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Extraction of soluble dyes from aqueous solutions with quaternary ammonium-based ionic liquids
Separation and Purification Technology 106 (2013) 105–109
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Extraction of soluble dyes from aqueous solutions with quaternary ammonium-based ionic liquids Xiaochun Chen, Fanlei Li, Charles Asumana, Guangren Yu ⇑ Beijing Key Laboratory of Membrane Science and Technology, Beijing University of Chemical Technology, 100029 Beijing, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2012 Received in revised form 11 December 2012 Accepted 4 January 2013 Available online 17 January 2013 Keywords: Ionic liquids Extract Methyl orange Methylene blue
a b s t r a c t In this work, four quaternary ammonium-based ionic liquids are used to extract methyl orange and methylene blue (major dye species in industry) from water. The influence of extraction time, temperature, salt effect, and pH on extraction efficiency is systematically investigated. Tricaprylmethylammonium thiocyanate ([N1888][SCN]) exhibits optimal ability for extracting both dyes; the extraction efficiency reaches 89.09% and 64.14% for methyl orange and methylene blue, respectively. It is also observed that the efficiency is remarkably increased by adding NaCl in the extraction system except in the extraction of methyl orange using [N1888][SCN]. This work shows that ionic liquids might provide new options in the disposal of dye-wastewater. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Synthetic dyes are widely used in many fields including textile dyeing, leather tanning, paper production, food technology and so on [1–5]. It is estimated that over 1 million tons of dyes are produced annually around the world, among which azo dyes account for nearly 50% [6,7]. Methyl orange is an intensely colored azo dye compound which is widely used for dyeing and printing of textiles and as indicator [8–10]. Methylene blue is a commonly used dyestuff with wider application domains including coloring paper, dyeing wools and temporary hair [11]. Both of them are toxic and represent the general chemical structure of anionic dyes and cationic dyes. In fact, typically about 1–2% of the dyes in production and 1–10% in use are discharged with the effluent [5]. The transparency of water is greatly reduced with the presence of dyes and the oxygen in water is consumed a lot, thus destroying the self-purification of water [12,13]. The products of azo dye degradation are mostly aryl amines which are more carcinogenic and toxic than the original effluents [6]. Many methods are used to treat dye wastewater, such as flocculation, chemical oxidation, air flotation processes, as well as aerobic and anaerobic processes [14]. Notwithstanding, the conventional engineering techniques are facing some challenges due to the enforcement of tighter regulations and cost ineffectiveness in removing dyes from water [15–17]. In recent years, ionic liquids (ILs) have attracted much attention in different areas of chemistry thanks to their novel physical and chemical properties such as non-volatility, thermal stability, wide ⇑ Corresponding author. Tel./Fax: +86 10 6443 3570.
liquidus range, non-flammability, and wide electrochemical window [18–21]. Application of ILs in separation and their mechanisms have been investigated by several research groups in the past few years. Rogers et al. [22] extracted metal ions from water with 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]); the distribution coefficients of most metal ions were low (about 0.05). Matsumoto et al. [23] studied the extraction of short-chain aliphatic carboxylic acids (acetic, glycolic, propanoic, lactic, pyruvic, and butyric) from water by [Cnmim][PF6]; the distribution coefficients were generally small (about 0.02–1.06) and varied slightly with the alkyl chain length of ILs. Vidal et al. [24] evaluated a series of [Cnmim][PF6] and [Cnmim][BF4] for the extraction of phenol, tyrosol and p-hydroxybenzoic acid from water. Recently, extraction of organic dyes from water by ILs was investigated. Vijayaraghavan et al. [25] used a hydrophobic IL, Nbutyl N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)amide, to extract acid blue and acid red dyes. Li et al. [26] used [C4mim][PF6] to extract acid dyes and reactive dyes from water, and studied the mechanism of the extraction. Pei [27] studied the influence of the extraction of anionic dyes with a series of imidazolium-based ILs. Fan et al. [28] investigated the ILs/water distribution ratios of azo dyes, where 5 ILs, viz, [C4mim][PF6], [C6mim][PF6], [C8mim][PF6], [C6mim][BF4] and [C8mim][BF4] were used to extract azo dyes, and found that the removal efficiency using [BF4] based ILs was higher than that of [PF6] based ILs. The results of extracting dyes using ILs are promising. However, these fluorine-containing ILs such as [BF4], [PF6], and [NTf2] species tend to release corrosive hydrogen fluoride (HF) upon decomposition [29,30].
