A rapid dispersive liquid-liquid microextraction based on hydrophobic deep eutectic solvent for selective and sensitive preconcentration of thorium in water and rock samples: A multivariate study

Article abstract of DOI:10.1016/j.molliq.2019.111242

Thorium [Th(IV)] and its compounds are highly toxic and cause severe damage to bone and kidney through long term exposures. Conventional methods of Th(IV) detection are complicated and costly. In this study, we developed a new dispersive liquid-liquid microextraction (DLLME) based on hydrophobic deep eutectic solvent (HDES) for preconcentration of Th(IV) in water and rock samples prior to UV–Vis. spectrophotometric detection. The microextraction procedure was carried out by applying thorin as a chelating agent, cethyltrimethyl ammonium bromide (CTAB) as an ion pairing agent and a DES consisted of 1-hexyl-3-methylimidazolium and salicylic acid components as an extraction solvent. Central composite design (CCD) was applied to obtain the best condition of the microextraction procedure. The developed method is a green, facile, fast, selective and sensitive that could detect Th(IV) in the concentration range of 10–600 ng mL^?1 with a limit of detection (LOD) and preconcentration factor of 2.1 ng mL^?1 and 59, respectively. The relative standard deviation of five replicate measurements of Th(IV) at 100 ng mL^?1 was 1.7%. Finally, the proposed method was successfully applied to the analysis of Th(IV) in water and rock samples, and desirable results were obtained.

Full text of DOI:10.1016/j.molliq.2019.111242

Accepted Manuscript
A rapid dispersive liquid-liquid microextraction based on
hydrophobic deep eutectic solvent for selective and sensitive
preconcentration of thorium in water and rock samples: A
multivariate study
Susan Sadeghi, Abdolbaset Davami
PII:
DOI:
Article Number:
Reference:
S0167-7322(19)32114-2
https://doi.org/10.1016/j.molliq.2019.111242
111242
MOLLIQ 111242
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Journal of Molecular Liquids
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Revised date:
Accepted date:
13 April 2019
9 June 2019
25 June 2019
Please cite this article as: S. Sadeghi and A. Davami, A rapid dispersive liquid-liquid
microextraction based on hydrophobic deep eutectic solvent for selective and sensitive
preconcentration of thorium in water and rock samples: A multivariate study, Journal of
Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111242
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ACCEPTED MANUSCRIPT
A rapid dispersive liquid-liquid microextraction based on hydrophobic
deep eutectic solvent for selective and sensitive preconcentration of
thorium in water and rock samples: A multivariate study
,a
Susan Sadeghi , Abdolbaset Davamia
aDepartment of Chemistry, Faculty of Science, University of Birjand, P.O. Box. 97175/615,
Birjand, Iran
Abstract
Thorium and its compounds are highly toxic and cause severe damage to bone and kidney
through long term exposures. Conventional methods of Th(IV) detection are complicated and
costly. In this study, we developed a new dispersive liquid-liquid microextraction (DLLME) based
on hydrophobic deep eutectic solvent (HDES) for preconcentration of Th(IV) in water and rock
samples prior to UV-Vis. spectrophotometric detection. The microextraction procedure was carried
out by applying thorin as a chelating agent, cethyltrimethyl ammonium bromide (CTAB) as an ion
pairing agent and a DES consisted of 1-hexyl-3-methylimidazolium and salicylic acid components
as an extraction solvent. Central composite design (CCD) was applied to obtain the best condition
of the microextraction procedure. The developed method is a green, facile, fast, selective and
sensitive that could detect Th(IV) in the concentration range of 10 - 600 ng mL-1 with a limit of
detection (LOD) and preconcentration factor of 2.1 ng mL-1 and 59, respectively. The relative
standard deviation of five replicates measurements of Th(IV) at 100 ng mL-1 was 1.7%. Finally, the
proposed method was successfully applied to the analysis of Th(IV) in water and rock samples, and
desirable results were obtained.
Keywords: Thorium; Dispersive liquid-liquid microextraction; Central composite design;
Hydrophobic deep eutectic solvent; Spectrophotometry

