Factors Affecting Ionic Liquids Based Removal of Anionic Dyes from

including methyl orange, eosin yellow, and orange G from aqueous solutions. The effects of extraction time, pH of aqueous phase, structure of the ioni...
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Environ. Sci. Technol. 2007, 41, 5090-5095

Factors Affecting Ionic Liquids Based Removal of Anionic Dyes from Water Y U A N C H A O P E I , †,‡ J I A N J I W A N G , * ,† XIAO PENG XUAN,† JING FAN,† AND M A O H O N G F A N * ,§ School of Chemistry and Environmental Science, Henan Key Laboratory of Environmental Pollution Control, Henan Normal University, Xinxiang, Henan 453007, P. R. China, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, P. R. China, and School of Materials Science and Technology, Georgia Institute of Technology, Atlanta, Georgia 30332

Liquid-liquid extraction with imidazolium based ionic liquidss[C4mim][PF6], [C6mim][PF6], [C6mim][BF4], and [C8mim][PF6]sis proposed for removal of anionic dyes including methyl orange, eosin yellow, and orange G from aqueous solutions. The effects of extraction time, pH of aqueous phase, structure of the ionic liquids, temperature, and KCl concentration on the extraction efficiencies have been studied. Extraction efficiencies of dyes were strongly affected by the pH of the aqueous phase. Under the optimized pH condition, 85-99% of methyl orange, almost 100% eosin yellows, and 69% of orange G in tested water samples were transferred into the ionic liquids in a single extraction. Extraction efficiency for a given dye was found to increase with increasing temperature and increasing alkyl chain length of cation of the ionic liquids. Presence of a small amount of KCl in the aqueous phase did not considerably improve the extraction efficiency of the dyes. Thermodynamic studies revealed that the extraction process was driven by hydrophobic interaction of the anionic dyes and the ionic liquids.

Introduction Large quantities of dyes are used in different industries. It is estimated that about 10-15% of the dyes utilized in these industries are discharged into wastewater streams (1). The effluent containing dyes are highly colored, resulting in major environmental problems. Therefore, these dyes should be removed from the large volume of aqueous effluent before being discharged. Each conventional dye removal method has its advantages and disadvantages. UV-ozone or UVH2O2 (2) is only effective for wastewater with low concentration of dyes. Adsorption methods (3-7) are effective; however, regeneration of spent adsorbents frequently makes adsorption approaches expensive and time-consuming. Biological methods are not effective due to the low biodegradability of dyes (8). Micellar enhanced ultrafiltration has recently been * Address correspondence to either author. Phone: 86-3733325805 (J.J.W.); 1-404-385-6725 (M.F.). Fax: 86-373-3326445 (J.J.W.); 1-404-3856725 (M.F.). E-mail: [email protected] (J.J.W.); [email protected] (M.F.). † Henan Normal University. ‡ Lanzhou University. § Georgia Institute of Technology. 5090

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used to remove methylene blue from wastewater (9), and about 93-94% retention of methylene blue was achieved using a hydrophilic cellulose membrane. Colloidal gas aphrons have been used by Roy et al. (10) for separation of organic dyes from wastewater, which is reportedly not very efficient compared to other known methods. Basu and Pandit (11-13) investigated the application of reverse micelles in the removal of ionic dyes from wastewater with a detailed examination of the effects of different process parameters on the removal efficiencies of the dyes. Muthuraman and Palanivelu (14) studied the separation of anionic dyes using a solution of tetrabutylammonium bromide in methylene chloride. Purkait et al. (15) separated toxic eosin dye from wastewater with cloud point extraction by using a nonionic surfactant Triton X-100. They achieved 87.5-100% eosin extraction efficiency and could recover the surfactant from the dilute phase by solvent extraction. In recent years, people are increasingly interested in using ionic liquids (ILs) for chemical synthesis, biocatalytic transformation, electrochemical device designs, and analytical and separation processes (16-29), due to the “green” characteristics of ionic liquids. In this work, imidazolium based ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6mim][PF6]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6mim][BF4]), and 1-octyl-3methylimidazolium hexafluorophosphate ([C8mim][PF6]) have been used for the first time to remove the anionic dyes methyl orange, eosin yellow, and orange G from wastewater. Different factors have been investigated for their effects on the removal of these dyes, and the changes of the thermodynamic properties during the phase transfer processes of dyes have been evaluated to provide the basis for the large scale demonstration and application of the ILs-based dye removal method. The chemical structures of the ILs and the anionic dyes used in the present work are shown in Figure 1.

