Extraction of Phenols from Water with Functionalized Ionic Liquids

Article pubs.acs.org/IECR

Extraction of Phenols from Water with Functionalized Ionic Liquids Yunchang Fan, Yun Li, Xing Dong, Guitao Hu, Shaofeng Hua, Juan Miao,* and Dongdong Zhou College of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, China S Supporting Information *

ABSTRACT: A series of hydroxyl-, benzyl-, and dialkyl-functionalized ionic liquids (ILs) were synthesized, and their extraction abilities for phenol, resorcinol, p-nitrophenol, guaiacol, and o-cresol were investigated. Results showed that the extraction efficiencies of the five phenols were significantly influenced by the pH values, salt added, phase ratio, and chemical structure of the IL. Phenols present in nonionized forms were preferable to transfer into IL phases. The anion/cation hydrogen-bonding characters of ILs were the main structural factors affecting the extraction efficiency, which also increased with increased hydrophobicity of phenols. These results are promising for the liquid−liquid extraction and enrichment of phenols in separation science and related industrial processes.

1. INTRODUCTION Phenols are intermediates widely used to manufacture medicines, resins, pesticides, dyes, and explosives and are considered as priority pollutants because of their high persistence and toxicity.1 Many techniques including solvent extraction,2 adsorption,1,3 catalytic oxidation,4 and biological degradation5 have been developed for the removal of phenols from water. Among these techniques, solvent extraction is considered to be the preferred option in the industrial separation process because of its ease of operation and lower cost. The most commonly used organic solvents, such as benzene, butyl acetate, and chloroform, are volatile, flammable, and toxic.2 Therefore, the development of environmentally benign solvents is a research hotspot. Ionic liquids (ILs) are regarded as more environmentally friendly solvents than traditional organic solvents.6 Several papers have reported the use of ILs for the extraction of phenols from water.7−16 Pletnev et al.7,8 used imidazoliumbased and quaternary ammonium-based ILs to extract phenols, which are preferably partitioned into ILs in nonionized forms. Huang et al.9 used a surfactant-anion-based IL, tetrabutylphosphonium dioctyl sulfosuccinate, to remove phenols. This IL exhibited highly efficient extraction of phenols because of the reduced energy at the IL−aqueous phase interface. Deng et al.12 synthesized a hydrophobic magnetic IL, trihexyltetradecylphosphonium tetrachloroferrate(III) ([3C6PC14][FeCl4]), for the removal of phenols. This IL exhibited better extraction ability than the traditional nonfunctionalized ILs, and the extraction efficiency was remarkably influenced by the aqueous pH values and the chemical structures of ILs and phenols. In our previous work, we have also used 1,3-dialkylimidazoliumbased ILs to extract phenols from water.10 We found that hydrogen-bonding and hydrophobic interactions play important roles in the extraction of phenols. The IL 1-methyl-3octylimidazolium tetrafluoroborate ([C8mim]BF4) is the most effective one for the extraction of phenols. Despite the progress made in this field, several issues remain to be address. First, given that hydrogen bonding is the main driving force, it is necessary to investigate whether the incorporation of a hydroxyl group into the IL cation can improve the extraction © 2014 American Chemical Society

efficiency and is necessary to investigate. Second, [C8mim]BF4 and water easily form an emulsification system under stirring, which is problematic for phase separation. Therefore, BF4−based ILs without emulsification in water are also necessary. Third and last, π−π stacking between imidazolium and benzene rings17,18 and that between neighboring aromatic rings19 are well established; thus, whether the introduction of a benzene ring into the IL cation can enhance the extraction ability is interesting to study. On the basis of these ideas, N-butylimidazole-derived ILs, incorporating hydroxyl, alkyl, and benzyl groups, were synthesized. The aim of this work was therefore to investigate the extraction abilities of these ILs for phenols.

2. EXPERIMENTAL SECTION 2.1. Materials. N-Butylimidazole (99%), 8-chloro-1-octanol (98%), and N-benzylimidazole (98%) were purchased from Alfa Chem., Ltd. (Berkshire, U.K.). 6-Chloro-1-hexanol (98%), 1-chlorododecane (99%), lithium bis(trifluoromethanesulfonyl)imide (98%), and potassium hexafluorophosphate (99%) were obtained from Energy Chemical Co. (Shanghai, China). 11-Bromo-1-undecanol (≥99%) was purchased from Fluka, Sigma-Aldrich Co. (St. Louis, MO). 1-Bromoheptane (98%), HPLC-grade acetonitrile, 1-bromobutane (>99%), phenol (≥99%), resorcinol (99%), p-nitrophenol (99%), guaiacol (>99%), and o-cresol (99%) were obtained from Aladdin Reagent Co. (Shanghai, China). 1-Chlorononane (98%) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). 1-Methyl-3-octylimidazolium hexafluorophosphate (99%, [C8mim]PF6), 1-methyl-3-octylimidazolium tetrafluoroborate (99%, [C8mim]BF4), and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide (99%, [C8mim]NTf2) were obtained from Lanzhou Institute of Chemical Physics of the Chinese Academy of Sciences (Lanzhou, China). All of the other chemicals were of analytical grade unless stated Received: Revised: Accepted: Published: 20024

August 30, 2014 December 1, 2014 December 4, 2014 December 4, 2014 dx.doi.org/10.1021/ie503432n | Ind. Eng. Chem. Res. 2014, 53, 20024−20031

Industrial & Engineering Chemistry Research

Article

otherwise. Ultrapure water (18.2 MΩ cm) produced by an Aquapro purification system (Aquapro International Co., Ltd., Dover, DE) was used throughout the experiments. 1-Butyl-3alkylimidazolium-based ILs were synthesized based on our previous work.10 Hydroxyl- and benzyl-functionalized ILs were prepared according to the literature.20−23 All of the synthesized ILs were characterized by 1H and 13C NMR spectroscopy (Bruker, AV-400, Karlsruhe, Germany) and elemental analysis (FLASH 2000 analyzer, Thermo Fisher Scientific, Belmont, MA). These data and the synthesis procedures are all shown in the Supporting Information (SI). The names and abbreviations of the ILs used in this work are listed in Table 1, and the chemical structures of the IL cations are shown in Figure 1.