Corresponding author. Tel/Fax: +98 5632202009
E-mail address: ssadeghi@birjand.ac.ir (S. Sadeghi)
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1. Introduction
Thorium [Th(IV)] as the radioactive element exists on monazite sand, soil, rock, plant,
atmosphere and water. It has extensive applications for various industries such as metallurgy, optic,
ceramic and especially in nuclear energy sources [1, 2]. People who are exposed to thorium through
food, air and water are at risk of bone fractures and liver and kidney failure. According to the
World Health Organization (WHO), concentration of Th(IV) in drinking water should be below 20
µg L-1 [3]. Hence, there is a considerable attention to develop, simple, sensitive and selective
methods for determination of Th(IV) in environmental samples. Conventional techniques for
detection of low concentration of Th(IV) are usually based on inductively coupled plasma-atomic
emission spectrometry (ICP-AES)[4], inductively coupled plasma-mass spectrometry (ICP-MS)[5],
capillary zone electrophoresis [6], ion chromatography [7] and X-ray fluorescence spectroscopy
[8]. Although these techniques provide reliable results, they require daily maintenance, well-
controlled experimental conditions, costly instruments and in some cases time consuming. Because
of simplicity, relatively low cost, and availability in most laboratories, UV-Vis. spectrophotometry
may be effectively used as a reliable and an alternative technique to determination of Th(IV).
However, it suffers from low sensitivity and matrix interferences in determination of analytes in
real samples. Due to these drawbacks, separation and preconcentration are trivial steps in trace
analysis of metal ions. Several preconcentration approaches such as solid phase extraction (SPE)
[9], liquid-liquid extraction (LLE) [10], cloud point extraction (CPE) [11] and flotation [12] have
been established for the determination of Th(IV). But, these methods are tedious, provide
insufficient preconcentration, consume a large amount of toxic organic solvents, time-consuming
and produce secondary labs’ waste that limits their applications. To overcome these drawbacks,
miniaturized extractions such as liquid phase microextraction (LPME) [13] and solid phase
microextraction (SPME)[14] have been developed. Until now, various miniaturized liquid phase
extraction was developed such as emulsification microextraction (EME), single-drop
microextraction (SDME), hollow-fiber LPME (HF-LPME), in situ solvent formation
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microextraction (ISFME) and dispersive liquid–liquid microextraction (DLLME) [15]. These
methods provides important benefits to extraction of analytes such as simplicity, less consumption
of toxic solvents, high preconcentration factor, being an environmental friend method and combine
with different analytical measurement techniques. Among the LPME methods, DLLME is well-
developed method to optimize microextration process. In this method, a mixture of a water-
immiscible solvent and a small amount of a water miscible solvent as an extraction and disperser
solvents, respectively, is injected rapidly into the sample tube to produce fine droplets of the
extractant which are dispersed entirely in the aqueous phase and increases mass transfer of the
analyte into the extraction solvent. Finally, the extraction phase can be settled in the bottom of the
tube which may be removed by a syringe [16]. Great efforts also were made in DLLME to use less
amount of organic extraction solvent or elimination of disperser solvent to simple phase separation
by air-assisted DLLME or ultrasound assisted DLLME. Also, to accelerate elmulsification
procedure, vortex assisted DLLME was introduced whereby the extraction solvents are dispersed
into aqueous samples by vortex mixing [15]. Thus, these methods simplify the operation and speed
up the extraction procedure. Instead of organic solvents, different types of extraction solvents such
as supramolecular solvents, switchable solvents, ionic liquids, and deep eutectic solvents have been
used in DLLME for extraction and preconcentration of heavy metal ions [17]. Utilization of new
DLLME based on surfactants as surfactant-assisted DLLME (SA-DLLME), ion pair-based
surfactant assisted microextraction (IP-SA-ME), and surfactant-enhanced emulsification
microextraction (SE-ME) combined with ultrasonic radiation, vortex agitation, and solidification
improvements have been reported [15, 17]. Few methods are reported in the literature for
separation and preconcentration of Th(IV) using various DLLME methods and several chelating
agents. Elmasry et al. used supramolecular solvent-based liquid-liquid microextraction method for
preconcentration of thorium(IV) in various environmental samples [13]. Veyseh and Niazi applied
flotation-assisted homogeneous LLME for determination of thorium in water samples [5]. Hydari
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and Ramazani reported a new stirring assisted LLME for the analysis of trace amount of thorium
ions [18].
Room temperature ionic liquids (RTILs) are a novel class of green solvents consisted of an
imidazolium or pyridinium cation and organic or inorganic anion which have been considered as an
alternative for the organic solvents [19]. Most of the hydrophobic RTILs are contained [PF6]- as an
hydrophobic anion but it produces toxic HF and POF3 compounds by hydrolysis. Besides, these
solvents are expensive and non-biodegradable [20]. In recent years, deep eutectic solvents (DESs)
have been used as green solvents for extraction of different compounds. DESs are consisted of two
or more components including a hydrogen bond donor (HBD) and a hydrogen bond acceptor
(HBA), which can be associated with each other at moderate temperature via hydrogen bonding
interaction result in a eutectic mixture with lower melting point than each component [21]. DESs
offer solvents with excellent physiochemical properties such as non-flammable, negligible vapor
pressure, extensive liquid range and thermal stability similar to RTILs. However, facile syntheses,
cost effective, not require to furthering purification, biodegradability and biocompatibility make
them potential alternatives to replace conventional organic solvents as well as traditional ILs [22-
24]. Most of the DESs are hydrophilic that limits their applications as extractants to recover of
analytes from aqueous solutions. Recently, some literatures have been reported on production of
hydrophobic DESs for extraction of inorganic or organic targets from water samples [21].
Habibollahi and coworker applied hydrophobic DESs as the extraction solvent in DLLME based on
the solidified deep eutectic solvent for extraction of heavy metal ions in soil and vegetables
irrigated with treated municipal wastewater [25]. Akramipour et al. extended the 1-octyl-3-
methylimidazolium chloride and 1-dodecanol as hydrophobic extraction solvent of vortex assisted
DLLME based on the freezing of deep eutectic solvent, and applied it for speciation of
organic/inorganic mercury and total mercury in blood samples [26]. Zhu et al. demonstrated that
hydrophobic DESs could be used as an extraction solvent in LLME to analysis of synthetic
pigments in beverages [27].
