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Simultaneous extraction of pesticides of different polarity applying aqueous biphasic systems based on ionic liquids
Accepted Manuscript Simultaneous extraction of pesticides of different polarity applying aqueous biphasic systems based on ionic liquids
Aleksandra Dimitrijević, Ljubiša Ignjatović, Aleksandar Tot, Milan Vraneš, Nebojša Zec, Slobodan Gadžurić, Tatjana TrtićPetrović PII: DOI: Reference:
S0167-7322(17)32626-0 doi: 10.1016/j.molliq.2017.08.077 MOLLIQ 7785
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
16 June 2017 1 August 2017 22 August 2017
Please cite this article as: Aleksandra Dimitrijević, Ljubiša Ignjatović, Aleksandar Tot, Milan Vraneš, Nebojša Zec, Slobodan Gadžurić, Tatjana Trtić-Petrović , Simultaneous extraction of pesticides of different polarity applying aqueous biphasic systems based on ionic liquids, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.077
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ACCEPTED MANUSCRIPT Simultaneous Extraction of Pesticides of Different Polarity Applying Aqueous Biphasic Systems Based on Ionic Liquids
Aleksandra Dimitrijevića, Ljubiša Ignjatovićb, Aleksandar Totc, Milan Vranešc, Nebojša Zecc,
Laboratory of Physics, Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box
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1
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Slobodan Gadžurićc, Tatjana Trtić-Petrovića*
b
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522, 11001 Belgrade, Serbia, [email protected], [email protected] Faculty of Physical Chemistry, University of Belgrade, Studentski trg 16, 11000 Belgrade,
Faculty of Sciences, University of Novi Sad, Department of Chemistry, Biochemistry and
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c
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Serbia, [email protected]
Environmental Protection, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia,
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[email protected], [email protected], [email protected],
Corresponding author:
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[email protected]
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TatjanaTrtić-Petrović, [email protected]
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Laboratory of Physics
Vinča Institute of Nuclear Sciences, University of Belgrade P.O. Box 522, 11001 Belgrade, Serbia Tel: +381 11 644 7700 Abbreviations: ABS – aqueous biphasic system, Ace – acetamiprid, Imi – imidacloprid, IL – ionic liquid, IPBE – ion-pair binding energies, Lin – linuron, NCI – non-covalent interactions, Sim – simazine, Teb – tebufenozide.
1
ACCEPTED MANUSCRIPT Abstract In this paper we report a simultaneous one-step extraction of five pesticides (acetamiprid, imidaclopride, simazin, linuron and tebufenozide) of different polarity using aqueous biphasic system based on 1-butyl-3(methyl or ethyl) substituted imidazolium or pyrrolidinium ionic liquids with bromide or dicyanamide anion and potassium carbonate as a
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salting-out agent. Experimentally data obtained for the ternary system {ionic liquid + K2CO3
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+ H2O} were fitted and correlated by Merchuk equation with satisfactory high correlation
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factor. The effect of the cation alkyl chain length and the variation of anions of the ionic liquid on the aqueous biphasic system formation and the efficacy of pesticide extraction were
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investigated. Complete extraction of all studied pesticides was obtained applying aqueous biphasic system based on 1-butyl-3-ethyl imidazolium dicyanamide. It was shown that
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simultaneous extraction of the different polarity pesticides is achieved in a single-step procedure applying properly tailored ionic liquids in the aqueous biphasic system
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formulation. In order to explain excellent extraction of the polar pesticides in the studied
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aqueous biphasic systems, molecular dynamics was applied and the binding energies and non-covalent interactions were calculated. It was found that 1-butyl-3-ethyl imidazolium
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dicyanamide achieves the strongest interactions with the polar pesticides (acetamiprid and imidaclopride) leading to the highest partition coefficents. It was shown that combination of
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experimental and computational approach can be successfully applied for the selection and design of suitable ionic liquids for efficient extraction of various polarity pesticides using simple aqueous biphasic ionic liquid based systems.
Keywords: Aqueous biphasic system; Binding energies; Extraction; Ionic liquids; Noncovalent interactions; Pesticides.
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ACCEPTED MANUSCRIPT 1
INTRODUCTION
It is known that current rates of agricultural production worldwide strongly depend on the use of pesticides. The presence of pesticides in the environment (soils, surface water, atmosphere) have been well documented [1,2]. It becomes an important environmental problem, due to harmful effects of the pesticides on sensitive non-target organisms such as
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humans and animals [3]. In order to analyze the effects of pesticides on the environment, it is
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necessary to clean-up and concentrate pesticides from various environmental samples. Also,
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industrial wastewaters originated from pesticides production industry before releasing to the receiving water streams have to be pre-treated and cleaned from residual pesticides. Different
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separation methods based on extraction have been applied for both of these purposes [4,5]. Most of the conventionally applied extraction procedures are based on utilization of the
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organic solvents which are highly volatile or semivolatile at room temperature, flammable and toxic. Additionally, polar pesticides have started to be frequently applied and to replace
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the non-polar ones and they cannot extract in significant rate using classical organic solvents.
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The room-temperature ionic liquids (ILs) are favorable alternative of organic solvents in liquid-liquid extractions, due to their distinctive tuned properties gained by careful
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selection of cation and anion, negligible vapor pressure and thus low flammability, broad liquidus range, high solvation ability, high chemical and thermal stability, good extractability
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and selectivity for organic and inorganic compounds [6,7]. Water immiscible ionic liquids with large, highly charge-delocalized anion easly can form biphasic systems with water and have been used for liquid phase micro-extraction of different nonpolar, hydrophobic organic compounds [8]. These systems are usefull for extraction of nonpolar pesticides [9-11], but they are unuseful for the extraction of polar compounds due to a low partition coefficient. Contrary, water miscible ILs with small and low charge-delocalized anion are able to form aqueous biphasic systems (ABS) due to a salting out effect upon addition of inorganic or 3
ACCEPTED MANUSCRIPT organic salts, polymers, carbohydrates [12]. ABS based on water miscible ILs are recognized as alternatives for extraction of more polar organic compunds such as bioactive alkaloid S(+)-glaucine from plant material [13], 1,3-propanediol [14], estrogens [15], pharmaceuticals [16], β-carotene [17], textiledyes [18,19] etc. Up to now, there was only one attempt to apply formation of aqueous biphasic systems based on ionic liquids for the extraction of the only
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one pesticide (pentachlorophenol) [20].
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The purpose of this work was simultanious one-step extraction of selected pesticides of
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different polarity (acetamiprid, imidaclopride, simazin, linuron and tebufenozide) applying ABS based on ionic liquids. The targeted pesticides were choosen as a model system to cover
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wide range of polarity from nonpolar to polar e.g. from hydrophobic to hydrophilic compounds. The influence of ILs cation and anion on the partitioning and extraction
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efficiency of the pesticide was investigated using the following ILs: 1-butyl-3methylimidazolium dicyanamide, [bmim][DCA], 1-butyl-3-ethylimidazolium dicyanamide, 1-butyl-3-ethylimidazolium
bromide,
[beim][Br]
and
1-butyl-1-
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[beim][DCA],
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methylpyrrolidinium dicyanamide, [bmpyr][DCA]. Additionnaly, molecular dynamic simulations were appled to describe main interactions of polar pesticides and the ILs in the
EXPERIMENTAL
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2
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ABSs which are important for their successul extraction.
2.1 Materials
The pesticides investigated in this work were: acetamiprid (Ace, N-[(6-chloro-3pyridyl)methyl]-N'-cyano-N-methyl-acetamidine),
imidacloprid
(Imi,
N-[1-[(6-chloro-3-
pyridyl)methyl]-4,5-dihydroimidazol-2-yl]nitramide), simazine (Sim, 6-Chloro-N,N'-diethyl1,3,5-triazine-2,4-diamine), linuron (Lin, 3-(3,4-dichlorophenyl)-1-methoxy-1-methylureum), and tebufenozide (Teb, N-tert-butyl-N'-(4-ethylbenzoyl)-3,5 dimethylbenzohydrazide). The 4
ACCEPTED MANUSCRIPT main properties of the selected pesticides are presented in Table 1. All pesticides (mass fraction purity 95%) were purchased from Galenika-Fitofarmacija A.D. Zemun, Serbia. Stock solutions of individual pesticides (1000 mg dm–3, except for Sim 100 mg dm–3) were prepared in methanol and stored at –20°C. Aqueous solutions containing 100 mg dm–3 of each pesticide except 10 mg dm–3 of Sim were prepared daily by diluting the stock solutions
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with Milli-Q water. Potassium carbonate (≥ 99%) and methanol (for HPLC ≥ 99.9%) were
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supplied from Sigma Aldrich (St. Louis, MO, USA).
