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631 
CHEMICAL ENGINEERING TRANSACTIONS Volume 21, 2010 

Editor J. J. Klemeš, H. L. Lam, P. S. Varbanov  

Copyright © 2010, AIDIC Servizi S.r.l., ISBN 978-88-95608-05-1 ISSN 1974-9791 

DOI: 10.3303/CET1021106 

 

Please cite this article as: Hernandez Fernandez F. J., Perez de los Rios A., Ginesta Anzola A., Sanchez Segado S., Lozano Blanco 

L. J. ,  Moreno Sánchez J. I.  and Godinez Seoane C., (2010), Use of ionic liquids as ‘green’ solvents for extraction of Zn
2+

, Cd
2+

, Fe
3+

 

and Cu
2+

 from aqueous solutions., Chemical Engineering Transactions, 21, 631-636, DOI: 10.3303/CET1021106. 

 

Use of ionic liquids as green solvents for extraction of Zn
2+

, 

Cd
2+, 

Fe
3+

 and Cu
2+

 from aqueous solutions 

Francisco José Hernández-Fernández *, Antonia Pérez de los Ríos*, A. Ginestá, Sergio 

Sánchez-Segado, Luis Javier Lozano, Juan Ignacio Moreno, C. Godínez 

Technical University of Cartagena, Dep. Chemical and Environmental Engineering, 

Campus Muralla del Mar,  
 C/ Doctor Fleming S/N, E-30202 Cartagena, Murcia, Spain 

fjherfer@upct.es 

 

This work analyzed the extraction of metal ions from aqueous solutions using ionic 

liquids in the absence of chelating agents. For this purpose, hydrochloride solutions of 

Zn
2+

, Cd
2+

,  Cu
2+

and Fe
3+ 

were used as model systems and ionic liquids based on 1-n-

alkyl-3-methylimidazolium and tetraalkyl ammonium cations combined with several 

anions (hexafluorophosphate, bis{(trifluoromethyl)sulfonyl}imide, tetrafluoroborate, 

and chloride) were used as extraction agents. It was found that the ionic liquid 

methyltrioctylammonium chloride, [MTOA
+
][Cl

-
], allowed almost the nearly complete 

removal of Zn
2+

, Cd
2+ 

and Fe
3+ 

and an extraction percentage around 80 % for Cu
2+

, from 

the aqueous solutions. Furthermore, efficient selective separation of Zn
2+

 and Cd
2+

 from 

Fe
3+ 

and  Cu
2+

 was achieved using 1-methyl-3-octylimidazolium tetrafluoroborate, 

[omim
+
][BF4

-
], since high extraction percentages were reached for Zn

2+ 
and Cd

2+
 (%E > 

90) while this parameter is very low for Fe
3+

 and  Cu
2+

. These results indicate that the 

use of ionic liquids as an alternative to traditional extraction agents in liquid/liquid 

extraction of heavy metal ions is very promising. 

1. Introduction 

The rapid increase in the use of heavy metals over the past few decades has inevitably 

involved an increased flux of metallic substances into natural water bodies or into soils. 

The presence of traces of heavy metals in the environment is particularly dangerous, 

since most of them are toxic and persistent. Several technologies can be used to remove 

toxic metals from liquid effluents, including precipitation, solvent extraction and ion 

exchange (Alonso et al., 2006; Lozano and Godínez, 2003). Among these technologies, 

solvent extraction is widely used in the recovery and separation of metals from aqueous 

solutions, where the extraction agent (di(2-ethylhexyl) phosphoric acid, tris(2-

ethylhexyl)amine, liquid phosphine oxides, etc.) is dissolved in an organic solvent 

(kerosene, toluene, etc.) which is used as the diluent.
 
