Impregnation of different ionic liquids onto cationic starch and their comparison in the extraction of Th(IV)

Article abstract of DOI:10.3906/kim-1506-65

Cationic starch (CS) was prepared by using epichlorohydrin (EPI) and 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTMA) and ionic liquids (ILs) having different anionic groups [1-butyl-3-methyl imidazolium tetrafluoroborate (BMI⁺ BF^-₄), ),lyhtem-3-lytub-1imidazolium (BMIIMB(⁺ Br Br^-), 1-butyl-3-methyl imidazolium hexafluorophosphate (BMI⁺ PF^-₆ ), and 1-butyl-3-methyl imidazolium bis-[(trifluoromethyl)sulfonyl]imide (BMI⁺ [(TF)₂ N]^-)] were impregnated onto CS. Thorium(IV) ions were preconcentrated by using IL impregnated CS. The effects of ILs were investigated for extraction of Th(IV) by CS and the results were compared. Th(IV) was preconcentrated with approximately 73% sorption capacity by CS and increased up to 98%-100% for IL impregnated sorbents at pH 7.0. The sorption capacities of Th(IV) were 0.453 mmol g^-1 , 0.399 mmol g^-1 , 0.281 mmol g^-1 , and 0.183 mmol g^-1 CS-BMI⁺ Br^- , CS-BMI⁺ [(TF)₂ N], CS-BMI⁺ PF^-₆ andCS-BMI⁺ BF^-₄ , respectively. The elution occurred with HCl and NaOH solutions at pH 7.0. for.

Full text of DOI:10.3906/kim-1506-65

Turk J Chem
Turkish Journal of Chemistry
(2016) 40: 364 – 372
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1506-65
Research Article
Impregnation of different ionic liquids onto cationic starch and their comparison
in the extraction of Th(IV)
Elif ANT BURSALI, Serap SEYHAN BOZKURT, Mu¨ru¨vvet YURDAKOC¸ ∗
˙
Department of Chemistry, Faculty of Science, Dokuz Eylu¨l University, Izmir, Turkey
Received: 23.06.2015
•
Accepted/Published Online: 27.10.2015
•
Final Version: 02.03.2016
Abstract: Cationic starch (CS) was prepared by using epichlorohydrin (EPI) and 3-chloro-2-hydroxypropyltrimethy-
lammonium chloride (CHPTMA) and ionic liquids (ILs) having different anionic groups [1-butyl-3-methyl imida-
zolium tetrafluoroborate (BMI+ BF4−), 1-butyl-3-methyl imidazolium bromide (BMI+ Br−), 1-butyl-3-methyl imida-
zolium hexafluorophosphate (BMI+ PF6−), and 1-butyl-3-methyl imidazolium bis-[(trifluoromethyl)sulfonyl]imide (BMI+
[(TF)2 N]−)] were impregnated onto CS. Thorium(IV) ions were preconcentrated by using IL impregnated CS. The ef-
fects of ILs were investigated for extraction of Th(IV) by CS and the results were compared. Th(IV) was preconcentrated
with approximately 73% sorption capacity by CS and increased up to 98%–100% for IL impregnated sorbents at pH 7.0.
The sorption capacities of Th(IV) were 0.453 mmol g−1 , 0.399 mmol g−1 , 0.281 mmol g−1 , and 0.183 mmol g−1 for
CS–BMI+ Br− , CS–BMI+ [(TF)2 N], CS–BMI+ PF6− and CS–BMI+ BF−4 , respectively. The elution occurred with HCl
and NaOH solutions at pH 7.0.
Key words: Thorium extraction, cationic starch, ionic liquids
1. Introduction
Starch has attracted attention in recent years because of its relative low price, renewability, and biodegradability.