Table 1. The chemical structure and the properties which influence the extraction of the selected pesticides
1
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Pesticide
Structure O
Imi
-
O
+
N
H N
N N
N
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Cl
pKa
2
logPow
11.2
0.46
0.7
1.55
1.62
2.28
12.13
3.12/ 3.09
10.89
4.38/ 3.97
Cl
CH3
Ace
N
N
N
N CH3
Sim
N H3C
NH
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Cl N
N
NH
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NH
Lin
O
N
Cl
H3C
CH3
O
CH3
Cl
O
1
H3C
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Teb
NH
N
H3C CH3
CH3
O
CH3 CH3
Calculated by ACD/Labs PhysChem program logP in octanol/water system at pH 6 and pH 11 calculated by ACD/Labs PhysChem program
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2
The ionic liquids: [bmim][DCA] and [bmpyr][DCA] were supplied by Iolitec GmbH (Denylingen, Germany) and Sigma Aldrich (St. Louis, MO, USA), respectively. Other two ionic liquids, [beim][Br] and [beim][DCA], were synthesized by the procedures published elsewhere [21]. Briefly, the equimolar amounts of 1-ethylimidazole and 1-bromobutane were mixed and stirred under nitrogen atmosphere for 72 h at low temperature (the mixture was cooled using the ice). Obtained [beim][Br] was purified by liquid-liquid extraction using 5
ACCEPTED MANUSCRIPT ethyl acetate. Ethyl acetate was removed from the samples by heating for 45 min at 343.15 K under the vacuum. When achieving a constant mass, the ionic liquid was additionally dried under the vacuum for the next 72 h. For synthesis of [beim][DCA], the equimolar amounts of [beim][Br] and sodium dicyanamide were mixed in the acetone. Precipitated sodium bromide was removed by filtration and obtained filtrate was dried at 343.15 K under the vacuum.
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Structure of the ionic liquids and the water content is confirmed by the NMR spectroscopy
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and Karl Fischer titration, respectively. The provenance and purity of the applied ILs are
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given in Table 2.
Chemical structure
[bmim][DCA]
Final mass fraction
Purification method
Water (ppm)
Iolitec GmbH
> 0.98a
None
189
Sigma Aldrich
> 0.99a
None
145
Synthesis
> 0.96b
98
Synthesis
> 0.99b
Evaporation and drying under vacuum Extraction and drying under vacuum
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[bmpyr][DCA]
Provenance
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Ionic Liquid
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Table 2.The provenance and purity of the applied ionic liquids.
[beim][DCA]
a
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[beim][Br]
89
Specified by supplier Confirmed by NMR spectroscopy
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b
2.2 Phase diagrams and tie-lines The ternary phase diagrams were determined applying the cloud point titration method at (296.15±1) K and atmospheric pressure (p = 0.1 MPa) [21-23]. The aqueous solution of K2CO3 (salt = 50%) was added drop by drop to the aqueous solution of IL (IL = 60%) until the mixture become cloudy, and the amount of the added salt solution was recorded using an analytical balance (CP224S, Sartorius) with an uncertainty of ±10−4 g. Then, water was added 6
ACCEPTED MANUSCRIPT into the mixture until the mixture gets clear and the amount of added water was measured using the analytical balance. After each addition, the mixture was shaken using a vortex agitator (Reax Top, Heidolph, Germany) at 2500 rpm. This procedure was repeated until enough points for the construction of binodal curve were obtained. The experimental data of the ternary phase diagrams were fitted by Merchuk equation [24]: )
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(
(1)
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where Y and X correspond to IL and salt mass fractions, respectively, and A, B and C, are
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constants obtained by least-squares regression.
The tie-lines (TL) which connect two nodes on the binodal curve were determined by a
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gravimetric method originally proposed by Merchuk [24]. The solution of predetermined quantity of IL and K2CO3 was prepared, vigorously stirred for 2 minutes and left for at least
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3h at 296.15 K to reach equilibrium. The phases were carefully separated and weighed. The mass fractions of IL (Y) and salt (X) in IL-rich (YIL and XIL) and salt-rich phases (YS and XS)
]
(2) (3) (4)
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[
]
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[
D
were calculated applying MathCad 15.0 program [19, 21] and using following equations:
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(5)
where subscript M denotes the mixture and α is determined mass ratio of the top phase and the mixture. Coefficients A, B and C were taken from the fitting parameters of binodal curve (Eq. 1). The tie line lengths (TLL) and slope of TLs (S TL) were calculated using the lever arm rule [25] to determine the mass relationship between the top phase and the overall system composition according following equations: √(
)
(
)
(6)
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ACCEPTED MANUSCRIPT (7)
2.3 Extraction of the pesticides by ABS A ternary mixture within the biphasic region was prepared containing approximately 22% of
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IL, 17% of K2CO3 and 61% of the mixture of pesticides and shaken for 2 min using a vortex agitator at 2500 rpm and left to equilibrate for 2 h. The final volume of the ternary system
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was 0.6 cm3 with the concentration of each pesticide of 33.3 mg dm–3, except in the case of
pesticides was quantified by HPLC-DAD method.
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simazine which was 10 mg dm–3. The phases were separated and the concentration of the
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The HPLC analysis of the samples was performed using Agilent 1100 liquid
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chromatograph with Zorbax XDB-C18 column (4.6 mm×250 mm, 3.5 μm particle size) and DAD detector at 220 nm (Sim and Teb), 254 nm (Ace and Lin) and 270 nm (Imi). The flow rate was 0.7 cm3 min–1 and an aliquot of 20 µL of the sample was injected into HPLC system.
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The mobile phase was a mixture of methanol (A) and deionized water (B) and the following gradient profile was run: 0.0 min 43% A and 57% B, then 7 min 70% A and 30% B, and 20
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min the initial composition. The system was controlled by the Chemstation software.
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2.4 Partition coefficient and extraction efficiency Partition coefficients of the pesticides in ABS based on ILs (PIL) were calculated as the ratio of the equilibrium concentration of the pesticides in the IL-rich and in the salt-rich phases. The extraction efficiency (E) was defined as the fraction of the initial amount of the pesticides which was extracted into the IL-rich phase. The dissociation constants, pKa, and n-octanol – water partition coefficients, Po/w, were calculated using the computer software ACD/Labs PhysChem Suite v12 (Advanced Chemistry Development Inc.). 8
ACCEPTED MANUSCRIPT 2.5 Computational details All simulations were obtained using Macro Model and Jaguar 8.5 program, as implemented in Schrödinger Material Suite 2015-1. Pre-optimization of pesticides and ionic liquids were performed using Macro Model Conformation Search, using force field OPLS_3. After initial optimization DFT geometrical optimization was performed employing B3LYP exchange-
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correlation functional with empirical correction for dispersion (B3LYP-D3) together with 6-
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31+G(d,p) basis set [26,27]. Ion-pair binding energies (IPBE) have been calculated including
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counterpoise correction for BSSE error according to the approach by Boys and Bernardi [28]. Intermolecular non-covalent interactions (NCI) have been investigated using the method of
RESULTS AND DISCUSSION
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3
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Johnson et al [29].
3.1 Phase diagrams and tie-line lengths of the aqueous biphasic systems
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The new phase diagrams of the ABSs based on the targeted ILs combined with inorganic salt (K2CO3) at (296.15±1) K and atmospheric pressure (p = 0.1 MPa) were determined in this work. The experimental binodal data for the ternary mixtures of the studied ILs are reported
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in Table S1 in the Supporting information of this manuscript. The phase diagrams (Fig. 1) are
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shown in molality units, moles of solute (IL or salt) per kilogram of solvent, which allow comparison of diagrams obtained for different ILs. The experimental data were fitted by empirical relation of Merchuk [24] and the regression parameters for this equation were estimated by a least-squares regression. The calculated coefficients of Merchuk equation (A, B and C) for the investigated ABS, corresponding standard deviations (σ) and correlations coefficients (R2) are given in Table S2 in the Supporting information of this manuscript. The high values of the regression parameters (R2 ≥ 0.9949) and low values of the standard deviations prove that Merchuk equation is suitable to fit the experimental data. The biphasic 9
ACCEPTED MANUSCRIPT region is localized on the right side of binodal curves in all phase diagrams. The effect of cation on ability of the targeted ILs with the same anion to form ABS follows the order: [beim][DCA] ~ [bmpyr][DCA] > [bmim][DCA]. Similar as we shown in our previous paper [19] IL with ethyl group in the imidazolium ion has higher ability to form ABS comparing to IL with the methyl group at the same position, due to higher hydrophobicity of the cation and
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lower affinity for water molecules. The ability of [beim][DCA] to form ABS is slightly
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higher than [bmpyr][DCA]. Also, Fig. 1 shows that [beim][DCA] has better ability to form
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ABS than [beim][Br] due to a lower affinity of DCA to bound hydrogen ion comparing to bromide ion.
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In order to compare extraction ability of the investigated ABS based on the selected ILs, TLs were determined so that the concentrations of IL and K2CO3 are approximately
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equal for all investigated systems (Fig. S1 in the Supporting information of this manuscript). The parameters of TLs such as total composition, IL-rich and salt-rich phases, TLL and slope
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are given in Table 3. TLs were chosen so that IL was dominantly presented in IL-rich phase,
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for example ABS based on [bmpyr][DCA] and [beim][DCA] mass fraction of IL in salt-rich phase is negligible (0.02%). Higher mass fraction of IL in salt-rich phase was found for ABS
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based on [beim][Br] due to poorer ability of this IL to form ABS.
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4
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[IL] / mol kg
-1
6
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2
0.0
0.5
1.0
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0
[K2CO3] / mol kg
1.5
2.0
-1
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Figure 1. Ternary phase diagrams of the studied {IL + K2CO3 + H2O} systems at T = 296.15 K and atmospheric pressure (p = 0.1 MPa). Legend: ▲ [beim][DCA], ● [bmpyr][DCA],
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D
■ [bmim][DCA], ▼[beim][Br].