Figure 1. Chemical structures of the IL cations used in this work; (a) [C4Cnim]+; (b) [C8mim]+; (c) [C4CnOHim]+; (d) [C4Beim]+.

Phenol-containing wastewater (pH 11.4) was supplied by a local coking plant (Jiaozuo, China); it was a clear yellow liquid with no sediment. After adjustment of the pH value to 7.0 with 85% (wt %) phosphoric acid, the water was directly used for extraction without any further pretreatment. The pH values of the aqueous solutions were adjusted with phosphate buffers (0.1 mol L−1) and measured with a pHS-3B digital pH-meter (Shanghai Leici Instrument Factory, Shanghai, China). 2.2. Measurements of Phenols. The concentrations of the phenols in aqueous solutions were determined with an Agilent 1200 high-performance liquid chromatograph (HPLC; Agilent, Santa Clara, CA). An Amethyst C18-H column (4.6 mm × 150 mm, 5 μm, Sepax Technologies Inc., Newark, DE) was used to separate the phenols. The mobile phase was a mixture of acetonitrile and a 0.20% (v/v) acetic acid aqueous solution (20:80, v/v), the flow rate was 0.90 mL min−1, the injection volume was 20 μL, and the detection wavelength was 280 nm. 2.3. Measurement of the Octanol−Water Partition Coefficients (Pow) of ILs and Phenols. Pow’s of ILs and phenols were measured as described in the literature.24,25 In a typical procedure, 10.0 mL of water saturated with octanol (containing a known amount of solute) and 10.0 mL of octanol saturated with water were mixed under stirring for 30 min at 298 K. The two phases were separated by centrifugation. A cation-exchange column (SuperIC-Cation/P, 4.6 mm × 15.0 cm, Tosoh Corp., Tokyo, Japan) was used to analyze the ILs. The aqueous phase was directly injected into the HPLC system. After being diluted five times with acetonitrile, the octanol phase was injected into the HPLC system. The injection volume was 20 μL, and the detection wavelength was 220 nm. For analyses of [C4C12im]-based ILs, the mobile phase was a mixture of acetonitrile and 2.0 × 10−3 mol L−1 of H2SO4 (70:30, v/v) and the flow rate was 0.6 mL min−1. For analyses of all other ILs, the composition of the mobile phase was acetonitrile and 2.0 × 10−3 mol L−1 of H2SO4 (50:50, v/v) and the flow rate was 0.5 mL min−1. For detection of the phenols in both the aqueous and octanol phases, the chromatographic conditions were the same as those mentioned in section 2.2, except that the injection volumes of both the aqueous and octanol (without dilution) phases were set at 2.0 μL. The IL concentrations were all 1.0 × 10−4 mol L−1, and the contents of the phenols were all 1.0 × 10−3 mol L−1. The P value was calculated by

Table 1. Abbreviations and Water Solubilities of the ILs Used in This Work

IL 1-methyl-3-octylimidazolium hexafluorophosphate 1-methyl-3-octylimidazolium tetrafluoroborate 1-butyl-3-(6-hydroxyhexyl)imidazolium hexafluorophosphate 1-butyl-3-(6-hydroxyhexyl)imidazolium tetrafluoroborate 1-butyl-3-heptylimidazolium hexafluorophosphate 1-butyl-3-heptylimidazolium tetrafluoroborate 1-butyl-3-(8-hydroxyoctyl)imidazolium hexafluorophosphate 1-butyl-3-(8-hydroxyoctyl)imidazolium tetrafluoroborate 1-butyl-3-nonylimidazolium hexafluorophosphate 1-butyl-3-nonylimidazolium tetrafluoroborate 1-butyl-3-(11-hydroxyundecyl)imidazolium hexafluorophosphate 1-butyl-3-(11-hydroxyundecyl)imidazolium tetrafluoroborate 1-butyl-3-dodecylimidazolium hexafluorophosphate 1-butyl-3-dodecylimidazolium tetrafluoroborate 1-butyl-3-benzylimidazolium hexafluorophosphate 1-butyl-3-benzylimidazolium tetrafluoroborate 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide 1-butyl-3-(6-hydroxyhexyl)imidazolium bis(trifluoromethylsulfonyl)imide 1-butyl-3-heptylimidazolium bis(trifluoromethylsulfonyl)imide 1-butyl-3-(8-hydroxyoctyl)imidazolium bis(trifluoromethylsulfonyl)imide 1-butyl-3-nonylimidazolium bis(trifluoromethylsulfonyl)imide 1-butyl-3-(11-hydroxyundecyl)imidazolium bis(trifluoromethylsulfonyl)imide 1-butyl-3-dodecylimidazolium bis(trifluoromethylsulfonyl)imide 1-butyl-3-benzylimidazolium bis(trifluoromethylsulfonyl)imide

abbreviation

water solubility (g L−1)

