Incorporating Size Selectivity into Synergistic Solvent Extraction: A Review of Crown...

Incorporating Size Selectivity into Synergistic Solvent Extraction:A Review of Crown Ether-Containing Systems

Andrew H. Bond,* Mark L. Dietz, and Renato Chiarizia

Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439

This paper provides a comprehensive review of synergistic solvent extraction using crown etherswith a focus on the role of both extractants in facilitating cation-specific separations. Anintroduction to the various equilibria affecting synergistic extraction using crown ethers isfollowed by a review of the work published in this field during the 1972-1999 time period. Theinfluence of various solvent extraction parameters on the potential for cation-selective synergismis critically examined. Those synergistic extractant combinations showing cation selectivity arehighlighted, as are the fundamental investigations that are the foundation of the currentunderstanding of synergistic solvent extraction using crown ethers.

Introduction

The transfer of metal cations from an aqueous me-dium into an organic phase most often requires the useof an extractant capable of satisfying the solvation andcoordination preferences of the cationic solute. In someinstances, a single extractant molecule can accomplishthis task, while in others, several extractant moleculesmust cooperate to effect phase transfer of the cation.The latter is frequently encountered in the extractionof cations by monoprotic chelating and/or acidic extrac-tants and may involve the formation of complexes inwhich the primary coordination sphere of the cation(Mm+) includes sufficient deprotonated extractant mol-ecules (Aa-) to maintain electroneutrality and one ormore neutral extractant molecules to saturate thecoordination environment of the cation:

Such self-adduct formation makes the complex moreorganophilic (i.e., increases extraction) but may behighly sensitive to steric repulsions because the solvat-ing Lewis base donors of the extractant are oftensurrounded by the bulky organic framework that im-parts organophilicity to the extractant. An alternativeto self-adduct formation is the use of two differentextractants that fulfill different objectives in the extrac-tion process. By example, one bulky acidic extractantmay serve to complex a metal cation and neutralizecharge, while another, perhaps less sterically demand-ing, extractant may serve to replace H2O or to occupyopen coordination sites. When two extractants cooperateto yield partitioning that is greater than the sum of eachextractant operating independently, the result is asynergistic extraction system.

The equilibrium treatment for synergistic extractionis discussed in several monographs on solvent extrac-tion,1-3 and four classes of synergistic extractant com-binations can be defined: (1) chelating extractant/

neutral extractant, (2) acidic extractant/neutral extrac-tant, (3) two neutral extractants, and (4) two chelatingextractants. The observed synergistic effect generallydecreases from class 1 to class 4,1 and it is clear thatpartitioning by chelating and/or acidic extractants suchas those shown in Figure 1 relies principally on elec-trostatic interactions.1 That the synergistic effects forcombinations of these extractants are electrostaticallydominated implies a general lack of selectivity, espe-cially for cations of the same valence.

The most versatile class of ion-specific extractants isarguably the crown ethers, in which the macrocycliccavity size, chelate ring size, macrocycle rigidity, andnumber and type of donor atoms may be tuned toprovide a high degree of metal ion selectivity.4,5 Despitethe clear potential of crown ethers in separation sciencethat was immediately evident in Pedersen’s initialreport in 1967,6 it was not until 12-13 years later thatthe use of crown ethers in synergistic solvent extractionwas first reported.7-10 Figure 2 provides examples ofsome of the various crown ethers used in synergisticsolvent extraction. In keeping with Pedersen’s nomen-clature,6 the number of atoms in the macrocyclic ringis followed here by the letter “C” for “crown” (to savespace) and last by the number of Lewis base donoratoms. Any ring substitutions precede the ring enu-meration and generally use conventional abbreviations.Mixed donor macrocycles are abbreviated on an indi-vidual basis.

As neutral extractants, the crown ethers (CE) requirethat sufficient aqueous phase anions (Xx-) accompanythe extracted cation complex to maintain electro-neutrality:

In most cases, Xx- is a mineral acid anion such as NO3-,

Cl-, or SO42- and the stoichiometric coefficient (q) for

the crown ether is 1. It is the requirement of anioncoextraction that places severe limitations on the utilityof separations using crown ethers, because the dehydra-tion and transfer of anions into hydrometallurgicaldiluents (e.g., paraffinic hydrocarbons and aromatics)is difficult. Four strategies have been devised to mini-

* To whom correspondence should be addressed. Presentaddress: PG Research Foundation, Inc., 8205 S. Cass Avenue,Suite 111, Darien, IL 60561. Telephone: 630-963-0320. Fax:630-963-0381. E-mail: [email protected].

Mm+ + pHAorg h M(A)m‚(p - m)HAorg + mH+ (1)

Mm+ + qCEorg + mX/xXx- h M(CE)qXm/org (2)

3442 Ind. Eng. Chem. Res. 2000, 39, 3442-3464

10.1021/ie000356j CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 09/08/2000

mize the adverse impact of anion coextraction in crownether-containing systems:11 (1) the use of high aqueousphase anion concentrations, (2) the use of phase modi-fiers to increase solvent polarity, (3) the use of organo-philic anions, and (4) the attachment of anionic func-tionalities to the macrocycle backbone. Strategies 1 and2 are the most widely employed, and significant progresshas been made in the design, synthesis, and character-ization of the ionizable macrocycles known as the lariatethers (strategy 4).12 Strategy 3 is classified as syner-gistic extraction and conveniently addresses both theshortcomings of synergistic solvent extraction (i.e., lackof cation specificity) and separations using crown ethers(i.e., necessity of anion coextraction).

The combination of two extractants in a single diluentintroduces a new set of experimental variables, and anunderstanding of the extraction equilibria will assist indefining the data needed to adequately characterize asynergistic system. Given the diversity of extractantswith which crown ethers have been combined (classes1-3), the respective extraction processes clearly cannotbe described by a single mechanism. The extractionreaction for the most often encountered monoproticchelating extractants (L-) (e.g., the â-diketones or the

acylpyrazolones shown in Figure 1) is

with the extraction constant (ignoring activity effects)in the absence of synergism defined as

In the presence of a crown ether, the synergisticextraction reaction becomes

with the synergistic extraction constant

Arbitrarily assuming a stepwise formation of theM(L)m‚(r - m)HLorg complex prior to coordination by the

Figure 1. Two-dimensional structures of some extractants used in synergistic solvent extraction.

Mm+ + rHLorg h M(L)m‚(r - m)HLorg + mH+ (3)

Kex )[M(L)m‚(r - m)HL]org[H

+]m

[Mm+][HL]orgr

(4)

Mm+ + qCEorg + mHLorg h M(CE)q(L)m,org + mH+

(5)

Kex,s )[M(CE)q(L)m]org[H

+]m

[Mm+][CE]orgq[HL]org

m(6)

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3443

neutral crown ether extractant,1 the synergistic organicphase reaction is

The synergistic constant is then defined by

For the general classes of alkylcarboxylic, alkylsul-fonic, and dialkylphosphoric acids shown in Figure 1,Kex in the absence of synergism is derived from reaction1. In nonpolar diluents and at low metal loading, thewidely used dialkylphosphoric acids may form thehydrogen-bonded dimers illustrated in Figure 1.2,3 Dur-ing cation extraction, these dimers lose a proton to formhydrogen-bonded chelating dimers. Synergistic extrac-tion by crown ethers and dialkylphosphoric acids innonpolar diluents may be represented as

with the synergistic constant defined by

At high metal loading of the solvent or in the presenceof polar phase modifiers (e.g., tri-n-butyl phosphate,TBP) or diluents (e.g., alcohols), the dialkylphosphoricacid aggregation equilibrium shifts to favor the mono-mer, although multiple species may still be present.

For the less frequently studied crown ether/neutralextractant combinations, coextraction of an aqueousphase anion is required to maintain electroneutrality:

which defines the synergistic constant as

Such systems may yield enhanced extraction consistentwith synergism, but care must be taken to discernbetween phase modifier effects and synergistic effects.For example, TBP is often employed at near molarconcentrations, which is in large excess with respect tothe crown ether concentration. Any enhanced partition-ing may then be due more to the increased polarity ofthe process solvent (i.e., phase modification) rather thanto an interaction of TBP with the primary coordinationsphere of the cation to make it more organophilic (i.e.,synergism).

An examination of the equilibria described aboveshows that the effects of many variables must be

Figure 2. Representative crown ethers used in synergistic solvent extraction.

M(L)m‚(r - m)HLorg + qCEorg h

M(CE)q(L)m,org + (r - m)HLorg (7)

Ks )[M(CE)q(L)m]org[HL]org

r-m

[M(L)m‚(r - m)HL]org[CE]orgq

)Kex,s

Kex(8)

M(HA2)m‚(p - m)(HA)org + qCEorg h

M(CE)q(HA2)m,org + (p - m)/2(HA)2,org (9)

Ks )[M(CE)q(HA2)m]org[(HA)2]org

(p-m)/2

[M(HA2)m‚(p - m)(HA)]org[CE]orgq

(10)

Mm+ + qCEorg + sSorg + m/xXx- h M(CE)qXm/x‚sSorg(11)

Ks )[M(CE)qXm/x‚sS]org

[Mm+][CE]orgq[S]org

s[Xx-]m/x(12)

3444 Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000

determined in order to understand and optimize asynergistic solvent extraction system. An excellentdiscussion of the experiments needed to characterizesuch systems, along with the limitations of slope analy-sis and computational equilibrium modeling of thesecomplicated mixtures, is reported.13 The recommendedexperiments are listed in Table 1, and examples of someof them are illustrated in the figures used in this paper.

Given the large number of experimental variables, itis not surprising that much of the literature reviewedin this paper is incomplete with respect to systemcharacterization. As a result, the extracted speciesdiscussed in this paper are formulated on the basis ofthe reported experiments and on the coordinationchemistry of the component species. Whenever sup-ported by experiment and/or the relevant coordinationchemistry, the extracted complexes are written as inreactions 5, 7, 9, and 11 to imply in-cavity coordinationto the crown ether. In other instances, the species arewritten as adducts, that is, in the form M(A or L)m(CE)q,where inner- or outer-sphere complexation is impliedand in-cavity complexation is unlikely or not supportedby experiment. Only in rare cases has in-cavity crownether complexation been directly probed in studies ofsynergistic extraction.14-18 Roman numerals are usedto specify the oxidation states of extracted cationsbecause little information regarding solution speciation(i.e., extraction as a fully hydrated ion, aqueous complex,etc.) is available.

Several reports discuss the thermodynamic basis forsynergism in solvent extraction19-22 and conclude thatthe dominant forces are system dependent but may beboth enthalpic (e.g., when a stronger Lewis base donorreplaces a weaker donor) or entropic (e.g., when severalH2O molecules are displaced as a result of the chelateand/or macrocyclic effects). In general, specific thermo-dynamic parameters are not discussed in this review,as they will be part of a future contribution describingthermodynamic drivers in synergistic extraction.23 Notealso that distribution ratios (DM (M ) solute cation))are infrequently cited in this review because theyseldom exceed 10 and are highly system dependent.

Literature Review

This review is limited to solvent extraction systemsin which a crown ether is combined with anotherconventional solvent extraction reagent to effect en-hanced metal cation partitioning into an organic phase.Notably absent from this review are systems involving

the related lariat ethers12 and cryptands,24 separationsinvolving the picrate anion,4,5,25 and analytical proce-dures using photoactive dye molecules to facilitatecation transport by crown ethers.25-34 A particularlyuseful aspect of this review is that the titles for eachreference appear in the Literature Cited section, allow-ing the reader to quickly identify those reports of thegreatest interest.