E-mail address: [email protected] (G. Yu). 1383-5866/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.01.002
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X. Chen et al. / Separation and Purification Technology 106 (2013) 105–109
R¼
C aq;0 C aq 100% C aq;0
ð1Þ
D¼
ðC aq;0 C aq Þ V aq R ¼K 1R C aq V 0
ð2Þ
In this work, the extraction of anionic dye methyl orange and cationic dye methylene blue from water using four non-fluorine quaternary ammonium-based ILs, i.e., tricaprylmethylammonium thiocyanate ([N1888][SCN]), tricaprylmethylammonium dicyanoamide ([N1888][DCA]), tricaprylmethylammonium benzoate ([N1888] [BA]), and tricaprylmethylammonium hexanoate ([N1888][Hex]) are investigated. These ILs have been characterized, as seen in the Supplementary data. Their distribution coefficients in IL/water were determined by means of UV–vis Spectrophotometry. The influence of extraction time, pH of aqueous phase, temperature, and concentration of NaCl on the distribution coefficient is studied.
where Caq,o and Caq represent the initial and final concentrations (mg/L) of a given dye in an aqueous phase respectively, and Vaq and Vo are the volumes of the aqueous phase and the ILs phase respectively. K = Vaq/Vo (extraction phase ratio) is the volume ratio of the aqueous and organic phases.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Effect of phase ratio
The synthesis of ILs was performed using published procedure [31,32]. The chemical structure of ILs used in the research is shown in Fig. 1. The chemicals (suppliers) are as follows: tricaprylmethylammonium chloride [N1888]Cl (Aldrich, Shanghai); sodium thiocyanate (NaSCN, Xilong Chemical Co., Guangdong); sodium dicyanamide (NaDCA, Yingkou Sanxin Chemical Co., Liaoning); benzoic acid (Xilong Chemical Co., Guangdong); hexanoic acid (Guangfu Fine Chemical Institute, Tianjin); sodium hydroxide (Beijing Chemical & Engineering Co.); hydrogen chloride (Beijing Chemical & Engineering Co.); methyl orange and methylene blue (Tianjin Damao Chemical Co.).
A series of systematic experiments show that the extraction process reaches equilibrium within 30 min under stirring at a speed of 200 rpm at a temperature of 20 °C. The influence of K on the distribution coefficient of methyl orange was investigated, as seen in Fig. 2. The value of D of methyl orange remained at about 2.9 with a change in the value of K, which has no significant influence on the distribution coefficient. The insensitivity of distribution coefficient to phase ratio indicates that the extraction is mainly dominated by physical interaction, where the distribution coefficient is determined by the temperature and nature of compound species [33,34]. An amount of 1 mL ILs and 5 mL dye solution is taken for each extraction experiment in order to economize on the ILs and easily measure the absorbance.
2.2. Extraction process The aqueous dye solutions were prepared by directly dissolving the dyes into deionized water. The original concentration of dye was about 20–40 mg/L. In a typical experiment, 5.0 mL dye solution and 1.0 mL IL were placed in a vial and then stirred for 30 min at a speed of 200 rpm at 20 °C. After that, the lower water phase was sucked with a straw and its absorbance was detected. 2.3. Method of analysis The partitioning of dyes in ILs was determined by UV–vis spectroscopy through the standard curves of concentration vs. absorbance. The initial absorbance and the absorbance of the aqueous methyl orange and methylene blue solutions were measured after each extraction with IL at different time intervals. The absorbance of methyl orange was measured at 510 nm and that of methylene blue at 660 nm. Extraction efficiency (R) and distribution coefficient (D) of dyes between the ILs and aqueous phases were respectively calculated as follows:
3.2. Effect of temperature The distribution coefficients of methyl orange and methylene blue in the four ILs/water system has been determined respectively at different temperatures (293.15–333.15 K) (Table 1). The [N1888][SCN] shows the highest efficiency for extracting methyl orange and methylene blue at room temperature, which is consistent with the observation that SCN anion can complex with metal ions and is commonly used to remove heavy metal ions [35,36]. This SCN is a stronger proton acceptor which can interact with the dye molecules through hydrogen bonds. So, the distribution coefficient of dyes in [N1888][SCN]/water is higher than the others. The extraction efficiency of methyl orange with four different ILs increases with increasing temperature, to the extent that it reaches
4.0
CH3 N+ H3C(H2C)7
X-
(CH2)7CH3 (CH2)7CH3
H3C
SO3Na+
N N
N
H3C Methyl Orange
X-=SCN=(CN)2N-
[N1888][SCN] [N1888][DCA]
=
O[N1888][BA]
=C5H11COO- [N1888][Hex]
H3C
ClN
S
CH3
3.0 2.5 2.0 1.5
N
O
Distribution Coefficient
3.5
N+
CH3
CH3 Methylene Blue
Fig. 1. Chemical structure of the ILs and dyes in this study.