Table 3. Experimental tie-lines data as mass fraction for the ternary system composed of {IL(Y) + K2CO3(X) + H2O} ABS at 296.15 K and pressure p = 0.1 MPa. IL-rich phase
Salt-rich phase
100
Slope
100 X
100 Y
100 X
100 Y
100 X
100 Y
TLL
17.08
22.48
1.30
57.45
27.08
0.29
62.71
–2.22
AC
[bmim][DCA]
CE
Total composition IL
[bmpyr][DCA]
17.16
22.29
0.83
57.04
27.62
0.02
62.99
–2.13
[beim][DCA]
16.99
22.66
0.56
58.76
27.23
0.02
60.01
–2.32
[beim][Br]
16.69
23.76
5.27
42.09
28.06
5.51
43.10
–1.60
3.2 Extraction of the pesticides Five pesticides of different polarity and chemical classes (Imi and Ace are neonicotinoid insecticides; Sim is triazine herbicide; Lin is phenylurea herbicide and Teb is diacylhydrazine 11
ACCEPTED MANUSCRIPT insecticide) were selected for extraction using ABS based on the studied ILs. The main properties of the pesticides are shown in Table 1. The choosen pesticides cover wide range of polarity from nonpolar to polar e.g. from hydrophobic to hydrophilic compounds. In order to compare extraction of the pesticides applying ABS based on ILs with classical liquid-liquid extraction based on immiscible liquids, the properties of the pesticides such as dissociation
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constants (given as logKa) and n-octanol/water distribution coefficients (express as logPo/w)
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which may influence the extraction, were calculated using the computer software ACD/Labs
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PhysChem Suite v12. Distribution coefficient were calculate at two pH values, at pH = 6 which is usually pH of laboratory and tap water, and at pH = 11 which is pH of the applying
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ABS based on the ILs and K2CO3. The slightly negative influence of pH on logPo/w was observed for Lin and Teb at pH = 11 due to their dissociation constants which are close to
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this pH. Based on the values of logPo/w, the pesticides can be divided into three groups: polar
log Po/w > 2 (Sim, Lin and Teb).
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(Imi) with logPo/w ˂ 1, medium polar (Ace) with 1 ˂ logPo/w ˂ 2 and low polar pesticides with
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The capability of the selected ILs [bmim][DCA], [beim][DCA], [beim][Br] and [bmpyrr][DCA] to extract pesticides of different polarity in ABS with K2CO3 as salting-out
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agent were studied. The ILs are chosen to allow investigation of effect of cation on extraction of the pesticides such as [bmim]+ and [bmpyrr]+ or effect of alkyl group in imidazolium ring
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([bmim]+ and [beim]+). Also, the influence of anion of the ILs (dicyanamide or bromide) on the extraction of the pesticides was studied.
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ACCEPTED MANUSCRIPT Table 4. The experimentally determined partition coefficients in the investigated ABSs based on the studied ILs. LogPIL [hmim][NTf2] + K2CO3
1.65
1.58
1.55
2.55
1.79
1.60
1.56
∞
2.86
1.81
–
2.05
3.07
∞
3.15
1.88
3.25
3.08
∞
∞
∞
2.63
4.60
∞
[beim][DCA]
[bmpyr][DCA]
[beim][Br]
Imi
2.36
∞
2.84
Ace
2.54
∞
Sim
2.85
Lin Teb
1
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[bmim][DCA]
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1
[hmim][NTf2]
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Pesticide
[11]
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In regard to a working solution of the pesticides in this study contents 50% of
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methanol, the influence of methanol on building ABS based on IL was examined (Fig. S2). [beim][Br] was chosen for this study due to the lowest capability to build ABS comparing to
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other ILs applied in this work. Methanol was added in aqueous solution of IL and K2CO3 so that the concentration was fixed at 16% of methanol. Fig. S2 shows that with addition of
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methanol decrease the quantity of K2CO3 which is necessary for ABS formation. Also Gutowski et al [30] described extraction of methanol in ABS based on [bmim][Cl] and K3PO4 with partition coefficient ranging from 4 to 12 at TLL from 43 to 83. Based on these results, methanol has a favorable influence on building ABS (higher biphasic region) and its coextraction with pesticides in IL-rich phase should be expected. Extraction of the targeted pesticides was performed at the point marked in Fig. S1 which corresponds to 22% of IL, 17% of K2CO3 and 61% of the pesticide mixture. This composition of ABS was chosen in regard to the properties of all investigated ABS. Namely, 13
ACCEPTED MANUSCRIPT Fig. S1 shows that for all studied ABSs based on the targeted ILs and K2CO3, the content of 22% of IL and 17% of K2CO3 guarantied formation of biphasic system and IL will be dominantly presented in IL-rich phase. The mixture of all components was stirring for 2 min, and left to phase separation for 3 hours. IL-rich and salt-rich phases were separated and the pesticides were quantified by HPLC in both phases (Fig. 2). Fig. 2 shows that after extraction
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of the selected pesticides in the investigated ABS, all pesticides were dominantly presented in
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IL-rich phase. In order to check possible interference of pesticides and ILs in HPLC analysis,
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chromatograms of [beim][DCA] and the pesticides mixture were shown in Fig. S3 in Supporting information file of this manuscript. The retention time of the IL was about 2.5
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min and no interference with the ionic liquid was observed during the quantification of the targeted pesticides using HPLC. The similar chromatograms and retention times of other
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investigated ILs were obtained.
The partition coefficients of the targeted pesticides in the ABS containing the studied
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ILs, K2CO3 and water are depicted in Fig. 3. All experiments were performed in triplicate and
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the mean concentrations were used for further calculation. It should be noted that when the complete extraction was reached (no detection of pesticides in the salt-rich phase) then PIL
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equals ∞ [18]. The results show that highest values of PIL were obtained in the ABS based on [beim][DCA], and four out of five studied pesticides were completely extracted in IL-rich
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phase. Partition coefficients higher than 200 were obtained for the ABSs based on [bmim][DCA] and [bmpyrr][DCA]. Lower partition coefficients for all targeted pesticides except Teb were obtained in ABS based on [beim][Br] due to significantly higher quantity of this IL presented in the salt-rich phase at applied ratio of IL and K2CO3. It is clear that extraction of the pesticides is achieved in a single-step procedure by a proper tailoring of the ionic liquid applied in the ABS formulation.
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ACCEPTED MANUSCRIPT
Lin
Lin
100
Imi
IL
Imi
IL
Ace
(b)
Ace
(a)
100
50
0 5
Sim
Signal
Sim
Signal
Teb
Teb
50
0 5 IL
0
Teb
Lin
Ace
Lin
Imi
0
Ace
Imi
IL
-5 15
20
0
5
10
Retention time / min
Lin
100
Imi
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IL
Imi
IL
50
0 5
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Signal
0 5
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Sim
-10
Teb
Imi
0 -5
Lin
IL
IL
0
Ace
Signal
Sim
Teb
50
Teb
(d)
Ace
(c)
20
Retention time / min
Ace
100
15
Lin
10
PT
5
Sim
0
-5
0
5
10
15
20
0
5
10
15
20
Retention time / min
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Retention time / min
Figure 2. Chromatograms of IL-rich (up) and salt-rich (down) phases after extraction of the
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selected pesticides in the ABS based on following ILs: (a) [bmim][DCA]; (b)
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[bmpyrr][DCA]; (c) [beim][DCA] and (d) [beim][Br].
In order to enhance partitioning of the selected pesticides in ABS based on [beim][Br],
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the effect of increasing the salt mass fraction was investigated and presented in Fig. 4. With
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increasing the salt concentration, the partition coefficients of the pesticides significantly increase. It should be noted that at the concentration of 21% K2CO3, the partition coefficients are still lower than the partition coefficient obtained in ABS based on [beim][DCA] and 17% of K2CO3.
15
ACCEPTED MANUSCRIPT 1000
PT
PIL
100
1
Imi
Sim
Ace
Lin
Teb
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Pesticide
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10
Figure 3. Partition coefficient of the targeted pesticides in the ABSs based on ILs: (blue)
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[bmim][DCA], (orange) [beim][DCA], (olive) [beim][Br] and (purple) [bmpyrr][DCA]. Experimental conditions: 22% of IL, 17% of K2CO3, total volume of ABS 0.6 cm3, temperature (296.15 ± 1) K. The error bars represent the standard deviation of the
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D
experimental results.
The comparison of determined logPIL for the targeted pesticides is given in Table 4.
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Table 4 also contents experimentally obtained distribution coefficient in the two-phase extraction system based on previously published water immiscible ILs [11]. The polar and
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medium polar compound (Imi and Ace) show significantly higher partitioning in the ABSs based on the studied ILs comparing to n-octanol/water system and liquid-liquid extraction based on water immiscible IL. Low polar pesticides (Lin and Teb) exhibit the similar partitioning in the ABS as in both classical liquid-liquid extraction systems based on noctanol or water immiscible IL ([hmim][NTf2]) as extractant. In order to investigate the influence of salt addition on partition of the pesticides in the extraction system with [hmim][NTf2], the extraction was performed under the same conditions as in the ABS: 22% of [hmim][NTf2] and 17% of K2CO3. The obtained results of logP of the studied pesticides 16
ACCEPTED MANUSCRIPT (Table 3) show that the addition of salt in this extraction system has no influence on partitioning of the pesticides.