Complementary search strategies in different elec-tronic databases were employed to provide maximumcoverage of the scientific literature from 1980 to 1999,inclusive. The electronic Science Citations Index wassearched using the Boolean strings “‘synergis*’ and‘extract*’” and “‘extract*’ and ‘crown’”. The CAS OnlineDatabase was searched using the strings “‘synergistic’and ‘crown’” and “‘synergism’ and ‘crown’” over the sametime period. The 1972-1981 (Sekine and Hasegawa’streatise2 covered the literature through 1972) GeneralSubjects Collective Indices of Chemical Abstracts weresearched manually for the topics “synergism”, “extrac-tion”, and “crown”. During the review of papers, thereference lists were examined to ensure that citedreferences were included in this review. While it is likelythat some references to synergistic solvent extractionusing crown ethers have been inadvertently overlooked,the authors believe that the following review encom-passes the vast majority of work that appears in theprimary scientific literature.

In the following discussion of synergistic solventextraction with crown ethers, systems using chelatingextractants are succeeded by acidic extractants andneutral extractants, respectively. Each section is furthersubdivided by extractant type and finally by cation type,with the alkali and alkaline-earth cations followedsuccessively by the transition, main group, and f-element cations. In all cases, the objective is to describethose systems showing the potential for cation selectiv-ity and to highlight those areas in which an improvedunderstanding is needed.

Chelating Extractant/Crown Ether Combina-tions. Significant effort has been dedicated to theinvestigation of synergistic extraction using crownethers and â-diketones. More work has been performedwith the acetylacetone class of â-diketone, probablybecause of the fact that the first report of synergisticextraction in 1954 used thenoyltrifluoroacetone (HTTA,Figure 1) and TBP in benzene to enhance the partition-ing of f-element cations.1 Synergistic extraction usingthe acylpyrazolones is discussed separately from the

Table 1. Recommended Solvent Extraction Experiments Required To Characterize a Synergistic Separation Systema

variablesb constants informationc

Single-Extractant ExperimentsDM vs [aqueous acid] diluent only, no extractants diluent blanksDM vs [HL, HA, or S] [aqueous acid] HL, HA, or S dependenceDM vs [aqueous acid] [HL, HA, or S] aqueous acid dependenceDM vs [CE] [aqueous acid] CE dependence

Two-Extractant (Synergistic Extraction) ExperimentsDM vs mole fraction of CE [aqueous acid], [HL, HA, or S] + [CE] ) constant ratio of CE to HL, HA or S

(if synergistic)DM vs [aqueous acid] [HL, HA, or S], [CE] aqueous acid dependence of

synergistic systemDM vs [HL, HA, or S] [aqueous acid], [CE] HL, HA, or S dependenceDM vs [CE] [aqueous acid], [HL, HA, or S] CE dependenceDM vs [CE + (HL, HA, or S)] [aqueous acid], [CE]:[HL, HA, or S] ratio constant aggregation effectsDM vs [M] [aqueous acid], [HL, HA, or S], [CE] metal loading dependence

a Adapted from Baes et al.13 b DM ) metal ion distribution ratio, HL ) chelating extractant, HA ) acidic extractant, S ) neutralsolvating extractant, CE ) crown ether extractant. c Examples of many of these experiments appear in Figures 7 and 8.


acetylacetone-type â-diketones because the steric con-straints of the former are significant and because thepyrazole heterocycle lowers the pKa to permit metal ionextraction at lower pH values.

(a) Acetylacetone-Type Extractants. Despite thevast body of literature describing separations of alkaliand alkaline-earth cations by crown ethers,4,5 only tworeports35,36 and one brief review37 describe crown ether/HTTA combinations for the extraction of these cations.The paucity of data probably relates more to thecomparatively weaker complexation of HTTA to mono-and divalent cations than to the polyvalent f elements.

Extraction of Li(I) by 12-crown-4 (12C4, Figure 2) andHTTA in o-dichlorobenzene is reported as part of ananalytical procedure for Li(I).35 While 12C4 approxi-mates the ideal cavity size for Li(I),4 the partitioncoefficient of this ligand is quite low, and the unsubsti-tuted (i.e., lacking significant alkyl or aryl functional-ization) crown ethers are generally recognized as goodligands but poor extractants.11,17,38 (In fact, the aqueoussolubility of unfunctionalized crown ethers has led totheir use in several different separation schemes.39-42)Similar studies have employed the more organophilicdibenzo-18-crown-6 (DB18C6, Figure 2) with HTTA ino-dichlorobenzene for partitioning of Ba(II), again foranalytical purposes.36

Cobalt(II) is the best characterized among the transi-tion metals,20,43 while Mn(II),44 Zn(II),45 Cd(II),45 andPb(II)46 have also been examined using combinationsof DB18C6 and HTTA for analytical separations. Co-balt(II) partitioning from ClO4

- media into CHCl3solutions of HTTA and crown ethers ranging in size from12C4 to 24-crown-8 (24C8) with different levels of arylor cycloalkyl substitution has been investigated.43 Thereported extraction sequence for the Co(TTA)2(CE)complex decreases in the order dicyclohexano-24-crown-8 (DCH24C8) > 18-crown-6 (18C6) > dicyclohex-ano-18-crown-6 (DCH18C6, Figure 2, isomeric compo-sition not specified) > 15-crown-5 (15C5) > DB18C6 >12C4. This sequence does not follow that predicted bythe cation size/cavity size relationship that often defineswhich macrocycles form the most stable complexes witha particular cation.4,5,47 The ordering is attributed to thedecreasing Lewis basicity of the oxygen donors in thesemacrocycles; however, the influence of aqueous phasesolubility of the unsubstituted crown ethers is notmentioned.

An important extension of Co(II) distribution studiesincluded calculation of various equilibrium constants(eqs 4, 6, and 8) for extraction of Co(TTA)2(DB18C6)from 0.1 M ClO4

- media into nitrobenzene/toluenemixtures.20,48 In agreement with previous discussionsof diluent effects,1 the synergistic constants (eq 8)decrease as the solvent polarity increases. A correlationbetween ∆H and ∆S for the extraction reaction suggeststhat the complexation thermodynamics of Co(TTA)2-(DB18C6) is controlled by H2O substitution in thehydrated Co(TTA)2 precursor.

Partitioning of Tc from an aqueous phase initiallycontaining KNO3 and NaBH4 into benzene solutions ofHTTA and 18C6 or DB18C6 is reported to undergosynergistic enhancement.49 Significantly more experi-ments are required to adequately characterize such acomplicated system, however, because macroconcentra-tions of K(I) may compete with the reduced Tc forcomplexation by the crown ether, extraction of [M(CE)]-[TcO4] is well-known,50,51 and the reduction of Tc(VII)

by BH4- is not well characterized in this system (e.g.,

the extraction kinetics for [K(DB18C6)][TcO4] may befaster than the Tc(VII) reduction kinetics).

Separations of the trivalent lanthanides and actinidesby combinations of HTTA and crown ethers will bediscussed together because of the many similarities inthe solution and separations chemistry of these cat-ions.52,53 Several of the earliest reports of synergism inthe extraction of trivalent f elements from ClO4

- mediainto CHCl3 assign to the extracted species the stoichi-ometry M(TTA)3(CE)2 (M ) Eu, Gd, Yb, Tm, or Am; CE) 15C5 or 18C6).48,54,55 Subsequent investigations oftrivalent f-element partitioning by HTTA and variouscrown ethers into cyclohexane,56,57 benzene,17,56,58 orCHCl359 suggest M(TTA)3(CE) as the extracted complex.This work prompted another report describing theformation of Eu(TTA)3(18C6)2 above 0.04 M 18C6 andEu(TTA)3(18C6) in the range 0.004-0.04 M,60 showinghow the crown ether concentration influences the com-plex stoichiometry. This work also concludes that theextraction kinetics of Eu(TTA)3(18C6) are controlled byinterfacial chemical reactions.

One study finds that partitioning of Pr(III), Gd(III),or Yb(III) by HTTA and benzo-15-crown-5 (B15C5) inCCl4, benzene, or CHCl3 involves M(TTA)3(B15C5)2.61

Additional data suggest that both Eu(TTA)3(B15C5) andEu(TTA)3(B15C5)2 may form in benzene after extractionfrom a 0.1 M ClO4

- medium.17 Formation of Am(TTA)3-(DCH18C6)2 in benzene at high crown ether concentra-tions is reported,62 but this formulation has not beenobserved in other studies in benzene that approach thesame concentration range.17,57,58

A number of reports have appeared describing “syn-ergistic ion pair extraction,” in which an aqueous phaseanion is part of the extracted complex.56,57,60,63-69 Co-extraction of NO3

- occurs in the partitioning of Eu(III)or Gd(III) by HTTA and 15C5 or 18C6 in benzene from0.2 M NO3

- solutions.66 An acid dependence of twowas further supported by a slope of 1 for a NO3

-

dependence, suggesting the extracted complex M(NO3)-(TTA)2(CE) (M ) Eu, Gd; CE ) 15C5, 18C6).

Anion coextraction is not unusual in polar diluentsthat are capable of solvating common inorganic acidanions, as observed in the extraction of trivalent lan-thanide or actinide cations from ClO4

- media into 1,2-dichloroethane containing HTTA and crown ethers.57,65,68

Figure 3 presents log D vs the eight-coordinate ionicradius70 of the lanthanide ions for extraction by 0.01 MHTTA + 0.01 M DCH18C6 in 1,2-dichloroethane.65 Forextraction by HTTA alone, Figure 3 shows the antici-pated increase in extraction commensurate with anincrease in the electrostatic bond strength between theoxygen donors of HTTA and the lanthanide ions ofincreasing charge density (because of the lanthanidecontraction). Upon addition of DCH18C6, however, thistrend is effectively reversed, and it is clear that the lightlanthanides, extracted as [M(TTA)2(DCH18C6)][ClO4](M ) La-Tb), are significantly better synergized byDCH18C6 than are the heavier lanthanides that areextracted as M(TTA)3(DCH18C6) (M ) Ho, Tm, Lu). The18C6 complex formation constants shown in Figure 3correlate well with the extraction sequence for M ) La-Gd, but such constants could not be determined for Tb-Lu because the reaction heats were too low to bemeasured calorimetrically.71 While these distributionratios are not large, the potential for intralanthanideseparations is evident. Separation factors for Nd(III)


from Pm(III)-Tb(III) (RNd/Ln) in a related HTTA/18C6/1,2-dichloroethane system are 2.6, 9.8, 25, 140, and 280,respectively.68

Some controversy exists regarding the nature of theinteraction of lanthanide ions with HTTA and 18C6. Anumber of reports propose that the size match betweenthe ionic diameter of the light lanthanides (radii inFigure 3) and the macrocycle cavity size of 18C6 (≈2.6-3.2 Å)4 favors in-cavity coordination and is responsiblefor the observed selectivity in trivalent lanthanideseparations.54,55,57,64,65,67,68 X-ray crystallographic studiesappear to lend support, as ample evidence exists forin-cavity 18C6 complexation of the Cl- and/or NO3

-

salts of La(III),72-75 Ce(III),73 Pr(III),73,76 Nd(III),77-80

Sm(III),81 Eu(III),73,75 Gd(III),73,81-83 Tb(III),81 andDy(III).80,82,84 Only second-sphere coordination throughhydrogen bonding in [M(NO3)3(OH2)3]‚18C6 is observedfor M ) Y and Tb-Lu.76

Other reports attribute variations in the trivalentf-element distribution to the oxygen donor basicity of18C6 and its dibenzo and dicyclohexano derivatives,56,62

although neither of these studies has directly probedthe solution structure of the extracted complex. Themost conclusive study utilized solvent extraction, solu-tion calorimetry, fluorescence measurements, and multi-nuclear NMR to conclude that La(III), Eu(III), U(VI)O2

2+,and Th(IV) extraction from ClO4

- media into benzenesolutions of HTTA and crown ethers ranging in size from12- to 24-membered rings with varying cyclohexano oraryl substitution is not clearly related to macrocyclecavity size.17 Karl Fischer H2O analyses and Eu fluo-rescence measurements support M(TTA)3(OH2)(18C6)(M ) Eu, Nd) as the extracted complex, while 1H and13C NMR studies show that the six oxygen donors of18C6 do not interact equally with the La(III) or Th(IV)metal centers. Conversely, the NMR spectra for theLa(III) complex with 15C5 indicate uniform bonding ofall ether oxygen atoms. Calorimetric titrations of theHTTA complexes of Eu(III), U(VI)O2

2+, and Th(IV) withbenzene solutions of crown ethers were also performed.