[C8mim]PF6

2.80

[C8mim]BF4

17.6

[C4C6OHim] PF6 [C4C6OHim] BF4 [C4C7im]PF6

16.2

[C4C7im]BF4

10.8


8.40

[C4C9im]BF4

3.10


1.60

[C4C12im]BF4

1.00

[C4Beim]PF6

3.40

[C4Beim]BF4

21.5

[C8mim]NTf2

1.00

[C4C6OHim] NTf2 [C4C7im]NTf2

4.70


2.10


0.410

[C4Beim]NTf2

1.40

miscible 3.00

26.4 0.510

10.5 0.390

0.360

0.180

Pow = Coctanol /Cwater

0.0600

(1)

where Coctanol and Cwater are the concentrations of an IL or a phenolic compound in the octanol and water-rich phases, respectively. 20025

dx.doi.org/10.1021/ie503432n | Ind. Eng. Chem. Res. 2014, 53, 20024−20031


Article

2.4. Solubility Measurements. The solubility of each IL in water was measured by spectrophotometry at 298 K according to the literature.26 Spectroscopic measurements were performed at 220 nm using a spectrophotometer (TU1810, Purkinje General Instrument Co., Ltd., Beijing, China). 2.5. Measurements of Fluoride (F−) in Water. The contents of F− were determined by an ion chromatograph (IC2001, Tosoh Corp., Tokyo, Japan) equipped with a conductivity detector; a TSKgel SuperIC-AZ anion exchange column (4.6 mm × 15.0 cm, 4 μm, Tosoh Corp., Tokyo, Japan) was used to analyze F−. The mobile phase was a mixture of NaHCO3 (6.3 × 10−3 mol L−1) + Na2CO3 (1.7 × 10−3 mol L−1); the flow rate was 0.7 mL min−1, the injection volume was 30 μL, and the detection mode was suppressed conductivity. 2.6. Extraction Procedure. Extraction of phenols was conducted with a constant-temperature magnetic stirrer (Yuhua Instrument Co., Ltd., Gongyi, China) at 298 ± 1 K. In a typical procedure, 0.50 mL of an IL was mixed with 5.0 mL of water (concentrations of the five phenols were all 1.0 × 10−3 mol L−1). The system was vigorously stirred for 5.0 min and then centrifuged to achieve phase separation. The extraction efficiency (E) was calculated using the following equation: E = (1 − Cw /Cw °) × 100%

shown in Figure 2. The extraction efficiencies of phenol, resorcinol, guaiacol, and o-cresol remain constant within the

Figure 2. pH dependence of the extraction efficiencies of the five phenols between [C4C8OHim]PF6 and water phases: (■) resorcinol; (●) phenol; (▲) guaiacol; (◆) o-cresol; (▼) p-nitrophenol.

range of pH 2.0−10 and then steeply decrease at pH > 10. A similar phenomenon is also observed for p-nitrophenol; i.e., its extraction efficiency almost remains a plateau within the range of pH 2.0−7.0 and decreases beyond this level. These results indicate that phenols existing in nonionized forms are preferable for extraction. Considering that the nonionized phenols have hydroxyl groups and ILs can form hydrogen bonding,29 transfer of the nonionized phenols to the IL phase can be attributed to hydrogen bonding between the phenols and ILs. When the pH values are higher than the pKa values, phenols lose the proton in the OH group and thus exist in anionic forms, thereby reducing hydrogen-bonding interactions between the phenols and ILs and resulting in lower extraction efficiencies. Given that pH 7 provides higher extraction efficiencies, it was selected as the optimal value for subsequent studies. 3.2. Thermodynamic Analysis. The driven forces involved in the transfer of phenols to the IL phase include van der Waals forces, electrostatic, hydrogen-bonding, and hydrophobic interactions, and π−π stacking. Thermodynamic parameters such as the enthalpy change (ΔH), Gibbs freeenergy change (ΔG), and entropy change (ΔS) are the main parameters for the determination of driven forces.30 The Gibbs free-energy change (ΔG) of the extraction process can be calculated from the distribution ratio using the following equation:30

(2)

where Cw° and Cw are the concentrations of phenols in the water phase before and after extraction, respectively. Additionally, the relationship of the extraction efficiency (E) and distribution ratio (D) can be described by the equation E = D /(D + Vw /VIL)

(3)

The distribution ratio (D) is defined as

D = C IL /Cw

(4)

where CIL and Cw are the concentrations of phenols in the IL and water phases, respectively. The concentrations of phenols in the water phase were determined by the aforementioned HPLC method, and the concentrations in the IL phase were calculated by mass balance. For accurate measurements of the D values, the aqueous and IL phases were mutually saturated with each other to reduce their volume changes.

3. RESULTS AND DISCUSSION The IL [C4C6OHim]BF4 is totally miscible with water, and [C4Beim]PF6 exists in the solid state at room temperature (melting point = 48−50 °C); thus, they were not used in this work. Preliminary experiments indicated that extraction equilibrium was achieved within 5 min. Therefore, a phase contact time of 5 min was used in all subsequent experiments. 3.1. Effect of the pH on the Extraction Efficiency. Phenols are present in different species because of ionization of their hydroxyl groups. The ionization constants (pKa) of phenol, guaiacol, p-nitrophenol, and o-cresol are 9.99, 9.99, 7.15, and 10.26, respectively.27 The pKa1 and pKa2 values of resorcinol are 9.8 and 11.3, respectively.28 Phenols exist in nonionized forms within the range of pH < pKa (pKa1 for resorcinol) and are mainly present in anionic forms above this level. Experimental data of the extraction efficiencies of the five phenols at different pH values are listed in Table S1 in the SI (volume ratio of water to IL phases = 10:1 and concentration of phenols = 1.0 × 10−3 mol L−1). As an example, the pH dependence of the extraction efficiencies of [C4C8OHim]PF6 is

ΔG = −RT ln D

(5)

where R is the gas constant. If ΔH and ΔS values do not vary significantly over the temperature range studied, they can be calculated with the van’t Hoff equation: ln D = −ΔH /RT + ΔS /R

(6)

The resulting ΔG, ΔH, and TΔS values along with the D values for [C4C11OHim]BF4 at pH 7.0 and the phase ratio of 10:1 are included in Table 2. The values of ΔG are negative, indicating that the extraction processes are spontaneous. The values of ΔH and ΔS for resorcinol are negative, which are the main features of hydrogen bonding, as suggested by Ross.31 This well explains the aforementioned results that the incorporation of hydroxyl on the IL cations can remarkably enhance the 20026