Materials and Methods Reagents. We distilled 1-Bromobutane, 1-bromohexane, 1-bromooctane, and 1-methylimidazolium (Shanghai Chemical Co.) twice at a reduced pressure, and middle fractions of the distillates were collected. Sodium hexafluorophosphate NaPF6 (Shanghai Chemical) was purified through a twice recrystallization process with deionized water. Methyl orange (C. I. Acid Orange 52), eosin yellow (C. I. Acid Red 87), and orange G (C. I. Acid Orange 10) used in the experiments were purchased from Shanghai Chemical and utilized as received. Hydrochloric acid and sodium hydroxide (Shanghai Chemical) were used to adjust pH of the aqueous phases. Water used in the research was always freshly deionized and distilled before use. Synthesis of Ionic Liquids. [C4mim][PF6], [C6mim][PF6], [C6mim][BF4], and [C8mim][PF6] ionic liquids were prepared and purified according to the methods described elsewhere (30, 31). 1H NMR spectra data of these ILs were determined by using a Bruker AV-400 Spectrometer, and they are in good agreement with those reported in literature (32). Experimental Methods. The aqueous dye solutions were prepared by directly dissolving the dyes into deionized water. Except for the tests on the effects of KCl, no electrolyte was added to aqueous solutions. Extraction experiments were performed in a water bath at 25 ( 0.1 °C. Preliminary study showed that dye concentration had little influence on the extraction. Therefore, 5.0 mL of aqueous solution of dye (1.0 mg‚mL-1) was contacted with 1.0 mL of a prepared IL. 10.1021/es062838d CCC: $37.00

 2007 American Chemical Society Published on Web 06/07/2007

FIGURE 1. Chemical structure of the ILs and the dyes used in the present work: (a) ILs; (b) Methyl orange; (c) Eosin yellow; (d) Orange G. The phase-contacting experiments were carried out in carefully stoppered 40 × 25 mm glass test tubes. The extraction system was vigorously stirred with magnetic stirrers. Usually, equilibrium was achieved within 20 min or less although the system was always allowed to equilibrate for 30 min. The phase separations quickly occurred after cessation of stirring process. However, a XYJ-802 centrifuge (Jiangsu Medical Instrument Factory) operated at 2000 rpm was still used to run for a period of 5 min in each test to ensure a complete phase separation. Then the sample was collected from the top aqueous phase and diluted for analyses. The pH values of aqueous phases were measured using an Orion 720 pH meter. Before and after each extraction, concentration of the studied dyes in the aqueous solution was determined with a Shanghai 752 UV-vis spectrophotometer at their maximum absorption wavelengths (methyl orange, 510 nm; eosin yellow, 517 nm; orange G, 472 nm). Partition coefficient (PIL/W) of the dyes between the ILs and aqueous phases and their extraction efficiencies (E%) were, respectively, calculated by using

PIL/W )

{

}

Ci - Cf Vaq Cf VIL

(1)

and

E% )

Ci - Cf × 100% Ci

(2)

where Ci and Cf represent, respectively, the initial and final concentrations (mol‚L-1) of a given dye in an aqueous phase, and Vaq and VIL are the volumes of aqueous phase and the ILs, respectively. Thus, a volume ratio is needed in the calculation of partition coefficients to account for the difference in volumes between two phases. The values of PIL/W were measured in duplicate with uncertainties less than 5%. Different IL stripping solutions were used to remove the dyes from the dye/IL mixtures so that the recovered ILs can be reused. The mass of the dye in 1 mL of dye/IL mixtures: methyl orange-[C4mim][PF6]; methyl orange-[C6mim][PF6]; methyl orange-[C8mim][PF6]; eosin yellow-[C4mim][PF6], eosin yellow-[C6mim][PF6]; eosin yellow-[C8mim][PF6]; orange G-[C4mim][PF6]; orange G-[C6mim][PF6]; and orange G-[C8mim][PF6], is 3.6, 3.8, 3.8, 0.5, 2.0, 3.8, 0.2, 0.5, and 3.1 mg, respectively. In each recovery test, 2 mL of dye/IL mixture and 1 mL of stripping solution was used. The stripping solutions for methyl orange-ILs, eosin yellow-

FIGURE 2. pH dependence of partition coefficients of methyl orange in ILs/aqueous systems at 25 °C.

