Comparison of Extraction Capacities Between Ionic Liquids and Dichloromethane

Comparison of Extraction Capacities Between Ionic Liquids and Dichloromethane

CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 34, Issue 5, May 2006 Online English edition of the Chinese language journal Cite this article as: Chi...

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CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 34, Issue 5, May 2006 Online English edition of the Chinese language journal

Cite this article as: Chin J Anal Chem, 2006, 34(5), 598−602.

RESEARCH PAPER

Comparison of Extraction Capacities Between Ionic Liquids and Dichloromethane Fu Xinmei, Dai Shugui*, Zhang Yu College of Environmental Science & Engineering, Nankai University, Tianjin 300071, China

Abstract: Extraction capacities between ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate, that is, C4, and 1-hexyl-3-methylimidazolium hexafluorophosphate, that is, C6) and dichloromethane were compared with respect to the extraction of 4-Nonylphenol (NP) and 4-tert-Octylphenol (OP) from water samples under different conditions. The results indicated that the extraction time leading to equilibrium was less in dichloromethane (20 min) than in ionic liquids C4 and C6 (60 min). When the pH in water increased from 1 to 13, the extraction efficiency of the ionic liquid C4 decreased from 72% to 35% for OP and from 61% to 34% for NP, the extraction efficiency of the ionic liquid C6 decreased from 68% to 25% for OP and from 56% to 28% for NP, and the extraction efficiency of dichloromethane decreased from 75% to 44% for OP and from 72% to 48% for NP. From the results of salt-out effect, it was concluded that the salt-out effect on the ionic liquids was less significant than that on dichloromethane. The extraction efficiencies were also influenced by the OP and NP concentrations and they decreased as the analyte concentration increased from 10 μg/l to 100 μg/l. The extraction capacities of dichloromethane (37.5 μg/l) on analytes were higher than those of ionic liquids (20 μg/l–30μg/l). The extraction efficiency of ionic liquids and dichloromethane increased as temperature rose from 25℃ to 55℃. Similar extraction properties were found in ionic liquids and dichloromethane when used to extract organic chemicals, that is, their extraction efficiency could be affected by acidity, temperature, and analyte concentrations. Key Words:

1

Ionic liquid; Dichloromethane; Extraction capacity; 4-Nonylphenol; 4-Tert-octylphenol

Introduction

In recent years, ionic liquids (ILs) have evoked increasing interest as environmental benign green solvents, primarily as replacements for environmentally damaging Volatile Organic Compounds (VOCs) in chemical processes under environmental pressure. Owing to their unique chemical and physical properties, ILs have been widely used in different areas, including organic synthesis[1], separation processes[2,3], and electrochemistry[4]. Traditional liquid-liquid extraction employs an organic solvent and an aqueous solution as the two immiscible phases. However, the organic solvents used are generally toxic, flammable and volatile. ILs are suitable and favorable novel solvents that could replace traditional VOCs in liquid-liquid extraction systems as a result of their unique properties, such as their negligible vapor pressure, nonflammability, and good solubility with organic and inorganic compounds.

ILs have already been applied to extract various organic compounds[2,5,6] and metal ions[7–10] from aqueous solutions, and the results of these studies demonstrated that the use of ILs as alternative solvents to replace traditional organic solvents in liquid-liquid extraction is very promising. However, there have been few researches to evaluate the differences of extraction capacities between ILs and traditional VOCs. This prevents the application of ILs in liquid-liquid extraction as alternative potential replacements for volatile organic solvents. There is considerable interest in the environmental endocrine disruptors (EEDs) since the 1990s, because of their deleterious effects on human health[11]. Among these EEDs, phenolic EEDs, such as 4-nonylphenol (4-NP) and 4-tert-octylphenol (4-t-OP), was the utmost concern to governments and scientists of developed nations because of their high production, widespread use, and ubiquitous occurrence in the environment[12]. So there was a need for the quantitative determination of 4-NP and 4-t-OP from

Received 20 June 2005; accepted 20 November 2005 * Corresponding author. Email: [email protected] This study was supported by the National Natural Science Foundation of China (20377025).

FU Xinmei et al. / Chinese Journal of Analytical Chemistry, 2006, 34(5): 598–602

environmental samples, and many related analytical methods have been reported. 1-butyl-3-methylimidazolium hexafluorophosphate, that is, C4, and 1-hexyl-3-methylimidazolium hexafluorophosphate, that is, C6 were the most widely used ILs, because of their unique properties, such as their existence in liquid state at room temperature, their air and water stability, immiscibility with water, and high solubility of organic compounds. Therefore, in the present study, C4, C6 and dichloromethane, one of the most widely used traditional VOCs, were selected as extraction solvents. To evaluate the potential use of ILs to replace traditional VOCs in liquid-liquid extraction, comparison of extraction capacities between ionic liquids (C4 and C6) and dichloromethane was made in the extraction of 4-Nonylphenol (NP) and 4-tert-Octylphenol (OP) from water samples under different conditions.