Article

cresol and p-nitrophenol) are extracted better and are in agreement with the results of thermodynamic analysis. 3.4. Effect of the Chemical Structures of the ILs. 3.4.1. Effect of the IL Solubility. To clearly depict the influence of the chemical structure of the ILs, the extraction efficiencies of resorcinol by ILs at pH 7.0, as an example, are shown in Figure 3A. For the convenience of comparison, the ILs were divided into eight groups based on the classification criteria that ILs in the same group had the same anion but different alkyl chain length on the cation: (I) [C8mim]NTf2, [C4C7im]NTf2, [C 4C 9 im]NTf 2 , and [C 4 C12 im]NTf 2 ; (II) [C 8mim]PF 6 , [C 4 C 7 im]PF 6 , [C 4 C 9 im]PF 6 , and [C 4 C 12 im]PF 6 ; (III) [C8mim]BF4, [C4C7im]BF4, [C4C9im]BF4, and [C4C12im]BF4; (IV) [C4C6OHim]NTf2, [C4C8OHim]NTf2, and [C4C11OHim]NTf2; (V) [C4C6OHim]PF6, [C4C8OHim]PF6, and [C 4 C 1 1 OHim]PF 6 ; (VI) [C 4 C 8 OHim]BF 4 and [C4C11OHim]BF4; (VII) [C4Beim]BF4; (VIII) [C4Beim]NTf2. From groups I−VI, ILs in the same group have similar extraction efficiencies for the five phenols. As far as group VI is concerned, the extraction ability of [C4C8OHim]BF4 is lower than that of [C4C11OHim]BF4, which resulted from the significant loss of [C4C8OHim]BF4 during extraction because of its high solubility. In order to confirm this assumption, the solubility of ILs was measured and is listed in Table 1. Meanwhile, in order to compensate for the soluble losses of ILs, the water phase was presaturated with each IL before extraction, and the resulting extraction efficiencies are listed in Table S2 in the SI. The extraction efficiencies of resorcinol, as a representative, is shown in Figure 3B. It is clearly shown that the extraction ability of [C4C8OHim]BF4 is close to that of [C4C11OHim]BF4 after elimination of its soluble loss, thereby proving the above assumption. 3.4.2. Effect of the IL Hydrophobicity. In order to quantitatively assess the hydrophobicity of the ILs, the log Pow values of the ILs between octanol and water were measured, and the correlation of the extraction efficiency of resorcinol and the IL hydrophobicity is shown in Figure 4. The hydrophobicity of the ILs increases by prolonging the alkyl chain length on the IL cation. However, the increase of the hydrophobicity has no significant effect on the extraction efficiencies. This appears to contradict the thermodynamic analysis, suggesting that, besides hydrophobicity, there must have been other driven forces. As mentioned in section 3.1,

Table 2. Thermodynamic Parameters for the Extraction of Phenols to [C4C11OHim]BF4 at pH 7.0 and Vw:VIL = 10:1 (Average Values, n = 3) analyte resorcinol

phenol

guaiacol

pnitrophenol

o-cresol

T (K)

D

ΔG (kJ mol−1)

ΔH (kJ mol−1)

TΔS (kJ mol−1)

298 308 318 328 298 308 318 328 298 308 318 328 298

40.0 33.9 29.7 25.4 64.0 60.2 57.6 54.0 60.6 60.5 60.5 59.0 286.0

−9.1 −9.0 −9.0 −8.8 −10.3 −10.5 −10.7 −10.9 −10.2 −10.5 −10.8 −11.1 −14.0

−12.1 −12.1 −12.1 −12.1 −4.5 −4.5 −4.5 −4.5 0.00 0.00 0.00 0.00 0.00

−3.0 −3.1 −3.2 −3.3 5.8 6.0 6.2 6.4 10.2 10.5 10.8 11.2 14.1

308 318 328 298 308 318 328

285.6 288.0 291.4 157.1 157.1 157.2 157.0

−14.5 −15.0 −15.5 −12.5 −12.9 −13.4 −13.8

0.00 0.00 0.00 0.00 0.00 0.00 0.00

14.6 15.1 15.5 12.6 13.1 13.5 13.9

extraction efficiency of resorcinol. As shown in Table 2, the ΔH values of the remaining four phenols are very small with some of them are even equal to zero, and the ΔS values are all positive with TΔS > |ΔH|; the data suggest that the extraction processes are driven by entropy terms, which is a characteristic of hydrophobic interactions.30,32 3.3. Effect of the Hydrophobicity of Phenols. The partition coefficient (log Pow) between octanol and water is often used as a measure of the hydrophobicity of a chemical substance. The log Pow values of o-cresol, p-nitrophenol, guaiacol, phenol, and resorcinol are 1.95, 1.78 1.37, 1.47, and 0.78, respectively. Data shown in Table S1 in the SI and Figure 2 clearly point out that the extraction efficiencies of the five phenols follow the order o-cresol ≈ p-nitrophenol > guaiacol ≈ phenol > resorcinol, such that the most hydrophobic ones (o-

Figure 3. Extraction efficiencies of the ILs for resorcinol at pH 7.0 and Vw:VIL = 10:1 without (A) and with (B) presaturation of the water phase. 20027