One disadvantage of solvent 

extraction is the loss of organic diluent via volatilisation, which has a detrimental 

impact on the environment and human health. Consequently, greener extraction 



632 

 

methods are being sought. The use of ionic liquids (ILs) could overcome this 

disadvantage. ILs are organic salts that are liquids over a temperature range around or 

below room temperature and normally consist of an organic cation, the most common 

used being dialkylimidazolium, N-alkylpyrindinium and tetraalkylammomium salts, and 

a polyatomic inorganic anion such as halides, [BF4
-
],  [PF6

−
], [NTf2

−
]. From an 

environmental point of view, the most important advantage of ILs is their non-

detectable vapour pressure, which makes them environmentally benign solvents 

compared with volatile organic solvents (VOCs). To date, ILs have recently been 

revealed as interesting clean alternatives to classical organic solvents in a wide range of 

processes (Ruiz et al., 2007; Hernández-Fernández et al., 2007) Over the last few years, 

extraction of several metal ions using ILs containing proper complexing agents such as 

crown ethers, Cyanex 923, dithizone and other organic ligands (Heitzman et al., 2006)  

has been carried out. 

The present work aims to study the extraction of Zn
2+

, Cd
2+

, Fe
3+ 

and Cu
2+ 

from 

hydrochloride aqueous solutions with ionic liquids in the absence of a chelating agent. 

For that, six ionic liquids with different cations (1-n-alkyl-3-methylimidazolium and 

tetraalkylamommium) and anions (bis{(trifluoromethyl)sulfonyl}imide, 

hexafluorophosphate, tetrafluoroborate and chloride)  were tested. The influence of the 

ionic liquid composition and the metal ion concentrations on the percentage extraction 

(% E) were evaluated. 

2. Experimental 

2.1. Chemicals  
The ionic liquids  1-butyl-3-methylimidazolium tetrafluoroborate [bmim

+
][BF4

-
] (purity 

>99 %), [bmim
+
][PF6

-
] (purity >99 %), 1-octyl-3-methylimidazolium 

hexafluorophosphate [omim
+
][PF6

-
] (purity >99 %), 1-butyl-3-methylimidazolium 

bis{(trifluoromethyl)sulfonyl}imide, [bmim
+
][NTf2

-
] (purity >99 %)  and 1-octyl-3-

methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [omim
+
][NTf2

-
] (purity  >99 

%)  were purchased from Iolitec GmbH (Denzlingen, Germany) and methyl trioctyl 

ammonium chloride (purity >97 %) was from Sigma-Aldrich-Fluka Chemical Co. 

(Madrid, Spain). The ILs were dried under anhydrous phosphorous pentoxide in vacuo 

and were stored in a desiccator to ensure a long-term stability of ionic liquids. Water 

content of the ionic liquids was measured by Karl Fischer tritation, been in all cases 

lower than 150 ppm. The solubility of the different ILs in the water was determined by 

placing, 2 mL of each phase in a test tube. The mixture was vigorously stirred, at room 

temperature for 10 min. To assure a better phase separation, the mixture was then 

centrifuged for 10 min at 5.000 rpm. Both phases were collected and the concentration 

of water in the IL was measured using a Karl-Fisher titrator. The stock aqueous 

solutions of metals were prepared by dissolving analytical grade chloride salts of the 

respective metals (from Sigma-Aldrich-Fluka Chemical Co, Madrid, Spain) in 

hydrochloric acid. 



633 

 

2.2. Extraction of Zn(II), Cd(II), Cu(II) and Fe(III) from hydrochloride aqueous 
solutions  

Zn(II), Cd(II),  Cu(II) and Fe(III) hydrochloride aqueous solutions were made up by 

dissolving solid ZnCl2, CdCl2, CuCl2 and FeCl3·6 H2O and in 1 M hydrochloric acid. 1 

mL of a metal aqueous solution containing a known concentration of metal ion was 

brought into contact with 1 mL of pure IL. The phase-contacting experiments were 

carried out in carefully stoppered centrifuge tubes. The mixture was shaken vigorously 

for 5 min to facilitate the transfer of compounds into the ionic liquid phase and 

centrifuged before being left at 303.15 K for 30 minutes in a thermostatic bath without 

stirring. Samples of the aqueous phase were taken from the tubes during at least three 

sampling events and the composition of the aqueous phase was analysed by atomic 

absorption spectrophotometry, as described in Section 2.3. Sampling ceased when the 

metal ion concentration stabilized. The efficiency of the extraction process was 

evaluated by the extraction percentage (% E), which was calculated by the following 

equation: 