FIGURE 3. pH dependence of partition coefficients of eosin yellow in ILs/aqueous systems at 25 °C. ILs, and orange G-ILs were 1.0 mol‚L-1 aqueous HCl, aspurchased 1-pentanol, and 1:1 (v/v) isopropyl alcohol-0.1 mol‚L-1 aqueous NaOH, respectively. These stripping solutions are immiscible with the ILs and miscible with the dyes. Therefore, the dyes can be separated and the ILs can be recovered for reuse.

Results and Discussion Effect of Phase Volume Ratio. The optimum extraction phase volume ratio is tested with a series of experiments. The results showed that the volume ratio has no significant effect on partition coefficients of the dyes within experimental error. To obtain the best extraction efficiency of dyes and the smallest amount of ionic liquids used, a volume ratio, 5:1, of an aqueous phase to an IL phase was selected in all the extraction experiments. Effect of pH of Aqueous Phases. The pH dependencies of partition coefficients of methyl orange, eosin yellow, and orange G in the ILs/aqueous systems are illustrated in Figures 2, 3, and 4. It is interesting to note that the effect of the pH values of the aqueous phases on the partition coefficients varies significantly from one anionic dye to another. The partition coefficients of methyl orange shown in Figure 2 VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. pH dependence of partition coefficients of orange G in ILs/aqueous systems at 25 °C. increased but those of eosin yellow decreased with pH at low pH ranges as indicated in Figure 3, and they appeared to be constants at high pH ranges. On the contrary, the partition coefficients of orange G presented in Figure 4 remained constant in most of the tested pH range and then dropped considerably. It is well-known that the pH of the aqueous phase affects the degree of ionization of a dye molecule. The charged state of a dye is determined by the pH of the aqueous solution and its dissociation constant (pKa). For instance, in the aqueous solution, the sulfonate groups of the methyl orange (DSO3Na) can be dissociated and converted to anionic form. As the pH decreases, its azo group (-NdN-) becomes protonated and the molecule exists in zwitterionic form as presented in Figure 5. Therefore, the anionic species of methyl orange dominates in the range of pH > pKa (3.46), but the zwitterionic species prevail in the range of pH < pKa. The maximum removal of methyl orange was observed in Figure 2 in the range of pH > pKa, which indicates that the anionic species of methyl orange prefers the ILs phase. From the viewpoint of molecular interactions, favorable hydrogen-bonding could be formed between -SO3- or/and -NdN- of the anionic species and the acidic 2H of imidazolium cation of the ILs compared with the zwitterionic species of methyl orange. Figure 3 indicates that quantitative extraction of eosin yellow occurred at low pHs. The extraction efficiency curves drop steeply in the pH range of 4-6, and then become plateaus under the condition of pH > 6.0. This pH dependency results from the variation of charged characteristics of eosin yellows with the aqueous phase pH. Figure 5 shows the ionization of eosin yellow at different pH ranges. It is clear that the concentrations of the three species (33), molecular form (EH2), monoanionic form (EH-), and dianionic form (E2-) of eosin yellow, change as a function of aqueous phase pH. The molecular form is the main species in the range of pH < pK1 (3.25), the monoanionic form and the dianionic form are predominant in the range of 3.25-3.8 (i.e., pK2 ) 3.80) and 3.8-13, respectively. Therefore, the molecular form of eosin yellow can be almost quantitatively extracted from the aqueous phase, and the preferential degree of the species of eosin yellow for the ILs phase follows the order: molecular form > mono-anionic form > dianionic form. The same conclusion has been drawn by Visser and co-workers (34) from studies of pH-dependent partitioning of thymol blue in ILs. From the chemical structure shown in Figure 5, it can be seen that there are two -OH groups in the molecular form 5092