The thermodynamic and solvent extraction measure-ments exhibit a nearly linear relationship between ∆Hand ∆S of extraction, which generally indicates acommon reaction source such as cation dehydration (asreported for Co(II) extraction43). Unfortunately, cationdehydration could not be identified as a limiting processbecause the ∆S data do not account for the complex-ation-induced loss of H2O by Nd(III), Eu(III), or Th(IV).17

This study concludes that the synergistic extraction oflanthanide and actinide cations by crown ether-adductformation likely derives from several contributing fac-tors and is not simply due to the cation size/cavity sizerelationship.

What is clear about the extraction of trivalent felements by mixtures of crown ethers and HTTA is thata variety of geometric (e.g., cavity size and/or stericrepulsion between extractant classes), enthalpic (e.g.,donor basicity), and entropic (e.g., cation dehydration)effects are involved. A better understanding of the originof the selectivity displayed in Figure 3 will require moredetailed investigations of the solution structure of thelanthanide(III)/HTTA/crown ether complexes.

The application of mixed hard/soft or soft Lewis basedonor macrocycles has seen only scant attention withHTTA, most notably in the enhanced distribution oftrivalent lanthanide and actinide cations between anaqueous acetate buffer and CHCl3.85 A di-N-decyl-substituted diaza-18-crown-6 (DA18C6, Figure 2) ex-tractant was combined with HTTA to synergize phasetransfer of Ce(III), Pm(III), Eu(III), Am(III), andCm(III). Little size selectivity is exhibited by thismacrocycle, but sufficient differences between lan-thanide and actinide extraction exist to suggest thepossibility of trivalent 4f/5f cation separations. Chelat-ing or weakly acidic extractants appear to be a goodmatch for use with the azacrown ethers in synergisticsystems, because separations may be accomplished atnear-neutral or alkaline pH values where competitionbetween H+ and solute cations for the basic nitrogendonor atoms is reduced.

Alterations to the Lewis basicity of the chelatingâ-diketone have been made, with several reports de-scribing the combination of monothiothenoyltrifluoro-acetone with various crown ethers for trivalent lan-thanide separations.37,86,87 Few studies have probed theinfluence of electron-withdrawing substituents in thechelating â-diketone, and trivalent lanthanide ex-traction by hexafluoroacetylacetone63 or 4,4,4-trifluoro-1-phenyl-1,3-butanedione69 combined with 18C6,DCH18C6, or DB18C6 essentially confirms ion pairextraction of light lanthanides as M(L)2(CE)+ into polardiluents63,69 and M(L)3(CE) into nonpolar media.63

More complete investigations of trivalent lanthanideand actinide extraction from ClO4

- media into 1,2-dichloroethane by 18C6 and either 1,1,1-trifluoroacetyl-acetone, HTTA, 4,4,4-trifluoro-1-phenyl-2,4-butanedi-one, or 4,4,4-trifluoro-1-(2-naphthoyl)-2,4-butanedioneindicate that steric effects in the â-diketone may influ-ence the extraction equilibria.64,68 Gadolinium(III) par-titioning into 1,2-dichloroethane containing 18C6 pro-ceeded by synergistic ion pair extraction of Gd(L)2(18C6)+

for L ) 1,1,1-trifluoroacetylacetone or HTTA, butGd(L)3(18C6) was observed for the more stericallydemanding phenyl- and naphthyl-substituted â-dike-tones. These observations are attributed to the stericbulk of the â-diketone that forces changes in thedenticity of 18C6.68 Given the lack of definitive solution

Figure 3. log DLn vs the eight-coordinate ionic radius70 of thetrivalent lanthanides. Organic phase: 0.01 M HTTA + 0.01 MDCH18C6 in 1,2-dichloroethane. Aqueous phase: 0.1 M Li(H)ClO4.Solvent extraction data adapted from Kitatsuji et al.65 and log Kdata from Izatt et al.71


speciation of the extracted complex in these M(III)/â-diketone/18C6 extraction systems and the complicationof ion pair extraction, the present understanding ofâ-diketone steric effects is limited.

The partitioning of U(VI)O22+ by mixtures of â-dike-

tones and various crown ethers has been investi-gated,17,88-90 but the extraction systems are not yet wellcharacterized. Macrocyclic rings of 12, 15, 18, or 24members, including the common cyclohexyl or benzoderivatives, were shown to synergize U(VI)O2

2+ extrac-tion into benzene.17,88 The more sterically demandingtert-butylcyclohexano-15-crown-5 (TBCH15C5, Figure 2)reportedly does not synergize U(VI)O2

2+ extraction fromNaNO3 media into CHCl3 solutions of HTTA.89 Basedon the synergism observed for macrocycles containing12 and 15 members,17,88 it appears that the cavity sizeis not an important factor in the synergism of U(VI)O2

2+

extraction by these crown ethers. For synergistic extrac-tion using 18C6 and its derivatives,17,88,90 in-cavitycomplexation of U(VI)O2

2+ is possible, because directinsertion of actinyl(V,VI) cations into 18-memberedmacrocyclic polyethers has been observed in the solidstate.91-93 However, U(VI)O2

2+ is also known to formnumerous second-sphere hydrogen-bonded interac-tions with crown ethers.94 More direct probes of theU(VI)O2

2+ primary coordination environment are neededbefore the crown ether synergized extraction of thiscation can be understood.

(b) Acylpyrazolone-Type Extractants. The gen-eral approach taken to characterizing synergistic ex-traction using acylpyrazolones is similar to that takenfor HTTA, where various solvent extraction dependen-cies are used to identify complex stoichiometries and,less frequently, thermodynamic parameters. As withHTTA, few studies of alkali or alkaline-earth cationextraction by crown ethers and acylpyrazolones havebeen undertaken,95 although several reports discuss theeffects of alkali cation interferents in the partitioningof Co(II) and Ni(II) between Cl- media and toluenesolutions of 18C6 or DCH18C6 and 4-benzoyl-3-methyl-1-phenylpyrazol-5-one (HPMBP, Figure 1).96 From aque-ous phases containing 1 M LiCl or (CH3)4NCl, Co(II) orNi(II) is extracted as M(PMBP)2(CE) (CE ) 18C6,DCH18C6). From 1 M KCl solutions, however, M )Co(II) or Ni(II) is extracted as the tris-PMBP- anions:[K(18C6)][M(PMBP)3]. Extraction of tris-PMBP- che-lates by organophilic alkylammonium cations is well-known, although the distribution ratios are generallylower in these crown ether systems. Studies whereacyclic polyethers extracted only Co(PMBP)2(PE) (PE) acyclic polyether) from KCl into toluene, 1,2-di-chloroethane, or CHCl3 confirm that the stability of[K(18C6)]+ is responsible for changing the extractedcomplex to [K(18C6)][M(PMBP)3] (M ) Ni, Co; CE )18C6, DCH18C6).97

A more complete investigation of aqueous phasecation effects probed Co(II) extraction from Li(I), Na(I),K(I), or Ba(II) aqueous phases into CHCl3 solutions ofB15C5, 18C6, DCH18C6, DB18C6, DCH24C8, or diben-zo-24-crown-8 (DB24C8, Figure 2) and HPMBP.98 Par-titioning of Co(II) and Cd(II) from KNO3 was effectedas the [K(CE)][M(PMBP)3] ion pair by 18-memberedmacrocycles because both of these transition-metalcations form stable tris-PMBP- chelate complexes. Thesmall cavity of B15C5 relative to the ionic diameter ofK(I) results in sandwich complex formation: [K(B15C5)2]-[Co(PMBP)3]. Cobalt(II) extraction by DB18C6 from an

aqueous phase containing Ba(II) was observed to pro-ceed by formation of Co(PMBP)2(DB18C6) and[Ba(DB18C6)][Co(PMBP)3]2.

An examination of Co(II) partitioning into 1,2-di-chloroethane, CH2Cl2, or CHCl3 from aqueous CsCl orCsNO3 by several 15-24-membered macrocyclic poly-ethers and 4-acyl-3-methyl-1-phenylpyrazol-5-ones (acyl) benzoyl, p-tert-butylbenzoyl, stearoyl) is also re-ported.99 The influence of the acyl group on the extrac-tion of [Cs(CE)q][Co(L)3] (q ) 1-2) derives principallyfrom changes in the pKa of the acylpyrazolone and notfrom the steric bulk. For ion-pair extraction of [Cs(CE)q]-[Co(PMBP)3], it can be expected that diluents of higherdielectric constant would yield the larger distributionratios; however, the values of DCo decrease in thesequence 1,2-dichloroethane > CH2Cl2 > CHCl3 (ε )10.4, 8.9, and 4.9, respectively100). While not discussedin the text, the synergistic extraction in CHCl3 may besuppressed by hydrogen-bonding interactions of thediluent with HPMBP.

Trivalent lanthanide and actinide extraction by crownethers and HPMBP has received considerably lessattention than HTTA systems. Partitioning of Pr(III),Gd(III), or Yb(III) by B15C5 and HPMBP in CCl4,benzene, or CHCl3 is reported and the number of B15C5molecules in the extracted complex shown to vary withthe diluent.101 Slope analyses indicate that M(PMBP)3-(B15C5) (M ) Pr, Gd, Yb) is extracted into CHCl3 orbenzene, while a mixture of the mono- and bis-B15C5adducts is observed in CCl4 over the B15C5 concentra-tion range ≈0.0025-0.01 M. As with the Co(II) distribu-tion studies99 using PMBP- and related studies ofPr(III) extraction by HPMBP and DB18C6,102 thedistribution is greatest in the least polar diluent (i.e.,the least able to hydrogen bond with the extractants)and the extent of synergism is generally highest inbenzene and lowest in CHCl3 (as expected). The stoi-chiometries of the trivalent lanthanide complexes ex-tracted by B15C5 and HPMBP compare favorably withthose observed in the HTTA/crown ether systems;however, an examination of the synergistic constantsshows that there is a general discontinuity in theextraction of Gd(III) (i.e., unusually low in CCl4 andunusually high in CHCl3) compared to Pr(III) andYb(III), which have larger and smaller ionic radii,respectively. No explanation is presented, although asimilar discontinuity is observed for Eu(III) extractioncompared to that of Nd(III) and Tm(III) by 15- and 18-membered macrocycles with 3-methyl-1-phenyl-4-(tri-fluoroacetyl)pyrazol-5-one (HPMTFP, Figure 1) inCHCl3.103

Tri-, tetra-, and hexavalent actinide extraction fromClO4

- media into toluene solutions of 12C4, 15C5, 18C6,DCH18C6, or DB18C6 and HPMBP did not correlatewith the macrocyclic cavity size but rather with theetheric oxygen donor basicity.104 Each crown ether isreported to synergize extraction of Am(PMBP)3,Th(PMBP)4, or UO2(PMBP)2, as previously observed inthe HTTA extraction systems.17 The synergistic con-stants for the 12C4 and 15C5 complexes generallydecrease with the decreasing effective charge of theactinide cation; that is, Th(IV) > U(VI)O2

2+ > Am(III)(the effective charge at the uranium center has beenestimated to be ≈3.2105,106). Conversely, the 18-mem-bered macrocycles follow the sequence Th(IV) g Am(III). U(VI)O2

2+. The latter sequence may be an artifact ofthe geometric constraints imposed by the linear dioxo


cation and the subsequent repulsion between the axialoxygen atoms of U(VI)O2

2+ and the ether oxygen atoms(despite the ability of U(VI)O2

2+ to fit in the cavity of18C6 derivatives91,92). An earlier study of U(VI)O2

2+

extraction by HPMTFP or HPMBP and DCH18C6 inbenzene107 affords synergistic constants similar to thatdescribed above104 for UO2(PMBP)2(DCH18C6) and at-tributes the low values to limitations of the interactionof U(VI)O2

2+ with crown ethers.Modifications to the acylpyrazolone backbone have

been made in attempts to assess the influence ofacyl group sterics and electron-withdrawing prop-erties on the synergistic extraction. Neither Co(II)99 norU(VI)O2

2+ 107 extraction is significantly influenced bythe steric bulk of the acylpyrazolone, but rather thedifferences in the extraction may generally be attributedto the substituent electron-withdrawing effects on thepKa of the acylpyrazolone.