However, the use of native starch is limited by its physicochemical properties such as water insolubility and
swelling power. The performance properties of starches can be altered through such physical or chemical
modifications to extend their usefulness in many applications in industrial processes.1
Cationic starches (CSs) are generally made by treating starch with reagents containing positively charged
groups. These starches have physicochemical properties that are significantly different from their raw materials.2−4
CSs were frequently used in adsorption and extraction studies of metal ions and due to having a cationic group
its mineral binding property has grown in importance.5,6
Ionic liquids (ILs) are inorganic and organic salts with melting points at or below 100 ◦ C. Most ILs
are composed of organic cations (e.g., imidazolium, pyridinium, pyrrolidium, ammonium, and phosphonium)
and organic (e.g., trifluoromethylsulfonate, trifluoroethanoate) or inorganic (e.g., Cl− , Br− , I− , PF6− , and
BF−4 ) anions.7,8 ILs have attracted interest as green solvents as a result of their exceptional properties. They
are nonvolatile and nonflammable, and have negligible vapor pressure, good conductivity, tunable viscosity, an
excellent feature of solvation, and high thermal stability. The physical and chemical properties of ILs can be
tuned through control of the nature and functionality of the cation or anion.9,10
∗Correspondence: m.yurdakoc@deu.edu.tr
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ILs have been the focus of many scientific investigations10 including analytical chemistry,11 synthesis,12
separation processes,13 green chemistry,14 spectroscopy,15 and electrochemistry.16 ILs have been recently
used in solid phase extraction studies by being impregnated onto support materials such as silica gel17 and
biopolymers. ILs could adsorb metal ions together with biopolymers or supporting materials having active
binding sites by electrostatic attraction.18
Th(IV) ion, a nonrenewable resource of nuclear energy present in nuclear fuel effluents, mine tailings,
seawater, and other sources, is an environmental health threat. Therefore, its preconcentration, extraction,
and separation from geological samples and waste sources by biopolymers is important in order to protect the
environment from this radioactive element.19−21
In the present study, starch was converted to CS by using epichlorohydrin (EPI) and 3-chloro-2-
hydroxypropyltrimethyl-ammonium chloride (CHPTMA). Then ILs containing different anion groups were im-
pregnated onto the CS and obtained sorbents were used for preconcentration of Th(IV) in aqueous solution.
The results obtained from the sorbents impregnated with different ILs were compared.
2. Results and discussion
2.1. FTIR analysis
FTIR spectra of the synthesized ILs were taken. In the spectra, aromatic =C–H and aliphatic –C–H stretching
vibrations varied between 3069 and 3167 cm−1 and 2860 and 2972 cm−1 for four ILs respectively. –C=N
stretching vibrations were seen between 1566 and 1573 cm−1 . S=O and S–N vibrations were also observed at
1348 cm−1 and 1182 cm−1 for BMI+ [(TF)2 N]− , respectively.
FTIR spectra of CS, CS–ILs and CS–ILs after Th(IV) sorption were so similar that only the spectra of
CS, CS–BMI+ Br− and CS–BMI+ Br–Th are given in Figure 1. In Figure 1a, an extremely broad band at 3400
cm−1 was due to the hydrogen-bonded hydroxyl groups of CS. Aliphatic C–H stretching vibrations associated
with the ring methine hydrogen atoms were observed at around 2928 cm−1 . The band at 1653 cm−1 , which
was due to water adsorbed in the amorphous regions of starch, and the bands located at 1460–1373 cm−1 region
were probably related to C–H bending vibrations. The bands assigned to C–O and C–C stretching vibrations
1,22
were observed at around 1018, 1080, and 1158 cm−1
.
The characteristic C–N bands related to quaternary
amine groups (R4 N+), at around 1460–1373 cm−1 and 1158–1018 cm−1 regions, probably overlapped with
C–H bending, and C–O and C–C stretching vibrations.
The spectra of CS, CS–BMI+ Br− , and CS–BMI+ Br− –Th (Figures 1a–1c) were very similar in shape
and the frequencies of the characteristic absorption bands did not change very much. Quite small changes
were observed in the intensities of the bands of CS and CS–BMI+ Br− , whereas CS–BMI+ Br− –Th showed
significant differences.
2.2. SEM analysis
SEM analysis was applied to CS–ILs and thorium (IV) treated CS–ILs. Images of the surface of the samples at
250× magnification are shown in Figures 2 and 3.
It was observed that different sized spherical-like particles were present in a sense among the coarse and
fine particles in the SEM image of CS (Figure 2a).
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Figure 1. FTIR spectra of CS (a) CS–BMI+ Br− (b) and CS–BMI+ Br− –Th (c).
Figure 2. SEM images of the surface of CS and CS–ILs: (a) CS, (b) CS–BMI+ [(TF)2 N]− , (c) CS–BMI+ Br− , (d)
CS–BMI+ BF4− , (e) CS–BMI+ PF−6
.