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1000
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PIL
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100
1
Imi
Ace
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10
Sim Pesticide
Lin
Teb
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Fig. 4. Partition coefficient of the selected pesticides in {[beim][Br] + K2CO3 + H2O} ABS at
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296.15 K, with 22% of [beim][Br] and different mass fraction of salt: (cyan) 14% of K2CO3, (violet) 17% of K2CO3 and (blue) 21% of K2CO3. The error bars represent the standard
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deviation of the experimental results.
Fig. S4 in the Supporting information file of this manuscript shows the effect of ILs on
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the extraction efficiency of the investigated pesticides under the same extraction conditions (22% of IL and 17% of K2CO3). The complete extraction (E = 100%) was not achieved only for partition coefficient lower than 100. The obtained results in Table 4 show similar trend of increasing logPIL for the targeted pesticides with increasing logPo/w, e.g. more hydrophobic pesticides are extracted easier from aqueous phase than more hydrophilic pesticides. Compounds with low values of logPo/w (Ace and Imi) were extracted quantitatively in ABS based on [beim][DCA] (logPIL = ∞), while
17
ACCEPTED MANUSCRIPT their logPIL in the ABS based on other targeted ILs were significantly lower. It should be emphasized that ABS based on the studied IL, especially [beim][DCA], completely extracted both hydrophilic and hydrophobic pesticides in one-step. These results imply that one-step extraction applying ABS based on the selected ILs can be successfully applied for easy and fast removal of various polarity pesticides (from highly hydrophilic to hydrophobic) either
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from natural samples in order to quantify them or from manufacturing waste waters in order
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to purify them. The recovery of the used ILs will be the subject of future research due to very
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low quantity of IL which used in these studies.
To explain extraction of the polar pesticides in the ABSs, the relationship between
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pesticide’s polarity, ionic liquid ability to form ABS and extraction efficiency, the binding energies and non-covalent interactions (NCI) between pesticides and ILs were calculated and
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presented in Table 5. It is clear that [beim][DCA] achieves the strongest interactions with Imi and Ace, which is certainly one of the reasons for their efficient extraction from the aqueous
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media. However, structurally similar ionic liquid [bmim][DCA], with the only difference of
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one –CH2 group in the alkyl substituent of the imidazolium ring, has significantly lower logPIL value. This can be explained from the calculated % of the hydrophilic surface of the
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pesticide molecules before and upon addition of IL (Table 6).
18
ACCEPTED MANUSCRIPT Table 5. Calculated binding energies (Ebin) and number of the non-covalent interactions between the studied pesticides and ILs. Pesticides
[bmim][DCA]
[beim][DCA]
[bmpyr][DCA]
[beim][Br]
[hmim][NTf2]
–28.33
–51.35
Imi
–50.61
–85.36
–46.23
Ace
–57.04
–129.66
–52.04
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Ebin (kJ·mol–1)
–29.59
–55.23
10
Ace
8
13
20
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9
10
14
15
14
14
NU
Imi
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Number of non-covalent interactions
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Table 6. Calculated hydrophilic surface area (%) of two pesticides (Imi and Ace) with or without addition od the ILs. without IL added 6.20
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Imi
[bmim][DCA]
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Pesticides
7.55
[beim][Br]
1.71
0
8.95
1.28
0
7.48
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Ace
[beim][DCA]
If the ionic liquid efficiently binds to the hydrophilic surface of the pesticide, its
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extraction from an aqueous medium will be facilitated. In the case of Ace and Imi, both molecules have in their structure the p-substituted chloropyridil group which is linked to the rest of the molecule through benzylic carbon atom. This carbon is sp3 hybridized and prevents complete delocalization of the π-electrons, separating thus different delocalized systems of the molecule (Figure S5 in the Supporting information of this manuscript). In the case of Imi, the benzylic carbon is associated through imidazolidine ring and the imino group with highly polar NO2– group, while in the structure of Ace benzylic carbon is connected to 19
ACCEPTED MANUSCRIPT nitrilo group through the amino and imino groups. These groups are the most hydrophilic parts of two pesticides. The second center of the hydrophilicity of both molecules is Cl and N atoms of chloropyridyl group. From Figures 5 (b and c) and 6 (b and c) it can be seen that ethyl group from [beim]+ forms NCI with nitrilo group in Ace and with NO2– group in Imi, but also with N and Cl from
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chloropyrydil group in both molecules. In contrast, [bmim]+ cation (Figures 5d and 6d) does
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not form any NCI with N and Cl in the case of Ace, and only one interaction with N in the case of Imi. Such interactions of hydrophobic alkyl substituents with chloropyridyl group
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reduce the ILs hydrophilicity and provide greater extraction ability of [beim]+ based ionic
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liquids. b
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D
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a
d
e
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c
f
Figure 5. Non-covalent interactions of: a) Ace; b) [beim][Br]+Ace; c) [beim][DCA]+Ace; d) [bmim][DCA]+Ace; e) [bmpyr][DCA]+Ace; f) [hmim][NTf2]+Ace.
20
ACCEPTED MANUSCRIPT b
c
d
e
f
D
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NU
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a
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Figure 6. Non-covalent interactions of: a) Imi; b) [beim][Br] + Imi; c) [beim][DCA] + Imi; d) [bmim][DCA] + Imi; e) [bmpyr][DCA] + Imi; f) [hmim][NTf2] + Imi.
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From the quantitative values of hydrophilic area presented in Table 5, it can be
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observed that in the case of [beim][DCA] after addition of Imi and Ace, the percentage of the available hydrophilic area is equal to zero, while in the case of [bmim][DCA] aproximately 1% of hydrophilic area remains available. However, if DCA anion is replaced with Br–, the extraction efficiency using [beim]+ based ionic liquid significantly decreases, although [beim]+ in the case of both pesticides forms NCI with Cl and N atoms of the chloropyridyl group. Also, it is evident from Table 4 that percentage of the hydrophilic area upon addition of [beim][Br] remains almost the same in the case of Ace, or slightly increases in the case of Imi compared to pure pesticides, resulting in the low logPIL values. Similarly, the substitution 21
ACCEPTED MANUSCRIPT of DCA with Br– ion decreases the value of binding energy (Table 4), indicating that the extraction efficiency of hydrophilic molecules from water strongly depends on the choice of both, cation and anion of the IL. It confirms that values obtained by DFT calculations (binding energy and the percentage of available hydrophilic area) can be used as a guidelines
CONCLUSIONS
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4
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for the choice of ionic liquids in the purpose of extraction.
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1-butyl-3(methyl or ethyl) substituted imidazolium or pyrrolidinium based ionic liquids with bromide and dicyanamide anions suitable for ABS formation were applied for simultaneous
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extraction of five pesticides of different polarity. The effect of cation on ability of the targeted ILs with the same anion to form ABS follows the order: [beim][DCA] ~
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[bmpyr][DCA] > [bmim][DCA]. Ionic liquids with ethyl group in the imidazolium ion have higher ability to form ABS, due to a higher hydrophobicity of the cation and lower affinity
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for water molecules. The results of the pesticides extraction in the studies ABSs show that
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highest values of partition coefficient were obtained in the ABS based on [beim][DCA]. The low and medium polar compound (Imi, Ace and Sim) show significantly higher partitioning
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in the ABSs based on the studied ILs comparing to n-octanol/water system and liquid-liquid extraction based on water immiscible IL. Low polar pesticides (Lin and Teb) exhibit the
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similar partitioning in the ABS as in both classical liquid-liquid extraction systems based on n-octanol or water immiscible IL as extractant. Also, the binding energies and non-covalent interactions between studied pesticides and ILs were calculated applying DFT approach. The main interaction between ILs and the polar pesticides which lead to their complete extraction were explained based on molecular simulation results. This study shows that ABS based on properly selected ILs give excellent opportunity for one-step simultaneous extraction of various polarity pesticides. It was also presented that computational approach together with
22
ACCEPTED MANUSCRIPT the experimental results can be efficiently applied for the selection of suitable ionic liquids for efficient extraction of polar pesticides which can be extended to other compounds.
Funding: This work was supported by the Ministry of Education, Science and Technological
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Development of Serbia under project contracts III 45006 and ON 172012.
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The authors have declared no conflict of interest.
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Action CM1206 – Exchange on Ionic Liquids.
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Acknowledgements: The authors would like to acknowledge the contribution of the COST
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CE
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D
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Conflicts of interest: none.
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ACCEPTED MANUSCRIPT
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights
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The new aqueous biphasic systems based on ionic liquids were investigated for pesticides extraction. One-step simultaneous extraction of different polarity pesticides was performed. The extraction of polar pesticides was explained by computational approach. The experimental and computational approaches for the selection of ionic liquid for efficient extraction.