A different modification to the geometrically con-strained â-diketone backbone is 4-benzoyl-3-phenyl-5-isoxazolone (a substituted gem-O,N-cyclopentyl ring),which exhibits trivalent lanthanide extraction behaviorsimilar to that of the acylpyrazolones.108 Most interest-ingly, however, there is no significant synergistic en-hancement of Eu(III) extraction by 18C6 compared toNd(III) or Tm(III), as discussed previously for theextraction of Eu(III) by acylpyrazolones.101,103 Based onthese data, it is not yet clear how the different stericrequirements of the isoxazolones and the acylpyrazo-lones influence synergistic effects.

The trivalent lanthanide complexes formed by thetrifluoroacetyl-substituted acylpyrazolones103,107,109,110

closely resemble the benzoyl-substituted derivatives,and this is further illustrated by a direct comparison ofthe thermodynamics of Am(III) extraction by B15C5 orDCH18C6 with HPMBP or HPMTFP in CHCl3.110 Thetemperature variation solvent extraction technique wasemployed to conclude that Am(L)3(HL)(DCH18C6) (thecharacterization of the extracted complex is reportedelsewhere109) for L ) HPMBP or HPMTFP is stabilizedby ∆S and opposed by ∆H, whereas the B15C5 adductsare driven by ∆H.

The similarities in the thermodynamic forces and thecoordination chemistry of acetylacetone and acylpyra-zolone-type â-diketones lead to very similar extractionbehavior. The most promising application of theseextractant combinations appears to be the intralan-thanide (or intra-actinide) separations shown in Figure3 by mixtures of DCH18C6 and HTTA in 1,2-dichloro-ethane.65 The separation of light lanthanides from heavylanthanides using a combination of size-selective syn-ergism and a change in the stoichiometry of the ex-tracted complex is potentially useful; however, signifi-cant fundamental work still remains because thecoordination environments of these complexes are notwell characterized and the extraction conditions havenot been optimized.

(c) Other Chelating Extractants. Two reports haveappeared that discuss the extraction of octahedral Co(II)complexes from NO3

- media by DB18C6 and 8-hydroxy-quinoline in CHCl3.111,112 The solvent extraction de-pendencies led to the formulation Co(L)2(DB18C6),112

and subsequent extraction kinetics experiments haveconcluded that the rate-limiting process is the aqueousphase complexation of Co(II) by 8-hydroxyquinoline.111

These findings contrast those reported for the Eu(III)/18C6/HTTA system in which interfacial chemical reac-

tions are identified as rate limiting,60 but the findingsare not unexpected considering that 8-hydroxyquinolineis less organophilic than HTTA. The rather low distri-bution constant (KD) of 8-hydroxyquinoline contributesto its limited utility as a chelating extractant.

Acidic Extractant/Crown Ether Combinations.In synergistic extraction using â-diketones, in-cavitycrown ether complexation and the potential for genuinesize selectivity are inhibited by the coordination strengthof the chelate complexes. In contrast, alkali and alkaline-earth cation partitioning by combinations of crownethers and acidic extractants provides an excellenttesting ground for size-selective separations because ofthe well-known coordination chemistry of these cationswith crown ethers.4,5 By example, Figure 4 presents aplot of log DM vs the six-coordinate ionic radius70 of thealkali cations for extraction by 15-, 18-, and 21-membered macrocycles and didodecylnaphthalenesulfon-ic acid (HDDNS, Figure 1) in toluene.38 In general, thefigure shows that the macrocycle with a cavity diameterapproaching the cation diameter affords the highestdistribution ratios, which provides strong evidence thatthe size selectivity exhibited by these systems is due toin-cavity crown ether complexation.

Three types of acidic organic extractants have beenutilized, and the following discussion is subdividedamong the alkylcarboxylic, alkylsulfonic, and dialkyl-phosphoric acid extractants. The use of organophilicinorganic anions is receiving consistent attention, andthis section concludes with a brief discussion of thesesynergistic systems.

(a) Alkylcarboxylic Acid Extractants. Before Ped-ersen’s initial report in 1967,6 separations of alkali oralkaline-earth cations generally utilized alkylphosphoricor alkylcarboxylic acids, respectively.2 These extractantsrely principally on electrostatic effects (i.e., hydrationor complexation preferences for alkali or alkaline-earthcations, respectively) and do not offer the ion specificityafforded by macrocycle-based separations. Several of the

Figure 4. log DM vs the six-coordinate ionic radius70 of the alkali-metal cations. Organic phase: 0.1 M HDDNS + 0.05 M CH15C5,DCH18C6, or DCH21C7 in toluene. Aqueous phase: 0.5 M M(NO3)(M ) Li, Na, K, Rb, Cs). Adapted from McDowell et al.38


earliest reports of size-selective synergism collectivelydescribe the results of survey studies that employedvarious organophilic crown ethers and alkylcarboxylic,alkylsulfonic, or alkylphosphoric acids.38,113,114 Theseworks are outstanding in their discussion of the factorsgoverning the design of process-scale synergistic solventextraction systems using crown ethers. Specifically, thediscussion of the factors influencing the selection ofextractants and diluents for both fundamental investi-gations and practical applications is worthy of consult.

Initial studies focused on the identification of thosemacrocycles that serve not only as effective organicphase ligands (i.e., the appropriate cavity size andfavorable complexation kinetics and thermodynamics)but also as good extractants (i.e., high KD values in theorganic phase to avoid extractant loss to the raffinate).38

The toluene solubilities of commonly encountered 12-24-membered macrocyclic polyethers with varying alkylor aryl functionalization are reported, as are the per-centages of each crown ether distributing to H2O. Thebenzo- and tert-butylcyclohexano-substituted crown ethersafford the highest KD values in toluene and, despite thedecreased flexibility and steric bulk of these macro-cycles, were predicted to be the best candidate extrac-tants for subsequent survey studies. In keeping withthe criteria established for the selection of crown etherextractants,38,113,114 the partitioning of a series of alkyl-carboxylic acids between H2O and toluene or n-dodecaneis also reported. A substituted alkylcarboxylic acid(Versatic acid, Figure 1) was used in the study of Ca(II),Sr(II), and Ba(II) extraction into toluene solutions ofseveral alkyl- or aryl-substituted crown ethers. Syner-gism of Ca(II) extraction is observed for TBCH15C5,whereas both Sr(II) and Ba(II) are synergized by 15-,18-, 21-, and 24-membered crown ethers. It is concludedthat the cation size/cavity size relationship is importantfor synergistic extraction of alkaline-earth cations inthese systems. Additional investigations using acyclicethers or tri-n-octylphosphine oxide (TOPO, Figure 1)indicate that the macrocyclic effect is indeed responsiblefor the observed synergism in these Versatic acid/crownether systems.38,113

A more detailed investigation of Sr(II) distributioninto CCl4 solutions of DCH18C6 and Versatic acid usednot only solvent extraction slope analyses but alsoviscometry, Karl Fischer H2O analysis, vapor pressureosmometry (VPO), and IR spectroscopy to probe mostaspects of the solvent extraction system.115 Precipitationof Sr(DCH18C6)(NO3)2 (DCH18C6 stereoisomers notspecified) is reported to occur at low organic phaseloading in the pH range 7.0-11.7. Above pH 11.7,Sr(DCH18C6)(A)2‚H2O is soluble, while a mixed anioncomplex is hypothesized for the pH range from 7.0 to11.7. Viscometric results show that the presence ofDCH18C6 effectively suppresses the increase in organicphase viscosity that often accompanies high metalloading of alkylcarboxylic acid extraction solvents.Presumably, the formation of Sr(DCH18C6)2+ reducesthe aggregation of Sr(A)2 that causes this increase inviscosity. Other investigations using small-angle neu-tron scattering (SANS) reach a similar conclusion forthe synergistic extraction of Sr(II) by di-n-octylphos-phoric acid (HDOP) and stereoisomers of DCH18C6 intoluene (discussed below).116

Attempts to design a synergistic extraction systemselective for Ra(II) were complicated by competingextraction equilibria.117 The DCH21C7 macrocycle is

sufficiently large (≈3.4-4.3 Å)4 to accommodate Ra(II)(12-coordinate ionic diameter ) 3.4 Å)70 and was usedwith 2-heptyl-2-methylnonanoic acid (HMHN, one com-ponent of Versatic acid) as the organophilic anion intoluene. Distribution ratios for Ra(II) increase from <0.1at pH 7 to >200 at pH g 11 from a 0.50 M NaNO3aqueous phase. Slight curvatures in the DCH21C7 andHMHN extractant dependencies discouraged the use ofslope analysis techniques and favored computationalfitting of the solution equilibria. Because optimal dis-tribution of Ra(II) is observed in the pH range 10-12,aggregation of the sodium salt of MHN- influences theextraction equilibria. Three organic phase species wereidentified in the computational analyses: Ra(DCH21C7)-(MHN)2‚Na(MHN), Ra(MHN)2‚Na(MHN), and DCH21C7‚2Na(MHN). This extraction system suffers from com-plications arising from aqueous phase cation coextractionas well as self-extractant and interextractant aggrega-tion. Nonetheless, Table 2 shows that the extractionsystem is selective for Ra(II) provided that the aqueousphase concentrations of other alkaline-earth cationsand/or Na(I) remain below ≈0.01 M. Interference byNa(I) is attributed to cation exchange with MHN- andis an artifact of the high pH required for the separation.

Only three reports have appeared that describe tri-valent f-element extraction by crown ether/alkylcar-boxylic acid extractant combinations.118-120 Distributionratios for Eu(III) and Am(III) in nitrobenzene-containing15C5 or 18C6 and an unspecified C7-C8 alkylcarboxylicacid (probably related to Versatic acid) are reported.118

None of the distribution ratios approach unity, and noother experiments were conducted to characterize theextraction system.

A single report describes the use of 1,4,10,13-tetrathia-7,16-diazacyclooctadecane (a tetrathiadiaza-18-crown-6) with 1-decanoic acid (lauric acid) in 1,2-dichloro-ethane for the extraction of trivalent lanthanides froma buffered aqueous phase at pH 6.0-6.5.120 Largesynergistic enhancements are reported (e.g., log Ks )5.6 for Eu(III)) for the extraction of macroconcentrations(≈10-4 M) of lanthanide ions by fairly dilute extractantsolutions (≈10-4-10-3 M in each of tetrathiadiaza-18-crown-6 and 1-decanoic acid). Unfortunately, the aque-ous phase partitioning of this unsubstituted crown etherwas not investigated, and matters are further compli-cated by the ability of some sulfur donor macrocyclesto bind cations in endo- or exocyclic modes. (In fact, thefree ligand conformation of tetrathiadiaza-18-crown-6in the solid state directs two sulfur atoms away fromthe macrocyclic cavity.121)

Pentadecafluorooctanoic acid has been combined with18C6 to facilitate extraction of La(III), Gd(III), or Lu(III)in benzene, CHCl3, 1,2-dichloroethane, or nitroben-zene.119 Various solvent extraction dependencies wereused to characterize the extracted complex as MA3-

Table 2. Separation Factors for Ra(II) from DifferentInterferent Cationsa

interferentcation M RRa/M

interferentcation M RRa/M

Li ≈8 × 103 Sr 12Cs 1.9 × 105 Ba 9.3Be ≈2 × 106 Zn 7.0 × 106

Mg ≈560 Th 3.7 × 106

Ca ≈58 U 6.2 × 106

a Organic phase: 0.1 M HMHN + 0.05 M DCH21C7 in toluene.Aqueous phase: 0.25 M NaOH + 0.25 M M(NO3)m (assumed fromtext).117


(18C6), with the pentadecafluorooctanoic acid and 18C6dependencies maximizing at ≈0.01 and ≈0.003 M,respectively. This behavior is attributed to self-extrac-tant and/or interextractant aggregation; however, thelow KD for 18C6 and possibly for pentadecafluoro-octanoic acid (KD for hexafluorobutyric acid from H2Ointo toluene is ≈0.4)38 has not been accounted for.Additional aqueous phase complexation arises from theuse of 10-3 M CH3CO2

- in the aqueous phase, whichwill mask extraction of the heavy lanthanide cations andartificially increase the intralanthanide separation fac-tors.