1.0 0
2
4
6
8
10
K Fig. 2. Relation between distribution coefficient of methyl orange and K.
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X. Chen et al. / Separation and Purification Technology 106 (2013) 105–109 Table 1 Distribution coefficient in IL/water at different temperatures. IL
Methyl orange
[N1888] [N1888] [N1888] [N1888] [N1888] [N1888] [N1888] [N1888]
Methylene blue
Distribution coefficient
[SCN] [DCA] [BA] [Hex] [SCN] [DCA] [BA] [Hex]
293.15 K
303.15 K
313.15 K
323.15 K
333.15 K
42.57 2.75 1.1 1.02 8.95 5.54 0.64 0.57
64.64 3.03 1.47 1.17 5.63 4.41 0.55 0.48
96.42 3.67 1.84 1.57 3.87 2.89 0.42 0.42
191.08 5.28 2.09 2.06 2.32 1.64 0.32 0.32
258.97 9.13 2.76 2.98 1.22 0.77 0.24 0.22
98% at 333.15 K with [N1888][SCN]. A similar phenomena was observed in Pei’s research [27]. However, a reverse conclusion relative to the methylene blue extraction is reached, i.e., the extraction efficiency decreases with increasing temperature. The extraction efficiency of methylene blue drops to below 5% with [N1888][BA] and [N1888][Hex] respectively at 333.15 K. From thermodynamics, extraction of a dye from the aqueous phase into a particular IL phase can be regarded as a dye transfer from the aqueous phase to the IL phase [37]. Assuming that the enthalpy change (DH0T ) of the dye molecules’ phase transfer process is constant within the experimental scope of the temperature, the value of enthalpy change can be calculated from the linear relations between the logarithm of the distribution coefficient (log D) and 1/T by the following formula:
log D ¼
2
[N1888][SCN], R =0.9904 2 [N1888][DCA], R =0.9595 2 [N1888][BA], R =0.9895 2 [N1888][Hex], R =0.9574
0.9 0.6
logD
Dye
0.3 0.0 -0.3 -0.6 0.0030
DH0T þA 2:3RT
The phase transfer enthalpy of methyl orange and methylene blue in different IL/water systems can be calculated from the slope of the line in Figs. 3 and 4 (Table 2). As can be seen, DH0T (methyl orange) > 0 but DH0T (methylene blue) < 0. Thus, the process is endothermic for the extraction of methyl orange, and the distribution coefficient increases with temperature; while it is exothermic for the extraction of methylene blue and the distribution coefficient decreases with temperature.
2.4 2.0 2
logD
[N1888][SCN], R =0.9881 2 [N1888][DCA], R =0.9131 2 [N1888][BA], R =0.9860 2 [N1888][Hex], R =0.9793
0.8 0.4 0.0 0.0030
0.0031
0.0032
0.0033
0.0034
1/T Fig. 3. Linear relation between the logarithm of distribution coefficient for methyl orange and 1/T in IL/water.
0.0033
0.0034
Fig. 4. Linear relation between the logarithm of distribution coefficient for methylene blue and 1/T in IL/water.
Table 2 Phase transfer enthalpy of dyes from water to IL. Dye
IL
Methyl orange
[N1888] [N1888] [N1888] [N1888] [N1888] [N1888] [N1888] [N1888]
Methylene blue
The relationship between the distribution coefficient and concentration of NaCl is shown in Figs. 5 and 6. The distribution coefficient of methyl orange increases after adding NaCl in [N1888][DCA]/water, [N1888][BA]/water and [N1888][Hex]/water.