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29
Aleksandra Dimitrijević, Ljubiša Ignjatović, Aleksandar Tot, Milan Vraneš, Nebojša Zec, Slobodan Gadžurić, Tatjana TrtićPetrović PII: DOI: Reference:
S0167-7322(17)32626-0 doi: 10.1016/j.molliq.2017.08.077 MOLLIQ 7785
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
16 June 2017 1 August 2017 22 August 2017
Please cite this article as: Aleksandra Dimitrijević, Ljubiša Ignjatović, Aleksandar Tot, Milan Vraneš, Nebojša Zec, Slobodan Gadžurić, Tatjana Trtić-Petrović , Simultaneous extraction of pesticides of different polarity applying aqueous biphasic systems based on ionic liquids, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.077
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ACCEPTED MANUSCRIPT Simultaneous Extraction of Pesticides of Different Polarity Applying Aqueous Biphasic Systems Based on Ionic Liquids
Aleksandra Dimitrijevića, Ljubiša Ignjatovićb, Aleksandar Totc, Milan Vranešc, Nebojša Zecc,
Laboratory of Physics, Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box
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1
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Slobodan Gadžurićc, Tatjana Trtić-Petrovića*
b
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522, 11001 Belgrade, Serbia, [email protected], [email protected] Faculty of Physical Chemistry, University of Belgrade, Studentski trg 16, 11000 Belgrade,
Faculty of Sciences, University of Novi Sad, Department of Chemistry, Biochemistry and
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c
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Serbia, [email protected]
Environmental Protection, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia,
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[email protected], [email protected], [email protected],
Corresponding author:
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[email protected]
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TatjanaTrtić-Petrović, [email protected]
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Laboratory of Physics
Vinča Institute of Nuclear Sciences, University of Belgrade P.O. Box 522, 11001 Belgrade, Serbia Tel: +381 11 644 7700 Abbreviations: ABS – aqueous biphasic system, Ace – acetamiprid, Imi – imidacloprid, IL – ionic liquid, IPBE – ion-pair binding energies, Lin – linuron, NCI – non-covalent interactions, Sim – simazine, Teb – tebufenozide.
1
ACCEPTED MANUSCRIPT Abstract In this paper we report a simultaneous one-step extraction of five pesticides (acetamiprid, imidaclopride, simazin, linuron and tebufenozide) of different polarity using aqueous biphasic system based on 1-butyl-3(methyl or ethyl) substituted imidazolium or pyrrolidinium ionic liquids with bromide or dicyanamide anion and potassium carbonate as a
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salting-out agent. Experimentally data obtained for the ternary system {ionic liquid + K2CO3
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+ H2O} were fitted and correlated by Merchuk equation with satisfactory high correlation
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factor. The effect of the cation alkyl chain length and the variation of anions of the ionic liquid on the aqueous biphasic system formation and the efficacy of pesticide extraction were
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investigated. Complete extraction of all studied pesticides was obtained applying aqueous biphasic system based on 1-butyl-3-ethyl imidazolium dicyanamide. It was shown that
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simultaneous extraction of the different polarity pesticides is achieved in a single-step procedure applying properly tailored ionic liquids in the aqueous biphasic system
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formulation. In order to explain excellent extraction of the polar pesticides in the studied
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aqueous biphasic systems, molecular dynamics was applied and the binding energies and non-covalent interactions were calculated. It was found that 1-butyl-3-ethyl imidazolium
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dicyanamide achieves the strongest interactions with the polar pesticides (acetamiprid and imidaclopride) leading to the highest partition coefficents. It was shown that combination of
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experimental and computational approach can be successfully applied for the selection and design of suitable ionic liquids for efficient extraction of various polarity pesticides using simple aqueous biphasic ionic liquid based systems.
Keywords: Aqueous biphasic system; Binding energies; Extraction; Ionic liquids; Noncovalent interactions; Pesticides.
2
ACCEPTED MANUSCRIPT 1
INTRODUCTION
It is known that current rates of agricultural production worldwide strongly depend on the use of pesticides. The presence of pesticides in the environment (soils, surface water, atmosphere) have been well documented [1,2]. It becomes an important environmental problem, due to harmful effects of the pesticides on sensitive non-target organisms such as
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humans and animals [3]. In order to analyze the effects of pesticides on the environment, it is
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necessary to clean-up and concentrate pesticides from various environmental samples. Also,
SC
industrial wastewaters originated from pesticides production industry before releasing to the receiving water streams have to be pre-treated and cleaned from residual pesticides. Different
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separation methods based on extraction have been applied for both of these purposes [4,5]. Most of the conventionally applied extraction procedures are based on utilization of the
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organic solvents which are highly volatile or semivolatile at room temperature, flammable and toxic. Additionally, polar pesticides have started to be frequently applied and to replace
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the non-polar ones and they cannot extract in significant rate using classical organic solvents.
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The room-temperature ionic liquids (ILs) are favorable alternative of organic solvents in liquid-liquid extractions, due to their distinctive tuned properties gained by careful
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selection of cation and anion, negligible vapor pressure and thus low flammability, broad liquidus range, high solvation ability, high chemical and thermal stability, good extractability
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and selectivity for organic and inorganic compounds [6,7]. Water immiscible ionic liquids with large, highly charge-delocalized anion easly can form biphasic systems with water and have been used for liquid phase micro-extraction of different nonpolar, hydrophobic organic compounds [8]. These systems are usefull for extraction of nonpolar pesticides [9-11], but they are unuseful for the extraction of polar compounds due to a low partition coefficient. Contrary, water miscible ILs with small and low charge-delocalized anion are able to form aqueous biphasic systems (ABS) due to a salting out effect upon addition of inorganic or 3
ACCEPTED MANUSCRIPT organic salts, polymers, carbohydrates [12]. ABS based on water miscible ILs are recognized as alternatives for extraction of more polar organic compunds such as bioactive alkaloid S(+)-glaucine from plant material [13], 1,3-propanediol [14], estrogens [15], pharmaceuticals [16], β-carotene [17], textiledyes [18,19] etc. Up to now, there was only one attempt to apply formation of aqueous biphasic systems based on ionic liquids for the extraction of the only
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one pesticide (pentachlorophenol) [20].
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The purpose of this work was simultanious one-step extraction of selected pesticides of
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different polarity (acetamiprid, imidaclopride, simazin, linuron and tebufenozide) applying ABS based on ionic liquids. The targeted pesticides were choosen as a model system to cover
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wide range of polarity from nonpolar to polar e.g. from hydrophobic to hydrophilic compounds. The influence of ILs cation and anion on the partitioning and extraction
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efficiency of the pesticide was investigated using the following ILs: 1-butyl-3methylimidazolium dicyanamide, [bmim][DCA], 1-butyl-3-ethylimidazolium dicyanamide, 1-butyl-3-ethylimidazolium
bromide,
[beim][Br]
and
1-butyl-1-
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[beim][DCA],
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methylpyrrolidinium dicyanamide, [bmpyr][DCA]. Additionnaly, molecular dynamic simulations were appled to describe main interactions of polar pesticides and the ILs in the
EXPERIMENTAL
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2
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ABSs which are important for their successul extraction.
2.1 Materials
The pesticides investigated in this work were: acetamiprid (Ace, N-[(6-chloro-3pyridyl)methyl]-N'-cyano-N-methyl-acetamidine),
imidacloprid
(Imi,
N-[1-[(6-chloro-3-
pyridyl)methyl]-4,5-dihydroimidazol-2-yl]nitramide), simazine (Sim, 6-Chloro-N,N'-diethyl1,3,5-triazine-2,4-diamine), linuron (Lin, 3-(3,4-dichlorophenyl)-1-methoxy-1-methylureum), and tebufenozide (Teb, N-tert-butyl-N'-(4-ethylbenzoyl)-3,5 dimethylbenzohydrazide). The 4
ACCEPTED MANUSCRIPT main properties of the selected pesticides are presented in Table 1. All pesticides (mass fraction purity 95%) were purchased from Galenika-Fitofarmacija A.D. Zemun, Serbia. Stock solutions of individual pesticides (1000 mg dm–3, except for Sim 100 mg dm–3) were prepared in methanol and stored at –20°C. Aqueous solutions containing 100 mg dm–3 of each pesticide except 10 mg dm–3 of Sim were prepared daily by diluting the stock solutions
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with Milli-Q water. Potassium carbonate (≥ 99%) and methanol (for HPLC ≥ 99.9%) were
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supplied from Sigma Aldrich (St. Louis, MO, USA).
Table 1. The chemical structure and the properties which influence the extraction of the selected pesticides
1
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Pesticide
Structure O
Imi
-
O
+
N
H N
N N
N
MA
Cl
pKa
2
logPow
11.2
0.46
0.7
1.55
1.62
2.28
12.13
3.12/ 3.09
10.89
4.38/ 3.97
Cl
CH3
Ace
N
N
N
N CH3
Sim
N H3C
NH
D
Cl N
N
NH
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NH
Lin
O
N
Cl
H3C
CH3
O
CH3
Cl
O
1
H3C
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Teb
NH
N
H3C CH3
CH3
O
CH3 CH3
Calculated by ACD/Labs PhysChem program logP in octanol/water system at pH 6 and pH 11 calculated by ACD/Labs PhysChem program
AC
2
The ionic liquids: [bmim][DCA] and [bmpyr][DCA] were supplied by Iolitec GmbH (Denylingen, Germany) and Sigma Aldrich (St. Louis, MO, USA), respectively. Other two ionic liquids, [beim][Br] and [beim][DCA], were synthesized by the procedures published elsewhere [21]. Briefly, the equimolar amounts of 1-ethylimidazole and 1-bromobutane were mixed and stirred under nitrogen atmosphere for 72 h at low temperature (the mixture was cooled using the ice). Obtained [beim][Br] was purified by liquid-liquid extraction using 5
ACCEPTED MANUSCRIPT ethyl acetate. Ethyl acetate was removed from the samples by heating for 45 min at 343.15 K under the vacuum. When achieving a constant mass, the ionic liquid was additionally dried under the vacuum for the next 72 h. For synthesis of [beim][DCA], the equimolar amounts of [beim][Br] and sodium dicyanamide were mixed in the acetone. Precipitated sodium bromide was removed by filtration and obtained filtrate was dried at 343.15 K under the vacuum.