Characterization of the alkylcarboxylic acid extractionsystems is often complicated by the need to performextractions at elevated pH. This requirement leads toaqueous phase cation coextraction, which competes withthe cation of interest for one or both extractants.Additional difficulties arising from the weakly acidiccharacter of the alkylcarboxylic acids are their tendencyto form self-adducts or aggregates, which may reducesynergistic effects1 and generally complicates the equi-librium analysis.

(b) Alkylsulfonic Acid Extractants. The synergis-tic extraction behavior of some crown ether/alkylsulfonicacid combinations is briefly covered,122 but all significantworks are detailed below. Alkylsulfonic acids generallyhave lower pKa values than the corresponding alkyl-carboxylic acids, thereby permitting extraction of cationsfrom considerably more acidic media (e0.5 H+). Underthese aqueous conditions, the hydrolysis of polyvalentcations is minimized and the extraction often involvesalkylsulfonic acid solvates rather than sodium saltadducts as observed for alkylcarboxylic acid extractionat elevated pH.

The earliest studies focused on the role of the crownether in the synergistic extraction of some alkali cationsfrom NO3

- media into toluene solutions of cyclohexano-substituted 15-, 18-, or 21-membered macrocyclic poly-ethers.38,114 Figure 4 shows the results of these inves-tigations and also shows the low distribution ratios andlack of selectivity afforded by extraction by HDDNSalone. Later investigations determined that: (1) thealkaline-earth cations show a slightly decreased depend-ence on the size match between the cation and themacrocycle, (2) the alkali cations are generally betterextracted and synergized by benzo-substituted crownethers, and (3) the extraction of the alkaline-earthcations is better facilitated by cyclohexano-substitutedcrown ethers.113,114 These observations still guide thedesign of inclusion-based separation systems for thesecations, as exemplified by the use of bis(tert-butylcyclo-hexano)-18-crown-6 for 90Sr(II) extraction in the SREXradioactive waste treatment process,123 the use of vari-ous benzo-substituted crown ethers for 137Cs(I) extrac-tion from acidic radioactive wastes,124 and the progres-sion from crown ethers to calixarenes (which have morearyl groups) for Cs(I) extraction.125

Following the initial studies, experiments targetinga better understanding of the extraction equilibria wereundertaken. Partitioning of cations by HDDNS aloneis recognized to involve reverse micelles (above thecritical micelle concentration that will vary by diluent),so that aggregation of at least one extractant is ex-pected to impact the equilibria.113,114 Extraction ofSr(DCH18C6)(DDNS)2‚2H2O and interextractant ag-gregation between HDDNS and TBCH15C5 was studiedby 1H NMR, IR, VPO, and computational modeling

methods.114 The complexity of Sr(II) extraction byDCH18C6 (stereoisomeric effects neglected) andHDDNS into CCl4 was also examined by solvent extrac-tion, IR, 1H NMR, VPO, Karl Fischer H2O analysis, andcomputational equilibrium analysis.14 Interaction of thehexameric aggregate (Sr(DDNS)2‚11H2O)6 with thereverse micelles of (HDDNS‚7.5H2O)11 yielded(Sr(DDNS)2)3(HDDNS)5‚xH2O for extraction into CCl4in the absence of a crown ether. Stoichiometric additionofDCH18C6yieldsSr(DCH18C6)(DDNS)2‚2H2O,whereassubstoichiometric conditions afford (Sr(DCH18C6)-(DDNS)2)2(Sr(DDNS)2)3‚xH2O. In each case, the spec-troscopic data indicate in-cavity complexation of Sr(II)with axial binding to the alkylsulfonate moieties. Op-posing synergistic extraction is interextractant aggrega-tion, namely, formation of (HDDNS)7(DCH18C6)‚xH2O.Formation of similar aggregate species was used toexplain the antagonism periodically observed in systemscontaining alkylsulfonic acids and oxygen donor crownethers.

Despite the complexity of these HDDNS extractionsystems, large-scale applications have been proposed.One extensive report targets Cs(I) and Sr(II) extractionin PUREX-like process solvents (i.e., ≈30% (v/v) TBPin kerosene126).127 A variety of dibenzo and dicyclohex-ano derivatives of 18C6 and 24C8 were surveyed and,in keeping with the apparent preference of alkali cationsfor benzo-substituted macrocycles, 4,4′(5′)-bis[(1-hy-droxyheptyl)benzo]-18-crown-6 was found to yield thebest extraction of Cs(I). Variation of the alkyl sidechains was also systematically investigated. Because ofthe rather low distribution of Cs(I) (DCs ≈ 2), thecomplexity of the extraction equilibria (i.e., a TBP +HDDNS + crown ether ternary extractant system), andthe ill-defined acid and radiation stability of the crownethers and HDDNS, no significant scale-up testing ofthis synergistic extraction process has ensued.

Building on this initial work targeting removal of137Cs(I) from acidic radioactive defense wastes,127 aseparation using 4,4′(5′)-bis(tert-butylbenzo)-21-crown-7(DTBB21C7) with HDDNS in toluene was developed foranalytical separations of Cs(I) from e1 M HNO3.128 Thesynthesis of the crown ether is reported, as are thevarious solvent extraction dependencies used to char-acterize the equilibria. Figure 5 shows the distributionratios for Na(I), K(I), Rb(I), and Cs(I) vs [HNO3] intotoluene containing HDDNS and DTBB21C7. The valuesof RCs/M relative to the competing matrix cations are alsolisted and point to the selectivity of this extractionsystem. Using computational equilibrium analysis, theextracted species are suggested to be Cs(DTBB21C7)-(DDNS), Cs(DDNS), and Cs(DTBB21C7)(DDNS)‚HDDNS. As with previous studies,14,114 an aggregatebetween three HDDNS molecules and one crown etherwas required to achieve convergence of the model.

The extraction behavior of the divalent first-rowtransition metals Mn-Zn is described in several re-ports.13,113,114,129 Initial studies reported that only Mn(II)and Zn(II), which respectively possess d5 and d10 outershell electron configurations, experience synergisticextraction by HDDNS and TBCH15C5 in toluene.Distribution of Fe(II), Co(II), Ni(II), and Cu(II) showedantagonism in the same systems, but no detailedexplanation is presented.113,114 Subsequent studies probedthe differences in Mn(II) partitioning into an HDDNSsolution containing either TBCH15C5 or (tert-butyl-benzo)-15-crown-5 (TBB15C5).13,129 Despite the virtually


identical cavity sizes of these two macrocycles, only theformer synergizes Mn(II) extraction in toluene. Man-ganese(II) was proposed to partition as Mn(TBCH15C5)-(DDNS)2‚4(HDDNS), whereas extraction in toluenesolutions of TBB15C5 was attributed strictly to cationexchange by the alkylsulfonic acid: Mn(DDNS)2‚8(HDDNS). To better explain the observed selectivityof the TBCH15C5/HDDNS extraction system, an IRspectroscopy study was performed15 prior to a compre-hensive equilibrium analysis.16 Examination of thecrown ether stretching bands in CCl4 solutions of 0.05M TBCH15C5 and 0.1 M HDDNS after contact with 0.5M NO3

- solutions of Ca(II), Mn(II), Fe(III), Co(II), Ni(II),or Zn(II) showed the characteristic C-O stretch at≈1088 cm-1 for only the Mn(II) complex with TBCH15C5.The spectrum of the extract containing Mn(II) comparedfavorably to a mineral oil mull of [Mn(15C5)(tri-n-butylmethanesulfonate)2], which had been crystallo-graphically characterized, and provides strong evidencefor in-cavity coordination of Mn(II) by TBCH15C5.Knowing the primary coordination environment aroundMn(II), the synergistic extraction results are explainedusing ligand field theory. Manganese(II) is better ableto accommodate the distorted pentagonal-bipyramidalcoordination geometry imposed by TBCH15C5 becausethe high-spin Mn(II) cation is not influenced by ligandfield stabilization to the same extent as Fe(II)-Cu(II).Other IR data on mulls of crystalline [Mn(CH15C5)]2+

salts afford similar spectral results, and the speciationinformation was used to more accurately define theextraction equilibria in these systems.16 Extraction ofMn(II) by (HDDNS)4 aggregates involves formation ofMn(DDNS)2‚2(HDDNS), and the synergistic extractionby TBCH15C5 is attributed to [Mn(TBCH15C5)]2+ thatis solvated by two to four molecules of HDDNS. Severalinterextractant aggregates are required to facilitateconvergence of the equilibrium models, with TBCH15C5potentially interacting with one, two, or four molecules

of HDDNS.16 The antagonistic effect of TBB15C5 men-tioned above is attributed to weak complexation withMn(II) and to an interaction with alkylsulfonic acid toform HDDNS‚TBB15C5 and (HDDNS)2‚TBB15C5.

After confirming in-cavity complexation of Mn(II) byTBCH15C5, the selectivity for Mn(II), and to a lesserextent Zn(II), in this HDDNS extraction system isattributed to both metal-based electronic effects andcrown ether size selectivity. This series of investiga-tions13,15,16,113,114,129 illustrates two important points: (1)differences in macrocyclic substituents can significantlyimpact selectivity (e.g., synergistic extraction of Mn(II)by TBCH15C5 and antagonism by TBB15C5) and (2)the coordination environment (e.g., an approximatelyplanar array of macrocyclic donors) imposed by thecrown ether can effect a geometrically-based selectivityfor certain cations. The former may derive from changesin the conformational energies of the macrocycle uponbinding,130 while the latter is at least partially relatedto the relative importance of orbital-based or electro-statically-based coordination preferences.

Soft donor macrocycles have been investigated assynergistic extractants for various transition-metalcations.18,131-133 The use of thiacrown ethers has theadvantage of a better match with the Lewis basicitypreferences of soft transition-metal cations, and thereduced polarity of the thioether linkages minimizeshydrogen-bonding interactions with alkylsulfonic acidextractants. Solvent extraction experiments usingtetrathia-14-crown-4 (TT14C4, Figure 2) and tetrathia-16-crown-4 (TT16C4) with HDDNS in toluene showapparent degradation of the thiacrown ethers by HNO3.Both Cu(II) and Ag(I) are synergistically extracted overMn(II), Fe(II), Fe(III), Co(II), Ni(II), and Zn(II).131 Giventhat certain thiacrown ethers may adopt exocyclicbinding modes, a series of cyclic and acyclic complexesof Cu(II) were investigated.132 Experiments involvingCu(II) extraction from H2SO4 in toluene-containingHDDNS and either TT14C4 or acyclic thioethers ofcomparable denticity and chelate ring size were per-formed. The highest distribution ratios are obtainedwith TT14C4, confirming the favorable influence of themacrocyclic effect. Subsequent experiments comparedTT14C4 with 12- and 16-membered tetrathia macro-cycles, and the 14-membered ring was shown to yieldthe largest distribution ratios, which is in agreementwith their aqueous complex formation constants. How-ever, the extraction curves plateau and/or decreasesharply in the region 0.3-6 M H2SO4 (attributed topossible acid extraction) and negates both the macro-cyclic and ring size effects.