Article

are all close to those of other BF4−-based ILs even after compensation for the soluble loss, thereby suggesting that the hydrogen-bonding interactions are the predominant factors compared with π−π stacking because of the stronger hydrogenbonding ability of BF4−. Another interesting phenomenon observed from Tables S1 and S2 in the SI and Figures 3 and 4 is that the extraction efficiencies of [C4Beim]NTf2 for the five phenols are higher than those of dialkylimidazolium-based ILs with NTf2− as the anion, which can be explained by the fact that, compared with BF4−, NTf2− has a lower hydrogenbonding ability, and π−π stacking is therefore the predominant factor. Finally, emulsification is not found for the BF4−-based ILs derived from N-butylimidazole, which is also an advantage over [C8mim]BF4. 3.5. Selection of ILs. As discussed, BF4−-based ILs exhibited the best extraction efficiency compared to their counterparts, PF6−- and NTf2−-based ILs; however, BF4− and PF6− can hydrolyze to release HF.37,38 The hydrolysis degrees of the IL anions were thus investigated, taking [C4C11OHim]NTf2, [C4C11OHim]PF6, and [C4C11OHim]BF4 as examples, by monitoring the content of F− in water. Experiments were consequently conducted by contacting 0.10 g of a given IL with 10 mL of pure water. Experimental results indicated that there is no F− to be found in the [C4C11OHim]NTf2 + water system, and the concentrations of F− in the [C4C11OHim]PF6 + water and [C4C11OHim]BF4 + water systems were 1.4 × 10−6 and 1.0 × 10−5 mol L−1, respectively, at room temperature over 30 days. After the above mixtures were heated at 348 K for 24 h, it was found that there was 1.1 × 10−5 mol L−1 of F− to be found in the [C4C11OHim]BF4 + water system, 9.6 × 10−5 mol L−1 of F− to be detected in the [C4C11OHim]PF6 + water system, and 1.0 × 10−6 mol L−1 of F− to be found in the [C4C11OHim] NTf2 + water system. The findings suggest that the effect of the temperature on the hydrolysis rate of [C4C11OHim]BF4 is negligible and the hydrolysis degree is lower; an increase of the temperature can accelerate the hydrolysis rate of [C4C11OHim]PF6, and [C4C11OHim]NTf2 can also hydrolyze to some extent at higher temperature (348 K). Generally, the NTf2−- and BF4−-based ILs are produced from LiNTf2 and NaBF4, respectively; the cost of LiNTf2 is much higher than that of NaBF4; [C4C11OHim]BF4 has low cost, low hydrolysis degree, and the best extraction ability and is thus a better choice and is used in the following experiments. 3.6. Effect of the Phase Ratio. The phase ratio is defined as the ratio of volumes of water to IL phases (Vw/VIL). As an example, the effect of the phase ratio on the extraction efficiencies of [C4C11OHim]BF4 at pH 7.0 was investigated, and the results are shown in Figure 5. The extraction efficiencies for the five phenols show an obvious decrease with increasing phase ratio. Reasonable interpretations mainly lie in two aspects: (1) with increasing phase ratio, a higher amount of a given IL dissolves in water, leaving a less bulky IL phase; (2) on the other hand, as described by eq 3 [E = D/(D + Vw/VIL)], an increase of the phase ratio results in a reduction of the extraction efficiencies. Considering the fact that the phase ratio of 10:1 has higher extraction efficiency and can reduce IL consumption, the ratio is selected as the optimal condition. 3.7. Effect of the Concentrations of Phenols. To investigate the effect of the concentrations of phenols on the extraction efficiency of [C4C11OHim]BF4, different levels of phenols (3.0 × 10−3, 1.0 × 10−3, 2.0 × 10−4, and 4.0 × 10−5 mol L−1, respectively) at pH 7.0 and Vw:VIL = 10:1 were used. The

Figure 4. Correlation of the log Pow values of the ILs and their extraction efficiencies for resorcinol.

hydrogen bonding played an important role during the extraction process and its effect is discussed in the next section. 3.4.3. Effect of the IL Anion. As indicated by the data in Tables S1 and S2 in the SI and Figures 3 and 4, ILs with BF4− have the highest extraction efficiencies compared to those of the corresponding ones with PF6− or NTf2−. Lungwitz and coworkers33 reported that, among the three anions, BF4− has the strongest hydrogen-bonding ability and the hydrogen-bonding ability of PF6− is slightly higher than that of NTf2−. Therefore, the strongest hydrogen-bonding interactions between BF4− and phenols will be expected, resulting in the highest extraction efficiencies accordingly. 3.4.4. Effect of the IL Cation. From the data shown in Table S1 and S2 in the SI and Figures 3 and 4, it is interesting to note that the ILs in group V have much higher extraction efficiencies compared to the ILs in group II; similar results are also observed for the comparison of groups I and IV, which indicates that the incorporation of a hydroxyl on the cations of the ILs with PF6− or NTf2− as the anion reinforces the hydrogen-bonding interactions between the ILs and phenols. Additionally, it is also found that the extraction efficiencies of [C4C8OHim]BF4 after compensation for its soluble loss and [C4C11OHim]BF4 for resorcinol are remarkably higher than those of the ILs in group III. The extraction efficiencies for the remaining four phenols are close to those of the dialkyl-based ILs with BF4−, indicating that the introduction of a hydroxyl on the IL cation enhances the hydrogen-bonding interaction between resorcinol and a given IL, thereby proving the results from thermodynamic analysis that hydrogen bonding is the main driven force for the extraction of resorcinol. For the other four phenols, hydrogen bonding between BF4− and phenols may predominate over those between the IL cations and phenols. 3.4.5. Role of π−π Stacking. Several papers suggested that π−π stacking between IL cations and aromatic rings of solutes also contributed to the transfer of solutes to the IL phase.34−36 The speculation is based on the X-ray crystal analysis and molecular dynamics simulation17,18 but lacks experimental data. As shown in Tables S1 and S2 in the SI and Figures 3 and 4, the extraction efficiencies of [C4Beim]BF4 for the five phenols 20028



Article

Figure 5. Effect of the phase ratio on the extraction efficiency of [C4C11OHim]BF4: (■) resorcinol; (●) phenol; (▲) guaiacol; (◆) ocresol; (▼) p-nitrophenol.

