Ionic Liquids as Green Solvents - ACS Publications - American

Chapter 42

Extraction of Chlorophenols from Water Using Room Temperature Ionic Liquids 1

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Evangelia Bekou , Dionysios D. Dionysiou *, Ru-Ying Qian , and Gregory D. Botsaris *

Downloaded by COLUMBIA UNIV on July 18, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch042

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Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, O H 45221-0071 Department of Chemical Engineering, Tufts University, Medford, MA 02155 *Correspond author: phone: 513-556-0724; fax 513-556-2599; email: [email protected] 2

ABSTRACT. This study deals with extraction of chlorinated phenols from aqueous solutions using two room temperature water immiscible ionic liquids, 1-ethyl-3methylimidazolium bis(perfluoroethylsulfonyl)imide, [emim]Beti, and 1-butyl-3methylimidazolium hexafluorophosphate, [bmim]PF . Partitioning of phenol, 2chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,5-tetrachlorophenol, and pentachlorophenol were measured in both aqueous and ionic liquid phases using HPLC. Extraction efficiency was found to be greater when [bmim]PF was used and when the pH of the aqueous solution was at least one unit below the value of the dissociation constant (pK ). Partitioning, for both ionic liquids, was increased as the number of chlorine atoms in the chlorophenol increased, illustrating the same behavior as 1-octanol-water partition coefficient. The ionic liquid-water distribution ratios for the extraction efficiency of chlorinated phenols using these two ionic liquids was one order of magnitude lower than the corresponding 1-octanol-water partition coefficients. The ionic strength of the aqueous phase had no significant effect on the ionic liquid-water distribution ratios of chlorinated phenols but had a dramatic effect on the solubility of ionic liquids in water. In addition, the ionic liquid-water distribution ratio of chlorinated phenols was not influenced significantly by the number of chlorophenols present in the aqueous phase. 6

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© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


545 1. INTRODUCTION Chlorophenols are detected in natural water and impose health risk to humans and hazard to biota. Heavy and acute exposure to chlorophenols can cause death, and small amounts or long-term exposure can cause liver and kidney problems to animals (/). The side effects of chlorophenols are perilous and the presence of such compounds in natural water should be nullified. Chlorophenols have been reported as toxic substances under emergency planning and community right-to-know act (EPCRA), according to US EPA (2). Herein, chlorophenols were separated from aqueous solutions using ionic liquids as extraction media. The goals of the study were to examine the feasibility of such process using two different room temperature ionic liquids and investigate the dependence of extraction efficiency on pH, ionic strength, and composition of aqueous solution. The use of ionic liquids in environmental applications is new. Nevertheless, the unique properties of ionic liquids attract the interest for exploring their use as solvents for the removal of contaminants from aqueous streams. Ionic liquids are characterized as neoteric solvents (3) and are considered as environmentally friendly compounds (4, 5), mainly because they are non-flammable and non-volatile. These two properties prompt the interest for replacing the well-known volatile organic solvents with ionic liquids (6). Other important properties are their wide liquid phase temperature range (around 300 °C), their high thermal stability (up to 400 °C), their miscibility or immiscibility with water depending on their chemical composition, and their good solvent characteristics for many organic, inorganic and polymeric materials (7). Some of the applications of ionic liquids as solvents and catalysts are related with Friedel-Craft reactions (8, 9), Diels-Alder reactions (10, 11), and Heck reactions (12-16). More detailed information relating their properties and applications can be found in published reviews dealing with ionic liquids (17-20). Ionic liquids have also been investigated as extraction media (21-30). Among the ionic liquids studied is l-ethyl-3-methylimidazolium chloride/aluminum trichloride,first-generationionic liquid (18), which was tested for the extraction of zicronium cluster [Zr (B)Cli ] * from a solid precursor (21). This ionic liquid can dissolve many cluster compounds and is redox stable over a wide electrochemical window, but its air and water sensitivity makes the extraction process very delicate, since it has to take place in inert atmosphere or under vacuum. The development of second-generation ionic liquids (31) made the extraction process easier and possible even for water-ionic liquid systems, because these ionic liquids are air and water stable. 1-buty 1-3-methylimidazolium hexafluorophosphate is a widely used secondgeneration ionic liquid and has been investigated for liquid-liquid (ionic liquidwater) extractions. Extraction of various substituted benzenes is one of the applications of this ionic liquid (22). The distribution ratio of the benzene compounds depends on the pH value of the aqueous solution and is high for neutral or uncharged species, and low for the charged species. pH dependence provides a flexibility in the extraction process, since by changing the pH value of the solution it is possible to extract a compound from water and then back extract it to a different solvent. Another example of partitioning dependence on pH is the preference of 5