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Investigation of main factors that control a process via minimal experiments associated with
confident results is desirable in each study. In the most conventional multifactor experiments,
optimization process is usually performed by changing a single factor in each step while all other
factors are set at constant levels (one factor at a time). Besides, a large number of experiments
should be performed that consuming high amount of materials and times without considering the
interactions between factors. To overcome these drawbacks, response surface methodology (RSM)
has been applied. The main benefits of RSM include determination of significant factors,
interactions among factors and reducing the number of experiments [28].
In the present study, we persuaded to evaluate a new hydrophobic DES as a green solvent in a
simple, selective and sensitive DLLME approach for the microextraction of Th(IV) in aqueous
solution for the first time. RSM based central composite design (CCD) was employed for
optimization of the most significant variables in the DLLME procedure. The validity of the
developed method was checked by determining Th(IV) in water and rock samples.
2. Experimental
2.1. Chemical and reagents
All of the reagents used in the present work were of analytical grade and supplied by Merck
(Darmstadt, Germany). 1-methylimidazole, 1-bromobutane, 1-bromohexane, salicylic acid (SA)
and benzoic acid (BA) were used to synthesize DESs. Th(NO3)4.5H2O, cetyltrimethylammonium
bromide (CTAB) and sodium salt of 2-(2-hydroxy-3, 6-disulfo-1-naphthylazo)-benzenearsonic acid
(thorin) were applied as the analyte, ion pairing and chelating agents, respectively. The stock
standard solution of Th(IV) (1000 mg L-1) was prepared by dissolving a suitable amount of thorium
salt in deionized water. The working standard solutions were freshly prepared by diluting the stock
solution with deionized water. The pH of the solution was adjusted with 0.1M HCl or NaOH.
2.2. Instrumentation and software
Absorption measurements were performed with a UV-Vis. spectrophotometer 2510-PC
(Shimadzu, Japan) equipped with a 1 cm quartz microcell (0.35 mL). The pH values were
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determined with BP3001 benchtop (Trans instruments, Singapore) pH meter with a combined glass
electrode. For the phase separation a centrifuge model EBA20 (Hettich, Germany) was used. A
Magnetic stirrer (Heidolph, Germany) and a R2062 rotary evaporator (Heidolph, Germany) were
employed in the synthesis of DESs. Deionized water was obtained from Aqua max water
purification system (Young–Lin, South Korea). The central composite design was accomplished
with the Design-Expert®Software Version 10-Stat-Ease (Minneapolis, USA).
2.3. Synthesis of DESs
In the first step, the 1-hexyl-3-methyl imidazolium bromide ([HMIM]Br) and 1-butyl-3-methyl
imidazolium bromide ([BMIM]Br) were synthesized according to the previous report [29]. Briefly,
an appropriate amount of 1-methylimidazole and an excess amount of 1-bromohexane or 1-
bromobutane were poured into a round bottom flask and the mixture refluxed at 80 ºC for 48 h until
a viscous liquid was produced. Finally, the products were washed three times with ethyl acetate and
dried under vacuum at 80 ºC. In the second step, to synthesize a DES, a HBA and a HBD in certain
molar ratio were mixed in a screw-capped bottle and stirred at room temperature until a
homogeneous liquid was formed. In this study, seven DESs (DES-1 to DES-7) were prepared by
employing two HBAs, namely [HMIM]Br and [BMIM]Br which could be combined with salicylic
acid (SA) or benzoic acid (BA) as HBDs with the different molar ratios (Table S1).
2.4. Preparation of real samples
Two types of water samples were used including river water (Band-e Dareh, Birjand, Iran) and
seawater (Oman Sea, Chabahar, Iran). These water samples were collected, filtered through a 0.45
µm membrane and finally stored at 4ºC before use.
The rock sample was collected from Shah-Kooh area (Nehbandan, South Khorasan, Iran). The
digestion of rock sample was performed as the following: 5 g of rock sample were weighted and
transferred into a Teflon reactor containing 50 mL concentrate HF. The reactor was heated at 100
ºC to digest the sample completely. Then, the sample was heated slowly to evaporate the solvent
and thereafter, cooled down to room temperature. In order to dissolution the residue, a mixture
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containing 5 mL HNO3 (65%, w/v) and 5 mL HCl (37%, w/v) was added. Finally, the mixture was
diluted to 50 mL with deionized water and was applied to the DLLME process.
2.5. DLLME procedure
An aliquot of 10 mL sample or standard solution containing 250 ng mL-1 of Th(IV) (pH 3.2),
thorin (60 µM) as chelating agent, CTAB (50 µM) as ion pairing agent and NaNO3 (0.5%, w/v) to
adjust ionic strength, were introduced to a conical bottom centrifuge tube. Then, 170 µL of
[HMIM]-SA (DES-4) as hydrophobic extraction solvent was injected into the solution and
immediately was shaken manually for 1 min. During shaking, fine droplets of the DES were
produced so that the red color ternary complex Th(IV)-thorin-CTAB could simplify transfer from
aqueous solution to the DES phase. To separate two phases, the cloudy solution was centrifuged at
3500 rpm for 5 min. Then, the aqueous phase was removed by a syringe, and the settled phase was
diluted to 500 µL with acidic ethanol and transferred to the cuvette. The absorbance was recorded
in the wavelength range from 450 to 650 nm against a blank solution which was prepared in the
same way without the analyte for control. The calibration curve was constructed based on the
measurement of absorbance against the different extracted concentration of Th(IV).
2.6. Experimental design methodology
In the present study, a design of experiments based on CCD was employed. Based on the
preliminary experiments, four independent variables were found that affected performance of the
microextraction process, i.e. sample pH, the concentration of thorin, the concentration of CTAB
and the amount of DES. In this design, the absorbance of the extracted complex of Th(IV)-thorin-
CTAB was considered as the analytical response. The effects of independent variables and their
interactions on the response were studied in five levels and three blocks. Other variables such as the
concentration of NaNO3 (0.5%, w/v), extraction time (1 min), centrifuge time (5 min) and
centrifuge speed (3500 rpm) were set at fixed values. The high and low levels of each variable were
selected according to the preliminary experiments (Table 1). The coded values of the variables in
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the CCD matrix and their corresponding responses are presented in Table S2. All experiments were
performed in a random manner using 10 mL sample solution containing 250 ng mL-1 Th(IV).
3. Results and discussion
In this study, thorin was used as a selective chelating agent for Th(IV) microextraction. The
hydrated free thorium is positive charge, and the charge of free thorin is negative due to the loss of
the sulfonic acid hydrogens in acidic media at pH< 6. Thus, the complex of Th(IV)-thorin has the
negative charge and could not extract into the hydrophobic extraction solvent. Therefore, the