After being impregnated with BMI+ Br− , the size of both particles and the holes inside the particles
were not changed so much when compared with surface morphology of CS. However, the sizes of these holes
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were greatly diminished due to aggregation in BMI+ [(TF)2 N]− , BMI+ BF−4 , and BMI+ PF6− impregnated
CSs (Figures 2b–2e). Especially in BMI+ BF4− and BMI+ PF−6 impregnated CSs, huge clusters were observed
similar to each other.
Figure 3. SEM images of the surface of CS–ILs treated with Th(IV): (a) CS–BMI+ [(TF)2 N]− –Th, (b) CS–BMI+ Br−
–
Th, (c) CS–BMI+ BF4− –Th, (d) CS–BMI+ PF−6 –Th.
The surface morphology of CS–ILs treated with Th(IV) ions (Figures 3a–3d) showed that aggregates
were dissociated into extremely small particles in all IL impregnated CSs.
2.3. Thermal stability
Thermogravimetric analysis (TGA) was used to determine the thermal decomposition behavior of CS, CS–
ILs, and CS–ILs–Th. The thermal behaviors of the CS–ILs and CS–ILs–Th(IV) showed similar results. The
mass losses temperatures of the CS–ILs and CS–ILs–Th were between 296 and 298 ◦ C and 304 and 307 ◦ C,
respectively. Thermal decomposition temperature of all the Cs–ILs after Th(IV) extraction increased about
8–9 ◦ C. TGA curves of CS, CS–BMI+ PF6− , and CS–BMI+ PF−6 –Th are given in Figure 4.
2.4. Interaction between cationic starch and ionic liquids
The schematic illustration of the interaction between CS and ILs is given in Figure 5.
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Figure 4. TGA curves for CS, CS–BMI+ PF6− , and CS–
BMI+ PF−6 –Th.
Figure 5. Hypothetically proposed interaction between
CS and ILs.
The anionic parts of the ILs interacted with positively charged groups of CS by the electrostatic inter-
action. Moreover, the imidazolium groups of ILs interacted with the lone pair of electrons of oxygen atoms in
the hydroxyl groups of CS.
2.5. Effect of pH on the sorption of thorium ion
The effect of pH on the preconcentration of thorium(IV) ion by CS was investigated and the results are given
in Figure 6. As seen from the figure, in the range of pH 6.0–8.0, thorium (IV) was preconcentrated with
approximately 73% sorption capacity by CS. The experiments were repeated for different IL impregnated CSs.
The results showed that sorption values were increased up to 98%–100% for all ILs. However, the IL difference
did not affect the pH of metal ion preconcentration. Therefore, the optimum pH value was accepted as pH 7.0
for all other experiments.
Figure 6. Effect of pH on Th (IV) ion preconcentration.
2.6. Sorption capacity
To determine the sorption capacity of the sorbents, different volumes of 5 µg mL−1 Th(IV) were passed
through the column. The loaded Th(IV) ions were eluted with stripping solutions from each sorbent and
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measured spectrophotometrically. The sorption capacities of Th(IV) were 0.453 mmol g−1 , 0.399 mmol g−1
,
0.281 mmol g−1 , and 0.183 mmol g−1 for CS–BMI+ Br− , CS–BMI+ [(TF)2 N], CS–BMI+ PF6− , and CS–
BMI+ BF−4 , respectively.
On the other hand, the sorption of Th(IV) reached 0.040 mmol g−1 for PAN/zeolite,23 0.082 mmol
g−1 for diatomite,24 0.215 mmol g−1 for γ –Al2 O3 ,25 0.001 mmol g−1 for SiO2 ,26 0.006 mmol g−1 for
Zr2 O(PO4)2 ,27 0.057 mmol g−1 for oxidized multiwall carbon nanotubes,28 and 0.58 mmol g−1 for graphene
oxide,29 respectively.
2.7. Stripping of thorium from CS–ILs
In order to determine the stripping of Th(IV) from sorbents, hydrochloric acid and sodium hydroxide with
concentrations of 0.1, 0.5, 1, and 2 mol L−1 were used. For quantitative recovery of Th(IV), 1.0 mol L−1 HCl
for CS–BMI+ Br− and CS–BMI+ [(TF)2 N]− , and 1.0 mol L−1 and 0.1 mol L−1 NaOH for CS–BMI+ BF4−
and CS–BMI+ PF−6 , respectively, were found sufficient.