Figure 7. Effect of the concentrations of phenols on the extraction efficiency of [C4C11OHim]BF4: (■) resorcinol; (●) phenol; (▲) guaiacol; (◆) o-cresol; (▼) p-nitrophenol.

results shown in Figure 6 clearly indicate that the extraction efficiencies of the five phenols are almost constant in the

hexafluorophosphate ([C4mim]PF6),7,11 tetrahexylammonium dihexylsulfosuccinate (THADHSS),8 tetrabutylphosphonium dioctyl sulfosuccinate ([4C4P][AOT]),9 and [3C6PC14][FeCl4]12 to extract phenols from water. The distribution ratios (D) of these ILs and those of [C4C11OHim]BF4 (in the presence of 30% NaCl at pH 7.0 and a phase ratio of 10:1) were consequently compared, and the results are listed Table 3. The result indicates that the extraction ability of [C4C11OHim]BF4 for p-nitrophenol is close to that of THADHSS and higher than those of [4C4P][AOT], [3C6PC14][FeCl4] and [C4mim][PF6]. As far as the remaining four phenols are concerned, their distribution ratios in [C4C11OHim]BF4/water are the highest compared to other IL/water systems. 3.10. Application for Water Treatment. The discussion on the previous sections suggested that the optimal extraction conditions are pH 7.0, 30% NaCl, and a phase ratio of 10:1; the combination is a better choice for the enrichment of phenols for analytical purposes; however, from the point of view of industrial application, the addition of 30% NaCl is not a better choice because NaCl should be removed from water, which will increase the cost. The consequent extraction of phenols from wastewater was conducted at pH 7.0, a phase ratio of 10:1, and without the addition of NaCl. HPLC analysis indicates that the wastewater contains 1.8 × 10−4 mol L−1 of resorcinol, 4.8 × 10−3 mol L−1 of phenol, and 2.2 × 10−4 mol L−1 of o-cresol, respectively. The extraction efficiencies of [C4C11OHim]BF4 for these phenols are shown in Table 4. To examine the effect of the wastewater matrix on the extraction efficiencies of guaiacol and p-nitrophenol, the wastewater was spiked with the two phenols (1.0 × 10−3 mol L−1 for each), and the resulting extraction efficiencies are also listed in Table 4. The extraction efficiencies of the five phenols are much higher and similar to those shown in Figure 5, indicating that the wastewater matrix does not affect the extraction efficiencies of the five phenols and [C4C11OHim]BF4 can thus be used to treat the wastewater containing phenols. 3.11. Reuse of [C4C11OHim]BF4 after Extraction. For the purpose of reusing [C4C11OHim]BF4, 5.0 mL of NaOH (0.10 mol L−1) was used to extract phenols in 1.0 mL of [C4C11OHim]BF4 under stirring; this procedure was repeated three times, and then 10 μL of 85% (wt %) phosphoric acid was added to react with the residual NaOH. The resulting IL phase

Figure 6. Effect of salt added on the extraction efficiency of [C4C11OHim]BF4: (■) resorcinol; (●) phenol; (▲) guaiacol; (◆) o-cresol; (▼) p-nitrophenol.

concentration range studied. The results suggest that [C4C11OHim]BF4 can be used to extract phenols over a wide concentration range. 3.8. Effect of Salt Added. The salt NaCl was added into the water phase to investigate its effect on the extraction efficiencies. As an example, the effect of NaCl on the extraction efficiency of [C4C11OHim]BF4 at pH 7.0 and Vw:VIL = 10:1 was determined, and the results are shown in Figure 7. As can be seen, the addition of NaCl improves remarkably the extraction efficiencies of phenols, which can be explained by the strong salting-out effect: with an increase of the salt concentration, some of the water molecules are attracted by the salt ions, which reduces the concentration of free water molecules, thereby increasing the relative concentrations of the phenols. As a result, phenols are preferably extracted into the IL phase.10,39 Because the extraction efficiencies of the five phenols are all above 98% with the addition of 30% NaCl, 30% NaCl is a better choice for analytical purposes. 3.9. Comparison with Reported Methods. Recently, several papers reported the use of 1-methyl-3-butylimidazolium 20029



Article

Table 3. Values of Distribution Ratios (log D) for the Phenols between [C4mim][PF6], THADHSS, [4C4P][AOT], [3C6PC14][FeCl4], [C4C11OHim]BF4, and Water analyte

[C4C11OHim]BF4

resorcinol phenol guaiacol p-nitrophenol o-cresol phase ratio (Vw:VIL) a

b

THADHSSa

2.9 3.0 2.8 3.5 3.7 10:1 c

[4C4P][AOT]c

[C4mim][PF6]

[3C6PC14][FeCl4]f

b 2.5

∼2.3

−0.046 1.1,d 1.2e

∼2.0

3.6

∼2.4

d

20:1 d

e

120:1 f

∼2.7 120:1

Reference 8. Not reperted. Reference 9. Reference 7. Reference 11. Reference 12.

resorcinol

phenol

guaiacol

p-nitrophenol

o-cresol

Wastewater a 82.4 89.8 96.6 Wastewater Spiked with Guaiacol and p-Nitrophenol (1.0 × 10−3 mol L−1 for Each) 84.4 89.4 89.3 95.3 95.9

Not detectable.

■

can be reused to extract phenols from water without losing its extraction efficiency.

