Chelate Extraction of Metals into Ionic Liquids

Naoki HIRAYAMA

Department of Chemistry, Faculty of Science, Toho University,

Miyama 2-2-1, Funabashi 274-8510, Japan

(Received February 1, 2011; Accepted February 25, 2011)

In the solvent extraction of metal cations into ionic liquids, use of Brønsted acid-type

chelate extractants has some advantages including simplicity in controlling extraction

selectivity by selecting suitable aqueous phase pH values and simple back-extraction

recovery of the extracted metals using acid solutions. In this review, various

fundamental and applied research on metal extraction using the “ionic liquid chelate

extraction system” since 2001 are described in detail.

1. Introduction

Solvent extraction of metal cations is a very important technique for their mutual separation,

recovery and preconcentration [1–6]. In solvent extraction, generally, various hydrophobic organic solvents

are used as extraction phase solvents. However, most of the solvents are toxic, flammable and volatile, and

it has been recommended that their use as solvents should be avoided.

Ionic liquids (ILs), hydrolytically stable salts with low melting points (ca. < 100 ˚C), have recently

been noted as ‘green’ solvents mainly in organic chemistry and catalysis chemistry [7–22]. In particular, a

series of ILs made from 1-alkyl-3-methylimidazolium cations (Cnmim+) and bulky fluorinated anions such

as hexafluorophosphate (PF6–), tetrafluoroborate (BF4

–) and bis(trifluoromethanesulfonyl)imide (Tf2N–)

have been assessed as valuable solvents because they have relative air and water stability and favorable

viscosity and density as solvents. Furthermore, some of them

such as [Cnmim][PF6] (1a, n ≥ 4) and [Cnmim][Tf2N] (1b, n ≥

2) are immiscible with water and, therefore, can possibly be

used as extraction solvents. In addition, many ILs have unique

physical and chemical properties as solvents, such as high

polarity and, therefore, are expected to be not only

‘alternative’ solvents but also ‘novel’ ones.

The use of ILs in solvent extraction was reported for organic materials by Huddleston et al. in 1998

[23] and for metal cations by Dai et al. in 1999 [24]. Many researchers have studied the use of ILs in

extraction, and various reviews have been published [25–40]. Many of the metal extraction studies have

NN +CnH2n+1 CH3

PF6–

1a

N–

SO2

SO2

CF3CF3NN +CnH2n+1 CH3

1b

Solvent Extraction Research and Development, Japan, Vol. 18, 1 – 14 (2011) – Reviews –

- 1 -

been performed using neutral extractants such as crown ethers and

organophosphorous compounds to form cationic complexes. The selection of

neutral extractants seems to be based on the simple idea that hydrophobic ILs

have enough hydrophobic IL anions to be used as counterions for ion-pair

extraction of cationic metal complexes. That is, it has been considered that

the ILs can act as good cation exchangers by releasing the imidazolium

cations to the aqueous phase. However, in actual fact, ILs may display

inadequate performance as ion exchangers. For example, the distribution of

some neutral aromatic carboxylic acids and aniline from the aqueous phase to

the [C4mim][PF6] phase is superior to that of the respective charged species

[23]. In addition, there are hydrogen bonds between the imidazolium cations

and the hydrophobic anions in the imidazolium-type ILs which stabilize the

ILs themselves [41–43]. Therefore, use of anionic ligands to form neutral or

low-charged metal complexes seems to be preferred for metal extraction to

the ILs

In addition, ILs are nonvolatile and it is essentially impossible to

recover the metals extracted to the ILs by evaporation. However, when using

neutral extractants, back-extraction (stripping) of the extracted metals is very

difficult. In fact, there are only a few reports in which back-extraction was

achieved. In most of them, interestingly, extractants having neutral

nitrogen-donor atom(s) were selected, i.e. N-alkylaza-18-crown-6 derivatives

(2) for Sr2+ and Cs+ [44], p-tert-butyl-O-(2-pyridylmethyl)calix[4]arene (3)

for Ag+ [45,46] and N,N,N’,N’-tetrakis(2-pyridylmethyl)ethylenediamine (4)

for Eu3+ [47]. In these cases, an aqueous acid solution is used for stripping

and back-extraction is achieved by a competitive Lewis acid-base reaction

between the metal cation and the proton.