% 100
)()(

)(





tHtIL

tIL

CC

C
E  (2) 

where CIL and CH(t) refer to the equilibrium concentration of the compounds in the ionic 

liquid and in the aqueous phases, respectively. The concentration of metal ions in the IL 

phase was deduced from the difference between the initial concentration of metal ions 

in the aqueous phase and the concentration of metal ions in the raffinate. It should be 

noted that the ILs employed in this work are immiscible with the aqueous phase. 

Determinations were made in triplicate to ensure the repeatability of the tests and mean 

values are reported. The repeatability of the assay, as measured by the relative standard 

deviation, was 3 % or less. 

2.3. Analytical method 
The uptake of metal ions was monitored by removing samples of the aqueous solutions 

periodically for analysis. Atomic absorption spectrophotometry (Varian spectra AA 

model) was used for the determination of metals (Zn, Cd, Cu and Fe). All aqueous 

solutions were prepared using deionised water.  

3. Results and discussion 

With the aim of evaluating the extraction of Zn(II), Cd(II), Fe(III) and Cu(II) by using 

ionic liquids, the extraction percentage values (%E) for 0.1 g/L of these metal ions from 

1M hydrochloride solutions at 303.15 K were determined. For this purpose, six different 

ionic liquids [MTOA
+
][Cl

-
], [omim

+
][BF4

-
], [omim

+
][PF6

-
], [bmim

+
][PF6

-
], 

[bmim
+
][NTf2

-
] and [omim

+
][NTf2

-
] were chosen on the basis of their low solubility in 

water, less than 1.4 %. The extraction percentage values (%E) for using the different ILs 

are presented in Figure 1. 



634 

 

 

 

Figure 1: Extraction of 0.1 g/L Zn(II), 0.1 g/L Cd(II),  0.1 g/L Cu(II),  and 0.1 g/L 

Fe(III) from 1M hydrochloride solutions using [MTOA
+
][Cl

-
], [omim

+
][BF4

-
], 

[omim
+
][PF6

-
], [bmim

+
][PF6

-
], [bmim

+
][NTf2

-
] and [omim

+
][NTf2

-
] ionic liquids. 

It is worthy of note that nearly complete removal of metal ions was achieved using 

[MTOA
+
][Cl

-
] for  Zn

2+
, Cd

2+ 
and Fe

3+
 and around 80% for Cu

2+
 . The ionic liquid 

[omim
+
][BF4

-
] also allows the quantitative extraction of Zn

2+
 and Cd

2+
. In contrast, non-

extraction of  Fe
3+

 and Cu
2+

 was observed by using the latter ionic liquid. Regarding the 

selective recovery of metal ions, important differences in the extraction percentage 

between Zn
2+

,Cd
2+ 

/ Cu
2+

,Fe
3+

 in [omim
+
][BF4

-
] suggesting that these ionic liquids could 

be used for selective separation of these metal ions (see Figure 1). High extraction 

efficiencies for Cd
2+ 

and less for Cu
2+

, Zn
2+ 

and Fe
3+

 were also reached using  

[omim
+
][PF6

-
]. With the rest of ionic liquids ([bmim

+
][PF6

-
], [bmim

+
][NTf2

-
],   

[omim
+
][NTf2

-
]), the maximum extraction percentages was obtained for Cd

2+ 
 which 

was around 25 %, being decreasing for the rest of ion metal as follows: Fe
3+

 >Cu
2+

 > 

Zn
2+

. Almost non extraction of Zn
2+ 

was found using [bmim
+
][PF6

-
], [bmim

+
][NTf2

-
] 