ASSOCIATED CONTENT

* Supporting Information S

Experimental details, 1H and 13C NMR spectra and elemental analysis data of synthesized ILs, and extraction efficiencies of ILs for phenols (Table S1) and those for resorcinol with presaturation of the water phase (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Archana, V.; Begum, K. M. M. S.; Anantharaman, N. Studies on Removal of Phenol using Ionic Liquid Immobilized Polymeric Microcapsules. Arab. J. Chem. 2013, http://dx.doi.org/10.1016/j.arabjc.2013. 03.017. (2) Jabrou, S. N. Extraction of Phenol from Industrial Water using Different Solvents. Res. J. Chem. Sci. 2012, 2, 1. (3) Gundogdu, A.; Duran, C.; Senturk, H. B.; Soylak, M.; Ozdes, D.; Serencam, H.; Imamoglu, M. Adsorption of Phenol from Aqueous Solution on a Low-Cost Activated Carbon Produced from Tea Industry Waste: Equilibrium, Kinetic, and Thermodynamic Study. J. Chem. Eng. Data 2012, 57, 2733. (4) Esguerra, K. V. N.; Fall, Y.; Petitjean, L.; Lumb, J. P. Controlling the Catalytic Aerobic Oxidation of Phenols. J. Am. Chem. Soc. 2014, 136, 7662. (5) Duan, Z. Microbial Degradation of Phenol by Activated Sludge in a Batch Reactor. Environ. Prot. Eng. 2011, 37, 53. (6) Abulhassani, J.; Manzoori, J. L.; Amjadi, M. Hollow Fiber BasedLiquid Phase Microextraction using Ionic Liquid Solvent for Preconcentration of Lead and Nickel from Environmental and Biological Samples Prior to Determination by Electrothermal Atomic Absorption Spectrometry. J. Hazard. Mater. 2010, 176, 481. (7) Khachatryan, K. S.; Smirnova, S. V.; Torocheshnikova, I. I.; Shvedene, N. V.; Formanovsky, A. A.; Pletnev, I. V. Solvent Extraction and Extraction−Voltammetric Determination of Phenols using Room Temperature Ionic Liquid. Anal. Bioanal. Chem. 2005, 381, 464. (8) Egorov, V. M.; Smirnova, S. V.; Pletnev, I. V. Highly Efficient Extraction of Phenols and Aromatic Amines into Novel Ionic Liquids Incorporating Quaternary Ammonium Cation. Sep. Purif. Technol. 2008, 63, 710. (9) Huang, F.; Berton, P.; Lu, C.; Siraj, N.; Wang, C.; Magut, P. K. S.; Warner, I. M. Surfactant-Based Ionic Liquids for Extraction of Phenolic Compounds Combined with Rapid Quantification using Capillary Electrophoresis. Electrophoresis 2014, 35, 2463. (10) Fan, J.; Fan, Y.; Pei, Y.; Wu, K.; Wang, J.; Fan, M. Solvent Extraction of Selected Endocrine-Disrupting Phenols using Ionic Liquids. Sep. Purif. Technol. 2008, 61, 324. (11) Inoue, G.; Shimoyama, Y.; Su, F.; Takada, S.; Iwai, Y.; Arai, Y. Measurement and Correlation of Partition Coefficients for Phenolic Compounds in the 1-Butyl-3-methylimidazolium Hexafluorophosphate/Water Two-Phase System. J. Chem. Eng. Data 2007, 52, 98.

4. CONCLUSIONS From the aforementioned results, some conclusions can be obtained: (1) The extraction efficiencies of phenol, resorcinol, p-nitrophenol, guaiacol, and o-cresol depend on the pH values of the water phase. (2) The extraction efficiencies of the five phenols increase with an increase of their hydrophobicities but have no certain correlation with the hydrophobicities of the ILs. (3) For the hydroxyl-functionalized ILs, if the anions are PF6− or NTf2−, the incorporation of a hydroxyl group on the IL cations can remarkably enhance their extraction abilities. For the ones with BF4−, their extraction abilities for resorcinol are also higher than those of dialkyl-based ILs with BF4−, while their extraction abilities for the other four phenols are close to those of the latter, suggesting that the hydrogen-bonding interactions between BF4− and phenols may predominate and resorcinol and the other four phenols has different extraction mechanisms. (4) Thermodynamic studies suggest that the driven force for the extraction of resorcinol is mainly hydrogen bonds and transfer of the other four phenols from water to IL phases is mainly driven by hydrophobic interactions.

■

■

ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (Grants 21307028 and 31372214), Basic and Cutting Edge Technology Research Projects of Henan Province (Grants 132300410293 and 122300410004), Young Backbone Teachers in Colleges and Universities of Henan Province (Grant 2013GGJS-053), Science and Technology Research Foundation Projects of Henan Province (Grant 142102210049), and Natural Science Foundation of the Education Department of Henan Province (Grant 14B150026).

Table 4. Extraction Efficiencies (E, %) of [C4C11OHim]BF4 for the Five Phenols in Wastewater and Spiked Wastewater

a

1.5d 1.4e 3:1,d 2:1e

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +863913987823. Fax: +863913987815. Notes

The authors declare no competing financial interest. 20030



Article

(32) Bianco-Peled, H.; Gryc, S. Binding of Amino Acids to “Smart” Sorbents: Where Does Hydrophobicity Come into Play? Langmuir 2004, 200, 169. (33) Lungwitz, R.; Strehmel, V.; Spange, S. The Dipolarity/ Polarisability of 1-Alkyl-3-Methylimidazolium Ionic Liquids as Function of Anion Structure and the Alkyl Chain Length. New J. Chem. 2010, 34, 1135. (34) Ventura, S. P.; Neves, C. M.; Freire, M. G.; Marrucho, I. M.; Oliveira, J.; Coutinho, J. A. Evaluation of Anion Influence on the Formation and Extraction Capacity of Ionic-Liquid-based Aqueous Biphasic Systems. J. Phys. Chem. B 2009, 113, 9304. (35) Tang, W.; Zhou, J.; Yang, Q.; Zheng, Q.; Wu, J. Determination of Methyl Parathion by Solid-Phase Extraction on an Ionic Liquid− Carbon Nanotube Composite Electrode. Anal. Methods 2014, 6, 5886. (36) Tomé, L. I. N.; Catambas, V. R.; Teles, A. R. R.; Freire, M. G.; Marrucho, I. M.; Coutinho, J. A. P. Tryptophan Extraction using Hydrophobic Ionic Liquids. Sep. Purif. Technol. 2010, 72, 167. (37) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Ionic Liquids Are Not Always Green: Hydrolysis of 1-Butyl-3-Methylimidazolium Hexafluorophosphate. Green Chem. 2003, 5, 361. (38) Freire, M. G.; Neves, C. M.; Marrucho, I. M.; Coutinho, J. A.; Fernandes, A. M. Hydrolysis of Tetrafluoroborate and Hexafluorophosphate Counter Ions in Imidazolium-Based Ionic Liquids. J. Phys. Chem. A 2010, 114, 3744. (39) Chen, W. S.; Chiang, W. C.; Wei, K. M. Recovery of Nitrotoluenes from Wastewater by Solvent Extraction Enhanced with Salting-out Effect. J. Hazard. Mater. 2007, 147, 197.

