S. Sadeghi, A.Z. Moghaddam / Journal of Molecular Liquids 221 (2016) 798–804
799
in 2003 by Rogers et al. [28]. Then, the IL-salt/aqueous biphasic systems
N-methylimidazole,
1-bromobutane,1-bromohexane,
and
1-
have been applied to the separation and preconcentration of organic
and inorganic compounds, and biomolecules in different matrices [29,
bromooctane were obtained from Merck (Darmstadt, Germany), and
were used for synthesis of salicylate and thiosalicylate based ILs. Stock
−
1
3
0]. Because, the unique combination of cations and anions influences
solutions of Cr(III) and Cr(VI) at a concentration of 1000 mg L were
freshly prepared by dissolving appropriate amounts of Cr(NO ·6H
and K Cr in DDW, respectively. The working standard solutions
the properties of ILs, they have been considered as the designer solvents
for extraction of various targets from aqueous environments [31]. A
biphasic system for extraction of metal cations should contain an ex-
tractant in the hydrophobic phase to ensure the complete and selective
removal of the metal ions from the aqueous phase. The hydrated nature
of most metal ions lowers their affinity for the hydrophobic phase, thus,
it is necessary to use either an organic ligand to provide a hydrophobic
complex with the metal ions or to find conditions under which the
metal ion species can be selectively extracted from aqueous phase con-
taining inorganic complexing ions.
Recently, the development of functionalized ionic liquids which is
referred to task-specific ionic liquids (TSILs) have been received much
attention due to specific chemical and/or physical properties of ILs in
which a functional group is incorporated as a part of their cation or
anion structures; enhancing their capacity for interaction with specific
solute types. Most of TSILs for extracting applications described in the
literature are based on imidazolium cations and fluorine containing an-
ions, whereas diverse functional groups are generally appended to the
cation [32]. An attractive alternative may be ILs, which contains
complexing anions, in particular, tetraalkyl ammonium carboxylates.
Recently, the newly ILs based on a hydrophobic, long chain tetraalkyl
ammonium cation with aliphatic and aromatic carboxylate anions
were synthesized [33]. The prepared ILs contain carboxylate, salicylate
or thiosalicylate were evaluated as potential extracting agents for
metal cations from different aqueous solutions [34–38].
)
3 3
2
O
2
2 7
O
were obtained by appropriate dilution of the stock standard solutions.
Acetic acid and sodium acetate were used to prepare buffer and the pH
−
1
adjustment was carried out with 1.0 mol L solutions of NaOH or HCl.
2.3. Synthesis of salicylate and thiosalicylate based ILs
Fig. 1 illustrates the general synthesis of salicylate and thiosalicylate
based ILs [39–41]. Firstly, 20 mmol of 1-bromobutane, 1-bromohexane,
or 1-bromooctane was added to 20 mmol of N-methylimidazole, and
the mixture was refluxed while being stirred at 140 °C for 30 min
until a yellow liquid with high viscosity was obtained. The prepared
ILs ([BMIM][Br], [HMIM][Br], or [OMIM][Br]) were extracted with
10 mL diethyl ether and washed with DDW, respectively, dried over
anhydrous sodium sulfate and evaporated under vacuum. Secondly, be-
cause the halide salts underwent metathesis reaction to give the desired
ionic liquid, 20 mmol of sodium salicylate was added to the obtained IL
in water, and the mixture was stirred at room temperature for 72 h until
the anion-exchange process was done. The water was removed with
rotary evaporator and the by-product salt of NaBr was removed by
filtration after addition of methanol. Finally, salicylate based IL as a
yellow liquid was dried under vacuum.
The preparation of thiosalicylate based IL as a green viscous liquid
was carried out with the same procedure except for the fact that sodium
thiosalicylate was used instead of sodium salicylate in the second step.
In the present work, alkyl derivatives of the 3-methylimidazolium
cation with salicylate or thiosalicylate anions as task specific ionic liq-
uids were prepared and aimed to develop a feasible extraction approach
to separate Cr species in an IL/aqueous biphasic system. Salicylate and
thiosalicylate ions have been chosen as anions, because ILs with anions
2.4. Procedure for Cr speciation using proposed IL/aqueous system
−
1
An aliquot of 10 mL aqueous solution containing 250 μg L of each
Cr(III) and Cr(VI) species in acetate buffer (pH = 4.7, 0.3 mol L− ) was
transferred into a 15.0 mL conical bottom centrifuge tube. Then, 220 μL
of salicylate based IL was added to the sample solution. To increase the
extraction efficiency, Triton X-114 and sodium nitrate were added at
concentrations of 0.05%(w/v) and 0.1%(w/v), respectively. Then, the
solution was stirred for 8.5 min and the resulted biphasic mixture was
centrifuged for 25 min at 4000 rpm, so that the IL phase containing
salicylate complex of Cr(III) was separated from aqueous phase. About
220 μL of IL would be separated. The separated IL was diluted up to
500 μL with ethanol: water mixture (50:50), and its Cr content as
Cr(III) was determined by FAAS. For the analysis of total Cr, the Cr(VI)
was converted to Cr(III) by addition of 100 μL of 10% (w/v) ascorbic
acid. In this step, the total Cr was determined according to the proce-
dure described above for extraction of Cr(III). The Cr(VI) content was
obtained by subtraction of Cr(III) from total Cr. The extraction efficiency
(EE%) of Cr species was obtained as the following:
1
containing fluoride like PF
6 4
and BF are poor chelating extractant for
metal cations and decompose in contact with water to produce a very
toxic by-product of HF. In addition, these ions have low solubility in
water to form better ABS. The factors that influenced the phase
formation and Cr extraction capacity of the proposed IL/aqueous
system, e.g. sample pH, type and concentration of coexisting ions in
aqueous phase, amount of added IL, and time, were discussed. These
factors were investigated by using preliminary experiments, and then
the significant factors were modeled by central composite design
(
CCD). The optimum condition was defined using non-linear Nelder–
Mead optimization, and then the proposed method was successfully
applied for speciation of Cr in water and urine samples.
2
. Materials and methods
2
.1. Apparatus
A Shimadzu Model AA-6300G flame atomic absorption spectrometer
Kyoto, Japan) was employed to determine the concentrations of Cr. A Cr
CILꢀVf
C0 ꢀ V0
EE% ¼
ꢀ 100
ð1Þ
(
hollow cathode lamp (Hama-Matsu Photonics, Japan) was used as a ra-
diation source at current of 10 mA and wavelength of 357.9 nm with slit
width of 0.7 nm. Deionized doubly distilled water was obtained from an
AquaMax water purification system (Younglin, Anyang, Korea). The pH
measurements were carried out using a SCHOTT pH meter (Mainz,
Germany) equipped with a combined glass electrode.
where CIL and C
0
are concentrations of Cr species in the IL and initial
0 f
aqueous phases, and V and V are the volumes of initial phase and
solvent added to the separated ionic liquid, respectively.
2.5. Experimental design
2
.2. Standard solutions and reagents
Experimental design methodology was used in this study to deter-
mine the main factors that influence the Cr(III) extraction by the pro-
posed IL/aqueous biphasic system. On the basis of the preliminary
experiments, the factors which had the greatest influence on EE% of
Cr(III) were selected. The important factors were applied by using CCD
in order to build a predictive model for the EE% of Cr(III) as the response.
All chemicals used were of the analytical grade, and all solutions were
−
1
prepared in deionized doubly distilled water (DDW; 18 MΩ cm ).
Sodium salicylate and sodium thiosalicylate were obtained from Sigma-
Aldrich(St. Louis, MO, USA) and were used without further purification.