Ionic Liquids in Separation of Metal Ions From Aqueous Solutions
Ionic Liquids in Separation of Metal Ions From Aqueous Solutions
Ionic Liquids in Separation of Metal Ions From Aqueous Solutions
1. Introduction
In 1992 the information on the first, stable in water and air, room temperature ionic liquid
was published. Since then the number of publications about ionic liquids (ILs) has been
rapidly growing. Only in the year 2005 alone more than one thousand articles concerning
ILs’ synthesis, analysis and applications appeared. Various fields of ILs application are
presented in Figure 1.
Alkylation, Lubricants,
sources of energy,
Nuclear wastewater
organic compounds,
Izomerisation, Plasticizers,
Acylation, Bactericides,
Production of selective
treatment,
Esterification, Fungicides,
Cracking, liquid membranes and Antielectrostatic agents,
Diels-Alder addition, sensors. Selective absorber of
Wittig reactions,
Polymerisation.
sulphur compounds
from gasoline and oils.
Fig. 1. Scheme of ILs’ applications (Adams, 2002; Holbrey & Seddon, 1999; Kosmulski et al.,
2002; Pernak, 2003; Seddon et al., 2000).
Ionic liquids (ILs) have become widely used as solvents for organic reactions; however,
recently they are more frequently used for separation of metal ions both in extraction,
membrane and adsorption systems. In this paper their current applications as solvents and
carriers in liquid-liquid extraction of metal ions are discussed and possible extraction
mechanisms in ILs are considered in the light of their further use and prospective
development.
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376 Applications of Ionic Liquids in Science and Technology
Hydrophobicity of
Cation Anion
anion
H3C R
+
N N + N
+ N
H3C CH3 R
R
R NO3-, Cl-, Br-, CuCl2-, ClO4-
+
P
,BF4-, PF6-, CF3SO3-,
N(SO2CF3)2- ,
R
R
N(SO2CF2CF3)2-
R
R
+
N + R
N
R
R
Table 1. Composition of ILs (Bradaric et al., 2003a, 2003b; Cocalia et al., 2005a; Del Sesto et
al., 2005; Han & Armstrong, 2007; Holbrey & Rogers, 2002; Hunddleston et al., 1998; Pernak
et al., 2005; Visser et al., 2001a; Visser et al., 2002a; Visser et al., 2003).
The wide range of their applications includes also those in extraction processes.
Imidazolium ILs are solvents of main interest and have been comprehensively described.
They are well defined and their synthesis is well established. Although they are employed
in several extraction systems, only a few of them with successful and efficient stripping are
described. Stripping from loaded organic phase containing IL is difficult because of strong
interactions among ions.
Some ammonium and phosphonium ILs have been also applied in extraction processes, and
are considered as prospective solvents and carriers in separation techniques (Bradaric et al.,
2003a, 2003b; Del Sesto et al., 2005; Pernak et al., 2005).
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Ionic Liquids in Separation of Metal Ions from Aqueous Solutions 377
3. Liquid-liquid extraction
3.1 Imidazolium ILs
3.1.1 Extraction systems
Ionic liquids have become widely used as solvents for organic reactions, however their use
as solvents in extraction systems seems much promising. Separation with imidazolium ILs is
best described and broadly investigated. In most cases ILs replace typical solvent extraction
diluents. The most frequently applied are 1-alkyl-3-methylimidazolium
hexafluorophosphate [Cnmim][PF6], tetrafluoroborate [Cnmim][BF4] and bis(trifluoromethyl-
sulphonyl)imide [Cnmim][N(SO2CF3)2] (anion also abbreviated as [Tf2N]) (Table 2).