From these facts, it is obvious that applying ‘chelate extraction’ to an IL extraction system can be

effective for extraction, separation and recovery of metals. Generally, chelate extraction is defined as a

metal extraction method using a Brønsted acid-type extractant that acts as an anionic chelator after

deprotonation. Furthermore, in chelate extraction, control of the aqueous phase pH value may result in

controlling extraction selectivity through the chelate formation ability between the anionic chelator and

each metal, and the extracted metals can be easily recovered by simple back-extraction into acid solution.

We call this system “ionic liquid chelate extraction system” [48]. In this review, various researches,

especially our own, on metal extraction using the IL chelate extraction system are described.

2. Possible Extraction Modes in Ionic Liquid Chelate Extraction

Hydrophobic ILs have an affinity not only for polar neutral species but also for hydrophobic charged

species and, in IL chelate extraction, various extraction modes should be considered [49]. In this section,

these modes are discussed on the extraction of an n-valent metal cation (Mn+) with a monobasic Brønsted

acid chelate extractant (HL) as an example.

O

N

OO

O

O CnH2n+1

2

3

N N

NN

NN

4

- 2 -

The simplest extraction mode is the “neutral complex extraction mode”. In this case, the neutral

complex formed is extracted, and its extraction equilibrium and the extraction constant (Kex) are described

as follows:

Mn+ + nHLIL MLn IL + nH+ (1)

Kex = [MLn]IL[H+]n / [Mn+][HL]ILn (2)

where subscript IL denotes the IL extraction phase. This mechanism is essentially the same as the

conventional chelate extraction into an organic solvent. In this mode, the IL acts simply as a polar

nonaqueous solvent.

On the other hand, ILs can act as liquid ion exchangers and charged complexes can be extracted into

ILs. This is the basis of metal extraction with neutral extractants into the IL extraction phase. Also in IL

chelate extraction, therefore, extraction of charged complexes should be considered. On extraction of

partly-neutralized cationic complexes, termed “cation-exchange mode”, the extraction equilibrium and Kex

are described as follows:

Mn+ + iHLIL + (n–i)IL-C+IL MLi

(n–i)+IL + iH+ + (n–i)IL-C+ (3)

Kex = [MLi(n–i)+]IL[H+]i[IL-C+]n–i / [Mn+][HL]IL

i[IL-C+]ILn–i (4)

(i < n)

where IL-C+ denotes an IL cation. In the case of over-neutralized anionic complexes (“anion-exchange

mode”), Eqs. (3) and (4) become:

Mn+ + iHLIL + (i–n)IL-A–IL MLi

(i–n)–IL + iH+ + (i–n)IL-A– (5)

Kex = [MLi(i–n)–]IL[H+]i[IL-A–]i–n / [Mn+][HL]IL

i[IL-A–]ILi–n (6)

(i > n)

where IL-A– denotes an IL anion. In addition, when IL ions are too hydrophobic to be partitioned to the

aqueous phase, ion-pair extraction of the charged complexes with opposite-charged hydrophobic ions

co-existing in the aqueous phase may occur instead of the ion-exchange reaction. (Actually, it was reported

that very hydrophobic [C10mim][Tf2N] shows a quite low cation-exchange ability [50].)

As shown in Eqs. (1), (3) and (5), all extraction modes are accompanied by the release of protons to

the aqueous phase. In other words, the extracted metals can be back-extracted into an acidic aqueous phase

irrespective of the extraction mode.