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FIGURE 5. Ionization of methyl orange and eosin yellow in aqueous solutions. of eosin yellow. According to IR, NMR, and molecular dynamic simulation studies by other researchers (35-37), hydrogen-bonding could probably be formed between these -OH groups and the F of the PF6- anion. With the increase of the aqueous phase pH, one of the -OH groups is ionized and the dye exists in anionic form. Accordingly, the Hbonding interactions between eosin yellow and the PF6anion become weak. In the range of pH > pK2, the dianionic form is predominant, and there is no -OH group available for the formation of H-bonding with the PF6- anion of the ILs. This explained the relatively low extraction efficiency of eosin yellow in this pH range. Therefore, it may be deduced that the extraction efficiency of different species of eosin yellow by a given IL was tuned by hydrogen-bonding interactions of the dye with anions of the ILs. Figure 4 shows that the extraction efficiency of orange G seems unaffected by the change of the aqueous phase pH when [C4mim][PF6] was used as organic phase, and decreased at very high pHs when [C6mim][PF6] and [C8mim][PF6] were used. This can be explained from the chemical structure of orange G. The molecule of orange G contains two sulfonate groups (D-SO3Na) and one hydroxyl group (-OH). In aqueous solution, the sulfonate groups are dissociated and become anionic. The -OH could form hydrogen bonding with F in PF6-, and this hydrogen-bonding is not very sensitive to the pH of the aqueous phase. However, in very basic aqueous solutions, H in the hydroxyl group would be removed, and no hydrogen-bonding interactions would be expected. Therefore, considerable decrease in extraction efficiency was observed in the pH range of 11-13. This suggests again that hydrogen-bonding interactions played an important role in the extraction of different species of the anionic dyes.

values are listed in Table 1. The PIL/W values in Table 1 show that the extraction efficiency of the dyes increases with the increase of temperature. From the viewpoint of thermodynamics, extraction of a dye into a particular IL from the aqueous phase can be regarded as a transfer process of a dye from the aqueous to the IL phases. The Gibbs energy (∆GT0) of such a transfer process at a given temperature can be calculated from the mole-fraction extraction equilibrium constant, Kx by using the following equation (39):

∆GT0 ) - RT ln Kx ) - RT [ln PIL/W + ln (Vm,IL/Vm,W)] (3)

FIGURE 6. Logarithm of partition coefficients for the dyes against the number of carbon atom in the alkyl chains of cation of the ILs at pH 6.0. Effect of Chemical Structure of the ILs. The relationships between the logarithm of partition coefficients of the anionic dyes at pH ) 6.0 and the number of carbon atoms, nc, in the alkyl chain of the cation of the ILs are shown in Figure 6. It is clear that, for a given dye, its partition coefficients in the ILs/aqueous systems follow the trend: P[C4mim][PF6]/w < P[C6mim][PF6]/w < P[C8mim][PF6]/w. Based on the fact that the hydrophobicity of the ionic liquids increase with the increase of length of alkyl substituent on the cation, this trend indicates that the partition coefficients of these dyes increased with increase of hydrophobicity of the ILs. As shown in Figure 6, good linear correlations between logPIL/W and nc exist for both methyl orange and eosin yellow. Similar linear correlations are also found at the other pHs investigated. In fact, the slopes of the linear correlations signify the degree of contribution of a -CH2 unit in the alkyl chains of imidazolium cation of the ILs to the value of logPIL/W. Calculations show that slopes of these linear correlations are not sensitive to the pH values of the aqueous phases, with the average values being 0.34 ( 0.03 for methyl orange and 0.51 ( 0.04 for eosin yellow, respectively. Therefore, the addition of a -CH2 unit to the cation of the ILs causes a significant increase in the partition coefficients of the anionic dyes. From these results, it is possible to tune the extraction efficiency of anionic dye by changing alkyl chain length of imidazolium cation of the ILs. As can be seen from Figure 2, the partition coefficients of methyl orange in [C6mim][BF4]/aqueous system are much higher than those in [C6mim][PF6]/aqueous system under the same conditions. Considering the fact that these two ILs have the same cations and different anions, this result implies that anion of the ILs played an important role in the extraction of the anionic dyes. Based on their IR spectroscopy measurements, Kristiansson et al. (35) and Cammarata et al. (38) found that the strength of hydrogen bonding between -OH group and anions increased in the order [PF6]- < [BF4]-. Combined with these findings, we conclude that the difference in the partition coefficients of methyl orange observed in [C6mim][BF4] and [C6mim][PF6] can be ascribed to the difference in the hydrogen-bonding strength of methyl orange with the anions of these ILs. Effect of Temperature and the Transfer Thermodynamic Properties. To assess the temperature effect of the dyes extraction by ILs, a series of extraction experiments were performed at 15, 25, 35, and 45 °C, and the obtained PIL/W