As shown in Table 2, alkali metals, UO22+, Cs+, Sr2+ and lanthanides are most frequently
extracted with imidazolium ILs. In most cases, extraction of metal ions into the hydrophobic
ionic liquid phase is insignificant because metal cations are strongly hydrated in the
aqueous phase and affinity of the IL phase to the aqueous one is too small. Thus, an
extractant or ligand must be applied, which is a substance that, when diluted in IL, forms
complexes with metal ions increasing their hydrophobicity and facilitating their transport to
the IL phase. Examples of such ligands are the following macrocyclic compounds:
pyridinecalix-4-arene, 18-crown-6 ether (18C6) or dicyclohexano-18-crown-6 (DCH18C6),
industrial extractants - TBP (tributyl phosphate), CMPO (octyl(phenyl)-N,N-
diisobutylcarbamoylmethylphosphine oxide), PAN (1-(2-pyridylazo)-2-naphthol), TAN (1-
(2-thiazolyl)-2-naphthol) or neutral - TODGA (N,N,N’,N’-tetra(octyl)diglycolamide) or
DEHEHP (di(2-ethylhexyl)2-ethylhexyl phosphonate) (Table 2). Significant improvement in
extraction efficiency of metal cations has been achieved when [Cnmim][Tf2N] replaces
molecular solvents, such as chloroform, dodecane or 1-octanol, in pyridinecalix-4-arene
(extraction of Ag+) (Shimojo & Goto, 2004), in CMPO – extraction of Ce3+, Eu3+, Y3+
(Nakashima et al., 2003, 2005) – in PAN and TAN (extraction of Hg2+) (Visser et al., 2001b)
by [Cnmim][PF6], and in crown ethers 18C6 and DCH18C6 (extraction of alkali metals and
Sr2+) (Dai et al., 1999; Dietz & Stepinski, 2005; Jensen et al., 2002; Luo et al., 2004a; Stepinski
et al., 2005). However, not always the presence of ILs in the organic phase instead of
conventional solvents increases the partitioning of the species to be extracted. For example,
uranium extraction into dodecane is more efficient than into [Cnmim][Tf2N] (Dietz &
Stepinski, 2008).
It is reported that Cs+ extraction with crown ethers or calixarene tends to increase with
shortening of the alkyl chain in IL cation (Luo et al., 2004b). However, a compromise should
be made between the extraction efficiency and the solubility of ligand in IL. In other words,
the shorter the alkyl chain the lower the solubility of a hydrophobic calixarene in the IL
phase.
Furthermore, sodium extraction with DCH18C6 increases in the presence of [Cnmim][Tf2N]
compared with 1-octanol (Table 3). However, it is noted that the partitioning of Na+ is
strongly affected by stereochemistry of the crown ether applied, which was not observed in
conventional solvents (Dietz et al., 2008). In general, the mechanism of extraction can be
tuned by changing the isomeric form of the extractant. The presence of trans isomers of
DCH18C6 make the neutral complex extraction dominate over the ion-exchange
mechanism. This mechanism is more environmentally friendly because of no release of IL to
the aqueous phase.
Papaiconomou et al. (2008) investigated extraction of Cu2+, Hg2+, Ag+ and Pd2+ from
aqueous chloride solutions at pH 7 with ten imidazolium, pyridinium, piperidinium and
pyrrolidinium ionic liquids comprising typical anions, i.e., [BF4-], [Tf2N-], trifluoromethyl
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378 Applications of Ionic Liquids in Science and Technology
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Ionic Liquids in Separation of Metal Ions from Aqueous Solutions 379
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380 Applications of Ionic Liquids in Science and Technology
sulphonate [TfO-] or nonafluorobutyl sulphonate [NfO-]. Only mercury has been efficiently
extracted (D > 24) with imidazolium and pyridinium ionic liquids, while the other metal
cations are not transferred to the IL phase.
Moreover, good extraction abilities of imidazolium ILs with [NfO-] have been confirmed in
the studies on Li+, Na+, Cs+, Ca2+, Sr2+ and La3+ extraction (Kozonoi & Ikeda, 2007).