presence of an ion pairing agent with the positive charge was necessary [30]. For this purpose,
CTAB was used as ion pairing agent that could facilitate interaction of Th(IV) with Thorin and
resulted in an association of Th(IV)-thorin-CTAB. This association was very insoluble in the
aqueous phase but could easily extract into hydrophobic extraction solvents. The UV-Vis.
absorption spectra of Thorin, Th(IV)-Thorin, Thorin-CTAB and Th(IV)-Thorin-CTAB complex
after DLLME are presented in Fig. 1. Thorin exhibited an absorption spectrum with a maximum
wavelength of 473 nm. It is obvious that Th(IV)-Thorin complex or Th(IV)-CTAB extracted poorly
in the hydrophobic solvent, while in the presence of the CTAB, an ion-pair complex of Th(IV)-
thorin-CTAB was produced which extracted into the DES; providing a significant increase in the
absorption with a red shift in the maximum wavelength to 515 nm. Thus, the hydrophobic DES
could extract the ternary complex efficiently.
3.1. Selection of DES
The choice of DES as the extraction solvent is a crucial step in designing the DLLME method.
The proper DES should have low solubility in aqueous solution, high affinity to extract the target
analyte and easily disperse into aqueous solution. In this work, four types of DES in similar molar
ratio (1:1) including [HMIM]-BA (DES-1), [BMIM]-BA (DES-2), [BMIM]-SA (DES-3) and
[HMIM]-SA (DES-4) were employed. The prepared hydrophobic DESs were evaluated for the
extraction of Th(IV) from diluted aqueous solutions. The results (Fig.2) demonstrated that
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[HMIM]-SA provided the highest extraction of Th(IV) among the used DESs. Therefore, [HMIM]-
SA was selected for further experiments.
After selection of an appropriate DES as extraction solvent, four DESs of [HMIM]-SA with
different molar ratios of [HMIM] to SA (1:1, 2:1, 1.5:1, 1:1.5; DES-4 to DES-7) were prepared and
examined for the extraction of Th(IV). As seen in Fig.3, changing the molar ratio of [HMIM]Br to
SA from 2:1 to 1:1 led to an increase in extraction efficiency and absorbance of the extracted and
remained nearly constant at molar ratio of 1:1.5. Therefore, the [HMIM]-SA with 1:1 molar ratio
(DES-4) was chosen as the best composition of extraction solvent in further experiments.
3.2. Statistical evaluation of the proposed procedure
The effectiveness of the developed regression model based on independent variables, and the
interactions between the variables was evaluated by the analysis of variance (ANOVA). The high
F-value (121.45) and low P-value (<0.0001) of ANOVA in Table S3 suggesting the model was
significant. The lack of fit (LOF) P-value of 0.14 was more than the critical value (0.05), meaning
that the LOF was not significant relatives to the pure error. The suggested second-order polynomial
equation that approximated the relationship between the independent variables and the response in
terms of coded variables could be expressed as the following equation (Eq. 1):
Absorbance = +0.520 + 0.019A + 0.034B + 0.012C + 0.022D + 0.019AB – 0.021BC – 0.013BD –
0.015CD – 0.074A2 – 0.025B2 – 0.025C2 – 0.020D2
(1)
where linear coefficients (A, B, C and D), quadratic coefficients (A2, B2, C2 and D2) and the
interaction coefficients (AB, BC, BD and CD) were significant in the microextraction process of
Th(IV). The value of the coefficient of determination suggests that the proposed polynomial model
is suitable to analyze the importance of individual variables, and interaction effects of the selected
variables in the response. According to ANOVA results, the predicted and adjusted R2 values
(0.9495 and 0.9797) were in reasonable agreement which implied an excellent relationship between
the response and the fitted model. The value of adequate precision, defined as a signal-to-noise
ratio, for the model was significantly greater than 4 (41.8), indicating to suitability of the
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model(Table S3).The predicted values versus actual data are also shown in Fig. 4A. As it can be
observed, the most actual values were lying on a straight line, which represented an excellent
relationship between the fitted model and actual data. Also, the random pattern of residual values
versus the predicted values indicated the high efficiency of the model for good presentation of
actual data (Fig. 4B). In addition, Fig. 4C illustrated a normal distribution of residuals in which
they were along a straight line approximately.
3.3. Interaction effects through response surface plots
Response surface plot shows relationship between independent and dependent (response)
variables and their effects on response graphically in two (2D) or three (3D) dimensions and
finding the optimum amounts of the variables that maximize the response. The response surface
plots were depicted the influence of two significant factors on the response in the experimental
range, while other factors were remained fixed at their central levels (Fig. 5). The observed
curvatures of the 3D response surface showed that there was a possible interaction between the
individual factors. Fig. 5A illustrates the effect of interaction of thorin concentration and pH on the
response.
The sample pH played a vital role in formation of the metal-chelate-ion pair complex which in turn
affects the extraction efficiency. In the acidic pH, the complex of thorin-Th(IV) is stable, so the pH
range of 2-4 on the formation the Th(IV)-thorin-CTAB complex was examined. As it can be seen
from Fig. 5A, the absorbance was increased with the elevation of pH from 2.0 to 3.2 and then
decreased from 3.2 to 4.0. This could be justified by the fact that at higher pH values than 3.2, the
complex was probably incompletely formed or hydrolyzed. Thus, pH of 3.2 was chosen for further
experiments. Also, the effect of thorin concentration as chelating agent on the absorbance of the
complex was explored. According to Fig. 5A, the absorbance was raised up with increasing of
thorin concentration from 25 to 60 µM in the pH range of 2.0-3.2 and thereafter, the absorbance
was nearly constant probably due to complete formation of the Th(IV)-thorin-CTAB complex in
this pH range. Consequently, the concentration of 60 µM thorin was selected as the optimum
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amount. CTAB as cationic surfactant enhanced the extraction of the complex to the DES extractor
through electrostatic interaction with Th(IV)-thorin complex. Thus, in the concentration range of
thorin from 25 to 60 µM, with increasing the CTAB concentration from 30 to 50 µM, the strength
of non-covalent interactions amplified, resulted in high extraction to the DES. The excess
concentration of CTAB more than 50 µM, diminished stability of the ternary complex and the
absorbance was decreased (Fig. 5B). As aforementioned, the ternary complex of Th(IV) was
hydrophobic and could extract from aqueous solution to the small volume of DES as an extractor
phase. Thus, volume of DES affects the concentration of the extracted complex. Fig. 5C shows the
interaction of DES volume and CTAB concentration on the response. According to Fig. 5C, with
increasing the concentration of CTAB from 30 to 50 µM, the absorbance raised up due to the
complete formation of the ternary complex of Th(IV)-thorin-CTAB and more extraction to the DES