where Vm,IL and Vm,W represent the molar volume of ILs and water, respectively. The molar volumes of water at different temperatures can be calculated from the density data reported by Kell (40) and from the molar mass of water. The density data for the tested ILs at different temperatures are not available in the literature. We determined these values by using an Anton Paar DMA 60/602 digital densimeter. Based on these data and the values of molar mass of ILs, molar volumes for the ILs were calculated to be 205.9, 207.6, 209.4, and 211.2 cm3‚mol-1 for [C4mim][PF6]; 240.1, 241.6, 243.1, and 244.6 cm3‚mol-1 for [C6mim][PF6]; and 274.0, 275.7, 277.4, and 278.1 cm3‚mol-1 for [C8mim][PF6] at 15, 25, 35, and 45 °C, respectively. If the transfer enthalpies (∆HT0) for a given dye are assumed to be constant over the short temperature range studied, their values can be calculated from the linear relationships (shown in Figures S1-S3 of the Supporting Information) between logPIL/W and 1/T expressed as follows:

∆HT0 log PIL/W ) +C 2.3 RT

(4)

The transfer enthalpies for the nine dyes’ extraction processes are determined by using the slopes (-∆HT0/2.3R) of the plots. Then according to the relationship among the transfer Gibbs energy, the transfer enthalpy and the transfer entropy (∆ST0), the values of T∆ST0 have been calculated with this equation:

T∆ST0 ) ∆HT0 - ∆GT0

(5)

The resultant ∆GT0, ∆HT0, and T∆ST0 values along with the experimental PIL/W data are included in Table 1. It can be seen that all the values of ∆HT0 and ∆ST0 are positive whereas those of ∆GT0 are negative. The negative values of ∆GT0 indicate that all the extraction processes are spontaneous. These thermodynamic data also suggest that all the extraction processes are driven by entropy terms, which is the characteristic of hydrophobic interactions (41, 42). Thus, the thermodynamic study revealed that hydrophobic interactions are the main driving force for ILs-based removal of the dyes from aqueous solutions. Effect of Salt Concentration. In actual textile dye bath effluent streams, dyes are frequently present along with different salts. To understand the effect of salt concentration on dye extraction, 5 mL of 1 mg‚mL-1 methyl orange solutions containing 3.049, 4.065, 5.081, 30.24, and 50.04 mg‚mL-1 KCl were prepared and were tested for their partition coefficients with 1 mL of prepared [C4mim][PF6] at pH 6.0 and 25 °C for a equilibrium period of 30 min. The obtained partition coefficients, PIL/W, of methyl orange between [C4mim][PF6] and aqueous phases with KCl concentrations of 3.049, 4.065, 5.081, 30.24, and 50.04 mg‚mL-1 were 27.5, 27.5, 27.7, 29.7, and 32.4, respectively. Obviously, the presence of KCl favors the improvement of partition coefficient of the methyl orange, which may be due to the reduction of the degree of hydration of methyl orange in KCl containing solutions. Pandit and VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. The Transfer Thermodynamic Properties for the Anionic Dyes from Aqueous Phase to Ionic Liquids at pH 6.0 [C4mim][PF6]

[C6mim][PF6]

∆GT0 T∆ST0 ∆HT0 (kJ‚mol-1) (kJ ‚mol-1) (kJ‚mol-1)

[C8mim][PF6]

∆GT0 T∆ST0 ∆HT0 (kJ‚mol-1) (kJ‚mol-1) (kJ‚ mol-1)

∆GT0 T∆ST0 ∆HT0 (kJ‚mol-1) (kJ‚mol-1) (kJ‚ mol-1)

T (°C)