1.2
0.0032
1/T
3.3. Effect of NaCl concentration
1.6
0.0031
ð3Þ
DH0T (kJ/mol) [SCN] [DCA] [BA] [Hex] [SCN] [DCA] [BA] [Hex]
38.99 24.54 18.20 22.50 40.46 40.99 20.79 19.18
The distribution coefficient increases from 2.75 to 31.79 in [N1888][DCA]/water when NaCl concentration increases from 0 g/L to 40 g/L. An opposite result was obtained for the extraction of methyl orange by [N1888][SCN], where the extractive performance of [N1888][SCN] decreases with increasing NaCl concentration. The dye solution salts out with the addition of electrolyte, e.g., NaCl, which ‘forces’ the methyl orange molecules to transfer from aqueous phase to ILs phase. When the salt concentration is increased, some of the water molecules are attracted by the salt ions, which decreases the number of water molecules available to interact with the charged part of the dyes, and then the concentration of dyes increases to be easily escaped from the solution [38]. The distribution coefficients of methylene blue in [N1888][BA]/water and [N1888][Hex]/water increase from 0.64 and 0.57 to 2.99 and 1.49 respectively when the concentration of NaCl increases from 0 to 40 g/L. This is remarkable so far as the extraction of cationic dyes by ILs with salt effect is concerned. There are many electrolytes in the real dye wastewater which could increase the efficiency of extracting dyes by ILs. Pandit and Basu [39] studied the effect of KCl on the partitioning of methyl orange in reverse micelles from
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X. Chen et al. / Separation and Purification Technology 106 (2013) 105–109
2.0
1.6
1.5 [N1888][SCN] [N1888][DCA] [N1888][BA] [N1888][Hex]
0.8
1.0
logD
logD
1.2
0.4
0.5 [N1888][SCN] [N1888][DCA]
0.0 -0.5
0.0 0
10
20
30
40
2
Concentration of NaCl (g/L)
4
6
8
10
12
pH
Fig. 5. Effect of the logarithm of distribution coefficient of methyl orange by the concentration of NaCl.
Fig. 7. Effect of the logarithm of methyl orange distribution coefficient by pH value.
1.0 1.2 0.8 [N1888][SCN] [N1888][DCA] [N1888][BA] [N1888][Hex]
0.6
logD
logD
0.9
0.6
0.3
[N1888][SCN] [N1888][DCA]
0.4 0.0 0.2
-0.3 0
10
20
30
40
2
4
6
8
10
12
pH
Concentration of NaCl (g/L) Fig. 6. Effect of the logarithm of distribution coefficient of methylene blue by the concentration of NaCl.
Fig. 8. Effect of the logarithm of methylene blue distribution coefficient by pH value.
water, and reported that the extraction of methyl orange increased with increasing concentration of KCl.
blue with acetone, hexane and chloroform. Only chloroform could extract some methylene blue from ILs; the recovery efficiency of methylene blue is 34.74% and 45.22% respectively with [N1888][SCN] and [N1888][DCA] in the first extraction, and reaches 56.37% and 65.81% in the second extraction. For industrial application, however, an ideal regeneration method of ILs is needed.
3.4. Effect of PH The effect of the pH of dyes solution on extraction equilibrium was also investigated. It can be seen from Figs. 7 and 8 that the pH has an obvious effect on the extraction equilibrium of methyl orange but no significant effect on the extraction of methylene blue. This might be ascribed to the reason that the chemical structure of methyl orange changes while methylene blue molecule does not at different pH values [40]. When the pH value is above the acid dissociation constant (pKa) of methyl orange, the distribution coefficient of methyl orange in the IL/water phase is larger because methyl orange molecules mainly exist in the form of anions. When the pH value is lower than the pKa, it is hard for methyl orange molecules to enter into ILs phase because they mainly exist in zwitterionic form. 3.5. Recovery of methyl orange and methylene blue To recover methyl orange from ILs, 0.1 mol/L HCl was used. The recovery efficiency is 92.47% for [N1888][SCN] and 95.58% for [N1888][DCA]. It was proven rather difficult to completely recover methylene blue from ILs. An effort was made to recover methylene
4. Conclusions In this paper, four quaternary ammonium-based hydrophobic ILs were synthesized and used to study the extraction efficiency of methyl orange and methylene blue from water. It was observed that temperature, salt effect, and pH have significant effects on the distribution coefficients of methyl orange and methylene blue between water and ILs, while phase ratio does not. The extraction efficiency reaches 89.09% and 64.14% for methyl orange and methylene blue, respectively using [N1888][SCN]. This work shows that ILs extraction may provide new options in the disposal of dye wastewater. The recovery of dyes from ILs is worthy of undergoing further studies. Acknowledgments This work was financially supported by National Natural Science Foundation of China (20806002, 20976005, 21176021, 21276020).