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In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


546 thymol blue on aqueous or ionic liquid phase in acidic, neutral, or basic solutions (23). Thymol blue is a dye and shows a preference for ionic liquid phase at low pH, when it is present in its neutral form, and for aqueous phase at higher pH, where it is present in its ionized form. In addition, the distribution ratio is an order of magnitude greater when 1-octyl-l-methylimidazolium hexafluorophosphate is used compared with l-butyl-3-methylimidazolium hexafluorophosphate, indicating the importance of the alkyl chain length of the imidazolium ring to the extraction process. Comparison of 1-octanol-water partition coefficient and ionic liquid-water distribution ratio demonstrates higher values for the 1-octanol-water system by one order of magnitude. This difference in magnitude is ascribed either to the less hydrophobic character of ionic liquid phase or to the fact that ionic liquids have strong interactions, since they are composed from ions (22). Extraction of aromatic compounds from mixtures of aromatics and paraffins to ionic liquids is also promising (24). The application of this process can lead to less complex and less expensive separation processes than existing aromatic/paraffin separations. The separation factor indicates that toluene can be extracted efficiently from heptane when toluene is present at low concentration. Various ionic liquids were examined for their ability to extract metal ions from aqueous solutions using crown ethers, l-(2-pyridylazo)-2-naphthol (PAN), or l-(2-thiazolylazo)-2-naphthol (TAN) as extractants (25-27). The presence of crown ethers proved necessary, because the distribution ratio is very small without their presence (25). The organic extractants PAN and TAN display pH dependence in their extraction efficiency for metal ions (26). In general, partitioning depends on the hydrophobicity of the extractant and on the species that are present in the aqueous phase. It has been shown that the configuration of ionic liquids has an effect on the distribution ratio since ionic liquids with anion bis[(trifluoromethyl)sulfonyl]imide show higher distribution ratio than those with hexafluorophosphate anion. In addition, the distribution ratio is larger when the imidazolium cation is not substituted in the second atom of the ring, since the hydrogen bonding between crown ethers and ionic liquid may be higher in this case resulting to higher partitioning (25). The problem is that highly acidic aqueous solution can decompose the hexafluorophosphate anion and can significantly increases the solubility of ionic liquid in the aqueous phase as well as the water content in the ionic liquid phase. On the other hand, low acid content in the aqueous phase decreases metal ion distribution ratio and water content in the ionic liquid phase. In addition, highly hydrated salts, such as A1(N0 ) and L i N 0 , salt out the ionic liquid ions and the crown ethers (27). The increase of metal ion transfer in ionic liquid phase has been reported to cause increase in the solubility of ionic liquid in water (28). The mechanism of metal transfer into ionic liquids was studied with extended X-ray absorption fine structure (EXAFS) measurements (29). 3

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547 Ionic liquids with various modifications of the imidazolium cations have been investigated for extraction of metal ions (30). The scope of these task specific ionic liquids was to examine simpler and more efficient metal ion extraction systems, since the conventional extractants (i.e., crown ethers) produced a complicated system for chemical analysis. The imidazolium cation modified by the addition of thiourea, thioether and urea into the alkyl chain of the cation, gives to the ionic liquid a double role of a hydrophobic medium and an extractant (30). The more efficient additions were those with thiourea or urea derivative with long chain; they were inserting sulfur or oxygen atoms, respectively, into the molecule. The anion of [emim]Beti contains both sulfur and oxygen atoms in contrast to [bmim]PF which has neither. The different molecular structure of these two ionic liquids, and in combination with the pH and the ionic strength of the aqueous solution, may affect the ionic liquid-water distribution ratio, as it is illustrated in this study.