PIL/W

15 25 35 45

21.5 27.2 29.8 32.7

-13.2 -14.3 -15.0 -15.7

10.3 10.3 10.3 10.3

23.5 24.6 25.3 26.0

119 121 136 158

Methyl orange -17.7 7.3 -18.3 7.3 -19.2 7.3 -20.3 7.3

25.0 25.6 26.5 27.6

483 555 613 657

-21.3 -22.4 -23.4 -24.4

7.8 7.8 7.8 7.8

29.1 30.2 31.2 32.2

15 25 35 45

0.179 0.431 0.576 0.627

-1.7 -4.0 -4.9 -5.2

31.2 31.2 31.2 31.2

32.9 32.9 32.9 32.9

1.92 4.14 6.82 10.1

Eosin yellow -7.8 41.9 -10.0 41.9 -11.6 41.9 -13.0 41.9

49.7 51.9 53.5 54.9

30.3 49.5 81.7 117

-14.7 -16.4 -18.3 -19.8

34.8 34.8 34.8 34.8

49.5 51.2 53.1 54.6

15 25 35 45

0.174 0.187 0.292 0.526

-1.6 -1.9 -3.1 -4.8

28.4 28.4 28.4 28.4

30.0 30.3 31.5 33.2

0.464 0.527 1.13 1.66

Orange G -4.4 -4.8 -7.0 -8.2

39.2 39.6 41.8 43.0

4.06 10.9 16.4 23.3

-9.9 -12.7 -14.2 -15.6

43.4 43.4 43.4 43.4

53.3 56.1 57.6 59.0

PIL/W

Basu (11) once studied the effect of KCl on the partitioning of methyl orange in reverse micelles from water. In their study, they also observed the improvement of the extraction efficiency of methyl orange with the increase in KCl concentration. Recovery of Ionic Liquids. In the use of ILs for a liquidliquid extraction process, recovery, and reuse of the ILs is of great importance. Consequently, a series of stripping solutions were tried to separate these dyes from the IL phase. For the ILs + methyl orange mixtures, 97.5% of methyl orange in [C4mim][PF6] and [C6mim][PF6], and 96.2% in [C8mim][PF6] could be precipitated from 2.0 mL of the mixtures with the addition of 0.2 mL aqueous HCl solution (1.0 mol‚L-1). The remaining methyl orange in the ILs could then be separated by back-extraction with 1.0 mol‚L-1 aqueous HCl. In a single back-extraction, nearly 99.5% of methyl orange in [C4mim][PF6], [C6mim][PF6], and [C8mim][PF6] have been separated. Eosin yellow could be stripped with 1-pentanol up to an extent of about 92% from [C4mim][PF6], 80% from [C6mim][PF6] and 36% from [C8mim][PF6] in a single stripping. After subsequent twice stripping by 1-pentanol, 97% of eosin yellow can be removed from [C4mim][PF6] and [C6mim][PF6], and 67% from [C8mim][PF6]. As it is expected, orange G was also possible to be stripped using a mixture of isopropyl alcohol with 0.1 mol‚L-1 aqueous NaOH. After twice stripping, nearly 98% of orange G in [C4mim][PF6] and [C6mim][PF6], and 89% in [C8mim][PF6] can be removed. Even though the partition coefficients of the dyes in [C4mim][PF6]/[C6mim][PF6]-aqueous solutions are lower than those in [C8mim][PF6]-aqueous solutions, [C4mim][PF6] and [C6mim][PF6] are preferable for removal of dyes due to their higher reusability. The issue of the loss of very small amounts of ILs into the aqueous phase should be addressed. At room temperature, the solubility of [C4mim][PF6], [C6mim][PF6], and [C8mim][PF6] in water is 1.88, 0.75, and 0.20 g‚100 mL-1 , respectively (32). Vijayaraghavan et al. (43) and Stepnowski (44) have successfully recovered the lost ILs from the aqueous phase by using ion exchange resins. However, this method is expensive. Therefore, alternative low-cost methods for recovering ILs from water need to be developed. Implications for Water Treatment. The aforementioned test results indicate that various dyes used in industry could be quickly and efficiently separated from water by using small quantities of ionic liquids. The simplicity and other characteristics of ILs-based dye separation procedures could render ILs as promising candidates for removal of dyes from water. However, two major follow-up research works need to be done before ILs can be actually utilized in water 5094

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34.8 34.8 34.8 34.8

PIL/W

treatment, which include a comprehensive study on the recovery of ILs with easily manageable and highly efficient procedures, and overall engineering cost analysis of ILs-based dye separation processes.

Acknowledgments We thank the National Natural Science Foundation of China for its financial support on the research under the grant contracts nos. 20273019 and 20573034.

Supporting Information Available Figures showing the linear relationships between logarithms of partition coefficients of different dyes and 1/T at pH 6.0. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review November 30, 2006. Revised manuscript received May 2, 2007. Accepted May 3, 2007. ES062838D

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