X. Chen et al. / Separation and Purification Technology 106 (2013) 105–109
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Extraction of soluble dyes from aqueous solutions with quaternary ammonium-based ionic liquids Xiaochun Chen, Fanlei Li, Charles Asumana, Guangren Yu ⇑ Beijing Key Laboratory of Membrane Science and Technology, Beijing University of Chemical Technology, 100029 Beijing, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2012 Received in revised form 11 December 2012 Accepted 4 January 2013 Available online 17 January 2013 Keywords: Ionic liquids Extract Methyl orange Methylene blue
a b s t r a c t In this work, four quaternary ammonium-based ionic liquids are used to extract methyl orange and methylene blue (major dye species in industry) from water. The influence of extraction time, temperature, salt effect, and pH on extraction efficiency is systematically investigated. Tricaprylmethylammonium thiocyanate ([N1888][SCN]) exhibits optimal ability for extracting both dyes; the extraction efficiency reaches 89.09% and 64.14% for methyl orange and methylene blue, respectively. It is also observed that the efficiency is remarkably increased by adding NaCl in the extraction system except in the extraction of methyl orange using [N1888][SCN]. This work shows that ionic liquids might provide new options in the disposal of dye-wastewater. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Synthetic dyes are widely used in many fields including textile dyeing, leather tanning, paper production, food technology and so on [1–5]. It is estimated that over 1 million tons of dyes are produced annually around the world, among which azo dyes account for nearly 50% [6,7]. Methyl orange is an intensely colored azo dye compound which is widely used for dyeing and printing of textiles and as indicator [8–10]. Methylene blue is a commonly used dyestuff with wider application domains including coloring paper, dyeing wools and temporary hair [11]. Both of them are toxic and represent the general chemical structure of anionic dyes and cationic dyes. In fact, typically about 1–2% of the dyes in production and 1–10% in use are discharged with the effluent [5]. The transparency of water is greatly reduced with the presence of dyes and the oxygen in water is consumed a lot, thus destroying the self-purification of water [12,13]. The products of azo dye degradation are mostly aryl amines which are more carcinogenic and toxic than the original effluents [6]. Many methods are used to treat dye wastewater, such as flocculation, chemical oxidation, air flotation processes, as well as aerobic and anaerobic processes [14]. Notwithstanding, the conventional engineering techniques are facing some challenges due to the enforcement of tighter regulations and cost ineffectiveness in removing dyes from water [15–17]. In recent years, ionic liquids (ILs) have attracted much attention in different areas of chemistry thanks to their novel physical and chemical properties such as non-volatility, thermal stability, wide ⇑ Corresponding author. Tel./Fax: +86 10 6443 3570.
liquidus range, non-flammability, and wide electrochemical window [18–21]. Application of ILs in separation and their mechanisms have been investigated by several research groups in the past few years. Rogers et al. [22] extracted metal ions from water with 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]); the distribution coefficients of most metal ions were low (about 0.05). Matsumoto et al. [23] studied the extraction of short-chain aliphatic carboxylic acids (acetic, glycolic, propanoic, lactic, pyruvic, and butyric) from water by [Cnmim][PF6]; the distribution coefficients were generally small (about 0.02–1.06) and varied slightly with the alkyl chain length of ILs. Vidal et al. [24] evaluated a series of [Cnmim][PF6] and [Cnmim][BF4] for the extraction of phenol, tyrosol and p-hydroxybenzoic acid from water. Recently, extraction of organic dyes from water by ILs was investigated. Vijayaraghavan et al. [25] used a hydrophobic IL, Nbutyl N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)amide, to extract acid blue and acid red dyes. Li et al. [26] used [C4mim][PF6] to extract acid dyes and reactive dyes from water, and studied the mechanism of the extraction. Pei [27] studied the influence of the extraction of anionic dyes with a series of imidazolium-based ILs. Fan et al. [28] investigated the ILs/water distribution ratios of azo dyes, where 5 ILs, viz, [C4mim][PF6], [C6mim][PF6], [C8mim][PF6], [C6mim][BF4] and [C8mim][BF4] were used to extract azo dyes, and found that the removal efficiency using [BF4] based ILs was higher than that of [PF6] based ILs. The results of extracting dyes using ILs are promising. However, these fluorine-containing ILs such as [BF4], [PF6], and [NTf2] species tend to release corrosive hydrogen fluoride (HF) upon decomposition [29,30].