and [omim
+
][NTf2

-
]. This fact allowed the selective separation of  Zn

2+
 from Fe

3+
 , Cu

2+
 

and Cd
2+

,  by using the latest ionic liquids. The above results have been analysed in 

order to evaluate the influence of the cation and anion composition of the ionic liquid on 

the recovery of metal ions. By comparing cation and anion effects in ionic liquid based 

on imidazolium cations, the greatest effect on the extraction percentage was observed 

by changing the nature of the anion of the ionic liquid. It can be concluded from the 

above results that it is possible to tailor ionic liquids for using as extraction agents for a 

specific metal ion separation. 

The effect of initial metal ion concentration, from 0.1 g/L to 5 g/L, in the aqueous 

solution on the efficiency of the extraction process for Zn
2+

, Cd
2+

, Fe
3+

 and Cu
2+ 

was 

also studied. It was observed  that  the extraction percentage of Zn
+2

 slightly decreases 

when metal ion concentrations increases when [MTOA
+
][Cl

-
] and [omim

+
][BF4

-
] were 

used as extraction agents. This effect is prominent for [omim
+
][PF6

-
]. The rest of ionic 

liquids were not suitable for the extraction of Zn
2+

 in all assayed conditions.  

In the case of Cd
2+

, almost complete recovery was reached with the ILs [MTOA
+
][Cl

-
] 

and [omim
+
][BF4

-
] for all metal ion concentrations. It was also observed that the 

extraction percentage decreases sharply for the rest of the ionic liquids. 



635 

 

On the other hand, a high efficiency of extraction of Fe
+3

 was reached with 

[MTOA
+
][Cl

-
], observing that this parameter slightly decreases with an increase in the 

metal ion concentration up to about 95 %. Nearly no recovery of Fe
3+ 

was achieved with 

[omim
+
][BF4

-
] in all assayed conditions. In the cases of [omim

+
][PF6

-
], [bmim

+
][PF6

-
], 

[bmim
+
][NTf2

-
] and [omim

+
][NTf2

-
], the maximum percentages of extraction reached 

was around 20%, decreasing with the increase in ion metal concentrations.  

In the case of Cu 
2+

, the extraction percentage decrease around 20% from 0.1g/L to 5g/L 

metal concentration for all ionic liquids assayed except for [omim
+
][PF6

-
] being  the 

decrease of 40%. 

Regarding the transfer mechanism of metal ions in ionic liquids, the ionic nature of 

these solvents can result in a variety of extraction mechanisms, including solvent ion-

pair extraction, ion exchange and simultaneous combinations of these.
 
In the case of 

[MTOA
+
][Cl

-
], which is a quaternary amine, the extraction mechanism could be ion 

exchange, as proposed by Juang et al. (2004) for the extraction as Zn(II) and Cd(II) 

from chloride solutions with Aliquat 336. Currently a great deal of interest is being 

shown in studying the interaction between metal ions and ionic liquids (Williams et al., 

2005; Dhumal and Krishnan, 2009) For instance, Williams et al. (2005) (Williams et al., 

2005) reported that room temperature ionic liquids composed of 

bis(trifluoromethylsulfonyl) imide anions and 1,3-ethylmethylimidazolium (EMI) 

cations stabilize monomeric ligand deficient transition metal complexes (iron, titanium 

and ruthenium) via four distinct binding modes: monodentate nitrogen or oxygen 

coordination and/or bidentate oxygen–oxygen  ́ or nitrogen–oxygen coordination. 

Clearly, an understanding of the transfer mechanism of metal ions in ionic liquids is of 

primary importance to be able to establish a predictive model that may be used in 

practical operations. For this reason, this issue is now being studied in our laboratories. 