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2. E X P E R I M E N T A L SECTION Materials: 1 -ethyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide, [emim] Beti (98% purity), was purchased from Covalent Associates, Inc. (Woburn, Massachusetts), while 1 -butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF (97% purity) was obtained from Sachem, Inc. (Austin, Texas). Both ionic liquids were used as received. All chlorophenols were obtained from Aldrich and their purities were between 97 and 99%. Sodium nitrate was purchased from Fisher and used as received. The aqueous solutions were prepared with double distilled deionized water (12 ΜΩ) and the pH was adjusted with H N 0 or NaOH whenever it was necessary. 6

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Extraction procedure: Two ml of ionic liquid ([bmim]PF or [emimJBeti) were placed in a 50 ml polypropylene tube and 10 ml of aqueous chlorophenol solution were added. The ionic liquid and water form two different phases, with the ionic liquid at the bottom of the tube and the aqueous solution at the top. The density of [emim]Beti is 1.57 g/cm (32) while that of [brnim]PF is 1.37 g/cm (33). The two phases were mixed vigorously for ten minutes in order to achieve equilibrium partitioning of chlorophenol between the two phases. This time was determined in preliminary experiments (34). After mixing, two distinct phases were formed, with ionic liquid being the bottom phase due to its higher density. However, small liquid droplets of the other phase could be seen in each phase. These liquid droplets were subsequently removed by centrifugation of the sample at 3578 g for 30 min. Clear solutions with no visible droplets of ionic liquid in aqueous phase or water in ionic liquid phase were obtained after centrifugation. 6

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548 The aqueous phase was analyzed using High Pressure Liquid Chromatography (HPLC), for the quantification of chlorophenols and ionic liquid, and by Total Organic Carbon (TOC) analyzer for the measurement of organic carbon in the sample. It has been previously published that the solubility of [bmim]PF is approximately 19 g/1 (55), which is in agreement with the results obtained using the HPLC method developed in this work. The upper phase was diluted with double distilled deionized water (12 ΜΩ) (1ml of the aqueous phase was placed into a 100 ml volumetric flask and deionized water was added), before analyzing a sample for the ionic liquid concentration using HPLC or TOC. The dilution contributes to better results, because the calibration curve for the ionic liquid in the HPLC is linear up to 0.5 g/1 and the amount of organic carbon in the sample must be below 0.5 g/1 for accuracy in TOC analysis. The ionic liquid phase was analyzed for its chlorophenol content using HPLC. The ionic liquid phase was also diluted (0.5 ml of the ionic liquid phase was diluted with 5 ml of acetonitrile), because ionic liquids are very viscous and may block the flow if injected undiluted into HPLC. All experiments and analyses were performed in triplicate. The average error in the analysis was 2 %. Analysis: An Agilent 1100 series HPLC equipped with a reverse phase amide column was used for the analysis. The mobile phase was acetonitrile and 0.01 Ν H S 0 aqueous solution (the ratio of the solvents was changing from 60:40 to 10:90 depending on the IL and the chlorophenol used). The absorbance wavelength was set at 212 nm for ionic liquids and at 197 nm or 212 nm for chlorophenols. The correlation coefficient of the calibration curves was 0.996 for ILs, 0.999 for chlorophenols, and 0.995 for phenol. Dilution of ionic liquid phase with acetonitrile was necessary because it reduces the viscosity of ionic liquid phase and enhances HPLC performance. Using these new HPLC methods, chlorophenol partitioning was quantified in both phases.