To use the IL chelate extraction system effectively, comprehension and detailed analysis of the

complicated extraction modes is required. Billard et al. [40] said, “This can be regarded as of huge potential

or of discouraging complexity.”

3. Fundamental Studies on Ionic Liquid Chelate Extraction

The first report on IL chelate extraction was published in 2001 [51]. Visser et al. studied the

distribution behavior of Co2+, Ni2+, Cd2+ and Fe3+ to

[C6mim][PF6] including 110–4 – 110–3 mol dm–3 of

1-(2-pyridylazo)-2-naphthol (PAN, 5a) or 1-(2-thiazoly-

lazo)-2-naphthol (TAN, 5b). From slope analysis, the

extracted species for Co2+, Ni2+ and Cd2+ were

determined as 1:2 neutral complexes (for the neutral

N

N

OH

N

S

NN

N

OH

5a 5b

- 3 -

complex extraction mode). Although they estimated that the extracted

species for Fe3+ with TAN was a 1:2 monohydroxo neutral complex, a

more detailed study seems to be necessary.

Wei et al. [52,53] reported the extraction of Ag+, Cu2+, Pb2+,

Cd2+ and Zn2+ to [C4mim][PF6] with dithizone (HDz, 6), 8-hydroxy-

quinoline (Hq, 7a) and PAN. From the UV-visible spectra of the IL

phase, the neutral complex extraction mode was suggested as the

extraction mechanism. Although Li et al. reported extraction of Pb2+

with HDz into [C8mim][PF6] [54] and a unique IL 1-butyl-3-trimethyl-

silylimidazolium hexafluorophosphate ([C4tmsim][PF6]) [55], the

extraction mechanism was not studied in detail. (They also used

[C4tmsim][PF6] for the chelate extraction of Ge4+ with methylbenzene-

azosalicylfluorone [56], Hg2+ with o-carboxyphenyldiazoamino-p-azo-

benzene [57] and Al3+ with 3,5-di-tert-

butylsalicylfluorone [58].) In addition,

Kumano et al. [59] reported the extraction

of Mn2+, Cu2+, Zn2+ and Cd2+ with Hq into

N,N-diethyl-N-methyl-N-(2-methoxyethyl)-

ammonium bis(trifluoromethanesulfonyl)-

imide, although the mechanism was not

discussed.

Hirayama studied the extraction

behavior of Ni2+, Cu2+, Zn2+ and Cd2+ into

[C4mim][PF6] with Hq and its derivatives

(7b–d) in detail [60,61]. In the IL chelate

extraction system, relatively high extract-

ability was obtained for all the metals,

especially for Zn2+ and Cd2+, compared

with the conventional organic solvent

system (Figure 1). From slope analysis, it

was found that not only Ni2+ and Cu2+ but

also Zn2+ and Cd2+ are extracted as 1:2

neutral complexes (in the neutral complex

extraction mode). The obtained log Kex

values [61] showed good correlation with

the pKa(–OH) values for the extractants, as

shown in Figure 2. In conventional organic

solvents as the extraction phase, it is well

known that Zn2+ and Cd2+ are extracted by

high concentrations of Hq (and its

S

HN

NH

NN

6

N

OH

R1

R2

7a (R1 = R2 =H) 7b (R1 = Cl, R2 =H) 7c (R1 = R2 =Cl) 7d (R1 = NO2, R2 =H)

Figure 1. Plots of the extraction ratios (%E) for Ni2+ (squares), Cu2+ (circles) and Zn2+ (triangles) with Hq for [C4mim][PF6] (solid symbols) and chloroform (open symbols) as a function of the aqueous phase pH [61]. Initial Hq concentration in the extraction phase was 1×10–3 mol dm–3.

Figure 2. Correlation between the log Kex values for Ni2+ (■), Cu2+ (●), Zn2+ (▲) and Cd2+ () for [C4mim][PF6] [61] and the pKa(–OH) values for the Hq derivatives. Broken lines were obtained from linear least-squares fitting.