phase in the volume range of 100-170 µL. At higher DES volumes over than 170 µL, a slight
decreasing in the absorbance was observed because of raising the sedimented phase volume.
Thereby, the optimum DES volume of 170 µL was set for the rest of experiments. Fig. 5D
demonstrated that increasing the concentration of thorin from 25 to 60 µM and DES volume from
100-170 was necessary for the complete extraction of the ternary complex into the extractor phase.
Based on the CCD results, the optimum conditions were: thorin concentration 60 µM,
concentration of CTAB 50 µM and DES volume 170 µL. The predicted response with the optimum
conditions of variables was 0.537. In order to confirm the predicted response, experiments were
repeated five times at the optimum conditions. The average response of Th(IV) with the confidence
level of 95% was found to be 0.549 ± 0.006, that was in good agreement with the predicted
response.
3.4. Analytical figures of merit
The analytical performance of the established method was evaluated in terms of the linearity,
precision and recovery. Under the optimal conditions, the calibration curve for the determination of
Th(IV) was linear in the concentration range from 10 to 600 ng mL-1 with the correlation
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coefficient of 0.9992 and a limit of detection (LOD) 2.1 ng mL-1 which was calculated based on
3Sb/m (where Sb and m are the standard deviation of the blank and the slope of calibration curve,
respectively). Also, the precision of the method described as relative standard deviation (RSD%)
for five replicate measurements of Th(IV) at 100 ng mL-1 was 1.7%. The preconcentration factor
which was calculated from the ratio of initial Th(IV) sample volume (10 mL) and the sedimented
DES volume (170 µL) obtained 59.
3.5. Interference studies
The influence of possible interferent cations and anions on the microextraction process was also
investigated. Binary solutions containing 250 ng mL-1 of Th(IV) and different concentrations of
interfering ions were prepared and analyzed by the proposed method. The highest concentration of
the added interfering ions that caused a variation of greater than ± 5% on the absorbance of the
analyte is defined as the tolerance limit. The results are available in Table 2. It can be observed that
a large excess of most cations and anions tested have no significant effects on the determination of
Th(IV). Zr4+ and Cu2+ ions didn’t interfere in the extraction and determination of Th(IV) up to 500
and 300 fold excess by using masking agents of tartaric acid and thiourea at 210-3 M, respectively.
Interference of Fe3+ ions was eliminated using ascorbic acid (510-3 M) in the recommended
procedure. Uranium didn’t interfere in the microextraction of Th(IV) up to 17500 ng mL-1
3.6. Analysis of real samples
To assess the applicability of the developed method, monitoring of Th(IV) in river and sea
waters, under optimum conditions were evaluated. Since there was no Th(IV) detected in the
samples, the waters were spiked with Th(IV) at four concentration levels of 20, 50, 100 and 300 ng
mL-1 and the analyses were performed in three replicates. The recoveries of Th(IV) ions in water
samples were ranged in 92.5 % - 109.4 % (Table 3), indicating that the matrices of the water
samples had a negligible effect on the performance of the developed method. Also, the developed
method was applied to find the amount of Th(IV) in rock sample for triplicate analyses and
assessed with the analysis by ICP-AES. The statistical significance of difference in the obtained
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results were evaluated by Fisher F-test and student t-test (Table 4). The comparison of the
precisions achieved by F-test indicated to not differ in those obtained in a statistically significant
manner. In addition, the mean concentration of Th(IV) in rock sample achieved by the two methods
was compared by t-test. The calculated t-value at 95% confidence level, 2.45, was lower than the
critical t-value (Table 4), meaning that there was no significant priority between the two available
methods. Thus, the proposed method was suitable for determination of Th(IV) in rock sample.
3.7. Comparison with other methods
The suggested method was compared with some reported extraction methods of Th(IV) based
on UV-Vis. spectrophotometric determination [13, 30-33] and the results are represented in Table
5. It is obvious that the developed method has a wide linear range, comparatively low detection
limits, high precision, and suitable preconcentration factor. As well, this method is a rapid, simple,
selective, sensitive and green microextraction method that doesn’t need to disperse the toxic
organic solvents by heating or ultrasonic treatment to accelerate and shorten the extraction process
which distinguishes it from other microextraction methods.
3.8. Conclusions
For the first time, a combination of a selective and straightforward DLLME method based on
using hydrophobic DES for microextraction of Th(IV) from water and rock samples coupled with
UV-Vis. spectrophotometric detection was developed. The hydrophobic DES was synthesized from
[HMIM]Br and SA that make it potential alternative to replace with the chlorinated organic solvent
as an extraction solvent. Surface response methodology based on CCD was successfully employed
for the simultaneous investigation of effective variables on the response. The proposed method is
rapid, easy, sensitive, cost effective and efficient for separation and preconcentration of thorium.
The performance of this method was tested for extraction and determination of Th(IV) in spiked
water and rock samples and the obtained results demonstrated the method is accurate and reliable
and suitable for the determination of Th(IV) in environmental samples at trace level.
Acknowledgments
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Authors are grateful to the Research Council of the University of Birjand for supporting this
work.
References
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Figure captions
Fig. 1 Absorption spectra of Thorin, Th(IV)-Thorin-CTAB, Thorin-CTAB, and Th(IV)-Thorin after
microextraction: Conditions: pH 3.2; Sample volume, 10 mL; Th(IV), 250 ng mL-1; Thorin, 60 µM; CTAB,
50 µM; DES, 170 µL, [HMIM]- SA DES with molar ratio of 1:1.
Fig. 2 Effect of type of DES on the microextraction of Th(IV) ions. Conditions: pH 3.2; Sample volume, 10
mL; Th(IV), 250 ng mL-1; Thorin, 60 µM; CTAB, 50 µM; DES, 170 µL, DES with molar ratio of 1:1.
Fig. 3 Effect of composition of [HMIM]-SA on the microextraction of Th(IV) ions. Conditions: pH 3.2;
Sample volume, 10 mL; Th(IV), 250 ng mL-1; Thorin, 60 µM; CTAB, 50 µM; DES, 170 µL.
Fig. 4 (A) A plot of predicted response against observed response. (B) A plot of the internally studentized
residuals vs. the predicted response. (C) Normal probability plot of residuals for the absorbance.
Fig. 5 The 3D central composite design curves for investigation of the effects of variables on analytical
response: (A) Effect of the concentration of thorin and pH on the absorbance. (B) Effect of the concentration
of CTAB and concentration of thorin on the absorbance. (C) Effect of the DES volume and concentration of
CTAB on the absorbance. (D) Effect of the DES volume and concentration of thorin on the absorbance.
Figure 1
18