(12) Deng, N.; Li, M.; Zhao, L.; Lu, C.; de Rooy, S. L.; Warner, I. M. Highly Efficient Extraction of Phenolic Compounds by Use of Magnetic Room Temperature Ionic Liquids for Environmental Remediation. J. Hazard. Mater. 2011, 192, 1350. (13) Ni, H. Q.; Dong, J.; Shi, J. J.; Wang, W. Ionic Liquid as Extraction Agent for Detection of Volatile Phenols in Wastewater and Its Regeneration. J. Sep. Sci. 2010, 33, 1356. (14) Balasubramanian, A.; Venkatesan, S. Removal of Phenolic Compounds from Aqueous Solutions by Emulsion Liquid Membrane Containing Ionic Liquid [BMIM]+[PF6]− in Tributyl Phosphate. Desalination 2012, 289, 27. (15) Balasubramanian, A.; Venkatesan, S. Optimization of Removal of Phenol from Aqueous Solution by Ionic Liquid-Based Emulsion Liquid Membrane Using Response Surface Methodology. Clean: Soil, Air, Water 2014, 42, 64. (16) Parham, H.; Saeed, S. Ultrasound-Assisted Solid Phase Extraction of Nitro- and Chloro-(Phenols) Using Magnetic Iron Oxide Nanoparticles and Aliquat 336 Ionic Liquid. J. Chromatogr. A 2014, 1336, 34. (17) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Liquid Clathrate Formation in Ionic Liquid−Aromatic Mixtures. Chem. Commun. 2003, 476. (18) Zhao, B.; Zhang, H. L.; Tang, W.; Yuan, W. G.; Xiong, F.; Jing, L. H.; Qin, D. B. Structure of Co-Crystals Formation from Imidazolium and Aromatic Ligands. Chin. J. Struct. Chem. 2014, 33, 415. (19) Martinez, C. R.; Iverson, B. L. Rethinking the Term “PiStacking. Chem. Sci. 2012, 3, 2191. (20) Branco, L. C.; Rosa, J. N.; Ramos, J. J. M.; Afonso, C. A. Preparation and Characterization of New Room Temperature Ionic Liquids. Chem.Eur. J. 2002, 8, 3671. (21) Zhang, S.; Qi, X.; Ma, X.; Lu, L.; Deng, Y. Hydroxyl Ionic Liquids: the Differentiating Effect of Hydroxyl on Polarity due to Ionic Hydrogen Bonds between Hydroxyl and Anions. J. Phys. Chem. B 2010, 114, 3912. (22) Mahurin, S. M.; Dai, T.; Yeary, J. S.; Luo, H.; Dai, S. BenzylFunctionalized Room Temperature Ionic Liquids for CO2 /N2 Separation. Ind. Eng. Chem. Res. 2011, 50, 14061. (23) Ab Rani, M. A.; Brant, A.; Crowhurst, L.; Dolan, A.; Lui, M.; Hassan, N. H.; Hallett, J. P.; Niedermeyer, H.; Perez-Arlandis, J. M.; Schrems, M.; Welton, T.; Wilding, R. Understanding the Polarity of Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 16831. (24) Lee, S. H.; Lee, S. B. Octanol/Water Partition Coefficients of Ionic Liquids. J. Chem. Technol. Biotechnol. 2009, 84, 202. (25) Xie, Y. J.; Liu, H.; Liu, H. X.; Zhai, Z. C.; Wang, Z. Y. Determination of Solubilities and n-Octanol/Water Partition Coefficients and QSPR Study for Substituted Phenols. Bull. Environ. Contam. Toxicol. 2008, 80, 319. (26) Freire, M. G.; Carvalho, P. J.; Silva, A. M. S.; Santos, L. M. N. B. F.; Rebelo, L. P. N.; Marrucho, I. M.; Coutinho, J. A. P. Ion Specific Effects on the Mutual Solubilities of Water and Hydrophobic Ionic Liquids. J. Phys. Chem. B 2009, 113, 202. (27) Dean, J. A. Lange’s Handbook of Chemistry, 14th ed.; McGrawHill: New York, 1992. (28) Vianello, F.; Miotto, G.; Cambria, M. T.; Lima, G. P. P.; Vanzani, P.; Di Paolo, M. L. Kinetic Role of a Histidine Residue in the T1 Copper Site of the Laccase from Rigidoporus Lignosus. J. Mol. Catal. B 2014, 99, 34. (29) Li, W.; Wu, P. On the Thermodynamic Phase Behavior of Poly(N-Vinylcaprolactam) Solution in the Presence of Different Ionic Liquids. Polym. Chem. 2014, 5, 761. (30) Pei, Y. C.; Wang, J. J.; Xuan, X. P.; Fan, J.; Fan, M. Factors Affecting Ionic Liquids Based Removal of Anionic Dyes from Water. Environ. Sci. Technol. 2007, 41, 5090. (31) Ross, P. D.; Subramanian, S. Thermodynamics of Protein Association Reactions: Forces Contributing to Stability. Biochemistry 1981, 20, 3096. 20031


Extraction of Phenols from Water with Functionalized Ionic Liquids

Recommend Documents