PT
Structure of the ionic liquids and the water content is confirmed by the NMR spectroscopy
RI
and Karl Fischer titration, respectively. The provenance and purity of the applied ILs are
SC
given in Table 2.
Chemical structure
[bmim][DCA]
Final mass fraction
Purification method
Water (ppm)
Iolitec GmbH
> 0.98a
None
189
Sigma Aldrich
> 0.99a
None
145
Synthesis
> 0.96b
98
Synthesis
> 0.99b
Evaporation and drying under vacuum Extraction and drying under vacuum
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D
[bmpyr][DCA]
Provenance
MA
Ionic Liquid
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Table 2.The provenance and purity of the applied ionic liquids.
[beim][DCA]
a
CE
[beim][Br]
89
Specified by supplier Confirmed by NMR spectroscopy
AC
b
2.2 Phase diagrams and tie-lines The ternary phase diagrams were determined applying the cloud point titration method at (296.15±1) K and atmospheric pressure (p = 0.1 MPa) [21-23]. The aqueous solution of K2CO3 (salt = 50%) was added drop by drop to the aqueous solution of IL (IL = 60%) until the mixture become cloudy, and the amount of the added salt solution was recorded using an analytical balance (CP224S, Sartorius) with an uncertainty of ±10−4 g. Then, water was added 6
ACCEPTED MANUSCRIPT into the mixture until the mixture gets clear and the amount of added water was measured using the analytical balance. After each addition, the mixture was shaken using a vortex agitator (Reax Top, Heidolph, Germany) at 2500 rpm. This procedure was repeated until enough points for the construction of binodal curve were obtained. The experimental data of the ternary phase diagrams were fitted by Merchuk equation [24]: )
PT
(
(1)
RI
where Y and X correspond to IL and salt mass fractions, respectively, and A, B and C, are
SC
constants obtained by least-squares regression.
The tie-lines (TL) which connect two nodes on the binodal curve were determined by a
NU
gravimetric method originally proposed by Merchuk [24]. The solution of predetermined quantity of IL and K2CO3 was prepared, vigorously stirred for 2 minutes and left for at least
MA
3h at 296.15 K to reach equilibrium. The phases were carefully separated and weighed. The mass fractions of IL (Y) and salt (X) in IL-rich (YIL and XIL) and salt-rich phases (YS and XS)
]
(2) (3) (4)
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[
]
PT E
[
D
were calculated applying MathCad 15.0 program [19, 21] and using following equations:
AC
(5)
where subscript M denotes the mixture and α is determined mass ratio of the top phase and the mixture. Coefficients A, B and C were taken from the fitting parameters of binodal curve (Eq. 1). The tie line lengths (TLL) and slope of TLs (S TL) were calculated using the lever arm rule [25] to determine the mass relationship between the top phase and the overall system composition according following equations: √(
)
(
)
(6)
7
ACCEPTED MANUSCRIPT (7)
2.3 Extraction of the pesticides by ABS A ternary mixture within the biphasic region was prepared containing approximately 22% of
PT
IL, 17% of K2CO3 and 61% of the mixture of pesticides and shaken for 2 min using a vortex agitator at 2500 rpm and left to equilibrate for 2 h. The final volume of the ternary system
RI
was 0.6 cm3 with the concentration of each pesticide of 33.3 mg dm–3, except in the case of
pesticides was quantified by HPLC-DAD method.
SC
simazine which was 10 mg dm–3. The phases were separated and the concentration of the
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The HPLC analysis of the samples was performed using Agilent 1100 liquid
MA
chromatograph with Zorbax XDB-C18 column (4.6 mm×250 mm, 3.5 μm particle size) and DAD detector at 220 nm (Sim and Teb), 254 nm (Ace and Lin) and 270 nm (Imi). The flow rate was 0.7 cm3 min–1 and an aliquot of 20 µL of the sample was injected into HPLC system.
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D
The mobile phase was a mixture of methanol (A) and deionized water (B) and the following gradient profile was run: 0.0 min 43% A and 57% B, then 7 min 70% A and 30% B, and 20
CE
min the initial composition. The system was controlled by the Chemstation software.
AC
2.4 Partition coefficient and extraction efficiency Partition coefficients of the pesticides in ABS based on ILs (PIL) were calculated as the ratio of the equilibrium concentration of the pesticides in the IL-rich and in the salt-rich phases. The extraction efficiency (E) was defined as the fraction of the initial amount of the pesticides which was extracted into the IL-rich phase. The dissociation constants, pKa, and n-octanol – water partition coefficients, Po/w, were calculated using the computer software ACD/Labs PhysChem Suite v12 (Advanced Chemistry Development Inc.). 8
ACCEPTED MANUSCRIPT 2.5 Computational details All simulations were obtained using Macro Model and Jaguar 8.5 program, as implemented in Schrödinger Material Suite 2015-1. Pre-optimization of pesticides and ionic liquids were performed using Macro Model Conformation Search, using force field OPLS_3. After initial optimization DFT geometrical optimization was performed employing B3LYP exchange-
PT
correlation functional with empirical correction for dispersion (B3LYP-D3) together with 6-
RI
31+G(d,p) basis set [26,27]. Ion-pair binding energies (IPBE) have been calculated including
SC
counterpoise correction for BSSE error according to the approach by Boys and Bernardi [28]. Intermolecular non-covalent interactions (NCI) have been investigated using the method of
RESULTS AND DISCUSSION
MA
3
NU
Johnson et al [29].
3.1 Phase diagrams and tie-line lengths of the aqueous biphasic systems
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D
The new phase diagrams of the ABSs based on the targeted ILs combined with inorganic salt (K2CO3) at (296.15±1) K and atmospheric pressure (p = 0.1 MPa) were determined in this work. The experimental binodal data for the ternary mixtures of the studied ILs are reported
CE
in Table S1 in the Supporting information of this manuscript. The phase diagrams (Fig. 1) are
AC
shown in molality units, moles of solute (IL or salt) per kilogram of solvent, which allow comparison of diagrams obtained for different ILs. The experimental data were fitted by empirical relation of Merchuk [24] and the regression parameters for this equation were estimated by a least-squares regression. The calculated coefficients of Merchuk equation (A, B and C) for the investigated ABS, corresponding standard deviations (σ) and correlations coefficients (R2) are given in Table S2 in the Supporting information of this manuscript. The high values of the regression parameters (R2 ≥ 0.9949) and low values of the standard deviations prove that Merchuk equation is suitable to fit the experimental data. The biphasic 9
ACCEPTED MANUSCRIPT region is localized on the right side of binodal curves in all phase diagrams. The effect of cation on ability of the targeted ILs with the same anion to form ABS follows the order: [beim][DCA] ~ [bmpyr][DCA] > [bmim][DCA]. Similar as we shown in our previous paper [19] IL with ethyl group in the imidazolium ion has higher ability to form ABS comparing to IL with the methyl group at the same position, due to higher hydrophobicity of the cation and
PT
lower affinity for water molecules. The ability of [beim][DCA] to form ABS is slightly
RI
higher than [bmpyr][DCA]. Also, Fig. 1 shows that [beim][DCA] has better ability to form
SC
ABS than [beim][Br] due to a lower affinity of DCA to bound hydrogen ion comparing to bromide ion.
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In order to compare extraction ability of the investigated ABS based on the selected ILs, TLs were determined so that the concentrations of IL and K2CO3 are approximately
MA
equal for all investigated systems (Fig. S1 in the Supporting information of this manuscript). The parameters of TLs such as total composition, IL-rich and salt-rich phases, TLL and slope
D
are given in Table 3. TLs were chosen so that IL was dominantly presented in IL-rich phase,
PT E
for example ABS based on [bmpyr][DCA] and [beim][DCA] mass fraction of IL in salt-rich phase is negligible (0.02%). Higher mass fraction of IL in salt-rich phase was found for ABS
AC
CE
based on [beim][Br] due to poorer ability of this IL to form ABS.
10
ACCEPTED MANUSCRIPT 8
PT
4
RI
[IL] / mol kg
-1
6
SC
2
0.0
0.5
1.0
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0
[K2CO3] / mol kg
1.5
2.0
-1
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Figure 1. Ternary phase diagrams of the studied {IL + K2CO3 + H2O} systems at T = 296.15 K and atmospheric pressure (p = 0.1 MPa). Legend: ▲ [beim][DCA], ● [bmpyr][DCA],
PT E
D
■ [bmim][DCA], ▼[beim][Br].