In-cavity complexation of Cu(II) by TT14C4 was laterconfirmed using UV-vis spectroscopy, and the resultswere incorporated into an equilibrium analysis.18 Theprincipal equilibria in the extraction of Cu(II) fromH2SO4 into toluene solutions of TT14C4 and HDDNSinvolve formation of Cu(TT14C4)(DDNS)2‚xHDDNS (x) 1, 2). Based on these modeling results, the interactionbetween HDDNS and TT14C4 was determined to beweak.

An exciting evolution of the synergistic solvent ex-traction-based approach for Cu(II) partitioning byTT14C4 and alkylsulfonic acids involves the sorptionof TT14C4 onto a sulfonic acid cation-exchange resin.133

Such an approach mirrors extraction chromatographictechniques, in which an extractant and diluent aresorbed onto a chromatographic support to combine the

Figure 5. DM vs [HNO3] for 0.025 M HDDNS + 0.025 MDTBB21C7 in toluene from an aqueous phase containing 0.1 MMNO3 (M ) Li, Na, K, Rb, Cs). Adapted from McDowell et al.128


selectivity of solvent extraction with the convenience ofresin-based separations.134 Figure 6a shows that theextraction of Cu(TT14C4)2+ by HDDNS micelles from≈0.03-1 M H2SO4 is similar to the uptake behaviordisplayed by a series of sulfonic acid cation-exchangeresins loaded with different percentages of TT14C4(Figure 6b). Both the liquid-liquid and solid-liquidsynergistic separations afford very high uptake at lowconcentrations of H2SO4. Stripping experiments wereperformed with an eye toward applications; however,≈10% of the thiacrown ether elutes during back extrac-tion, which prompted the synthesis and testing of moreorganophilic thiacrown ethers.

Studies of the synergistic extraction of f elements bycrown ethers and alkylsulfonic acids are limited to threereports involving the trivalent lanthanides.135-137 Ex-traction of trivalent Ce, Pm, Eu, and Tm into toluenesolutions of NaDDNS and TBCH15C5 displays syner-gism for all but Tm(III).136 Extraction of Pm(III) isenhanced considerably more than that of Ce(III) orEu(III), which is not readily explained using eitherelectrostatic or steric arguments. Significant antago-nism is observed above ≈10-4 M TBCH15C5 that maybe related to the interaction of ≈5 × 10-4 M NaDDNSwith TBCH15C5. A related investigation showed thatPm(III) is effectively synergized by NaDDNS in toluene-containing DCH18C6, DB18C6, or DCH24C8.137 Theimpact of the crown ethers on the ability of NaDDNSto form micelles at low concentrations is mentioned.

Dinonylnaphthalenesulfonic acid (HDNNS) has beenused only sparingly in studies of synergistic solvent

extraction with crown ethers.138-140 In an addition tothe report of 137Cs(I) extraction from acidic radioactivewastes by crown ethers and HDDNS discussed above,127

process-specific conditions and numerous fission productdistribution ratios in the related HDNNS/crown ethersystem are listed.138 In PUREX-like solvents, HDNNSbehaves similarly to HDDNS in the extraction of Sr(II)and Cs(I) by different bis(alkylcyclohexano)- and bis-(alkylbenzo)-substituted 18C6 derivatives. Coextractionof Ba(II) and Zr(IV) interferents by the proposed crownether-containing process solvent was observed andsignificantly reduces the selectivity of the extractionsystem. More troublesome, however, is that strippingof Cs(I) and Sr(II) is incomplete even after severalcontact stages.

The development of solvent extraction-41,42 and resin-based40 methods for the separation and preconcentra-tion of Ra(II) from environmental samples using crownethers initially present in the aqueous phase is reported.These investigations have used H2O-soluble crownethers and differ from those previously discussed forHDDNS that used functionalized, organophilic crownethers. The solvent extraction technique employs o-xylene as a diluent with HDNNS and 15C5, 18C6, or21C7 as extractants. Table 3 shows the high distributionratios for Ra(II) exhibited by this system as well as theRRa/M for various alkaline-earth cation interferents.Separation of Ra(II) from Sr(II) and Ba(II) is bestachieved using 15C5, whereas convenient separation ofSr(II), Ba(II), and Ra(II) from Ca(II) can be accom-plished with 18C6. Several detailed separation schemesusing the solvent extraction approach41,42 or a sulfonicacid cation-exchange resin are presented.40,42

In a study of Eu(III) extraction by HDNNS, theaddition of DCH18C6, DB24C8, or TOPO was found tohave antagonistic effects.139 By contrast to the previ-ously discussed works in which HDDNS micelles facili-tated extraction, submillimolar benzene solutions ofHDNNS were employed in this study. Dependencies oflog DEu vs [HDNNS] afford a slope of 3 from NaClO4media, whereas more concentrated solutions of alkyl-sulfonic acids generally afford near unit slopes due to amicellar extraction mechanism.18,141,142 The antagonismintroduced by addition of DCH18C6, DB24C8, or TOPOis likely attributable to hydrogen-bonding interactionswith HDDNS, as observed for the HDDNS/crown ethersystems, that effectively suppress extraction when[crown ether] > [alkylsulfonic acid]. That synergism isobserved for trivalent lanthanide extraction by crownethers and HDDNS in toluene136,137 when [crown ether]< [HDDNS] is suggestive that the antagonism observed

Figure 6. (a) DCu vs [H2SO4] for 0.01 M HDDNS or 0.01 MHDDNS + 0.005 M TT14C4 in toluene. (b) Weight distributionratios (Dw) for Cu(II) vs [H2SO4] for polystyrene-2% divinylben-zene cross-linked sulfonic acid resins impregnated with differentweight percents of TT14C4. Adapted from Moyer et al.133

Table 3. Separation Factors for Ra(II) from AlkalineEartha

interferentcation M DM RRa/M

interferentcation M DM RRa/M

15C5Ca 6.1 41 Ba 210 1.2Sr 40 6.2 Ra 250 1

18C6Ca 16 140 Ba 1200 1.9Sr 810 2.8 Ra 2300 1

21C7Ca 6.8 350 Ba 880 2.7Sr 440 5.5 Ra 2400 1

a Organic phase: 0.01 F HDNNS + 0.001 M crown ether ino-xylene. Aqueous phase: 0.1 M HCl.41,117


in this study using HDNNS is an artifact of theextractant concentrations.

A soft donor macrocycle, 3-n-octyltrithia-10-crown-3(OTT10C3), was investigated in the HDNNS extractionof Pd(II) from HNO3 into n-dodecane.140 Millimolarconcentrations of HDNNS were employed, and presum-ably monomeric or only slightly aggregated HDNNScooperated with OTT10C3 to enhance Pd(II) extraction.A number of larger thiacrown ethers were investigated,but no studies probing the Pd(II) coordination environ-ment or the possibility of exocyclic binding by thevarious thiamacrocycles were undertaken.

As with the alkylcarboxylic acids, aggregation ofalkylsulfonic acids complicates, attenuates, and periodi-cally negates synergistic effects. Contrasting the behav-ior of the alkylcarboxylic acids, where only self-aggregation is reported, is the formation of alkylsulfonicacid/crown ether interextractant aggregates that havebeen shown to oppose synergism in many of the separa-tions described here. This phenomenon has been over-come by the use of thiacrown ethers, where the hydrogen-bonding interactions with the alkylsulfonic acids arequite weak; however, the chemical instability in contactwith HNO3, the potential for exocyclic complexation, andthe slow metal/ligand dissociation rates5 (important forstripping) limit the application of thiacrown ethers.

(c) Dialkylphosphoric Acid Extractants. The util-ity of bis(2-ethylhexyl)phosphoric acid (HDEHP, Figure1) as an extractant has been widely exploited,2,3 par-ticularly in the extraction of the trivalent f elementswhere very large separation factors for the heavylanthanides or actinides from their lighter relatives canbe achieved.52,53 The dialkylphosphoric acids have seenmoderate application in crown ether-containing syner-gistic systems, and the majority of the work in-volves the extraction of alkali or alkaline-earthcations7-9,113,116,143-149 with lesser efforts dedicated totransition or main group metals150-152 and the felements.118,153-155 The limited study of lanthanide andactinide extraction by crown ethers and dialkylphos-phoric acids may be related to the very efficient extrac-tion of the tri-, tetra-, and hexavalent f elements byHDEHP alone, which increases the denominator of eq10 and reduces the overall synergistic effect.

Solvent extraction and supported liquid membrane(SLM) transport of Li(I) by dibenzo-14-crown-4 (DB14C4)combined with HDEHP are reported to be synergisticin nature.145 The selectivity for Li(I) over Na(I) in theSLM procedure is reversed when DB14C4 is replacedby B15C5. The electrostatically-based selectivity ofHDEHP for Li(I) over its heavier congeners is effectivelynegated by addition of DCH18C6 to the solvent.8,9,113,143

Benzene solutions of DCH18C6 most effectively syner-gize K(I) extraction from an aqueous phase 0.1 M eachin MNO3 (M ) Li, Na, K, Rb, Cs) at pH ≈ 5 (corre-sponding to ≈50% H(DEHP)2

-). The synergistic extrac-tion of K(I) by DCH18C6 and HDEHP in tolueneexhibits a slight negative dependence as a function oftemperature, implying an exothermic reaction that isconsistent with temperature-dependent perturbationsto the pKa of HDEHP and to the formation constant ofthe [K(DCH18C6)]+ complex.144

Work comparing alkali and alkaline-earth cationextraction in benzene solutions of different crown ethersand HDEHP143 exhibits trends that compare favorablyto published results using picric acid as the organophilicanion. Increasing the aqueous phase NaNO3 concentra-

tion over the range 0-4 M effects a regular decrease inCs(I) distribution that is attributed to competitiveextraction of Na(I) in the DCH18C6 + HDEHP extrac-tion solvent. The alkaline-earth cations show a lesserdependence on the NaNO3 concentration because theyform considerably stronger complexes with bothHDEHP and the crown ethers than does the Na(I)interferent. A series of extractant and acid dependencieswere used in slope analyses to deduce M(DCH18C6)-(H(DEHP)2) as the extracted complex.

Recent investigations into the extraction of alkaline-earth cations by DCH18C6 and HDOP have probed theinfluence of DCH18C6 stereoisomerism on the magni-tudes of the synergistic effects.146-149 An examinationof the formation constants47 for Sr(II) complexes of thecis-syn-cis (log K ) 3.24) and cis-anti-cis (log K )2.64) stereoisomers of DCH18C6 (Figure 2) in H2O at25 °C shows a significant difference in stability that isstrictly attributable to stereoisomer effects. Almost allprevious studies using DCH18C6 have neglected stereo-isomeric effects by using mixtures of the primarily cis-DCH18C6 stereoisomers, and it is clear that differentstereoisomeric compositions will influence the extractionefficiency. The partitioning of Ca(II), Sr(II), and Ba(II)in solvent extraction systems comprising HDOP andindividual stereoisomers of DCH18C6 in toluene wasinvestigated.146-149 Results of VPO experiments as wellas the continuous variation studies, single and doubleextractant dependencies, and acid dependencies recom-mended in Table 1 were used to determine the stoichi-ometries of the extracted complexes. Examples of thevarious solvent extraction dependencies for Sr(II) par-titioning by HDOP and the individual stereoisomers ofDCH18C6 are presented in Figure 7, and Figure 8depicts the acid dependencies for Ca(II), Sr(II), andBa(II) partitioning by the same extractants. No syner-gism is observed for extraction of Ca(II) in the presenceof DCH18C6 (Figure 8), presumably because of thestrong complexation of Ca(II) with HDOP and thestrained complexes this cation forms with DCH18C6.In the absence of DCH18C6, Sr(II) and Ba(II) areextracted as M(H(DOP)2)2‚2HDOP (M ) Sr or Ba). Uponaddition of DCH18C6, the extracted complexes areformulated as M(DCH18C6)(H(DOP)2)2 (M ) Sr or Ba),and Ba(II) shows greater synergistic effects than Sr(II).Figures 7 and 8 show that the effectiveness of thestereoisomers of DCH18C6 as synergists decreases inthe order cis-syn-cis > cis-anti-cis > cis-trans >trans-syn-trans > trans-anti- trans. This sequenceis explained by correlating the logarithm of the syner-gistic constants with the ligand strain energies (∆Ureorg,calculated using molecular mechanics methods) of theDCH18C6 stereoisomers. The ligand strain energiesprovide a measure of the degree of binding site pre-organization provided by each stereoisomer, where morehighly organized cavities yield more stable complexes.130

In Figure 8, those stereoisomers having the largestligand strain energies afford the lowest distributionratios. Figure 9 presents the inverse linear relationshipbetween log Ks and ∆Ureorg that has been establishedfor both the Sr(II) and Ba(II) extraction systems.146-148

Steric effects in dialkylphosphoric acid have beeninvestigated in connection with DCH18C6 stereoisomereffects.147-149 The di-n-octyl-, bis(2-ethylhexyl)-, and bis-(diisobutylmethyl)-substituted phosphoric acids gener-ally afford the same sequence of decreasing effectivenessof the DCH18C6 stereoisomers as synergists, suggesting


that the differing steric bulk of these diakylphosphoricacids is only of minor significance in these synergisticextraction systems.