According to the authors the metal ions with greater charge are more easily transferred to
the [Cnmim][NfO] phase.
Another point of view is represented by Wei et al. (2003a, 2003b) and Domanska & Rekawek
(2009). They propose to complex metal ions in the aqueous phase with dithizone, 8-
hydroxyquinoline or 1-(2-pyridylazo)-2-naphthol and next, to extract such metal complexes
with imidazolium ILs. The [Cneim] ILs (Domanska & Rekawek, 2009) show better extraction
efficiency of Ag+ and Pb2+ than chloroform, however, it decreases with increasing alkyl
chain length in the cation and with increasing hydrophobicity of an anion (i.e., [Tf2N-],
[PF6-]). The extraction with [C4mim][PF6] (Wei et al. 2003a, 2003b) of various metal ions (e.g.,
Ag+, Cu2+, Pb2+, Cd2+ and Zn2+) is strongly dependent on pH and allows selective separation
of Cu2+ from Pb2+ and Zn2+ at pH 2 and from Cd2+ at pH 1.9. Furthermore, Ag+ is selectively
separated from Pb2+ also at pH 1.9. The dependence on pH is advantageous for stripping
because metal ion can be stripped from the loaded IL phase by dissociation of metal-
dithizone complex with acid solution. 0.1 M HNO3 is used to regenerate IL and the
reproducibility of extraction is confirmed in five cycles of recycling (extraction-stripping).
Not always an additional ligand is necessary to extract metal ions. For example, Zn2+ and
Fe3+ can be transported directly to [C8mim][BF4] phase. The extraction tends to increase in
the following order: [Tf2N-]<[PF6-]<[BF4-] that corresponds to decreasing hydrophobicity of
the anions studied (Perez de los Rios et al., 2010). In parallel, the same authors observed
increasing extraction efficiency with lengthening of the alkyl chain of imidazolium cation
contrary to Cs+ extraction studied by Luo et al. (2004b). Similarly, efficient extraction of Ce4+
from HNO3 is shown in pure [C8mim][PF6] (Zuo et al., 2008). However, its application for
the recovery of Ce4+ from bastnasite leaching liquor, containing Th4+ and rare earth metals
(RE), is limited by the presence of F- that negatively affects extraction efficiency. Because of
this, neutral extractant (DEHEHP) has been added to [C8mim][PF6] to overcome this
problem and the extraction efficiency of metal ions is compared to that in the traditional
DEHEHP-heptane system (Table 3) (Zuo et al., 2009). The selectivity of extraction in both IL
and heptane systems can be ordered as follows: Ce4+ > Th4+ > RE3+.
The higher capacity for Ce4+ of DEHEHP-[C8mim]PF6 than of DEHEHP-heptane indicates
that both DEHEHP and [C8mim]PF6 may act as extractants. The mechanism of extraction is
presented in section 3.2.2.
Further, ILs dedicated to very specific extractions have been synthesized and called task
specific ionic liquids (TSILs). Elimination of the use of additional extractant or ligand from
the organic phase can be pointed out as a consequence of imidazolium cation modification
with other compounds. However, according to Abbot et al. (2011), the term TSIL should be
changed into ‘functionalised ILs’, as now most of ILs are designed and synthesised for a
dedicated application.
N-(3-butylimidazolium propyl)aza-18-crown-6 ether bis[(trifluoromethyl)sulphonyl]imide
[C4mim18C6][Tf2N] illustrates TSIL with the IL-cation structure modified with aza-crown
ether via covalent bonds (Luo et al., 2006a). Another type of TSIL, based on thioether,
thiourea and urea derivatives, are involved as carriers in Hg2+ and Cd2+ liquid-liquid
extraction (Visser et al., 2001c, 2002b). Correspondingly, ILs composed of a functional
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Ionic Liquids in Separation of Metal Ions from Aqueous Solutions 381
Conventional
IL system D or E D Ref.