Table 3. Experimental tie-lines data as mass fraction for the ternary system composed of {IL(Y) + K2CO3(X) + H2O} ABS at 296.15 K and pressure p = 0.1 MPa. IL-rich phase
Salt-rich phase
100
Slope
100 X
100 Y
100 X
100 Y
100 X
100 Y
TLL
17.08
22.48
1.30
57.45
27.08
0.29
62.71
–2.22
AC
[bmim][DCA]
CE
Total composition IL
[bmpyr][DCA]
17.16
22.29
0.83
57.04
27.62
0.02
62.99
–2.13
[beim][DCA]
16.99
22.66
0.56
58.76
27.23
0.02
60.01
–2.32
[beim][Br]
16.69
23.76
5.27
42.09
28.06
5.51
43.10
–1.60
3.2 Extraction of the pesticides Five pesticides of different polarity and chemical classes (Imi and Ace are neonicotinoid insecticides; Sim is triazine herbicide; Lin is phenylurea herbicide and Teb is diacylhydrazine 11
ACCEPTED MANUSCRIPT insecticide) were selected for extraction using ABS based on the studied ILs. The main properties of the pesticides are shown in Table 1. The choosen pesticides cover wide range of polarity from nonpolar to polar e.g. from hydrophobic to hydrophilic compounds. In order to compare extraction of the pesticides applying ABS based on ILs with classical liquid-liquid extraction based on immiscible liquids, the properties of the pesticides such as dissociation
PT
constants (given as logKa) and n-octanol/water distribution coefficients (express as logPo/w)
RI
which may influence the extraction, were calculated using the computer software ACD/Labs
SC
PhysChem Suite v12. Distribution coefficient were calculate at two pH values, at pH = 6 which is usually pH of laboratory and tap water, and at pH = 11 which is pH of the applying
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ABS based on the ILs and K2CO3. The slightly negative influence of pH on logPo/w was observed for Lin and Teb at pH = 11 due to their dissociation constants which are close to
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this pH. Based on the values of logPo/w, the pesticides can be divided into three groups: polar
log Po/w > 2 (Sim, Lin and Teb).
D
(Imi) with logPo/w ˂ 1, medium polar (Ace) with 1 ˂ logPo/w ˂ 2 and low polar pesticides with
PT E
The capability of the selected ILs [bmim][DCA], [beim][DCA], [beim][Br] and [bmpyrr][DCA] to extract pesticides of different polarity in ABS with K2CO3 as salting-out
CE
agent were studied. The ILs are chosen to allow investigation of effect of cation on extraction of the pesticides such as [bmim]+ and [bmpyrr]+ or effect of alkyl group in imidazolium ring
AC
([bmim]+ and [beim]+). Also, the influence of anion of the ILs (dicyanamide or bromide) on the extraction of the pesticides was studied.
12
ACCEPTED MANUSCRIPT Table 4. The experimentally determined partition coefficients in the investigated ABSs based on the studied ILs. LogPIL [hmim][NTf2] + K2CO3
1.65
1.58
1.55
2.55
1.79
1.60
1.56
∞
2.86
1.81
–
2.05
3.07
∞
3.15
1.88
3.25
3.08
∞
∞
∞
2.63
4.60
∞
[beim][DCA]
[bmpyr][DCA]
[beim][Br]
Imi
2.36
∞
2.84
Ace
2.54
∞
Sim
2.85
Lin Teb
1
NU
SC
RI
[bmim][DCA]
MA
1
[hmim][NTf2]
PT
Pesticide
[11]
D
In regard to a working solution of the pesticides in this study contents 50% of
PT E
methanol, the influence of methanol on building ABS based on IL was examined (Fig. S2). [beim][Br] was chosen for this study due to the lowest capability to build ABS comparing to
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other ILs applied in this work. Methanol was added in aqueous solution of IL and K2CO3 so that the concentration was fixed at 16% of methanol. Fig. S2 shows that with addition of
AC
methanol decrease the quantity of K2CO3 which is necessary for ABS formation. Also Gutowski et al [30] described extraction of methanol in ABS based on [bmim][Cl] and K3PO4 with partition coefficient ranging from 4 to 12 at TLL from 43 to 83. Based on these results, methanol has a favorable influence on building ABS (higher biphasic region) and its coextraction with pesticides in IL-rich phase should be expected. Extraction of the targeted pesticides was performed at the point marked in Fig. S1 which corresponds to 22% of IL, 17% of K2CO3 and 61% of the pesticide mixture. This composition of ABS was chosen in regard to the properties of all investigated ABS. Namely, 13
ACCEPTED MANUSCRIPT Fig. S1 shows that for all studied ABSs based on the targeted ILs and K2CO3, the content of 22% of IL and 17% of K2CO3 guarantied formation of biphasic system and IL will be dominantly presented in IL-rich phase. The mixture of all components was stirring for 2 min, and left to phase separation for 3 hours. IL-rich and salt-rich phases were separated and the pesticides were quantified by HPLC in both phases (Fig. 2). Fig. 2 shows that after extraction
PT
of the selected pesticides in the investigated ABS, all pesticides were dominantly presented in
RI
IL-rich phase. In order to check possible interference of pesticides and ILs in HPLC analysis,
SC
chromatograms of [beim][DCA] and the pesticides mixture were shown in Fig. S3 in Supporting information file of this manuscript. The retention time of the IL was about 2.5
NU
min and no interference with the ionic liquid was observed during the quantification of the targeted pesticides using HPLC. The similar chromatograms and retention times of other
MA
investigated ILs were obtained.
The partition coefficients of the targeted pesticides in the ABS containing the studied
D
ILs, K2CO3 and water are depicted in Fig. 3. All experiments were performed in triplicate and
PT E
the mean concentrations were used for further calculation. It should be noted that when the complete extraction was reached (no detection of pesticides in the salt-rich phase) then PIL
CE
equals ∞ [18]. The results show that highest values of PIL were obtained in the ABS based on [beim][DCA], and four out of five studied pesticides were completely extracted in IL-rich
AC
phase. Partition coefficients higher than 200 were obtained for the ABSs based on [bmim][DCA] and [bmpyrr][DCA]. Lower partition coefficients for all targeted pesticides except Teb were obtained in ABS based on [beim][Br] due to significantly higher quantity of this IL presented in the salt-rich phase at applied ratio of IL and K2CO3. It is clear that extraction of the pesticides is achieved in a single-step procedure by a proper tailoring of the ionic liquid applied in the ABS formulation.
14
ACCEPTED MANUSCRIPT
Lin
Lin
100
Imi
IL
Imi
IL
Ace
(b)
Ace
(a)
100
50
0 5
Sim
Signal
Sim
Signal
Teb
Teb
50
0 5 IL
0
Teb
Lin
Ace
Lin
Imi
0
Ace
Imi
IL
-5 15
20
0
5
10
Retention time / min
Lin
100
Imi
RI
IL
Imi
IL
50
0 5
SC
Signal
0 5
NU
Sim
-10
Teb
Imi
0 -5
Lin
IL
IL
0
Ace
Signal
Sim
Teb
50
Teb
(d)
Ace
(c)
20
Retention time / min
Ace
100
15
Lin
10
PT
5
Sim
0
-5
0
5
10
15
20
0
5
10
15
20
Retention time / min
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Retention time / min
Figure 2. Chromatograms of IL-rich (up) and salt-rich (down) phases after extraction of the
D
selected pesticides in the ABS based on following ILs: (a) [bmim][DCA]; (b)
PT E
[bmpyrr][DCA]; (c) [beim][DCA] and (d) [beim][Br].
In order to enhance partitioning of the selected pesticides in ABS based on [beim][Br],
CE
the effect of increasing the salt mass fraction was investigated and presented in Fig. 4. With
AC
increasing the salt concentration, the partition coefficients of the pesticides significantly increase. It should be noted that at the concentration of 21% K2CO3, the partition coefficients are still lower than the partition coefficient obtained in ABS based on [beim][DCA] and 17% of K2CO3.
15
ACCEPTED MANUSCRIPT 1000
PT
PIL
100
1
Imi
Sim
Ace
Lin
Teb
NU
Pesticide
SC
RI
10
Figure 3. Partition coefficient of the targeted pesticides in the ABSs based on ILs: (blue)
MA
[bmim][DCA], (orange) [beim][DCA], (olive) [beim][Br] and (purple) [bmpyrr][DCA]. Experimental conditions: 22% of IL, 17% of K2CO3, total volume of ABS 0.6 cm3, temperature (296.15 ± 1) K. The error bars represent the standard deviation of the
PT E
D
experimental results.
The comparison of determined logPIL for the targeted pesticides is given in Table 4.
CE
Table 4 also contents experimentally obtained distribution coefficient in the two-phase extraction system based on previously published water immiscible ILs [11]. The polar and
AC
medium polar compound (Imi and Ace) show significantly higher partitioning in the ABSs based on the studied ILs comparing to n-octanol/water system and liquid-liquid extraction based on water immiscible IL. Low polar pesticides (Lin and Teb) exhibit the similar partitioning in the ABS as in both classical liquid-liquid extraction systems based on noctanol or water immiscible IL ([hmim][NTf2]) as extractant. In order to investigate the influence of salt addition on partition of the pesticides in the extraction system with [hmim][NTf2], the extraction was performed under the same conditions as in the ABS: 22% of [hmim][NTf2] and 17% of K2CO3. The obtained results of logP of the studied pesticides 16
ACCEPTED MANUSCRIPT (Table 3) show that the addition of salt in this extraction system has no influence on partitioning of the pesticides.