The synergistic extraction of Sr(II) by HDOP andeither cis-syn-cis- or cis-anti-cis-DCH18C6 in tolu-ene has been probed using SANS techniques.116 Extrac-tion of HDOP alone involves Sr(H(DOP)2)2‚2HDOP withthe concomitant formation of small quantities of verylarge aggregates. Upon addition of either the cis-syn-cis or the cis-anti-cis stereoisomer of DCH18C6, theformation of large aggregates is inhibited. Interestingly,the cis-anti-cis stereoisomer, which both extractsSr(II) more weakly146-149 and forms weaker complexes47

than cis-syn-cis-DCH18C6, more effectively sup-

presses aggregate formation. Overall, these resultscompare very favorably to the previously discussed workinvolving Sr(II)/DCH18C6/Versatic acid synergistic sys-tems.115 Recall that in organic phases containing alkyl-carboxylic acids the viscosity generally increases at highmetal loading in the absence of the crown ether;however, the viscosity did not increase when DCH18C6was added to the Sr(II)/Versatic acid mixture. It wasproposed that DCH18C6 interrupted the Sr(II)/Versaticacid aggregation process, thereby precluding an increasein the organic phase viscosity. As hypothesized in theSANS studies of HDOP mixtures with DCH18C6 ste-reoisomers, complexation of Sr(II) by DCH18C6 inhibitsaggregate formation by occupying Sr(II) coordination

Figure 7. DSr vs mole fraction of DCH18C6, [HDOP], [DCH18C6], and [DCH18C6 + HDOP]. Note the different scales and the differentaqueous phase for the continuous variation study. Adapted from Bond et al.146

Figure 8. DM vs pHeq for Ca(II), Sr(II), and Ba(II) extraction by HDOP alone and in the presence of stereoisomers of DCH18C6. Theleast-squares lines have a fixed slope of 2. Adapted from Bond et al.146


sites that would otherwise be filled by terminal and/orbridging acidic ligands. More structural studies of highlyaggregated (e.g., micellar) extraction systems are war-ranted, because few reports describe the synergisticsystems in sufficient detail to permit a clear under-standing of structure/function relationships.

As discussed for the alkylsulfonic acids, the use ofHDEHP and dibenzo- or dicyclohexano-substituted crownethers in PUREX-like process solvents has been inves-tigated.7,127 For a kerosene diluent containing TBP,HDEHP, and 4,4′(5′)-bis(tert-butylbenzo)-24-crown-8,distribution ratios of 1.4 and 200 were obtained for Cs(I)and Sr(II), respectively, above pH 3.7 Unfortunately,PUREX processing employs HNO3 concentrations of ≈3M,126 which prevents Cs(I) and Sr(II) extraction becauseHDEHP is ineffective at this acidity. A decrease in theaqueous acidity of the PUREX process to accommodatethis synergistic extractant combination is unacceptablebecause high nitrate concentrations are required tofacilitate partitioning of Pu(IV) and U(VI)O2

2+,126 Pu(IV)hydrolysis commences above pH ≈ 1,106 and the pres-ence of acidic extractants (e.g., TBP degradation prod-ucts such as di-n-butylphosphoric acid) adversely affectsactinide stripping.126

The synergistic extraction of soft Lewis acid transitionand main group cations by dialkylphosphoric acids hasbeen studied using oxygen,144,151 nitrogen,151 and sul-fur151,152 donor macrocycles. Distribution ratios for Ag(I)and Tl(I) exceed unity in a toluene phase containingDCH18C6 and HDEHP.151 The distribution of Cd(II)was low (DCd ) 0.23(2)) and probably due to its smallsize, which is more amenable to 15C5,156,157 althoughcomplexes of Cd(II) with 18C6 are known.157 In accordwith the high stability of the [Pb(DCH18C6)]2+ com-plexes,47 the synergistic extraction of Pb(II) by DCH18C6and HDEHP is exceptional, affording potentially usefuldistribution ratios of 47 from 10-4 M Pb(NO3)2.144,151 Theazacrown ether DA18C6 was shown to be nearly aseffective a synergist as DCH18C6, whereas dithia- and

hexathia-18-crown-6 caused antagonistic behavior thatis attributed to interactions of HDEHP with the thia-crown ethers.151 Studies discussed above report onlynegligible interextractant aggregation of thiacrownethers with acidic extractants,18,131 and perhaps theineffectiveness of these macrocycles stems from exocyclicbinding or from conformational rearrangements priorto cation complexation.

Silver(I) extraction is reportedly synergized by amixture of HDEHP and a substituted dibenzo-dithia-18-crown-6 in benzene.152 The synergistic effect is quitelow; however, as the aqueous phase contained 2 MHNO3.

Two reports have briefly investigated trivalent lan-thanide and actinide partitioning into nitrobenzene by15C5 or 18C6 and a fluoro-substituted dithiophosphoricacid.118,153 No significant synergism was observed, whichmay be an artifact of the marginal organophilicity ofthese unsubstituted crown ethers and the polar diluent,which may suppress the synergistic effects.1

Extraction of U(VI)O22+ by various crown ethers is

also reported.154,155 Of the six crown ethers examined,DCH18C6 was shown to be the most effective synergistfor extraction of U(VI)O2

2+ by HDEHP into nitroben-zene.

Dialkylphosphoric acid/crown ether combinations canyield high distribution ratios for the larger alkaline-earth cations (Figure 8), although the details of the sizeselectivity in these systems are not yet well character-ized. Aggregation of the acidic extractant is againobserved; however, the tendency toward aggregation ofdialkylphosphoric acids at high metal loading can befurther suppressed by the addition of DCH18C6 to theorganic phase. The effects of crown ether stereoisomer-ization in separations can be significant, and themagnitudes and causes of these effects have onlyrecently begun to be addressed.

Synergistic extraction involving crown ethers andalkylcarboxylic, alkylsulfonic, or alkylphosphoric acids

Figure 9. Ks vs ∆Ureorg for different stereoisomers of DCH18C6. Organic phase: 0.10 M HDOP + 0.025 M DCH18C6 in toluene. Aqueousphase: 0.001 M M(NO3)2 (M ) Sr, Ba). Adapted from Bond et al.146


appears to have significant potential for cation selectiv-ity, as evidenced by the work presented in Tables 2 and3 and Figures 4-6 and 8. The selectivity exhibited by asynergistic extractant combination is most accuratelydetermined by competitive extraction experiments inwhich interfering matrix cations are present with thetarget cation in the aqueous phase, but very few reportshave adopted this approach. The H3O+ cation, which iscapable of binding with a variety of macrocycles,94

should also be included in matrix ion studies. Theinteraction of the acidic extractant with the crown ether,most significant for alkylcarboxylic and alkylsulfonicacids, thermodynamically opposes the synergistic ex-traction process and should be considered in the designof synergistic separations with these extractants. Aswith synergistic extraction using the monoprotic chelat-ing extractants, separations by acidic extractants alsoshow an inverse dependence on [H+] that provides aconvenient means of stripping target solutes from theloaded organic phase.

(d) Organophilic Inorganic Anion Extractants.A variety of inorganic anions ranging in size and com-plexity have been studied as alternatives to the use ofcarbon-based organophilic acids. Various organoanti-monic acids,158 SbCl6

-,159,160 poly(oxometalates),159-162

and cobalt(III) bis(dicarbollide)118,163-178 have been com-bined with crown ethers in order to synergize cationextraction.

Alkali metal,158 and particularly Cs(I),159,160 extractionby antimony compounds has been investigated. Extrac-tion of Cs(I) was targeted because of its importance inradioactive waste processing, and DB21C7 was com-bined with SbCl6

- in nitrobenzene or chlorocarbondiluents to facilitate removal of Cs(I) from medium-levelradioactive waste simulants.159 A synergistic factor(distribution ratio in the synergistic system divided bythe sum of the distribution ratios obtained for eachextractant individually) as high as 1200 was obtainedfor Cs(I) extraction by DB21C7 and NaSbCl6 in 1,2-dichloroethane. This system was so promising that itwas tested in a five-stage mixer-settler laboratorydemonstration.160 Significant drawbacks to this processinclude the loss of SbCl6

- to the raffinate and theadverse consequences of its eventual hydrolysis andprecipitation during large-scale continuous operation.

Poly(oxometalates), oxide clusters of controlled sizeand structure, have been combined with crown ethers,again for the removal of Cs(I) from radioactive wastesimulants.159,160 A large number of variables wereinvestigated, including the crown ether cavity size andfunctionalization, diluent effects, aqueous phase matrixion effects, and extractant dependencies.159 From thesestudies, DB21C7 was shown to be the most effectivesynergist for Cs(I) extraction in nitrobenzene, andvarious operational parameters are defined for themultistage contact described above for SbCl6

-.160 Both[PMo12O40]3- and [PW12O40]3- extract Cs(I) from aradioactive waste simulant in nitrobenzene, affordingDCs ≈1.2 up to ≈2 × 10-3 M [PM12O40]3- (M ) Mo, W)where precipitation commences. Upon addition ofDB21C7, however, precipitation and loss of poly(oxo-metalate) to the aqueous raffinate is suppressed whilethe extraction of Cs(I) is synergized. Regeneration of theCs(I)-containing extract is accomplished by contact with3 M NH4NO3 or 1 M KNO3, which saturate DB21C7causing release of Cs(I) to the aqueous stream. Theshortcomings of stripping into a concentrated salt phase

are acknowledged, and concerns about the possiblecoextraction of f elements have been assuaged by arelated study.161,162 Actinide ions are known to interactwith poly(oxometalates) in liquid-liquid distributionsystems,39 but neither Eu(III) nor U(VI)O2

2+ is extractedfrom a radioactive waste simulant into nitrobenzene-containing DB21C7 and [PW12O40]3-.162

Carborane complexes of Co(III) may also serve asorganophilic anions and have been investigated on boththe laboratory- and pilot-scales for the treatment ofradioactive wastes. The cobalt(III) bis(nido-1,2-C2B9H11)complex, commonly known as cobalt(III) bis(dicarbol-lide) or ambiguously as cobalt dicarbollide (Figure 1),has received the greatest attention. Cobalt(III) bis-(dicarbollide) has been combined primarily with 15C5,18C6, or DB18C6 in nitrobenzene for the extraction ofalkali or alkaline-earth cations,163-172,178 lead(II),173,174

and f elements.118,175-177 A number of these papers arequite similar and report synergistic extraction con-stants, most of which are derived solely from crownether extractant dependencies using computationalmethods. These equilibrium constants should be closelyexamined prior to use, because the species used in theircalculation are, in some cases, experimentally un-founded or their existence is chemically suspect. Someof the difficulties encountered in modeling these systemsmost likely derive from the insufficient number ofexperiments used to characterize the extraction systemand from the aqueous phase solubility of the unsubsti-tuted 15C5 and 18C6 extractants.

