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625 
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/CET1021105 

 

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

Sánchez J. I and Godinez Seoane C., (2010), Selective separation of Zn(II) over Fe(III) from acidic media using ionic liquids as sole 

extraction agents., Chemical Engineering Transactions, 21, 625-630, DOI: 10.3303/CET1021105. 

 

Selective separation of Zn(II) over Fe(III) from acidic 

media using ionic liquids as sole extraction agents 

Antonia Pérez de los Ríos*, Francisco José Hernández-Fernández, 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 

antonia.perez@upct.es 

 

This work analysed the extraction of Zn
2+ 

and Fe
3+ 

from hydrochloride aqueous 

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

liquids based on 1-n-alkyl-3-methylimidazolium and tetraalkyl ammonium cations and 

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

tetrafluoroborate, and chloride) were tested. It was found that the ionic liquid 

methyltrioctylammonium chloride, [MTOA
+
][Cl

-
], allowed almost complete removal 

(%E > 94) of Zn
2+

 and Fe
3+ 

from the aqueous solutions. Furthermore, efficient selective 

separation of Zn
2+

/Fe
3+ 

was achieved using 1-methyl-3-octylimidazolium 

tetrafluoroborate, [omim
+
][BF4

-
], since high extraction percentages were reached for 

Zn
2+ 

(%E > 90) while this parameter is very low for Fe
3+

.  

1. Introduction 

Separation/concentration of metals not only involves environmental but also economical 

profits. One example of the economic profit from recovery of metals is the iron 

rejection in the zinc industry. Most operating zinc refineries use precipitation methods 

for zinc purification, in which iron are usually precipitated as residues, followed by zinc 

recovery by electrowinning. This purification method has several problems from an 

environmental point of view. Firstly, the residues generated are voluminous. For 

example, from a solution containing 25 to 45 % iron, 0.5 tonne jarosite (solid residue) 

are produced per tonne zinc metal. Secondly, the metal impurities entrained with these 

precipitates classify the residues as hazardous industrial waste. Consequently, as 

environmental regulations become increasingly more stringent it is necessary to develop 

new environmentally friendly purification methods. Among the alternative 

technologies, solvent extraction is being considered (Lozano and Godinez, 2003). Until 

now, a number of extractants have been proposed for the extraction of iron from zinc 

process solutions, such as D2EHPA and M2EHPA (organophosphorus acids), NED 

(hydroxamic acid), TEHA (tris-(2-ethylhexyl)amine) and EU2 (aminophosphonic acid) 

dissolved in  organic diluents (Agrawal at al., 2008). One disadvantage of the solvent 

extraction is the loss of organic diluent via volatilization which has a detrimental impact 



626 

 

on the environment and human health. The use of ionic liquids (ILs), which are liquid in 

a temperature range around or below room temperature, could overcome this 

disadvantage (de los Ríos et al., 2009). ILs are fused salts containing an organic cation, 

such as dialkylimidazolium cation or N-alkylpyridinium cation, and an inorganic anion, 

such as halides, [BF4
-
], [PF6

−
], [NTf2

−
] or other complex anions. From an environmental 

point of view, the most important properties of ionic liquids are their negligible vapour 

pressure and their good chemical and thermal stabilities. 

The aim of the present work was the study of the selective separation of Zn
+2

 and Fe
+3

  

from hydrochloride aqueous solution using ionic liquid as extraction agent in 

replacement of conventional organic solvents. For that, six ionic liquids with different 

cation (1-n-alkyl-3-methylimidazolium and tetraalkyl ammonium) and anion 

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

chloride)  compositions were tested for the selective separation of  the target metal ions. 

The influence of the ionic liquid composition, the metal ion concentrations and the pH 

of the aqueous solutions on the percentage extraction and selectivity 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 5000 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. 

2.2. Extraction of Zn(II) and Fe(III) from hydrochloride aqueous solutions  
Zn(II), Fe(III) hydrochloride aqueous solutions were made up by dissolving solid 

FeCl3·6 H2O and ZnCl2 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 



627 

 

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  (1) 

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 and Fe). All aqueous solutions 

were prepared using deionised water.  

3. Results and Discussion 

With the aim of evaluating the extraction of Zn(II) and Fe(III) 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 ([MTOA
+
][Cl

-
] < 0.02 % (v/v),   [omim

+
][BF4

-
] < 1.4 % (v/v), [omim

+
][PF6

-
] < 

0.04 % (v/v), [bmim
+
][PF6

-
] < 0.501 % (v/v), , [bmim

+
][NTf2

-
] < 0.280 % (v/v), 

[omim
+
][NTf2

-
] <  0.1 % (v/v)). The extraction percentage values (%E) for using the 

different ILs are presented in Figure 1.  

MTOA Cl omim BF4 omim PF6 bmim PF6 bmim NTf2 omim NTf2

%
 E

x
tr

a
c
ti

o
n

 

0

20

40

60

80

100

Zn (II)

Fe (III)

 



628 

 

Figure 1: Extraction of 0.1 g/L Zn(II) and 0.1 g/L Fe(III) from 1M hydrochloride 

solutions using the different ionic liquids. 

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

[MTOA
+
][Cl

-
] for the two metal ions (Zn

2+
 and Fe

3+
) and with [omim

+
][BF4

-
] for Zn

2+
. 