2

Experimental

2.1

Apparatus and reagents

Equipment Constant-temperature vortex THZ-C was supplied by Taicang instrument manufacturer in Jiangsu province, China. Vibration speed and temperature was set at 120 rpm and 25 ℃, respectively, in all the tests. The HPLC systems LC-10AT (Shimadzu, Tokyo, Japan) were equipped with a fluorescence detector RF-10A XL (Shimadzu, Tokyo, Japan) set at 223-nm excitation and 302-nm emission. The chromatography was carried out on a Shimadzu Shim-pack VP-ODS column (150 mm×4.6 mm I.D., 5 μm particle size). The mobile phase was acetonitrile–water (80:20, v/v) at the rate of 1 ml/min. HPLC-grade methanol and acetonitrile was bought from Scharlau (Barcelona, Spain). Reagents for synthesis of ILs C4 and C6 including 1-methylimidazole (99%), 1-chlorobutane (99%), 1-bromohexane (98%), and hexafluorophosphoric acid (60 wt% solution in water) were obtained from Acros Organics (Geel, Belgium) and used directly. Hexafluorophosphoric acid was a corrosive and toxic chemical and therefore requires careful handling. A mixture of isomers, 4-NP (100%) and 4-t-OP (99%) was purchased from Dr. Ehrenstorfer (Ausburg, Germany). All the other chemicals were analytical grade reagents. Ultrapure water was produced on the system of Millipore S.A.S. 67120 Molsheim (France). The pH value of aqueous solutions was adjusted using HCl or NaOH. ILs C4 and C6 (shown in Fig. 1) were prepared according to the published literature[17] and were identified using FT-IR, 1H NMR spectroscopies. 2.2

4-t-OP were spiked) was transferred to a 20-ml graduated centrifuge tube with a cap and 0.4 ml of organic solvent (dichloromethane, C4 or C6) was added, and the tube was capped. The mixture was shaken mechanically (vortex, 120 rpm) for a prescribed time. The organic layer was allowed to separate from the water phase completely. The water phase was removed using a disposable glass pipette, without disturbing the organic phase. If dichloromethane was used as an extraction solvent, it was evaporated to dryness using a gentle stream of clean, dry nitrogen. This step could be omitted in ILs. After extraction, acetonitrile was added to the extract to a final volume of 1.0 ml (calibration of the volume employed in the test was checked in advance). In the case of dichloromethane, 1.0 ml of acetonitrile was added. Then 20 μl of the sample extract was injected into the HPLC system for analysis. The amount of analytes in the aqueous phase was calculated by subtracting the amount of analytes in the organic phase from the original total amount of analytes.

Extraction procedure

Twenty milliliters of water sample (50μg/l of 4-NP and

Fig. 1 Structures of ionic liquids C4(A) and C6(B)

3 3.1

Results and discussion Comparison of extraction time

Two kinds of solvents including C4 and C6, the typical ILs, and dichloromethane, a typical traditional organic solvent, were compared as extraction solvents at different extraction times (5, 10, 20, 30, 60, 90 and 120 min) in this study. Results (extracting 20 ml of water sample containing 4-NP and 4-t-OP at the 50 μg/l level with 0.4 ml of organic solvent) in Fig. 2 and Fig. 3 show the effect of organic solvents and extraction time on the extraction efficiency. The extraction efficiency is defined as the fraction of the analyte extracted from the water sample into the extraction solvent, that is, no/na, where no is the amount of analyte in the organic phase after extraction, and na is the original total amount of analyte in the aqueous phase. As can be seen in Fig. 1, when C4 and C6 were used as extraction solvents, the extraction efficiencies increase with an increase in extraction time up to 60 min, above which the extraction efficiency remained constant because the extraction equilibrium was reached between the two phases. However, for dichloromethane, the extraction efficiency increased with the increase in extraction time up to 20 min, remained unchanged between 20 min and 30 min, and then decreased gradually with longer extraction time. This is probably because dichloromethane is volatile, and the volume of the existing dichloromethane decreases significantly after extraction equilibrium (20–30 min), leading to the reduction

FU Xinmei et al. / Chinese Journal of Analytical Chemistry, 2006, 34(5): 598–602

of the extraction efficiency. Extraction equilibrium time in the dichloromethane-water system was much less than that in the [C4MIM][PF6],-water system, which was because of the fact that the viscosity of dichloromethane (0.43 mPa s) was much lower than that of C4 (312 mPa s)[13] or C6 (586 mPa s)[14], and thus the mass transfer rate was faster in dichloromethane than in C4 and C6. It is clear from Fig. 2 that dichloromethane possessed a higher extraction

efficiency for both 4-NP and 4-t-OP than C4 and C6. However, being volatile and toxic, dichloromethane could pollute air and could lead to harmful effects on the people handling such chemicals. When dichloromethane was used as the extracting solvent, it was necessary to evaporate dichloromethane to dryness using nitrogen just before injecting the sample extract into HPLC for analysis, whereas this operation was not needed in ILs.