To optimize the stripping volume, different volumes of stripping solutions were tested for each sorbent.
It was observed that the stripping volume was 8 mL for CS–BMI+ Br− , 3 mL for CS–BMI+ [(TF)2 N]− , 5 mL
for BMI+ BF−4 , and 7 mL for CS–BMI+ PF6−
.
3. Conclusion
In this work, CS-based solid phase extraction sorbents having different ILs were used to separate Th(IV) ions
from aqueous solutions and for preconcentration of this ion.
The optimum pH value for preconcentration of Th(IV) ions was pH 7.0. Th(IV) was preconcentrated
with approximately 73% sorption capacity by CS but this value increased up to 98%–100% for IL impregnated
sorbents. The IL difference did not affect the pH.
Sorption capacities for Th(IV) ions were 0.453 mmol g−1 and 0.399 mmol g−1 for CS–BMI+ Br− and
CS–BMI+ [(TF)2 N]− sorbents, respectively, with the highest sorption values. It could be concluded that, due to
the excess of cavities in the sorbent impregnated with IL including Br− as anionic group, the sorption capacity
was higher than that of the other ILs. Furthermore, in CS–BMI+ [(TF)2 N]− sorbent not only the cavities but
also the structure of the anionic group acting as a ligand by itself affect the increase in sorption capacity of
Th(IV) ion.
In the case of stripping of Th(IV) ion from the sorbents, NaOH solutions were used for CS–BMI+ BF4−
Th and CS–BMI+ PF6− –Th due to the fact that the ILs have low electron density on the central atom, X−
On the other hand, HCl solutions were used for CS–BMI+ [(TF)2 N]− Th and CS–BMI+ Br− Th because the
–
.
ILs have high electron density on the central atom, X−
.
4. Experimental
4.1. Materials, reagents, solvents, and measurements
Thorium concentrations were determined spectrophotometrically by arsenazo–III method with a Shimadzu 1601
UV–Vis spectrophotometer.30−32
FTIR spectra of the obtained sorbents and ILs were recorded with a PerkinElmer Spectrum BX-II Model
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ANT BURSALI et al./Turk J Chem
Fourier Transform IR spectrometer using KBr pellets in the range of 4000 and 400 cm−1 , at a resolution of 4
cm−1 , and with an average of 50 scans.
Morphological analyses of samples were performed with an emission scanning electronic microscope
(SEM), JEOL JSM 6300F, operated at an acceleration voltage of 10 kV.
Thermal analyses of the samples were carried out using a PerkinElmer Diamond TG/DTA instrument.
The analysis was performed under nitrogen flow from 30 ◦ C to 600 ◦ C at a heating rate of 10 ◦ C/min.
A Denver 215 model pH meter for adjustment of pH and a Heidolph MR standard magnetic stirrer were
used for the preparation of the sorbents. The solvents during synthesis of ILs were evaporated by a Buchi
Rotary evaporator. Millipore Milli-Q system ultrapure water equipment was used during the study. A Watson
Marlow 323i model peristaltic pump was used in the preconcentration process. For solid phase experiments
a Varian cartridge (plastic container, 1.0 cm × 10.0 cm) equipped with 20-mm polypropylene frits was used.
PTFE tubing with i.d. of 0.5 mm was used for all connections.
N-methyl imidazole (Aldrich), butyl bromide (Aldrich), ammonium hexafluorophosphate (Aldrich), am-
monium tetrafluoroborate (Aldrich), lithium bis(trifluoromethanesulfonyl)imide (Merck), potato starch (Fluka),
epichlorohydrin (Aldrich), 3-chloro-2-hydroxypropyl-trimethyl-ammonium chloride (Fluka), ammonium hydrox-
ide solution (Fluka), sodium hydroxide (Fluka), hydrochloric acid (Riedel-de Ha¨en), and all other reagents used
were of analytical reagent grade and were used without any further purification. Potato starch was dried in an
oven for 2 h at 105 ◦ C before use. The water used throughout the study was deionized.
The stock thorium solutions were prepared by dissolving analytical reagent grade nitrate salt of tho-
rium(IV) (Merck) in ultrapure water. Working solutions were prepared by appropriate dilution of the stock
solutions. For pH adjustments, NaOH and HCl solutions were used.
