A enolate structure and this is clearly not involved in the antisynergism. It is also likely that the formation of the ketohydrate structure CITAI) is involved. This structure probably contains attached water but in any case it is very much influenced by the presence of water; it may or may not contain S. Formation of a metal/TTA complex containing a hydrate would be expected to give lower partition coefficients because the normal coordination energy of the hydrate water with other water is lost on extraction; the complex is also more hydrophillic. O n the other hand, while one possibility is formation of a M(TTA)xSy(H20 ) this species would have to be formed at the same aqueous activity as the anhydrous M(TTA)xSy and this seems less likely. Antisynergism caused by addition of another donor solvent to a M(TTA)~Sy synergistic system such as addition of methyl isobutyl ketone to Pm(TTA)3(TBP)2 can also be similarly attributed to the lowering of the HTTA activity [T/A,] and the formation of [TTAt].. Ultra-violet evidence also indicates that changing from one "inert" diluent to another, also causes [TTA.]-[TTAI] differences, that is changes in the [TTAo] activity. The reduction of I-TTAe] activity is obviously bound up with the organic water content and water is necessary for formation of the [TTAd species.

Versatility of the synergistic-antisynergistic phenomena in metal or group separations In view of the difference in complexes formed in the synergistic extraction of metals, use could be made of these properties in the separation of metallic species. For example, the rare earths Eu, Pro, with complexes such as Eu(TTA)a(TBP)2 form different complexes from other rare earths such as thulium which forms a complex Tm(TTA)3TBP. Similarly, separations could be made of certain species by taking advantage of different antisynergistic properties of either the donor or non-donor antisynergists. Altogether, the available scope for variations in extraction procedures opened up by the synergistic and antisynergistic phenomena offers many possibilities for separation techniques and for methods of analysis.