Fig. 2 Variation curve of extraction efficiency versus time of ionic liquids C4 and C6 (a) and dichloromethane (b) a: ◆ C4-OP; ■ C4-NP; ▲ C6-OP; × C6-NP; b: ◆ OP; ■ NP

3.2

Effect of pH value

To compare the extraction capacities of ILs and the traditional solvent (dichloromethane) under a different pH value, the pH in water was increased from 1 to 13. The influence of pH value on the extraction of 4-NP and 4-t-OP with ILs and dichloromethane is shown in Fig. 3. All F curves exhibited a similar trend, where the extraction efficiencies decreased with the increase in pH from 1 to 13. Being weak acids, 4-NP and 4-t-OP were mainly in their neutral forms at a low pH that preferred to distribute in the

organic phase. This led to high extraction efficiency at a low pH. On the contrary, low extraction efficiency was obtained at high pH, because 4-NP and 4-t-OP were partially in their deprotonated forms that prefer partition in the aqueous phase. When pH values were above 13, the extraction efficiencies of ILs and dichloromethane were both less than 45%. It could be concluded that both kinds of biphasic systems showed similar behavior and the extraction was strongly dependent on the pH value of the extraction system. This phenomenon was in agreement with that reported in a previous literature[2].

Fig. 3 Variation curve of extraction efficiency versus pH of ionic liquids C4 and C6 (a) and dichloromethane (b) a: ◆ C4-OP; ■ C4-NP; ▲ C6-OP; × C6-NP; b: ◆ OP; ■ NP

3.3

Salt-out effect

The addition of a salt could often improve the extraction efficiency of many compounds when conventional VOC based LLEs were used[15,16]. Sodium chloride (NaCl) was normally used in this regard. In the current study, the effect of salt-out on ILs and dichloromethane based LLE was investigated by adding different amounts of sodium chloride (NaCl) to the water sample. The results shown in Fig. 4a indicated that the increase of NaCl concentration in aqueous phase, that is, 0, 5, 10 and 20g/100 ml, had no significant influence on the extraction of 4-NP and 4-t-OP with ILs. However, for

dichloromethane, the results (Fig. 4b) showed an initial increase of extraction efficiency with an increase in salt concentration, with a maximum of 90% or so being reached at 5 g/100 ml, followed by an unchanging extraction efficiency with a further increase in salt concentration to 20 g/100 ml. On the basis of the above observations, it seemed that the salt-out effect on ILs was less significant than on dichloromethane. 3.4 Effect of concentration of analytes in the water sample The effect of the concentration of 4-NP and 4-t-OP in the water sample on the extraction efficiency of ILs and

FU Xinmei et al. / Chinese Journal of Analytical Chemistry, 2006, 34(5): 598–602

dichloromethane based LLE was studied. It could be seen from Fig. 5 that an increase of analyte concentration from 10 μg/l to 100 μg/l correspondingly decreased the extraction efficiency of the three extraction solvents. On the basis of the above experiments, the extraction capacities were calculated when the extraction efficiency was above 90%, and the phase ratio (the aqueous phase:organic phase ratio) was 50:1, that is, extracting 20 ml of water sample with 0.4 ml of organic solvent. The extraction capacities of 1 ml of dichloromethane for 4-NP and 4-t-OP were both 37.5 μg/l. The extraction capacities of 1 ml C4 for 4-NP and 4-t-OP were 26.7 μg/l and 30 μg/l, respectively, and the extraction capacities of 1 ml C6 for 4-NP and 4-t-OP were 20.5 μg/l and 24.3 μg/l, respectively. 3.5

Effect of temperature

Temperature is an important parameter in LLE. If an extraction thermodynamics process was assumed to be a macroscopic process, equilibrium distribution ratios are described as follows: lnD = C+(–ΔH/(RT)), where D is the ratio of the analyte concentration in the organic phase to that in the aqueous phase at the extraction equilibrium point, C is the constant value, ΔH is the enthalpy change in the extraction process, R is the molar gas constant(8.314 J mol–1 K–1), and T is the absolute temperature (K). The thermodynamics of extracting 4-NP and 4-t-OP with ILs and dichloromethane was studied. By investigating the influence of temperature (298, 308, 318, 328K) on equilibrium distribution ratios; the relationship between them is shown in