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3. RESULTS AND DISCUSSION 3.1 Single solute extraction using [emimJBeti Phenol, 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,5-tetrachlorophenol and pentachlorophenol were extracted from their aqueous solution using [emim]Beti. Their distribution ratios are compared with 1-octanol-water partition coefficient (36) and are illustrated in Figure 1. The pH of all the initial aqueous chlorophenol solutions was at least one unit lower than the pK values of the compounds (37), and the dominant species in the solution were the neutral species. Most of the initial solutions had a pH value close to 5.5, which is at least one unit lower than the pK value of the phenol (9.99), 2-chlorophenol (8.55), and 2,4-dichlorophenol (7.85). The pH of the aqueous solution containing 2,4,6-trichlorophenol, or pentachlorophenol was adjusted at 3.0 with H S0 , because the pK values of these two chlorophenols are 6.23, and 4.75, respectively. All D values were calculated based on a volume ratio 1:5; 2 ml of ionic liquid phase and 10 ml of aqueous chlorophenol solution. All chlorophenols were used at initial concentration in water of 50 mg/1, except for pentachlorophenol, which was used at an initial concentration of 4 mg/1 due to its lower solubility in a

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549 water. After the extraction process, the detection of pentachlorophenol in the aqueous phase was not possible using this HPLC method, when a volume ratio 1:5 was used. The volume ratio was altered to 1:10, 1:20, 1:30 and 1:40 of [emim]Beti to aqueous phase, and the distribution ratio for these conditions was calculated at 1468 (std. dev. 308). The trends of chlorophenol partitioning are similar for water-1-octanol and water-[emim]Beti extraction systems. Specifically, the [emim]Beti-water distribution ratio, D , increases with the increase of chlorine atoms in the solute, like 1-octanolwater partition coefficient, P . However, P is an order of magnitude higher than D , indicating that [emimJBeti is not as hydrophobic as 1-octanol as also discussed in previous studies (22). bw

ow

ow


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3.2 Single solute extraction using [bmim]PF Phenol, 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 2,3,4,5-tetrachlorophenol were extracted as single solutes from aqueous solution using [bmim]PF . The [bmim]PF distribution ratio, D , of these experiments was compared with the P and the results are illustrated in Figure 2. It is evident that the [bmim]PF -water extraction system demonstrates similar behavior as the 1-octanolwater and [emim]Beti-water extraction systems; increasing D for increasing number of chlorine atoms of the chlorophenols. Furthermore, P values are higher than D values by one order of magnitude. 6

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p w

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p w

o w

p w

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SINGLE-COMPOUND EXTRACTION •pcp-pH:3&pKa:4.75 1,000

•2,3,4,5-tcp - pH:3 & pKa.5.64 A2,4,6-tcp - pH:3 & pKa:6.23 0 2 . 4 « f c p - pH:6 & pKa:7.85

too

02-cp-pH:5&pKa:8.55 Opnenol - pH:5 & pKa:9.99

100

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10.000

100,000

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1-Octanol-Water Partition Coefficient, Ρ«*

Figure 1: Extraction of chlorophenols using [emimJBeti


550 The detection of pentachlorophenol in the aqueous phase after the extraction process was again not possible with HPLC methods used in this study for volume ratio 1:5; indicating that pentachlorophenol may be completely removed from the aqueous phase with this process by both ionic liquids. It should be noted that the detection limit of the method was 0.001 mg/L. It was noticed that pH adjustment of the initial aqueous chlorophenol solution was not necessary for the extraction process with [bmim]PF . It was observed that after extraction with [bmim]PF , the aqueous phase had a pH around 3.0. On the other hand, the pH of the aqueous phase after extraction with [emim]Beti was around 5.0. This means that the pH of the aqueous phase after the extraction can be affected by the presence of ionic liquid and perhaps any impurities they contain. The solubilities of [bmim]PF and [emim]Beti were measured with HPLC and were close to 20 g/1 and 5 g/1, respectively. The solubility of [bmim]PF as determined using the HPLC method described in this study is close to a published value of 18.8 g/L (35). 6