E-mail address: [email protected] (G. Yu). 1383-5866/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.01.002
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X. Chen et al. / Separation and Purification Technology 106 (2013) 105–109
R¼
C aq;0 C aq 100% C aq;0
ð1Þ
D¼
ðC aq;0 C aq Þ V aq R ¼K 1R C aq V 0
ð2Þ
In this work, the extraction of anionic dye methyl orange and cationic dye methylene blue from water using four non-fluorine quaternary ammonium-based ILs, i.e., tricaprylmethylammonium thiocyanate ([N1888][SCN]), tricaprylmethylammonium dicyanoamide ([N1888][DCA]), tricaprylmethylammonium benzoate ([N1888] [BA]), and tricaprylmethylammonium hexanoate ([N1888][Hex]) are investigated. These ILs have been characterized, as seen in the Supplementary data. Their distribution coefficients in IL/water were determined by means of UV–vis Spectrophotometry. The influence of extraction time, pH of aqueous phase, temperature, and concentration of NaCl on the distribution coefficient is studied.
where Caq,o and Caq represent the initial and final concentrations (mg/L) of a given dye in an aqueous phase respectively, and Vaq and Vo are the volumes of the aqueous phase and the ILs phase respectively. K = Vaq/Vo (extraction phase ratio) is the volume ratio of the aqueous and organic phases.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Effect of phase ratio
The synthesis of ILs was performed using published procedure [31,32]. The chemical structure of ILs used in the research is shown in Fig. 1. The chemicals (suppliers) are as follows: tricaprylmethylammonium chloride [N1888]Cl (Aldrich, Shanghai); sodium thiocyanate (NaSCN, Xilong Chemical Co., Guangdong); sodium dicyanamide (NaDCA, Yingkou Sanxin Chemical Co., Liaoning); benzoic acid (Xilong Chemical Co., Guangdong); hexanoic acid (Guangfu Fine Chemical Institute, Tianjin); sodium hydroxide (Beijing Chemical & Engineering Co.); hydrogen chloride (Beijing Chemical & Engineering Co.); methyl orange and methylene blue (Tianjin Damao Chemical Co.).
A series of systematic experiments show that the extraction process reaches equilibrium within 30 min under stirring at a speed of 200 rpm at a temperature of 20 °C. The influence of K on the distribution coefficient of methyl orange was investigated, as seen in Fig. 2. The value of D of methyl orange remained at about 2.9 with a change in the value of K, which has no significant influence on the distribution coefficient. The insensitivity of distribution coefficient to phase ratio indicates that the extraction is mainly dominated by physical interaction, where the distribution coefficient is determined by the temperature and nature of compound species [33,34]. An amount of 1 mL ILs and 5 mL dye solution is taken for each extraction experiment in order to economize on the ILs and easily measure the absorbance.
2.2. Extraction process The aqueous dye solutions were prepared by directly dissolving the dyes into deionized water. The original concentration of dye was about 20–40 mg/L. In a typical experiment, 5.0 mL dye solution and 1.0 mL IL were placed in a vial and then stirred for 30 min at a speed of 200 rpm at 20 °C. After that, the lower water phase was sucked with a straw and its absorbance was detected. 2.3. Method of analysis The partitioning of dyes in ILs was determined by UV–vis spectroscopy through the standard curves of concentration vs. absorbance. The initial absorbance and the absorbance of the aqueous methyl orange and methylene blue solutions were measured after each extraction with IL at different time intervals. The absorbance of methyl orange was measured at 510 nm and that of methylene blue at 660 nm. Extraction efficiency (R) and distribution coefficient (D) of dyes between the ILs and aqueous phases were respectively calculated as follows:
3.2. Effect of temperature The distribution coefficients of methyl orange and methylene blue in the four ILs/water system has been determined respectively at different temperatures (293.15–333.15 K) (Table 1). The [N1888][SCN] shows the highest efficiency for extracting methyl orange and methylene blue at room temperature, which is consistent with the observation that SCN anion can complex with metal ions and is commonly used to remove heavy metal ions [35,36]. This SCN is a stronger proton acceptor which can interact with the dye molecules through hydrogen bonds. So, the distribution coefficient of dyes in [N1888][SCN]/water is higher than the others. The extraction efficiency of methyl orange with four different ILs increases with increasing temperature, to the extent that it reaches
4.0
CH3 N+ H3C(H2C)7
X-
(CH2)7CH3 (CH2)7CH3
H3C
SO3Na+
N N
N
H3C Methyl Orange
X-=SCN=(CN)2N-
[N1888][SCN] [N1888][DCA]
=
O[N1888][BA]
=C5H11COO- [N1888][Hex]
H3C
ClN
S
CH3
3.0 2.5 2.0 1.5
N
O
Distribution Coefficient
3.5
N+
CH3
CH3 Methylene Blue
Fig. 1. Chemical structure of the ILs and dyes in this study.