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0.6
0.5
0.4
0.3
0.2
0.1
0
Thorin
Th(IV) - Thorin - CTAB
Thorin - CTAB
Th(IV) - Thorin
450
475
500
525
550
575
600
625
650
Wavelength (nm)
Figure 2
0.6
0.5
0.4
0.3
0.2
0.1
0
[HMIM]-SA [HMIM]-BA [BMIM]-SA [BMIM]-BA
Extraction solvent
19

ACCEPTED MANUSCRIPT
Figure 3
0.6
0.5
0.4
0.3
0.2
0.1
0
1:1
1.5:1
1:1.5
2:1
Molar ratio of HBD/HBA
Figure 4
0.60
0.50
A
0.40
0.30
0.20
0.10
0.10
0.20
0.30
0.40
0.50
0.60
Actual
20

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4.00
3.77363
B
2.00
0
0.00
-2.00
-3.77363
-4.00
0.10
0.20
0.30
0.40
0.50
0.60
Predicted
99
C
95
90
80
70
50
30
20
10
5
1
-3.00
-2.00
-1.00
0.00
1.00
2.00
3.00
Externally Studentized Residuals
Figure 5
21

ACCEPTED MANUSCRIPT
(B)
(A)
0.550
0.488
0.425
0.362
0.550
0.488
0.425
0.362
0.300
0.300
65.0
4.0
80.0
65.0
55.0
3.5
67.5
55.0
45.0
3.0
55.0
45.0
2.5
B:Thorin Conc. (µM)35.0
A: pH
42.5
C:CTAB Conc. (µM)
35.0B:Thorin Conc. (µM)
25.0 2.0
30.0 25.0
(D)
(C)
0.550
0.488
0.425
0.362
0.550
0.488
0.425
0.362
0.300
200.0
0.300
200.0
80.0
65.0
175.0
67.5
175.0
55.0
45.0
150.0
55.0
150.0
125.0
125.0
D: DES volume (µL)
42.5C:CTAB Conc. (µM)
D: DES volume (µL)
35.0B:Thorin Conc. (µM)
100.0 30.0
100.0 25.0
22