Fig. 6. It was calculated from Fig. 6 that ΔH was 20.435 kJ mol–1 for OP and 23.519 kJ mol–1 for NP when C4 was the extraction solvent, and in the case of C6, it was 103. For OP it was 717 kJ mol–1 and 85.78 kJ mol–1 for NP. From this it could be seen that the extraction process of 4-NP and 4-t-OP with ILs was an endothermic reaction process and it was beneficial to increase temperature during the extraction process. Compared with ILs, dichloromethane did not present any obvious role in the thermodynamic extraction process as its D had no certain rule (Table 1) because it was easily volatile (its boiling point is 40.2℃). Table 1 Variation of extraction efficiency versus temperature of dichloromethane T/℃

OP extraction efficiency (%)

25

69

NP extraction efficiency (%) 70

30

76

78

35

70

73

From the results above mentioned, it could be concluded that LLE application areas of ILs with a wide range of liquid state (400℃)[1] are more than those of traditional VOCs with volatility, especially under high temperatures. After the systematic investigations had been done, as above, the results showed that the extraction efficiencies of dichloromethane for 4-NP and 4-t-OP were both higher than those of the ILs, and those of the ILs for 4-t-OP were slightly higher than those for 4-NP, whereas those of dichloromethane for 4-t-OP were the same as those for 4-NP.

Fig. 4 Variation curve of extraction efficiency vs. NaCl concentration of ionic liquids C4 and C6 (a) and dichloromethane (b) a: ◆ C4-OP; ■ C4-NP; ▲ C6-OP; × C6-NP; b: ◆ OP; ■ NP

Fig. 5 Variation curve of extraction efficiency of ionic liquids C4 and C6 (a) and dichloromethane (b) vs. NP and OP concentrations a: ◆ C4-OP; ■ C4-NP; ▲ C6-OP; × C6-NP; b: ◆ OP; ■ NP

FU Xinmei et al. / Chinese Journal of Analytical Chemistry, 2006, 34(5): 598–602

[2]

Huddleston J G, Willauer H D, Swatloski R P, Visser A E, Rogers R D. Chem. Commun., 1998: 1765–1766

[3]

Liu J F, Jonsson J A, Jiang G B. Trends in Analytical Chemistry, 2005, 24(1): 20–27

[4]

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[5] Fig. 6 Variation curve of equilibrium distribution ratio versus

[6]

temperature of ionic liquids C4 and C6 ◆ C4-OP; ■ C4-NP; ▲ C6-OP; × C6-NP

Fadeev A G, Meagher M M. Chem. Commun., 2001: 295–296 Visser A E, Holbrey J D, Rogers R D. Chem. Commun., 2001: 2484–2485

[7]

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4

Conclusions

[8]

Chun S K, Dzyuba S V, Bartsch R A. Anal. Chem., 2001, 73: 3737–3741

After a comparative study of the extraction effect of NP and OP in water by ILs and the typical traditional organic solvent dichloromethane, the results showed that similar extraction behaviors were found between ILs and dichloromethane when they were used to extract typical organic chemicals under different influencing factors such as acidity, temperature, analyte concentrations etc. On account of their unique properties such as the negligible vapor pressure, nonflammability, and a long liquid range, ILs have very important advantages over traditional VOCs as extraction solvents in LLE systems. Owing to lesser degree of pollution in the environment, ILs are receiving a surge of interest as green solvents, within the field of “Green Chemistry”, to replace the traditional organic solvents.

[9]

Visser A E, Swatloski R P, Griffin S T. Separation Science and Technology, 2001, 36(5–6): 785–804

[10] Visser A E, Rogers R D. Journal of Solid State Chemistry, 2003, 171: 109–113 [11] Sharpe R M, Tumer K J, Sumpter J P. Environ. Health Perspect, 1998, 106(5): 220–221 [12] Ying G G, Williams B, Kookana R. Environment International, 2002, 28: 215–226 [13] Bonhote P, Dias A P, Papageorigiou N. Inorg. Chem., 1996, 35: 1168–1178 [14] Jonathan G, Huddleston A E, Visser W, Reichert M, Willauer H D, Broker G A, Rogers R D. Green Chemistry, 2001, 3: 156–164 [15] Zhang A L, Zhou J T, Han M, Teng L M, Huang L P, Wang D, Wang J. Chinese J. Chromatogr., 2001, 19(2): 144-146 [16] Cancho B, Ventura F, Galceran M T. J. Chromatogr. A, 2001,

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