4.2. Preparation of cationic starch
In order to prepare CS, 1.62 g (10 mmol) of potato starch was mixed with EPI as cross-linking agent and
also with CHPTMA in alkali medium of NH4 OH (25% solution) and NaOH. The molar ratios used were 0.030
(EPI):0.010 (NH4 OH):0.050 (NaOH):0.372 (H2 O):0.030 (CHPTMA). The mixture was stirred for 24 h at room
temperature at 500 rpm. The product was filtered and washed with deionized water up to neutral pH. Then
the product was washed further with ethanol and acetone. Finally it was dried at 45 ◦ C for 3 h.5
4.3. Preparation of ionic liquids
ILs having different anionic groups (Figure 7) were synthesized and impregnated onto CS. ILs were mainly
synthesized according to Ozdemir et al. and Ceyhan et al.34 Therefore, the preparation procedures given
33
¨
here were shortly and modified in the following parts.
Figure 7. The ILs used in the study.
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4.3.1. 1-Butyl-3-methyl imidazolium bromide (BMI+ Br−
)
A mixture of 0.04 mol N-methyl imidazole, 0.04 mol butyl bromide, and 25 mL of toluene was refluxed at 60–70
◦ C for 2 h. After cooling to room temperature, the lower phase, which contained the product, was separated
from the upper phase. The product was washed a few more times with toluene and yellowish viscous liquid was
obtained in 95% yield.
4.3.2. 1-Butyl-3-methyl imidazolium tetrafluoroborate (BMI+ BF4−
)
First 0.02 mol imidazolium bromide salt, 0.02 mol NH4 BF4 , and 50 mL of dichloromethane solvent were mixed
at room temperature for 24 h. Then ammonium bromide was filtered and the solvent was evaporated using a
rotary evaporator. The yield was calculated as 75%.
4.3.3. 1-Butyl-3-methyl imidazolium hexafluorophosphate (BMI+ PF6−
)
First 0.02 mol imidazolium bromide salt and 0.02 mol NH4 PF6 were added to 50 mL of dichloromethane. The
mixture was then stirred at room temperature for 24 h. At end of the reaction, ammonium bromide was filtered
and dichloromethane was evaporated using a rotary evaporator. The yield was calculated as 75%.
4.3.4. 1-Butyl-3-methyl imidazoliumbis-[(trifluoromethyl)sulfonyl]imide BMI+ [(TF)2 N]−
)
First 0.02 mol imidazolium bromide salt was added to 50 mL of dichloromethane and mixed. Then 0.02 mol
bis(trifluoromethanesulfonyl)imide salt of lithium was added to the stirring solution of imidazolium bromide
salt. The mixture was left stirring for about 24 h at room temperature. Then the lithium bromide salt was
filtered and the concentrated AgNO3 solution was added to the solution. The resulting solution was washed
with pure water so that AgBr was allowed to pass into the water phase. Then dichloromethane was evaporated
with a rotary evaporator. The yield was calculated as 60%.
4.4. Preparation of ionic liquid impregnated cationic starch
First 0.5 g of CS was added to a solution of 0.5 g of ILs (BMI+ Br− , BMI+ BF4− , BMI+ PF−6 , and BMI+ [(TF)2 N]−)
in acetone. The mixture was then stirred for 24 h at room temperature, filtered, and washed with deionized
water. The resulting material was dried at 45 ◦ C for 3 h.
4.5. Preconcentration method
First, 0.1 g of CS–IL sorbent was wetted in 5 mL of methanol and 5 mL of ultrapure water was added. The
mixture was transferred to the solid phase extraction column. Later, 10 mL of methanol:water (10:90) was
passed through the column. In the preconcentration experiments 50 mL of 5 ppm Th(IV) solution was used.
The pH of the solution was adjusted to 7.0 and then passed through the column at a flow rate of 1.0 mL min−1
.
Th(IV) ions were eluted from the column by 8 mL of 1.0 mol L−1 HCl for CS–BMI+ Br− , 3 mL of 1.0 mol
L−1 HCl for CS–BMI+ [(TF)2 N]− , 5 mL of 1.0 mol L−1 NaOH for CS–BMI+ BF−4 , and 7 mL of 0.1 mol L−1
NaOH for CS–BMI+ PF6− sorbents. Eluted ions were determined spectrophotometrically at 667.5 nm after the
addition of 1 mL of KCl/HCl buffer and 0.2 mL of arsenazo III in a total volume of 5 mL.
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