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Table 1. Variables and levels used in the central composite design
Coded variable levels
Variable
Symbol
-α
1
-1
0
+1
4
+α
5
pH
A
B
C
D
2
3
Concentration of thorin (µM)
Concentration of CTAB (µM)
Amount of DES (µL)
5
25
30
100
45
55
150
65
80
200
85
5
105
250
50
Table 2. Effect of interfering ions on the absorbance 250 ng mL-1 Th(IV) from the aqueous solution
Ion
Na+ NaNO3
K+
Added as
Tolerance Limit
1000
Ion
Added as
Tolerance Limit
Cd2+
Co2+
Cd(NO3)2.6H2O
Co(NO3)2.6H2O
Na2SO4
800
800
700
500
500
500
400
300
300
250
250
150
120
120
KNO3
1000
Ca2+ Ca(NO3)2.4H2O
Mg2+ Mg(NO3)2.6H2O
Ba2+ Ba(NO3)2
1000
SO4
2-
1000
cZr4+
MoO4
ZrCl4
2-
1000
Na2MoO4.2H2O
Na2WO4.2H20
NaVO3.H2O
Cu(NO3)2.3H2O
TaCl5
Al3+ Al(NO3)3.2H2O
Mn2+ Mn(NO3)2
1000
WO4
2-
-
1000
VO3
Ni2+ Ni(NO3)2.6H2O
Pb2+ Pb(NO3)2
1000
dCu2+
Ta5+
Gd3+
Er3+
1000
Cr3+ Cr(NO3)3.9H2O
Fe2+ Fe(NO3)2.9H2O
Zn2+ Zn(NO3)2.6H2O
bFe3+ Fe(NO3)3.9H2O
1000
Gd2O3
1000
Er2O3
1000
Ho3+
Ce3+
La3+
Ho(NO3)3.6H2O
Ce(NO3)3.6H2O
La(NO3)3.6H2O
1000
-
NO3 NaNO3
1000
23