- 4 -

derivatives) as neutral self-adducts M(q)2(Hq)2 [62]. These facts suggest that the

IL extraction phase has some advantage in the extraction of hydrated neutral

complexes such as M(q)2(H2O)2. ILs have polarities comparable to light alcohols

[63,64] which supports the above hypothesis.

Jensen et al. performed a detailed study of the

extraction of Eu3+ and Nd3+ into [C4mim][Tf2N] with

2-thenoyltrifluoroacetone (Htta, 8). At high Htta

concentrations (> ca. 0.1 mol dm–3 in [C4mim][Tf2N]), it

was found from slope analysis and EXAFS that these

metals were extracted as 1:4 anionic complexes (in the

anion-exchange mode) [65]. In contrast, at lower Htta

concentrations, they were extracted as 1:3 neutral

complexes (in the neutral complex extraction mode) [66].

These results were confirmed also for other lanthanoids by

Hirayama et al. [67,68]

Hirayama and his co-researchers investigated the

extraction behavior of various divalent metals with Htta

into some imidazolium-type ILs including [Cnmim][PF6] (n = 4, 5, 6 and 8) and [Cnmim][Tf2N] (n = 4, 6

and 8) in detail [49,61,69,70]. Figure 3 shows the extraction behavior of Cu2+ and Zn2+ into [C4mim][PF6]

and [C6mim][PF6] as examples [49]. Interestingly, [C4mim][PF6] showed a higher extractability for Cu2+

than [C6mim][PF6], whereas this order was reversed for Zn2+. From slope analysis, it was found that Cu2+ is

extracted as neutral Cu(tta)2, whereas Mn2+, Co2+, Zn2+ and Cd2+ are extracted as anionic M(tta)3–.

Furthermore, the extracted species for Ni2+ was determined as neutral Ni(tta)2 on extraction into

[Cnmim][PF6] and anionic Ni(tta)3– on extraction into [C6mim][Tf2N] and [C8mim][Tf2N]. On extraction

SCF3

OH O

8

0

50

100

1 3 5 7

%E

pH Figure 3. Plots of the extracted ratios (%E) for Cu2+ (circles) and Zn2+ (triangles) with Htta to [C4mim][PF6] (solid symbols) and [C8mim][PF6] (open symbols) as a function of the aqueous phase pH [49]. Initial Htta concentration in the extraction phase was 1×10–2 mol dm–3.

-6

-4

-2

0

2

log

Kex

(M(t

ta) 2)

[C4

mim

][X]

[C8

mim

][X]

[C6

mim

][X]

[C5

mim

][X]

Ni

Cu

-12

-10

-8

-6

-4

log

Kex

'(M(t

ta) 3- )

[C4

mim

][X]

[C8

mim

][X]

[C6

mim

][X]

[C5

mim

][X]

Ni

Cd

Mn

Co

Zn

Figure 4. Obtained log Kex values for neutral M(tta)2 (left) and log Kex’ values for anionic M(tta)3

– (right) in [Cnmim][X] systems [49,61,70]. Solid symbols, X– =PF6

–; open symbols, X– = Tf2N–.

- 5 -

into [C4mim][Tf2N], additionally, Ni2+ was extracted as Ni(tta)2 and

Ni(tta)3– competitively. Figure 4 shows the log Kex values obtained for

neutral M(tta)2 and the log Kex’ (= log Kex[X–]IL/[X–], X– = PF6

– or Tf2N–)

values for anionic M(tta)3– [49,61,70]. In general, neutral M(tta)2 favored

more hydrophilic ILs, whereas M(tta)3– favored more hydrophobic ones.