solvent system
0.1 M in[C4mim][Tf2N] 0.1 M in
[diC2hist18C6][Tf2N] DCs=14.5 [C4mim][Tf2N] Cs+=25.7 Luo et al.,
DSr=447 N-octylaza18C6 DSr=1070 2006a; Luo
[C4mim18C6][Tf2N] DCs=23.9 Cs+=380 et al., 2006b
DSr=213 DCH18C6 DSr=935
Luo et al.,
DCs=576
8 mM BOBCalixC6 in 0.01 M BOBCalixC6 DCs 2004b;
DK=8.4
[C4mim][Tf2N] in 1,2-dichloroethane negligible Haverlock
DSr=0
et al., 2000
0.15 M DCH18C6 in 0.15 M DCH18C6 in
Dai et al.,
[C2mim][Tf2N] DSr=1100 toluene DSr=0.76
1999
[C4mim][PF6] DSr=2.4 chloroform DSr=0.77
0.1 M DCH18C6 in
0.1 M DCH18C6 in Dietz et al.,
[C5mim] [Tf2N] DSr ~8
1-octanol DSr=1 2003
[C10mim][Tf2N] DSr ~4
in 1 M
0.1 M DCH18C6 in in 1 M Dietz &
HNO3 0.1 M DCH18C6 in
[C5mim] [Tf2N] HNO3 Stepinski,
DNa~0.11 1-octanol
[C10mim][Tf2N] DNa ~0.06 2005
DNa ~0.1
0.1 M CMPO in 0.1 M CMPO in Visser &
DUO2=1000 DUO2=100
[C4mim][PF6] dodecane Rogers,
0.6 mM TODGA in 5 mM TODGA in Shimojo et
DLa =100 DLa=0.01
[C2mim][Tf2N] isooctane al., 2008
ECe(IV)=99% ECe(IV)=96%
0.34 M DEHEHP in 0.34 M DEHEHP in Zuo et al.,
ECe(III)=2% ECe(III)=2%
[C8mim][PF6] heptane 2009
ETh=49% ETh=30%
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382 Applications of Ionic Liquids in Science and Technology
[C4mim][Tf2N]. The selectivity of Sr2+/Cs+ can be tuned by the choice of IL anion. Sr2+
extraction prevails over that of Cs+ with increasing anion hydrophobicity for [C4mim] cation
(Luo et al., 2006b).
The synergism observed in the presence of ILs is attributed to ion-recognition capabilities of
complexing ligands, unique ionic solvation environment and ion-exchange capabilities of
ILs (Luo et al., 2006a).
Reaction Ref.
Ln(3w) 3TODGA( o ) 3[C nmim ]( o )
3
3[C nmim ]( o )
Shimojo et
LnTODGA3( o) (1)
al., 2008
Ag(w ) t Bu[4]CH 2 Py( o ) [C 8mim ]( o ) Ag t Bu[4]CH 2 Py(o ) [C 8mim ]( w ) (2)
Shimojo &
Goto, 2004
Dietz &
Sr(2w) CE( o ) 2[C 5mim ]( o ) Sr (CE)(2o) 2[C 5mim ]( w )
Dzielawa,
(3)
2001; Jensen et
al., 2002
M(3w) 3CMPO( o ) 3[C 4 mim ]( o ) M(CMPO )33(o ) 3[C 4 mim ]( w )
Nakashima et
(4)
al., 2005
Ln(3w) Htta( o ) [C 4 mim][Tf 2 N ]( o ) [C 4 mim][ Ln(tta)4 ]( o ) 4 H(w ) [Tf 2 N ]( Jensen et al.,
(5)
2003
Ce(4w) 6 NO3(
w ) 2[C 8 mim ][ PF4 ]( o ) [C 8 mim ]2 [Ce( NO3 )6 ]( o ) 2[ PF6 ]( w )
Zuo et al.,
(6)
2008
Ce(4w) 4 NO3(
w ) HF( w ) DEHEHP( o ) Ce( HF )( NO3 )4 DEHEHP( o )
Zuo et al.,
(7)
2009
Bu[4]CH2Py - pyridinecalix-4-arene; CE – crown ether; (w) and (o) denote aqueous and organic phase.