1.0 0
2
4
6
8
10
K Fig. 2. Relation between distribution coefficient of methyl orange and K.
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X. Chen et al. / Separation and Purification Technology 106 (2013) 105–109 Table 1 Distribution coefficient in IL/water at different temperatures. IL
Methyl orange
[N1888] [N1888] [N1888] [N1888] [N1888] [N1888] [N1888] [N1888]
Methylene blue
Distribution coefficient
[SCN] [DCA] [BA] [Hex] [SCN] [DCA] [BA] [Hex]
293.15 K
303.15 K
313.15 K
323.15 K
333.15 K
42.57 2.75 1.1 1.02 8.95 5.54 0.64 0.57
64.64 3.03 1.47 1.17 5.63 4.41 0.55 0.48
96.42 3.67 1.84 1.57 3.87 2.89 0.42 0.42
191.08 5.28 2.09 2.06 2.32 1.64 0.32 0.32
258.97 9.13 2.76 2.98 1.22 0.77 0.24 0.22
98% at 333.15 K with [N1888][SCN]. A similar phenomena was observed in Pei’s research [27]. However, a reverse conclusion relative to the methylene blue extraction is reached, i.e., the extraction efficiency decreases with increasing temperature. The extraction efficiency of methylene blue drops to below 5% with [N1888][BA] and [N1888][Hex] respectively at 333.15 K. From thermodynamics, extraction of a dye from the aqueous phase into a particular IL phase can be regarded as a dye transfer from the aqueous phase to the IL phase [37]. Assuming that the enthalpy change (DH0T ) of the dye molecules’ phase transfer process is constant within the experimental scope of the temperature, the value of enthalpy change can be calculated from the linear relations between the logarithm of the distribution coefficient (log D) and 1/T by the following formula:
log D ¼
2
[N1888][SCN], R =0.9904 2 [N1888][DCA], R =0.9595 2 [N1888][BA], R =0.9895 2 [N1888][Hex], R =0.9574
0.9 0.6
logD
Dye
0.3 0.0 -0.3 -0.6 0.0030
DH0T þA 2:3RT
The phase transfer enthalpy of methyl orange and methylene blue in different IL/water systems can be calculated from the slope of the line in Figs. 3 and 4 (Table 2). As can be seen, DH0T (methyl orange) > 0 but DH0T (methylene blue) < 0. Thus, the process is endothermic for the extraction of methyl orange, and the distribution coefficient increases with temperature; while it is exothermic for the extraction of methylene blue and the distribution coefficient decreases with temperature.
2.4 2.0 2
logD
[N1888][SCN], R =0.9881 2 [N1888][DCA], R =0.9131 2 [N1888][BA], R =0.9860 2 [N1888][Hex], R =0.9793
0.8 0.4 0.0 0.0030
0.0031
0.0032
0.0033
0.0034
1/T Fig. 3. Linear relation between the logarithm of distribution coefficient for methyl orange and 1/T in IL/water.
0.0033
0.0034
Fig. 4. Linear relation between the logarithm of distribution coefficient for methylene blue and 1/T in IL/water.
Table 2 Phase transfer enthalpy of dyes from water to IL. Dye
IL
Methyl orange
[N1888] [N1888] [N1888] [N1888] [N1888] [N1888] [N1888] [N1888]
Methylene blue
The relationship between the distribution coefficient and concentration of NaCl is shown in Figs. 5 and 6. The distribution coefficient of methyl orange increases after adding NaCl in [N1888][DCA]/water, [N1888][BA]/water and [N1888][Hex]/water.