Recently, Kidani and Imura [70] analyzed the extraction behavior of neutral

Cu(tta)2 and Ni(tta)2 into [Cnmim][Tf2N] numerically using regular solution

theory [71], and it was found that the extraction of Cu(tta)2 into the ILs is

similar to that into nonpolar organic solvents, whereas that of Ni(tta)2

shows a large deviation similar to oxygen-containing solvents such as

ethers and ketones. In addition, the extraction behavior for a halogen-free

IL, tetraoctylammonium dodecylsulfate, was also investigated [72].

The extraction behavior of divalent metals into [C4mim][PF6] with

other trifluorinated -diketones including 2-naphthoyltrifluoroacetone

(Hnta, 9a), benzoyltrifluoroacetone (Hbfa, 9b) and trifluoroacetylacetone

(Htaa, 9c) was also studied [73]. The order of metal extractability was Hnta

Hbfa Htta » Htaa, except for Ni2+, and use of these extractants resulted

in the same extraction modes with the exception of neutral Co(nta)2.

In the IL chelate extraction studies discussed above, extraction of

anionic complexes (based on the anion-exchange mode) occurred only

when trifluorinated -diketones were used as the extractants. As described

in Section 1, generally hydrophobic ILs have fluorine-containing anions such as PF6−, Tf2N

– and several

monoanions having a perfluoroalkyl group. Furthermore, several trialkylammonium

perfluoroalkyl--diketonates behave like ILs [74]. From these facts, it was expected that anionic complexes

with fluorine-containing extractants may have a high affinity for ILs. Ajioka et al. [48] studied the

extraction behavior of Cu2+, Co2+, Zn2+ and Cd2+ into [C4mim][PF6] with three 8-sulfonamidoquinoline

derivatives (HRsq) namely 8-(p-toluenesulfonamido)quinoline (H(C7H7)sq, 10a), 8-methane-

CF3

HOO

9a

CF3

HOO

9b

CF3

HOO

9c

N

NHO2S

R

10a (R = p-CH3(C6H4)) 10b (R = CH3) 10c (R = CF3)

-10

-6

-2

2

-11 -7 -3 1

log

Kex

(M(R

sq) 2

)

log Kex(M(q)2) Figure 5. Correlation between log Kex values for Cu2+, Zn2+ and Cd2+ for [C4mim][PF6] with H(C7H7)sq (□), H(CF3)sq (○) and H(CF3)sq (●) [48] and those with Hq [61]. Broken lines were obtained from linear least-squares fitting.

0

50

100

1 3 5 7 9

%E

pH

Figure 6. Plots of the extraction ratios (%E) for Cu2+ (circles) and Cd2+ (diamonds) for [C4mim][PF6] with H(CF3)sq (solid symbols) and H(CH3)sq (open symbols) as a function of the aqueous phase pH [48]. Initial extractant concentration in the [C4mim][PF6] phase was 1×10–3 mol dm–3.

- 6 -

sulfonamidoquinoline (H(CH3)sq, 10b) and 8-trifluoromethanesulfonamidoquinoline (H(CF3)sq, 10c). The

HRsq derivatives are structural analogs of Hq and, in most cases, the metals were extracted in the neutral

complex extraction mode. Furthermore, log Kex values for M(Rsq)2 showed a good correlation with those

for M(q)2 (Figure 5). Although H(CF3)sq showed a relatively low extractability compared with H(CH3)sq

because of steric inhibition of complex formation by the bulky trifluoromethyl group, the extractability of

bulky Cd2+ with H(CF3)sq was higher than that with H(CH3)sq (Figure 6) because of the formation of

anionic Cd((CF3)sq)3–. This result suggests that, in anionic complexes with fluorine- containing extractants,

the fluorine atoms show some IL-philic effect in IL chelate extraction.

Recently, Kubota et al. [75] studied the extraction of Y3+ and Eu3+ into [C4mim][Tf2N] with

N,N-dioctyldiglycol amic acid (DODGAA, 11). These metals were

extracted as 1:3 neutral complexes (in the neutral complex extraction mode).