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3.3 Extraction using solution with multiple chlorophenols The same extraction process was repeated with an initial aqueous solution containing 50 mg/1 each of 2-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol using [emim]Beti or [bmim]PF . The objective of these experiments was to examine the influence of the aqueous composition of multi-component mixture on the extraction efficiency. The latter is defined here as the ratio of the amount of chlorophenol transferred into ionic liquid phase versus the maximum theoretical amount that can be transferred. The extraction efficiencies of the multi-compound solution were 6

10.000 SINGLE-COMPOUND EXTRACTION •pcp-pH:3&pKa:4.75 1,000

•2,3A5-!cp - pH:3 & pKa:5.04 Δ 2 , 4 . 6 4 φ · pH.3 & pKa:6.23 02,4-dcp - pH:3 & pKa:7.85

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•2-cp-pH:3&pKa:8.55 OpnenoJ-pH:3&pKa:9.99

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10,000

100.000 1.000,000

1-Octanol-Water Partition Coefficient, Pow

Figure 2: Extraction of chlorophenols using [bmim]PF

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551 1.000

I

i


.S

•

2.4.6-tcp & {bnim]PF6

Δ

2.4-dcp & (bmim]PF6

Ο

2-cp & (bmimJPF6

m

2.4.6-tcp & [emim]Beti

Α

2.4-clcp & (emim]Beti

2-cp & (cmim)BcU

1.000 D,L in the single-compound solution W

Figure 3: Extraction of aqueous solution with three chlorophenols

slightly lower than those of the single solute systems: 87% vs. 91% for 2-cp, 89% vs. 94% for 2,4-dcp, and 90% vs. 98% for 2,4,6-tcp. As shown in Figure 3, the decrease of the extraction efficiency is not significant on the ionic liquid-water distribution ratio, Dn^wA similar experiment was performed with a different initial solution containing 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol. The extraction efficiencies of the multi-component mixture and single-solute solutions were the same. The corresponding values were 88, 94 and 98 % for 4-cp, 2,4-dcp, and 2,4,6-tcp, respectively. The DILW was not changed in the multi-component and single-component solutions. 3.4 Effect of acidic and basic solution The influence of the pH on the extraction was best demonstrated by the extraction of a solution with five different chlorophenols with varying pK values. Specifically, a solution with 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichloro phenol, and pentachlorophenol was used for the extraction process using [emimJBeti or [bmim]PF . Eight experiments were performed with this solution. The following parameters were varied: (a) the pH of the initial solution (3.0 or 5.0), and (b) the volume ratio of the two phases (1:5 or 1:30). The pH value affects the presence of the neutral and ionized species, while the volume ratio affects the final concentration of pentachlorophenol in the aqueous phase and thus its detection with HPLC. The initial pH solution was adjusted at 3.0 with H S 0 whenever it was necessary. The initial concentration for each of the chlorophenols was 50 mg/1, except for pentachlorophenol, which was 4 mg/1. The results of this series of experiments are presented in Figure 4. It is apparent from Figure 4 that the highest effect of the pH is a

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552 on pentachlorophenol, which has the lowest pK value of these compounds (4.75). The distribution ratio remained intact for 2-chlorophenol, 4-chlorophenol and 2,4dichlorophenol with the change of pH from 5.0 to 3.0. The distribution ratio was affected for 2,4,6-trichlorophenol and pentachlorophenol, since their pK values are 6.23 and 4.75, respectively. These results corroborate that the transferred species to ionic liquids is the neutral and not the ionized. a

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10.000 -to[bmimJPF6pH:3.1


OtoibmimJPF6pH:3.0 Xto[bmtm]PF6pH:4.l6 Ato|bmim]PF6pH:3.3 XtofemimJBeUpH.3.1 Ο to [etram]Bed pH:3. ] + to{cmim]BetipH:5.6 •to [eiramJBeti pH:5.8 2-cp