1.2
0.0032
1/T
3.3. Effect of NaCl concentration
1.6
0.0031
ð3Þ
DH0T (kJ/mol) [SCN] [DCA] [BA] [Hex] [SCN] [DCA] [BA] [Hex]
38.99 24.54 18.20 22.50 40.46 40.99 20.79 19.18
The distribution coefficient increases from 2.75 to 31.79 in [N1888][DCA]/water when NaCl concentration increases from 0 g/L to 40 g/L. An opposite result was obtained for the extraction of methyl orange by [N1888][SCN], where the extractive performance of [N1888][SCN] decreases with increasing NaCl concentration. The dye solution salts out with the addition of electrolyte, e.g., NaCl, which ‘forces’ the methyl orange molecules to transfer from aqueous phase to ILs phase. When the salt concentration is increased, some of the water molecules are attracted by the salt ions, which decreases the number of water molecules available to interact with the charged part of the dyes, and then the concentration of dyes increases to be easily escaped from the solution [38]. The distribution coefficients of methylene blue in [N1888][BA]/water and [N1888][Hex]/water increase from 0.64 and 0.57 to 2.99 and 1.49 respectively when the concentration of NaCl increases from 0 to 40 g/L. This is remarkable so far as the extraction of cationic dyes by ILs with salt effect is concerned. There are many electrolytes in the real dye wastewater which could increase the efficiency of extracting dyes by ILs. Pandit and Basu [39] studied the effect of KCl on the partitioning of methyl orange in reverse micelles from
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X. Chen et al. / Separation and Purification Technology 106 (2013) 105–109
2.0
1.6
1.5 [N1888][SCN] [N1888][DCA] [N1888][BA] [N1888][Hex]
0.8
1.0
logD
logD
1.2
0.4
0.5 [N1888][SCN] [N1888][DCA]
0.0 -0.5
0.0 0
10
20
30
40
2
Concentration of NaCl (g/L)
4
6
8
10
12
pH
Fig. 5. Effect of the logarithm of distribution coefficient of methyl orange by the concentration of NaCl.
Fig. 7. Effect of the logarithm of methyl orange distribution coefficient by pH value.
1.0 1.2 0.8 [N1888][SCN] [N1888][DCA] [N1888][BA] [N1888][Hex]
0.6
logD
logD
0.9
0.6
0.3
[N1888][SCN] [N1888][DCA]
0.4 0.0 0.2
-0.3 0
10
20
30
40
2
4
6
8
10
12
pH
Concentration of NaCl (g/L) Fig. 6. Effect of the logarithm of distribution coefficient of methylene blue by the concentration of NaCl.
Fig. 8. Effect of the logarithm of methylene blue distribution coefficient by pH value.
water, and reported that the extraction of methyl orange increased with increasing concentration of KCl.
blue with acetone, hexane and chloroform. Only chloroform could extract some methylene blue from ILs; the recovery efficiency of methylene blue is 34.74% and 45.22% respectively with [N1888][SCN] and [N1888][DCA] in the first extraction, and reaches 56.37% and 65.81% in the second extraction. For industrial application, however, an ideal regeneration method of ILs is needed.
3.4. Effect of PH The effect of the pH of dyes solution on extraction equilibrium was also investigated. It can be seen from Figs. 7 and 8 that the pH has an obvious effect on the extraction equilibrium of methyl orange but no significant effect on the extraction of methylene blue. This might be ascribed to the reason that the chemical structure of methyl orange changes while methylene blue molecule does not at different pH values [40]. When the pH value is above the acid dissociation constant (pKa) of methyl orange, the distribution coefficient of methyl orange in the IL/water phase is larger because methyl orange molecules mainly exist in the form of anions. When the pH value is lower than the pKa, it is hard for methyl orange molecules to enter into ILs phase because they mainly exist in zwitterionic form. 3.5. Recovery of methyl orange and methylene blue To recover methyl orange from ILs, 0.1 mol/L HCl was used. The recovery efficiency is 92.47% for [N1888][SCN] and 95.58% for [N1888][DCA]. It was proven rather difficult to completely recover methylene blue from ILs. An effort was made to recover methylene
4. Conclusions In this paper, four quaternary ammonium-based hydrophobic ILs were synthesized and used to study the extraction efficiency of methyl orange and methylene blue from water. It was observed that temperature, salt effect, and pH have significant effects on the distribution coefficients of methyl orange and methylene blue between water and ILs, while phase ratio does not. The extraction efficiency reaches 89.09% and 64.14% for methyl orange and methylene blue, respectively using [N1888][SCN]. This work shows that ILs extraction may provide new options in the disposal of dye wastewater. The recovery of dyes from ILs is worthy of undergoing further studies. Acknowledgments This work was financially supported by National Natural Science Foundation of China (20806002, 20976005, 21176021, 21276020).
X. Chen et al. / Separation and Purification Technology 106 (2013) 105–109
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