This system was applied to highly stable supported liquid membrane (SLM)

based on [C8mim][Tf2N]. This preliminary research suggests the possible

use of the IL chelate extraction system in industry.

4. Specific Extraction Systems Using Ionic Liquid Chelate Extraction

In conventional chelate extraction into organic

solvents, addition of a neutral Lewis base to form

adducts often results in increasing the

hydrophobicity and the extractability of the

complexes. This is commonly referred to as a

‘synergistic effect’. In particular, the use of

macrocycles such as crown ethers as the Lewis base

often results in a change of extraction selectivity

between metals originating from their

size-recognition ability. Hirayama et al. [67,68]

investigated the synergistic effect of crown ethers

such as 18-crown-6 (18C6, 12a) and

dicyclohexano-18-crown-6 (DC18C6, 12b) on the

extraction of trivalent lanthanoids (Ln3+) with Htta

into [C4mim][Tf2N]. The pH values (pHD=1) at

which the distribution ratio of the metal is unity are

shown in Figure 7. The addition of 18C6 or

DC18C6 resulted in an enhancement of light Ln

extractability and an inversion of extraction selectivity between the Lns. From slope analysis, the following

synergistic extraction equilibria were confirmed:

Ln3+ + 2HttaIL + CEIL + C4mim+IL Ln(tta)2(CE)+

IL + 2H+ + C4mim+ (7)

Ln3+ + HttaIL + CEIL + 2C4mim+IL Ln(tta)(CE)2+

IL + 2H+ + 2C4mim+ (8)

where CE denotes 18C6 or DC18C6. Namely, these ternary complexes were extracted through the

cation-exchange mode, and this synergistic extraction system was named “ionic liquid synergistic

O

O

OH

NOC8H17

C8H17

11

O

O

O

OO

O

O

O

O

OO

O

12a 12b

2

3

4

5

pH

D=

1

La3+ Lu3+

Figure 7. Obtained pHD=1 values for the extraction of Ln3+ for [C4mim][Tf2N] containing Htta alone (●), Htta–18C6 (○) and Htta-DC18C6 ( ) [68]. Initial extractant concentration in [C4mim][Tf2N] was 1×10–2 mol dm–3.

- 7 -

cation-exchange system” [67].

Shimojo et al. [76] proposed a concept of

“intramolecular synergistic effect” in the IL

extraction system. They used a -diketone-

introduced diaza-18-crown-6 (H2DA18C6, 13)

for the extraction of Sr2+ into [C2mim][Tf2N]. This

extractant showed a higher extractability than the ‘simple’

synergistic extraction system using both

1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP) and

N,N’-dibenzyl-4,13-diaza-18-crown-6 (DBzDA18C6).

As is well known, ILs are generally immiscible in

nonpolar aliphatic organic solvents. In other words, a

hydrophobic IL, water and a nonpolar organic solvent can

form three separate phases [77]. Takata and Hirayama [78]

made a preliminary study on the use of [C4mim][PF6]/

water/cyclohexane triphasic system for the fractional

extraction separation of divalent metal cations using Hq

and trioctylphosphine oxide (TOPO) as competitive

extractants. In this system, Ni2+ was extracted into the

[C4mim][PF6] phase as neutral Ni(q)2 chelate, whereas

Mn2+, Zn2+, Cd2+ and Pb2+ were extracted into the

cyclohexane phase as an ion-pair between the cationic

M(TOPO)32+ and two PF6

– anions (Figure 8).

5. Task-Specific Ionic Liquids for Chelate Extraction

Task-specific ionic liquids (TSILs) are, originally, ILs with functional groups which impart particular

properties or reactivities [79]. Nowadays, also the salts made by introducing the functional groups to ILs

are called TSILs, although the salts themselves are not ILs. Strictly speaking, however, the latter materials

should be called task-specific onium salts (TSOSs) [80].