4. Conclusions 

In this work, preliminary results about the extraction of Zn
+2

, Cd
+2

, Fe
+3

 and Cu
+2 

from 

hydrochloride aqueous solutions in ionic liquids in the absence of chelating agents are 

shown. It is worthy of note that nearly complete removal of metal ions was achieved 

using [MTOA
+
][Cl

-
] for Zn

2+
, Cd

2+
 and Fe

3+
 and with [omim

+
][BF4

-
] for Zn

2+ 
and Cd

2+
. 

High extraction efficiences for Cd
2+ 

and less for
 

Zn
2+ 

were also reached using 

[omim
+
][PF6

-
]. In the case of Cu

2+
, the highest extraction percentage (80%) was 

reached using [MTOA
+
][Cl

-
].  The composition of the ionic liquid used as the extraction 

agent was seen to strongly influence the efficiency of the extraction process. By 

comparing cation and anion effects in ionic liquid based on imidazolium cations, the 

greatest effect on the extraction percentage was observed by changing the nature of the 

anion of the ionic liquid. Regarding the effect of the metal ion concentrations, it was 

found that the extraction percentage increased with the decrease in the metal ions 

concentrations of the aqueous solutions.  

In conclusion, this work demonstrates the exciting potential of ionic liquids for use as 

extraction agents in green extraction processes, since it is possible to design a specific 

ionic liquid for each metal ion by changing either the anion or the cation in order to 

create more efficient processes. 



636 

 

Acknowledgment 

This work was partially supported by the CICYT ENE2006-09395 grant and SENECA 

Foundation 05649/PI/07grants.  

References 

Alonso, M., Delgado, A. L., Sastre, A. M. and Alguacil, F. J., 2006, Kinetic modelling 

of the facilitated transport of cadmium (II) using Cyanex 923 as ionophore, 

Chemical Engineering Journal 118,  213-219. 

Lozano, L. J. and Godínez, C., 2003, Comparative study of solvent extraction of 

vanadium from sulphate solutions by primene 81R and alamine, Minerals 

Engineering 16, 291-294. 

Ruiz, A., de los Ríos, A. P., Hernández, F. J., Janssen, M. H. A.,
 
Schoevaart, R., van 

Rantwijk, F. and Sheldon, R. A., 2007, A cross-linked enzyme aggregate of Candida 

antarctica lipase B is active in denaturing ionic liquids, Enzyme and Microbial 

Technology 40, 1095-1099. 

Hernández-Fernández, F. J., de los Ríos, A. P., Rubio, M., Tomás-Alonso, F., Gómez 

D. and Víllora, G., 2007, A novel application of supported liquid membranes based 

on ionic liquids to the selective simultaneous separation of the substrates and 

products of a transesterification reaction, J. Membrane Sci 293, 73-80. 

Heitzman, H., Young, B. A., Rausch, D. J., Rickert, P., Stepinski, D. C. and Dietz, M. 

L., 2006, Fluorous ionic liquids as solvents for the liquid-liquid extraction of metal 

ions by macrocyclic polyethers, Talanta 69, 527-531. 

Juang, R. S., Kao, H. C. and Wu, W. H., 2004, Analysis of liquid membrane extraction 

of binary Zn(II) and Cd(II) from chloride media with Aliquat 336 based on 

thermodynamic equilibrium models, J. Membr. Sci. 228, 169-177. 

Williams, D. B., Stoll, M. E., Scott, B. L., Costa, D. A., Oldham, W. J., Coordination 

chemistry of the bis(trifluoromethylsulfonyl)imide anion: molecular interactions in 

room temperature ionic liquids, Chem. Commun. 2005, 1438–1440. 

Dhumal, N. R. and Krishnan, D., 2009, Molecular interactions and normal vibrations of 

Fe–bis(trifluoromethanesulfonyl)imide and 1-ethyl-3 methylimidazolium–Fe–

bis(trifluoromethanesulfonyl)imide ionic liquids: A density functional study, Journal 

of Molecular Structure: THEOCHEM 906, 78-82.