t
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Ionic Liquids in Separation of Metal Ions from Aqueous Solutions 383
Equations (2)-(4) can be combined into one equation of extraction according to cation-
exchange mechanism, where L is a ligand complexing metal cation in the organic phase:
Metal cation binds the ligand contained in the organic phase, and then it is exchanged for
imidazolium cation [Cnmim+] of the ionic liquid. When metal cation becomes, as a result of
extraction, a part of ionic liquid and is strongly bound in the organic phase, its stripping is
very difficult. Additionally, the loss of IL cation to the aqueous phase is not advantageous
for high cost and environmental impact (Dietz, 2006). However, the change in the Sr2+ and
Cs+ extraction mechanism from cation-exchange to extraction of neutral complexes,
observed with increasing alkyl chain in imidazolium cation (Dietz et al., 2003), causes a
reduction in IL loss to the aqueous phase. Alternatively, to overcome the loss of IL cation
Luo et al. (2004b) have proposed addition of organophilic species (NaBPh4) to control a
transfer of imidazolium cations to the aqueous phase. Its addition decreases the loss of IL
cation by 24%.
The knowledge of extraction mechanism with participation of ILs has been continuously
extended. Recently, Dietz & Stepinski (2005) have reported a complex extraction process of
Na+ from nitrate solution with crown ether in [Cnmim][Tf2N], described by a combination of
three processes:
sodium nitrato-crown ether complex partitioning:
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384 Applications of Ionic Liquids in Science and Technology
equilibrium of reactions (5-7). At the same time pure cerium as CeF3 nano-particles or
Ce2(SO4)3 solutions are obtained as products after regeneration of the [C8mim][PF6] phase.
However, it is proven by Rickert et al. (2007) that the presence of certain solutes (e.g., crown
ethers) in a hydrophobic IL, even in the absence of metal ion in the extraction system, can
significantly increase the solubility of the ionic liquid in acidic aqueous media. Thus, it is
still an open question whether they are environmentally friendly and can replace traditional
organic solvents.
Apart from small amounts of synthesised ILs and their solubility in the aqueous phase, there
is one more issue that must be indicated as limiting the ILs use for liquid-liquid extraction,
which is their hydrolysis in the contact with an acidic solution (Swatloski et al., 2003). It is
particularly risky to use [PF6-] containing ILs because the decomposition reaction of the
anion leads to a toxic and corrosive product HF. Additionally, during acidic reactions gas
HF may be released. Thus, Swatloski et al. (2003) propose to consider the list of non-toxic
pharmaceutically acceptable anions when designing ILs as solvents for extraction.
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Ionic Liquids in Separation of Metal Ions from Aqueous Solutions 385
chloride, respectively. [A336][Sal] efficiently extracts Fe3+ and Cu2+ (99 and 89%). The
mechanism proposed for the extraction of Fe3+ may be represented by equation (12) given in
Table 5.
Reaction Ref.
Fe(3w) 2[ A336][ HSal ]( o ) HSO4(
w ) [ A336][ FeSal2 ]( o ) Egorov et
[ A336][ HSO4 ]( o ) 2 H(w )
(12)
al., 2010
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386 Applications of Ionic Liquids in Science and Technology
CH3
H3C
CH3
C14H29 H3C
C14H29 O
Cl- P+ C6H13 -
P CH3
P+ O
CH3
C6H13 C6H13 C6H13
CH3
C6H13
C6H13 CH3
Cyphos IL 104,
Cyphos IL 101,
Trihexyl(tetradecyl)phosphonium bis(2,4,4-
Trihexyl(tetradecyl)phosphonium chloride
trimethylpentyl)phosphinate
[QP][Cl]
[QP][Bis]
O O
S BF4- C14H29
C14H29
CF3
-N
CF3 P+
P+
C6H13 S
H13C6 C6H13
C6H13
C6H13 O O C6H13
Cyphos IL 109,
Cyphos IL 111,
Trihexyl(tetradecyl)phosphonium
Trihexyl(tetradecyl)phosphonium
bis(trifluoromethylsulphonyl)imide
tetrafluoroborate
[QP][Tf2N]
[QP][BF4]
Reaction Ref.