TSILs (or TSOSs) for metal extraction have complexation sites. These salts can be used as extraction

solvents themselves, or as novel extractants with

high IL-philicity. The first report on the TSILs

with neutral complexation sites binding to cationic

imidazolium groups was published in 2001 [81].

Unfortunately, however, both the imidazolium

units and the complexed parts of TSILs have

positive charges, which lowers complexation

stability with metal cations due to electrostatic

repulsion [82].

The first TSILs with anionic (Brønsted

NN + NH

HO

X–

14a (X– = Tf2N–), 14b (X– = PF6

–)

N

NHSO2

N

N + Cl–

N

NHSO2

15a 15b

O

N

OO

O

N

O

N

N

HO

Ph

OOH

N

NPh

13

0

50

100

%E

to

cycl

ohe

xane

0

50

100

1 3 5 7%

E to

[C4m

im][P

F6]

pH

Figure 8. Plots of the extraction ratios (%E) for Ni2+ (●) and Pb2+ (■) to [C4mim][PF6] and cyclohexane phases with Hq and TOPO as a function of the aqueous phase pH in the triphasic extraction system [77]. Initial Hq and TOPO concentrations in the cyclohexane phase were 1×10–2 and 1×10–3 mol dm–3, respectively.

- 8 -

acid-type) chelation sites are 1-butyl-3-[3-(2-

hydroxybenzylamino)propyl]imidazolium salts (14a

and 14b) [83] for the extraction of Am3+ by Ouadi et

al. Furthermore, Morita et al. [84] reported

1-methyl-3-[2-(8-quinolinylaminosulfonyl)ethyl]imi-

dazolium chloride ([Hmimesq]Cl, 15a), having an

8-sulfonamidoquinoline complexation site, for the

extraction of divalent metal cations into

[C4mim][PF6]. Actually, as shown in Figure 9,

[Hmimesq]Cl showed higher extractability for the

metals than its imidazolium-free analog, 8-ethane-

sulfonamidoquinoline (Hesq, 15b).

Choice of a deprotonated Brønsted acid-type

chelator as the IL anion is another effective approach to develop a novel TSIL for IL chelate extraction.

Kogelnig et al. [85] synthesized trioctylmethylammonium thiosalicylate ([TOMA][HTSal], 16). This IL

showed high efficiency and selectivity for the extraction of Cd2+ to the IL phase. Similarly, Egorov et al.

[86] synthesized trioctylmethylammonium salicylate ([TOMA][HSal], 17) and its extraction performance

for Fe3+, Cu2+ and Ni2+ was investigated. Not only are both HTSal– and HSal– stable species over a wide pH

range because of their intramolecular

hydrogen bonding but also they release a

further proton on complexation to metal

cations. From these facts, they are excellent

selections as the IL-anions for the TSIL.

6. Use of Ionic Liquid Chelate Extraction for Liquid-Liquid Microextraction

In the use of solvent extraction for concentration of trace materials, extraction phase volume should

be minimized. Such a system, termed liquid-liquid microextraction (LLME), was firstly reported in 1995

[87]. ILs are nonvolatile materials and have relatively high viscosities compared to conventional organic

solvents, and thus are suitable as extraction phase solvents in LLME. Furthermore, ILs are very expensive

solvents and it is advantageous for IL extraction to minimize the IL phase volume. The first report of the

use of an IL for LLME was published in 2003 [88]. The number of papers on IL-LLME studies has

increased explosively [36,37,39].

LLME is categorized according to phase separation technique. Single drop microextraction (SDME)

is the simplest one. In the above-mentioned first report [88], SDME was investigated. Manzoori et al.

combined the SDME technique to IL chelate extraction for Mn2+ [89] and Pb2+ [90] or Co2+ [91] using TAN

and ammonium pyrrolidinedithiocarbamate (APDC, 18a), respectively, as the extractant. Furthermore,

hollow-fiber liquid-phase microextraction (HF-LPME) [92], in

which the extraction phase is immobilized onto a hollow-fiber, was

applied by them for IL chelate extraction [93] for Pb2+ and Ni2+

using APDC.