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2+
Cl-
NaCl
1000
800
UO2
UO2(NO3)2.6H2O
70
Sr2+ Sr(NO3)2
aAmount of foreign ion/amount of Th(IV).
b Masked with ascorbic acid 0.005 M.
c Masked with tartaric acid 0.002 M.
d Masked with thiourea 0.002 M.
Table 3. Analytical results for Th(IV) monitoring in water sample
Sample
Added (ng mL-1)
Founded (ng mL-1)
Recovery %
92.5
20
50
18.5 ± 1.5
River water
50.3 ± 1.6
100.6
100
300
95.3 ± 2.1
95.3
311.5± 2.9
103.8
20
50
20.1 ± 0.9
54.7 ± 2.3
106.2 ± 2.8
320.0 ± 3.0
100.5
109.4
106.2
106.7
Seawater
100
300
Table 4. Determination of Th(IV) in rock sample by the proposed method and comparison with the
ICP-AES method
Sample
Rock
Th(IV) found (µg g-1)
Relative error (%)
Proposed method
ICP-AES method
55.51 ± 1.41
57.70 ± 0.90
3.79
t Crit. = 2.78 ; t Calc. = 2.26
FCrit. = 39.0 ; FCalc.= 2.45
24

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Table 5. A comparison between the analytical data of the suggested method with the other
extraction methods based on UV-Vis. spectrophotometry for the determination of Th(IV) ion.
Preconcentration
technique
Extraction
solvent
LOD
(ng mL −1)
0.40
7.50
-
Linear range
(ng mL −1)
20-100
RSD Extraction time Ref.
(%)
(min)
SsLLMEa
VALLMEb
CPEc
1-decanol/THF
Chloroform
1.4
2.8
2-5
1.6
1.8
1.7
5
3
[13]
20-240
[30]
Triton X-114
Triton X-100
Triton X-114
[HMIM]-SA
10-1000
0.5-15
130
5
[31]
CPE
0.16
1.40
2.10
[32]
CPE
5-100
15
1
[33]
DES-DLLME
10-600
This work
aSupramolecular solvent-based liquid-liquid microextraction
bVortex-assisted liquid–liquid microextraction.
cCloud point extraction
25

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Graphical abstract
Research Highlights
 New hydrophobic, low cost and nontoxic DESs were synthesized.
 For the first time, the using of DESs as extraction solvent for DLLME of
Th(IV) has been reported.
 Fast, excellent selectivity, sensitivity and green microextraction are
prominent properties of this method.
 RSM based on CCD was applied for optimization of significant variables.
26

Figure 1

Figure 2

Figure 3