With the rest of ionic liquids ([omim
+
][PF6

-
], [bmim

+
][PF6

-
], [bmim

+
][NTf2

-
],  

[omim
+
][NTf2

-
])  the extraction percentages of Fe

3+ 
were around 25 %, except for Fe

3+ 

using [bmim
+
][PF6

-
] in which case the extraction efficiency was lower. Almost no 

extraction of Zn
2+ 

was found using [bmim
+
][PF6

-
], [bmim

+
][NTf2

-
] and [omim

+
][NTf2

-
], 

being the extraction with [omim
+
][PF6

-
] near 40%. Regarding the selective recovery of 

metal ions, important differences in the extraction percentage between Zn
2+

/Fe
3+ 

were 

observed when [omim
+
][BF4

-
] were used as extraction agents, suggesting that these 

ionic liquids could be used for selective separation of these metal ions (see Figure 1). 

Furthermore, appreciable extraction percentage differences were observed between Fe
3+ 

and Zn
+2

 in [bmim
+
][NTf2

-
] and [omim

+
][NTf2

-
], since theses ionic liquids do not extract 

the Fe
3+ 

cation. The effect of initial metal ion concentration in the aqueous solution on 

the efficiency of the extraction process for Zn
2+

 and Fe
3+

 have been also studied. An 

example is shown in Figure 2, which shows the effect of ionic liquid composition and 

metal ion concentration on the extraction percentage (%E) of Zn(II). 

MTOA Cl omim BF4 omim PF6 bmim PF6 bmim NTf2 omim NTf2

%
 E

x
tr

a
c

ti
o

n

0

20

40

60

80

100
0.1 g/L

0.5 g/L

1 g/L

5 g/L

 

Figure 2: Effect of ionic liquid composition and metal ion concentration on the 

extraction percentage (%E) of Zn(II) from hydrochloride solutions. 

As can be seen from Figure 2, 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.  

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 % (Figure not shown). 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.  



629 

 

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. As can be observed from Figures 1 and 2 the extraction percentage for Zn
+2

 

using imidazolium based ionic liquids increased in the sequence: [NTf2
-
] < [PF6

-
] < 

[BF4
-
], which is in agreement with the anion hydrophilicity sequence (de los Ríos et al,  

2008; Ropel et al. 2005). Furthermore, the increase in the chain length of the alkyl 

substituent of the 3-methylimidazolium cation from [bmim
+
] to [omim

+
] for the anion 

[PF6
-
] resulted in an increased extraction efficiency for both metal ions. Similar results 

were found by Germani et al. (2007) in the extraction of Hg
+2

 with these ionic liquids. 

In the case of Fe
3+

, it is worthy to note that higher efficiencies were reached with the 

tetraalkylammonium cation than that obtained with imidazolium cations. Among 

imidazolium based ionic liquids, higher extraction percentages of Fe
3+ 

were reached 

with ionic liquids based on [PF6
-
] anions respect to those based on [NTf2

-
] anions. 

The influence on the extraction percentage of Fe
3+

 and Zn
2+

 of the HCl concentration in 

the aqueous media was also studied in the case of [omim
+
][PF6

-
].  For that. a metal 

solution of 0.1g/L was used and the HCl concentration of the aqueous phases was range 

from 0.1M  to 5M. The increased in the HCl  concentration (Figure not shown) involves 

an  increased in the extraction percentage, reaching maximum value around 80% and 

near 100% for  Zn
2+ 

and Fe
3+

, respectively. 

4. Conclusions 

In this work, the extraction of Zn
+2

 and Fe
+3

 from hydrochloride aqueous solutions in 

ionic liquids in the absence of chelating agents are shown. It was found that the ionic 

liquid methyltrioctylammonium chloride, [MTOA
+
][Cl

-
], allowed almost the complete 

removal of Zn
2+

 and Fe
3+ 

from the aqueous solutions. Furthermore, efficient selective 

separation of Zn
2+

/Fe
3+

 was achieved using 1-methyl-3-octylimidazolium 

tetrafluoroborate, [omim
+
][BF4

-
], since high extraction percentages were reached for 

Zn
2+ 

 while this parameter was very low for Fe
3+

. The ionic liquids [bmim
+
][PF6

-
], 

[bmim
+
][NTf2

-
] and [omim

+
][NTf2

-
] made also possible the selective recovery of 

Fe
+3

/Zn
+2

.  It was also found that the extraction percentage increased with the increase  

in the HCl concentration. To sum up, in this study we highlight the possibility of using 

ionic liquids as sole extraction agents for the selective separation of Zn
+2

 and Fe
+3

 from 

hydrochloride aqueous solutions. 

Acknowledgment 

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

Foundation 05649/PI/07grants.  

References 

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. 



630 

 

Agrawal, A., Kumari, S. and Sahu, K. K., 2008, Liquid–liquid extraction of sulphuric 

acid from zinc bleed stream, Hydrometallurgy 92, 42-47. 

de los Ríos, A. P., Hernández-Fernández, F. J., Presa, H., Gómez, D. and Víllora, G., 

Tailoring supported ionic liquid membranes for the selective separation of 

transesterification reaction compounds, 2009, J. Membrane Sci., 328, 81-85. 

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

D. and Víllora, G., 2008, On the importance of the nature of the ionic liquids in the 

selective simultaneous separation of the substrates and products of a 

transesterification reaction through supported ionic liquid membranes, J. Membrane 

Sci. 307, 233-238. 

Ropel, L., Belvèze, L. S., Aki, S. N. V. K., Stadtherr, M. A. and Brennecke, J. F., 2005, 

Octanol-water partition coefficients of imidazolium-based ionic liquids, Green. 

Chem. 7, 83-90. 

Germani, R., Mancini, M. V., Savelli, G. and Spreti, N., 2007, Mercury extraction by 

ionic liquids: temperature and alkyl chain length effect, Tetrahedron Lett. 48, 1767-

1769.