[QP ][Cl ]( o ) [QP ][ FeCl ]( o ) Cl(w )
Kogelnig et
FeCl4( w) (16)
al., 2010
Na(w ) TcO4(
w ) CE( o ) Na CE TcO4( o )
Stepinski et
(17)
al., 2010
Table 7. Reactions of metal ions with phosphonium Ils.
Recently, phosphonium ILs have been reported as solvents for DCH18C6 to extract TcO4-
and ReO4- from NaOH and/or NH4OH solutions (Stepinski et al., 2010). The use of
[QP][Tf2N] ensures the highest distribution of TcO4- both in the presence and absence of a
crown ether and prefers extraction of TcO4- over ReO4-. The most important is that, unlike
imidazolium ILs, [QP][Tf2N] extracts without IL loss (negligible amounts of [Tf2N-]
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Ionic Liquids in Separation of Metal Ions from Aqueous Solutions 387
determined) to the aqueous phase. Owing to this, it has been suggested by Stepinski et al.
that ‘these solvents may provide the basis for improved approaches to the extraction and
recovery of a variety of anions’. The dominant extraction mechanism is the ion pair transfer
according to eq. (17) included in Table 7. The enhancement in TcO4- extraction with crown
ether, compared with conventional solvents, is attributed to improvement in the solvation
properties of IL.
The authors of the chapter have studied Cyphos ILs as extractants for regeneration of spent
pickling solutions from hot-dip galvanizing plants (Marszalkowska et al., 2010; Nowak et
al., 2010; Regel-Rosocka et al. 2006, 2007; Regel-Rosocka, 2009, 2010).
Among the studied extractants trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101),
trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104),
trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulphonyl)imide (Cyphos IL 109), and
trihexyl(tetradecyl)phosphonium tetrafluoroborate (Cyphos IL 111) in mixture with toluene
have been investigated as reagents to extract Zn2+, Fe3+ or Fe2+ from chloride media. Toluene
has been applied to overcome some drawbacks caused by the high viscosity of ILs. In some
cases alkylene carbonates (propylene or butylene carbonate) as novel diluents in extraction
of Zn2+ and Fe3+ have been used replacing toluene. In addition, TBP has been used to modify
the organic phase properties.
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388 Applications of Ionic Liquids in Science and Technology
40
IL 101 in toluene
30
IL 104 in toluene
3
[Zn]*o, g/dm
IL 109 in TBP
IL 109 in toluene
20
10
0
0 10 20 30 40 50 60 70 80 90
3
[Zn]*w, g/dm
100
80
60
EZn, %
40
20
0
100% 50% 50% 50% 50%
IL101 IL101 IL109 IL109+TBP IL104
Fig. 3. Percentage extraction of Zn2+ with various phosphonium ILs from feed containing