C8H17

N+C8H17

CO O–

SH

C8H17CH3

C8H17

N+C8H17

CO O–

OH

C8H17CH3

16 17

0

50

100

1 3 5 7 9

%E

pH

Figure 9. Plots of the extraction ratios (%E) for Cu2+ (circles), Zn2+ (triangles) and Cd2+ (diamonds) as a function of the aqueous phase pH in the [Hmimesq]Cl (solid symbols) and Hesq (open symbols) systems [56]. Initial reagent concentration in the [C4mim][PF6] phase was 1×10–3 mol dm–3.

N C

S– NH4+

S

N C

S– Na+

S

C2H5

C2H5

18a 18b

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Dispersion of the extraction phase by ultrasound [94] or

disperser solvent [95] is an efficient way to obtain high extraction

efficiency. This technique, termed dispersive liquid-liquid

microextraction (DLLME), has been widely investigated.

Ultrasound-assisted DLLME (USA-DLLME) was firstly applied to

IL chelate extraction by Li et al. [96] for Cd2+ using sodium diethyldithiocarbamate (NaDDTC, 18b), and

they used the same technique to the extraction of Cr(VI) with APDC [97]. Recently, Molaakbari et al. [98]

applied USA-DLLME to the extraction preconcentration of Rh3+ with 2-(5-bromo-2-pyridylazo)-5-diethyl-

aminophenol (5-Br-PADAP, 19). In contrast, application of DLLME by disperser solvent to IL chelate

extraction has been studied in depth by Abdolmohammad-Zadeh et al. [99–101], Shemirani et al.

[102–104] and Berton et al. [105]

Cold-induced aggregation microextraction (CIAME), developed by Baghdadi and Shemirani [106] is

an LLME technique based on the temperature-dependency of the solubility of an IL in water. CIAME has

been applied to IL chelate extraction by many researchers [107–111]. Furthermore, another LLME

technique termed in situ solvent formation microextraction (ISFME) has been developed [112]. In this

technique, the IL extraction phase is formed by association between the IL cation and the IL anion in the

aqueous phase. This method was also applied to IL chelate extraction for Cd2+ with

O,O-diethyldithiophosphate [113].

The micro volume back-extraction approach for concentration has been also studied. Martinis et al.

investigated micro volume back-extraction of Cd2+ extracted into [C4mim][PF6] as its 5-Br-PADAP

complex [114], which was combined with a flow injection analysis technique [115,116]. Dadfarnia et al.

[117] used a similar approach for the Ni2+–PAN system.

In all the literature referred to this section, unfortunately, the extracted metal species are not

discussed at all. Various extraction modes are an important feature in IL chelate extraction, and detailed

analysis of the extraction mechanism should result in further development of LLME and other

microextraction techniques.

7. Conclusion

In this article, research on the use of ILs as the extraction phase for the chelate extraction of metal

cations has been reviewed. Although many researchers have reported on IL chelate extraction from various

viewpoints as mentioned above, the potential of ILs as chelate extraction solvents is still not fully

understood. In particular, specific properties of the ILs compared with conventional organic solvents have

not yet been understood in detail. More detailed fundamental investigations on IL chelate extraction system

seem to be necessary for comprehension and development of such systems.

Acknowledgements

For our research progress shown in this paper, I am grateful for financial support by a Grant-in-Aid

for Scientific Research (Nos. 13640420, 18550070 and 22550071) from the Japanese Society for the

Promotion of Science, Mitsubishi Chemical Corporation Fund, Reimei Research Program from Japan

Atomic Energy Research Institute (Japan Atomic Energy Agency at present), and the Start-up Research

N

N N

OH

N

C2H5

C2H5

Br

19

- 10 -

Fund from Faculty of Science, Toho University.

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