The utility of cobalt(III) bis(dicarbollide) in alkalication extraction is widely acknowledged. For example,partitioning of Cs(I) by this anion alone is quite effective,and when combined with cyclic or acyclic polyethers, theprocess solvent also synergistically extracts Sr(II). Abrief review of the use of cobalt(III) bis(dicarbollide) inradioactive waste treatment flowsheet testing and acomparison with competing technologies are avail-able.179

The extraction of trivalent lanthanide and actinidecations by cobalt(III) bis(dicarbollide) in the pres-ence of 15C5,118,175 18C6,118,175-177 DCH18C6,176 andDB18C6176 has attracted considerable attention. Forexample, RAm/Eu increased from 1.3(2) to 3.0(2) onaddition of 0.3 M 18C6 to a 0.3 M solution of cobalt(III)bis(dicarbollide) in nitrobenzene.175 The RAm/Eu obtainedusing 15C5 started at 4.5(4) at tracer concentrations ofEu(NO3)3, reached a minimum of 3.8(3) at 0.01 M, andincreased steadily to a maximum of 6.0(4) in 0.5 MEu(NO3)3. The observed minimum and the subsequentincrease in partitioning are likely attributable to activitycoefficient effects. The highest RAm/Eu reported in thiswork, 6.4(4), was obtained from a mixture of 0.5 MHNO3 + 2.8 M Al(NO3)3, which prompted a subsequentinvestigation probing aqueous phase effects on theextraction of Ce(III), Eu(III), Am(III), or Cm(III) intonitrobenzene-containing 18C6 and a chlorinated co-balt(III) bis(dicarbollide) derivative.175 The distributionratios for Ce(III) from 0.5 M HNO3 + 2 M Al(NO3)3decrease as the mole fraction of 18C6 decreases, andthe absence of a maximum in distribution vs molefraction implies a lack of synergism in this system. Adecrease in extraction above ≈0.2 M 18C6 was at-tributed to the formation of [(H3O)(18C6)][cobalt(III) bis-(dicarbollide)], although the aqueous phase partitioningof 18C6 and its metal complexes may also contribute tothe diminution in extraction.


The partitioning of the trivalent lanthanide ions,except Pm(III) and including Sc(III), Y(III), and La(III),by cobalt(III) bis(dicarbollide) and crown ethers innitrobenzene is discussed.176 The extraction of thesecations by cobalt(III) bis(dicarbollide) alone decreaseswith an increase in the ionic radii, consistent withnonselective partitioning into an organic phase contain-ing a noncoordinating anion. Addition of 15C5 affordsonly a slight synergistic effect, whereas 18C6, DCH18C6,and DB18C6 generally cause antagonistic effects. De-spite the antagonistic effect, the distribution ratios intoa nitrobenzene solution of 0.0488 M cobalt(III) bis-(dicarbollide) + 0.0171 M DCH18C6 vary in a mannerquite similar to that shown in Figure 3.65 The interest-ing leveling of the extraction near Tb(III) in these twosystems is not yet fully understood and will be discussedin greater detail in a forthcoming paper.23

A limitation to the use of inorganic anions in syner-gistic solvent extraction is the need for polar diluents(to maintain the solubility of the poly(oxometalate) orthe carborane extractants), which often serves to in-crease extraction but also to diminish the magnitudeof Ks. Furthermore, careful control of the crown ether/inorganic anion ratio is required to maintain suitableextraction behavior, although no clear guidelines for thiscontrol exist. Last, the apparent thermodynamic driversin solvent extraction using cobalt(III) bis(dicarbollide)derive from the ∆Ghyd of the solute cation rather thanfrom an interaction of the cation with this organophilicanion. The probability of cation-selective synergisticextraction is raised by addition of crown ethers tosolutions of organophilic inorganic anions, but suchcombinations appear to offer lower selectivity and lessversatility than synergistic extraction by two extract-ants that interact with the primary coordination sphereof the target cation.

Neutral Extractant/Crown Ether Combinations.Synergistic solvent extraction by crown ethers andneutral solvating alkylphosphorus extractants is re-ported,180-184 as is the combination of crown ethers withcalixarene extractants.185,186 In the absence of organo-philic anions such as picrate, trialkylphosphates andtrialkylphosphine oxides are reported to show little orno synergism.9,143 Very slight synergistic effects arereported for the extraction of alkali cations by 12C4,15C5, or B15C5 combined with TBP or TOPO180-182 andpicrate as the organophilic anion. In general, synergisticextraction of the alkali-metal cations was shown to peakwhen the cation diameter approaches the cavity size,181

and the extracted species changes from M(CE)2(picrate)to M(CE)(picrate)(TBP) (M ) Rb, Cs; CE ) 12C4, 15C5,B15C5) upon addition of the neutral solvating extrac-tant. Rubidium(I) is extracted slightly better thanCs(I),180,181 and B15C5 generally affords larger syner-gistic extraction constants than 15C5, although it is notclear how the aqueous phase solubility of the latter wastreated in the equilibrium analysis.

A single investigation probed the effect of basicitydifferences in the alkylphosphorus coextractant.182 Inthis work, benzene solutions of TOPO were shown tomore effectively synergize Rb(I) and Cs(I) extraction by12C4, 15C5, or B15C5 than TBP. In general, thesynergistic constants are considerably larger for TOPOthan for TBP, and again Rb(I) is better extracted thanCs(I). It is clear that the more basic trialkylphosphineoxide is a more effective extractant than TBP and thatthe more charge dense Rb(I) should be better extracted

than Cs(I) by these neutral alkylphosphorus extractant/crown ether combinations. Unfortunately, the synergis-tic effects are quite low, and little information regardingalkali cation selectivity in these systems can be derivedfrom these data.

Synergistic extraction of Ag(I) by DB18C6 andTOPO184 is reported, as are the extraction equilibria forCu(II) and Zn(II) partitioning by various crown ethersand TBP.183 The stoichiometries of the Cu(II) or Zn(II)species extracted into CHCl3 by 12C4 or 15C5 and TBPare all 1:1:1. The TBP adduct formation was strongerfor the 12C4 complexes than for 15C5, possibly becauseof the greater need for solvation in the stericallyunsaturated 12C4 complexes. The overall extraction wasgreater for the 15C5-containing systems, but the dis-tribution ratios seldom exceed 0.01.

p-tert-Butylcalix[4]arene was combined with B15C5,18C6, DCH18C6, or DB18C6 to facilitate alkali cationpartitioning into 1,2-dichloroethane.185,186 Extractionwas only observed above pH ≈ 8, where an alcoholicproton is removed from the lower rim of the p-tert-butylcalix[4]arene to form an organophilic anion. Thehighest synergistic extraction constants are observed forNa(I) and K(I) in systems using 15- or 18-memberedmacrocycles, respectively, which indicates a correlationwith the macrocycle cavity size. In such systems, itappears that the interaction of the cation with the crownether is responsible for the selectivity and that thecalixarene is relegated to the role of an organophilicanion.

The utility of solvent extraction using crown ethersdepends on the coextraction of anions to maintainelectroneutrality. In keeping with this tenet, the syn-ergistic extraction exhibited by combinations of crownethers and neutral alkylphosphorus solvating extract-ants are low and such combinations will likely be oflimited practical utility. The behavior of “two-host”extraction systems (e.g., crown ethers and calixarenes)has received very little attention and again requires thepresence of an organophilic anion to maintain electro-neutrality. It seems that organophilic anions less costlythan the calixarenes are better suited to this task;however, the “two-host” systems combined with organo-philic anions may be an interesting approach to thesynergistic extraction of different groups of cations,although the complexity of such a ternary extractantsystem is acknowledged.

Conclusions

The introduction of cation selectivity into synergisticsolvent extraction is best accomplished by the use ofcrown ethers that both form stable in-cavity (or nearlyso) complexes with the target cation and have adequatefunctionalization to impart organophilicity (but not tocause adverse steric or conformational effects). Whenthese two criteria are met, the crown ether is both aneffective ligand and an effective extractant. The formercriterion requires some understanding of the coordina-tion chemistry and, particularly, a knowledge of thesolution speciation of the cation and extractants, butremarkably few studies of synergistic extraction haveprobed the cation coordination environment. The use ofunfunctionalized crown ethers (e.g., 12C4, 15C5, 18C6,etc.) in studies of synergistic extraction is widespread,but the partitioning of these macrocycles to the aqueousphase often is not adequately addressed. More carefulselection of the crown ether extractants is required so


that aqueous phase complexation and, thus, maskingeffects do not influence the selectivity or equilibriumconstants reported for a given synergistic system.

The generally accepted explanation for synergisticextraction is that the organophilicity of the extractedsolute is increased by adduct formation with organo-philic extractants. The thermodynamic factors are col-lectively described for several synergistic extractionsystems22 and seem to suggest that the enthalpic andentropic contributions should be most favorable forcharge dense cations that both form strong complexeswith extractants and possess large hydration numbers(all hydration shells). Unfortunately, charge densecations also may be effectively partitioned by thechelating or acidic extractants alone (Kex), which willreduce both the selectivity and synergistic effects (Ks).

Competition between the two extractants also impactsthe synergistic equilibrium (e.g., eq 8), because a metalcomplex involving an extractant that alone is highlyeffective for a particular cation (i.e., Kex is large) willgain little thermodynamic advantage by adduct forma-tion with other extractants.1 When Kex is very large, Ksis small and the partitioning is dominated by a singleextractant rather than by synergism of the two extrac-tants. Balancing the partitioning efficiencies of the twoextractants in a synergistic system requires an under-standing of the coordinative interactions of the cationwith both extractants, separately and collectively, andcareful selection of the remaining system parameters.

As with conventional solvent extraction, the choice ofdiluent can significantly influence the efficiency and/orselectivity in a synergistic separation. Some of the morepromising fundamental studies discussed here haveemployed polar chlorinated diluents that are not ac-ceptable for use in practical applications.38,113,114 Fur-thermore, the cooperative effect (Ks) between extract-ants is generally largest in nonpolar diluents, which areineffective with respect to ion solvation and which donot interact (e.g., hydrogen bond) appreciably with theextractant molecules to oppose synergism. Such non-polar diluents include the aliphatic hydrocarbons (e.g.,kerosene) that see widespread application in hydro-metallurgy. Unfortunately, the high synergistic effectsand selectivity often obtained in nonpolar diluentscoincide with distribution ratios that are considerablylower than those obtained using polar diluents. Thereasons behind these observations are not well-defined,especially when considering that the extractant/diluentinteractions opposing synergism are likely to be largerfor polar diluents. It is likely that higher dielectricconstant diluents more readily permit synergistic ion-pair extraction and aqueous phase anion coextractionand/or increase the organic phase H2O content. Under-standing the role of the diluent in such complicatedextraction systems is an area that will benefit from moreresearch and may even be the key to increasing distri-bution ratios without the loss of synergism or selectivity.

The ability of synergistic systems to yield enhanceddistribution at reagent concentrations below those ofsingle extractant solvents is distinctly advantageous. Alower extractant concentration equates to lower capitalinvestment in reagents (especially important for thecomparatively expensive crown ethers) and most likelywill result in lower extractant losses to the raffinate.Synergistic extraction with crown ethers also enablesthe use of inexpensive, nonselective extractants that areotherwise of limited utility.

Synergistic extraction systems offer unique benefitsin cation separations, and it can be argued that suchsystems hold the best promise for the large-scale ap-plication of crown ethers in separation science. Signifi-cant fundamental and developmental research is stillrequired; however, only with well-planned and carefullyexecuted research will the full potential of size-selectivesynergism be realized.

Acknowledgment

This work was performed under the auspices of theOffice of Basic Energy Sciences, Division of ChemicalSciences (manuscript preparation), and the Environ-mental Management Sciences Program of the Officesof Energy Research and Environmental Management(literature review), U.S. Department of Energy, underContract No. W-31-109-ENG-38.

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Received for review March 8, 2000Revised manuscript received May 25, 2000

Accepted May 25, 2000

IE000356J


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