0.58 M HCl () and without HCl () (Regel-Rosocka et al., 2006).
[QP][Cl] is selected as the most effective extractant among the phosphonium ILs studied
(Figs. 2 and 3). Extraction equilibrium is achieved in 5 minutes. It transfers more than 95% of
Zn2+, and up to 80% Fe2+ from the individual metal ion solutions. Twofold molar excess of
the extractant over Zn2+ is necessary for efficient extraction (100%). Moreover, Zn2+
extraction is preferred over Fe2+ when both are present in a mixture. The kinetics of both
Zn2+ and Fe2+ extraction is very fast, and can be successfully applied to separate Zn2+ from
Fe2+ when Zn2+ exceeds Fe2+ content in the feed. The presence of HCl in the feed enhances
Zn2+ extraction (Fig. 3). The following reactions of Zn2+ extraction mechanism are proposed:
ZnCl 24(w) 2[QP ][Cl ]( o ) [QP ]2 [ZnCl4 ]( o ) 2Cl(w)
(19)
Moreover, the studies on stripping and regeneration of IL phase have revealed that
sulphuric acid is the best stripping solution from among those studied. The ability to reuse
the [QP][Cl]/toluene mixture in several cycles of Zn2+ extraction-stripping has been proven.
However, Zn2+ recovery from the organic phase needs three steps (Regel-Rosocka, 2009).
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Ionic Liquids in Separation of Metal Ions from Aqueous Solutions 389
60
50
40
0.1 M HCl
30 3 M HCl
20
10
0
0 5 10 15 20 25 30
Time, min
Fig. 4. Effect of contact time on Pd2+ extraction with [QP][Cl]: (feed: 5 mM Pd2+, 0.1 or 0.3 M
HCl; organic: 5 mM [QP][Cl] in toluene) (Cieszynska & Wisniewski, 2010).
0.006
0.005
0.004
[Pd]o, mol/L
0.003
0.002
0.001
0.000
0.000 0.001 0.002 0.003 0.004 0.005
[Pd]a, mol/L
Fig. 5. The isotherms of Pd2+ extraction from 0.1 (■,) and 3 M HCl (▲, ) with [QP][Cl] (■,
▲) and [QP][Bis] (, ); (feed: 1 – 8 mM Pd2+; organic: 5 mM [QP][Cl] in toluene)
(Cieszynska, 2010; Cieszynska & Wisniewski, 2011).
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390 Applications of Ionic Liquids in Science and Technology
As a result of our investigation, the ion exchange mechanism has been proposed to describe
the extraction of Pd2+ from 0.1 and 3 M HCl:
On this basis, [QP][PdCl3] and [QP]2[PdCl4] are proposed as ion pairs formed in the organic
phase by Pd2+. Consequently, the ionic bonds in the IL phase are very strong, which is a
drawback when stripping the metal ions from the loaded IL with water. Different stripping
phases have been examined (Table 8). Ammonia solution is found to be an effective one,
resulting in almost 100% Pd2+ stripped from the loaded IL in one step.
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Ionic Liquids in Separation of Metal Ions from Aqueous Solutions 391
[QP][Cl] [QP][Bis]
Aqueous phase
DCo(II) DNi(II) SCo(II)/Ni(II) DCo(II) DNi(II) SCo(II)/Ni(II)
Ni + Co; without HCl 0.15 0.05 3.00 20.4 0.29 70.4
Ni + Co; 0.1 M HCl 0.15 0.05 3.00 0.18 0.03 6.00
Ni + Co; 0.5 M HCl 0.18 0.19 0.95 0.19 0.00 0.00
Table 9. Distribution coefficients of metal ion between organic and aqueous phase and the
selectivity of Co2+ over Ni2+
An increase in HCl concentration in the feed aqueous phases containing metal ions,
negatively influences the extraction efficiency and selectivity (shown in Table 9). The
stripping efficiency of Co2+ decreases in the following order of stripping phases: 2 M H2SO4
(97%) > 0.5 M HCl (96%) > 0.25 M H2SO4 (84%) > 4 M HCl (61%) > H2O (39%), and for Ni2+:
2 M H2SO4 (65%) > 0.25 M H2SO4 (55%) > distilled water (39%), while HCl does not strip
Ni2+ ions from the loaded organic phase. On this basis sulphuric acid is selected as the most
efficient stripping phase.