4-cp

2,4-dcp

2,4,6-tcp

pep

Figure 4: Extraction of aqueous solution with five chlorophenols

The influence of high pH was demonstrated with the extraction of a solution that contained 50 mg/1 phenol, 2-chlorophenol, or 2,4-dichlorophenol. The results are presented in Table-I. The pH of the initial solution was adjusted at 11.0 with NaOH and extracted using [bmim]PF or [emimJBeti. The D was calculated to be 0.7 and 0.2 for phenol and 2,4-dichlorophenol, respectively. The D was calculated at 1.0, 0.1, and 0.05 for phenol, 2-chlorophenol, and 2,4-dichlorophenol, respectively. Further work on the pH effect of initial aqueous solution on the distribution ratio was performed for the system 2-chlorophenol/[bmim]PF . Five ml of [bmim]PF and 25 ml of aqueous solution with high 2-cp concentration of 400 mg/1 were mixed well in a 60 ml bottle by a magnetic stirrer at 800 rpm for 4 minutes. A stirring time longer than 4 minutes was found to have no effect on extraction in the range of 4-150 minutes for the case of "neutral" 2-cp solution. The acidity of the solution was adjusted as above by adding dilute H S 0 solution while the alkalinity was adjusted by dilute NaOH solution. The chlorophenol concentration in the aqueous solution was determined by the well-established 4-aminoantipyrine colorimetric method (37). HP8452A diode array UV/Vis spectrometer was used. The results are listed in Table II. 6

p w

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2

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H2SO4

0.00114N 49.7

H2SO4

0.0114N 46.1 50.7

None

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NaOH 0.0032N 1.16

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NaOH 0.0096N 0.061

NaOH 0.048N 0.015*

Extraction with [bmim]PF Ph 2-cp 2,4-dcp 17 70 180 17 68 172 0.7 0.2 76 92 98 76 92 97 11 3

Table-II: The effect of acid and alkaline in aqueous 2-cp solution on the distribution ratio of [bmim]PF

Extraction with [emimjBeti ph 2-cp 2,4-dcp 14 46 105 13 46 106 1.0 0.1 0.05 73 90 95 68 92 94 17 2.0 0.9

* 10 ml [bmtm]PF and 5 ml aqueous 2-cp solution were used.

Concentration of H S0 or NaOH Distribution Ratio

LW

D : Neutral pH:3 pH:11 Efficiency: Neutral pH:3 pH:11

Characteristics

Table-I: Influence of pH on the distribution ratio of ionic liquids


NaOH 0.48N 0.038*

554 The pH-dependence of thymol blue in ionic liquids was reported (23). In this work the dramatic decrease of distribution ratio to a very low value in alkaline solution is very significant since it could provide an effective technology for back extraction of chlorophenol to an aqueous phase and regeneration of the ionic liquid. In such a technology, the volume of the alkaline back-extract phase should be less than that of ionic liquid in order to achieve concentration of chlorophenol. Thus, 10 ml [bmim]PF and 5 ml aqueous solution were used for the determination of distribution ratio at NaOH concentration of 0.048 or 0.48 N.


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3.5 Effect of ionic strength The effect of the ionic strength was examined for three different solutions that contained 50 mg/1 phenol, 2-chlorophenol or 2,4-dichlorophenol. The extraction was performed using [bmim]PF or [emim]Beti and the ionic strength was varied form 0 to 500 mM NaN0 . The results are illustrated in Figure 5. It is obvious from Figure 5, that the ionic strength does not affect the distribution ratio. It only seems to be a small decrease in the distribution ratio for ionic strength 500 mM, when 2,4dichlorophenol is extracted using [bmim]PF . Even though the distribution ratio is not affected by the ionic strength, the solubility of ionic liquids in water increases significantly, which is in agreement with previously published results (28). The solubility of [bmim]PF and [emimjBeti in water increases with an increase in the ionic strength. The solubilities of [bmim]PF and [emimjBeti increase from 20 g/1 and 4.2 g/1 at zero ionic strength to 25 g/1 and 5.3 g/1 at 500 mM NaN0 , respectively. 6

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1,000.0 a2.4-€fcp-[bmim]PFe 100.0

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Ionic Liquids as Green Solvents - ACS Publications - American

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