PT
1000
SC
PIL
RI
100
1
Imi
Ace
MA
NU
10
Sim Pesticide
Lin
Teb
D
Fig. 4. Partition coefficient of the selected pesticides in {[beim][Br] + K2CO3 + H2O} ABS at
PT E
296.15 K, with 22% of [beim][Br] and different mass fraction of salt: (cyan) 14% of K2CO3, (violet) 17% of K2CO3 and (blue) 21% of K2CO3. The error bars represent the standard
CE
deviation of the experimental results.
Fig. S4 in the Supporting information file of this manuscript shows the effect of ILs on
AC
the extraction efficiency of the investigated pesticides under the same extraction conditions (22% of IL and 17% of K2CO3). The complete extraction (E = 100%) was not achieved only for partition coefficient lower than 100. The obtained results in Table 4 show similar trend of increasing logPIL for the targeted pesticides with increasing logPo/w, e.g. more hydrophobic pesticides are extracted easier from aqueous phase than more hydrophilic pesticides. Compounds with low values of logPo/w (Ace and Imi) were extracted quantitatively in ABS based on [beim][DCA] (logPIL = ∞), while
17
ACCEPTED MANUSCRIPT their logPIL in the ABS based on other targeted ILs were significantly lower. It should be emphasized that ABS based on the studied IL, especially [beim][DCA], completely extracted both hydrophilic and hydrophobic pesticides in one-step. These results imply that one-step extraction applying ABS based on the selected ILs can be successfully applied for easy and fast removal of various polarity pesticides (from highly hydrophilic to hydrophobic) either
PT
from natural samples in order to quantify them or from manufacturing waste waters in order
RI
to purify them. The recovery of the used ILs will be the subject of future research due to very
SC
low quantity of IL which used in these studies.
To explain extraction of the polar pesticides in the ABSs, the relationship between
NU
pesticide’s polarity, ionic liquid ability to form ABS and extraction efficiency, the binding energies and non-covalent interactions (NCI) between pesticides and ILs were calculated and
MA
presented in Table 5. It is clear that [beim][DCA] achieves the strongest interactions with Imi and Ace, which is certainly one of the reasons for their efficient extraction from the aqueous
D
media. However, structurally similar ionic liquid [bmim][DCA], with the only difference of
PT E
one –CH2 group in the alkyl substituent of the imidazolium ring, has significantly lower logPIL value. This can be explained from the calculated % of the hydrophilic surface of the
AC
CE
pesticide molecules before and upon addition of IL (Table 6).
18
ACCEPTED MANUSCRIPT Table 5. Calculated binding energies (Ebin) and number of the non-covalent interactions between the studied pesticides and ILs. Pesticides
[bmim][DCA]
[beim][DCA]
[bmpyr][DCA]
[beim][Br]
[hmim][NTf2]
–28.33
–51.35
Imi
–50.61
–85.36
–46.23
Ace
–57.04
–129.66
–52.04
PT
Ebin (kJ·mol–1)
–29.59
–55.23
10
Ace
8
13
20
SC
9
10
14
15
14
14
NU
Imi
RI
Number of non-covalent interactions
MA
Table 6. Calculated hydrophilic surface area (%) of two pesticides (Imi and Ace) with or without addition od the ILs. without IL added 6.20
PT E
Imi
[bmim][DCA]
D
Pesticides
7.55
[beim][Br]
1.71
0
8.95
1.28
0
7.48
CE
Ace
[beim][DCA]
If the ionic liquid efficiently binds to the hydrophilic surface of the pesticide, its
AC
extraction from an aqueous medium will be facilitated. In the case of Ace and Imi, both molecules have in their structure the p-substituted chloropyridil group which is linked to the rest of the molecule through benzylic carbon atom. This carbon is sp3 hybridized and prevents complete delocalization of the π-electrons, separating thus different delocalized systems of the molecule (Figure S5 in the Supporting information of this manuscript). In the case of Imi, the benzylic carbon is associated through imidazolidine ring and the imino group with highly polar NO2– group, while in the structure of Ace benzylic carbon is connected to 19
ACCEPTED MANUSCRIPT nitrilo group through the amino and imino groups. These groups are the most hydrophilic parts of two pesticides. The second center of the hydrophilicity of both molecules is Cl and N atoms of chloropyridyl group. From Figures 5 (b and c) and 6 (b and c) it can be seen that ethyl group from [beim]+ forms NCI with nitrilo group in Ace and with NO2– group in Imi, but also with N and Cl from
PT
chloropyrydil group in both molecules. In contrast, [bmim]+ cation (Figures 5d and 6d) does
RI
not form any NCI with N and Cl in the case of Ace, and only one interaction with N in the case of Imi. Such interactions of hydrophobic alkyl substituents with chloropyridyl group
SC
reduce the ILs hydrophilicity and provide greater extraction ability of [beim]+ based ionic
NU
liquids. b
PT E
D
MA
a
d
e
AC
CE
c
f
Figure 5. Non-covalent interactions of: a) Ace; b) [beim][Br]+Ace; c) [beim][DCA]+Ace; d) [bmim][DCA]+Ace; e) [bmpyr][DCA]+Ace; f) [hmim][NTf2]+Ace.
20
ACCEPTED MANUSCRIPT b
c
d
e
f
D
MA
NU
SC
RI
PT
a
PT E
Figure 6. Non-covalent interactions of: a) Imi; b) [beim][Br] + Imi; c) [beim][DCA] + Imi; d) [bmim][DCA] + Imi; e) [bmpyr][DCA] + Imi; f) [hmim][NTf2] + Imi.
CE
From the quantitative values of hydrophilic area presented in Table 5, it can be
AC
observed that in the case of [beim][DCA] after addition of Imi and Ace, the percentage of the available hydrophilic area is equal to zero, while in the case of [bmim][DCA] aproximately 1% of hydrophilic area remains available. However, if DCA anion is replaced with Br–, the extraction efficiency using [beim]+ based ionic liquid significantly decreases, although [beim]+ in the case of both pesticides forms NCI with Cl and N atoms of the chloropyridyl group. Also, it is evident from Table 4 that percentage of the hydrophilic area upon addition of [beim][Br] remains almost the same in the case of Ace, or slightly increases in the case of Imi compared to pure pesticides, resulting in the low logPIL values. Similarly, the substitution 21
ACCEPTED MANUSCRIPT of DCA with Br– ion decreases the value of binding energy (Table 4), indicating that the extraction efficiency of hydrophilic molecules from water strongly depends on the choice of both, cation and anion of the IL. It confirms that values obtained by DFT calculations (binding energy and the percentage of available hydrophilic area) can be used as a guidelines
CONCLUSIONS
RI
4
PT
for the choice of ionic liquids in the purpose of extraction.
SC
1-butyl-3(methyl or ethyl) substituted imidazolium or pyrrolidinium based ionic liquids with bromide and dicyanamide anions suitable for ABS formation were applied for simultaneous
NU
extraction of five pesticides of different polarity. The effect of cation on ability of the targeted ILs with the same anion to form ABS follows the order: [beim][DCA] ~
MA
[bmpyr][DCA] > [bmim][DCA]. Ionic liquids with ethyl group in the imidazolium ion have higher ability to form ABS, due to a higher hydrophobicity of the cation and lower affinity
D
for water molecules. The results of the pesticides extraction in the studies ABSs show that
PT E
highest values of partition coefficient were obtained in the ABS based on [beim][DCA]. The low and medium polar compound (Imi, Ace and Sim) show significantly higher partitioning
CE
in the ABSs based on the studied ILs comparing to n-octanol/water system and liquid-liquid extraction based on water immiscible IL. Low polar pesticides (Lin and Teb) exhibit the
AC
similar partitioning in the ABS as in both classical liquid-liquid extraction systems based on n-octanol or water immiscible IL as extractant. Also, the binding energies and non-covalent interactions between studied pesticides and ILs were calculated applying DFT approach. The main interaction between ILs and the polar pesticides which lead to their complete extraction were explained based on molecular simulation results. This study shows that ABS based on properly selected ILs give excellent opportunity for one-step simultaneous extraction of various polarity pesticides. It was also presented that computational approach together with
22
ACCEPTED MANUSCRIPT the experimental results can be efficiently applied for the selection of suitable ionic liquids for efficient extraction of polar pesticides which can be extended to other compounds.
Funding: This work was supported by the Ministry of Education, Science and Technological
PT
Development of Serbia under project contracts III 45006 and ON 172012.
RI
The authors have declared no conflict of interest.
NU
Action CM1206 – Exchange on Ionic Liquids.
SC
Acknowledgements: The authors would like to acknowledge the contribution of the COST
AC
CE
PT E
D
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Conflicts of interest: none.
23
ACCEPTED MANUSCRIPT REFERENCES [1]
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The new aqueous biphasic systems based on ionic liquids were investigated for pesticides extraction. One-step simultaneous extraction of different polarity pesticides was performed. The extraction of polar pesticides was explained by computational approach. The experimental and computational approaches for the selection of ionic liquid for efficient extraction.
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