Spectrophotometric analysis allows the coordination of Co2+ complexes formed in the
presence of chlorides to be determined and indicates a change from an octahedral complex
in the aqueous phase into tetrahedral one, existing in the organic phase (Ma et al., 2008).
[QP][Bis] is very selective for Co2+ over Ni2+ in the aqueous solutions without HCl (ECo(II) =
95% at pH 6). The same transformation of metal complex coordination is observed during
extraction from a mixture of metal ions.
The proposed equations describing Co2+ extraction mechanism are as follows:
Nevertheless, further studies should be carried out on the mechanism of Co2+ extraction
with [QP][Bis]. It is known that CoCl42- is not stable in the aqueous phase, so the extraction
could proceed in another way.
4. Conclusions
Ionic liquids, considered as ‘green solvents’, have been studied as potential solvents or
carriers of metal ions in liquid-liquid extraction. Although recently their ‘greenness’ is
questioned because of possible hydrolysis with formation of toxic HF or partial loss to the
aqueous phase, they are still interesting and important compounds in metal processing.
Further, mutual solubility of imidazolium ILs and aqueous phase, i.e., problems with
extraction but also (when the solubility is too high) with loss of ILs must be pointed out as
their limitations. Hence, a compromise between IL hydrophobicity and its extraction power
must be achieved. Their potential lies in improvement of extraction efficiency, vast
possibilities to design ‘functionalised ILs’, significant reduction in the volume of the low
concentration aqueous streams and process intensification.
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392 Applications of Ionic Liquids in Science and Technology
Looking at literature data, it is obvious that imidazolium ILs are the best described and
applied for a variety of metal ion systems. However, research on extraction with ammonium
and phosphonium ILs has been developed in the last five years, and indicates successful
application of some of them for separation of metal ions. There are still plenty of research to
be pursued in the field of separation with phosphonium and new ammonium ILs, to
describe their extraction behaviour, mechanism of metal ion, water and other species
transfer to the organic phase. Additionally, their stability and regeneration in separation
processes, particularly those operating in strongly acidic or basic solutions, should be
investigated.
The mechanism of extraction with ILs in most cases differs from that in conventional
solvents, and seems to be more complex. ILs prefer extraction of charged species and, as a
result, most metal ions are transported to the IL phase according to the cation or anion-
exchange mechanism. It can be attributed to the unique ionic solvation environment and
the ion-exchange capabilities of ILs that influence their specific extraction behaviour.
Finally, regeneration of ILs is recently highlighted as an important issue affecting their
greenness. Not only stripping with various solutions but pH change and even
electrowinning of metal ions are proposed. Nonetheless, it seems that ILs will be rather
applied for special separation processes, but not on a large scale.
5. Acknowledgment
We thank Anna Cieszynska for an agreement to present some results from her doctoral
thesis. We thank Cytec Industries Inc. for providing us with free samples of Cyphos ILs.
This work was supported by the grant No. 32/067/11/DS.
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Applications of Ionic Liquids in Science and Technology
Edited by Prof. Scott Handy
ISBN 978-953-307-605-8
Hard cover, 516 pages
Publisher InTech
Published online 22, September, 2011
Published in print edition September, 2011
This volume, of a two volume set on ionic liquids, focuses on the applications of ionic liquids in a growing range
of areas. Throughout the 1990s, it seemed that most of the attention in the area of ionic liquids applications
was directed toward their use as solvents for organic and transition-metal-catalyzed reactions. Certainly, this
interest continues on to the present date, but the most innovative uses of ionic liquids span a much more
diverse field than just synthesis. Some of the main topics of coverage include the application of RTILs in
various electronic applications (batteries, capacitors, and light-emitting materials), polymers (synthesis and
functionalization), nanomaterials (synthesis and stabilization), and separations. More unusual applications can
be noted in the fields of biomass utilization, spectroscopy, optics, lubricants, fuels, and refrigerants. It is hoped
that the diversity of this volume will serve as an inspiration for even further advances in the use of RTILs.
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