Solvent Extraction Research and Development, Japan, Vol. 21, No 1, (2014)

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1 Solvent Extraction Research and Development, Japan, Vol. 1, No 1, (14) Notes Salting-out Phase Separation System of Water Tetrahydrofuran with Co-using 1-Butyl-3-methylimidazolium Chloride and Sodium Chloride for Possible Extraction Separation of Chloro-complexes Naoki HIRAYAMA, 1, * Takaaki HIGO, 3 and Hisanori IMURA 4 1 Department of Chemistry, Faculty of Science, Toho University, Miyama --1, Funabashi , Japan Research Center for Materials with Integrated Properties, Toho University, Miyama --1, Funabashi , Japan 3 Division of Material Chemistry, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 9-119, Japan 4 Division of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 9-119, Japan (Received September 1, 13; Accepted October, 13) The salting-out phase separation behavior of the water tetrahydrofuran (THF) homogeneous system with co-adding a hydrophilic ionic liquid, 1-butyl-3-methylimidazolium chloride (), and was investigated. In the water THF/ biphasic system, the lower phase showed a relatively large volume and low polarity compared with that in the water THF/ system, because of the coexistence of THF and. In the water THF/ mixed salting-out agent system, the lower phase polarity depended on the composition of the salting-out agents. Furthermore, the distribution behavior of Fe(III) and Zn(II) chloro complexes in the mixed salting-out agent system was investigated to show the potential for application of the system for the extraction separation of metals as their chloro-complexes. 1. Introduction In solvent extraction, the distribution behavior of solutes depends mainly on the difference in the polarities of the two phases. Hydrophobic organic solvents have relatively low polarities and, therefore, it is clearly not easy to extract polar or ionic solutes from the aqueous phase into the extraction (organic) phase without the use of a hydrophobic reagent. Although high-polar organic solvents such as lower alcohols, acetonitrile and tetrahydrofuran (THF) are water-miscible, it is well known that adding large amount of salts, including inorganic chlorides [1 3] and tetramethylammonium chloride [4], to the water high-polar organic solvent mixture results in phase separation based on salting-out. Although the aqueous biphasic system has some superiority in the extraction of polar or ionic species, the combination of organic solvent and salt to form the biphasic system is limited. Recently, use of hydrophobic ionic liquids (ILs) as the extraction phase in solvent extraction has widely been investigated [5 7]. ILs are salts with low melting points and can act as high-polar solvents

2 Some researchers reported that water hydrophilic IL mixtures can form immiscible separate phases in the presence of some inorganic salts [8]. In contrast, hydrophilic ILs can be also used as salting-out agents. In previous work, Hu et al. reported the phase separation behavior of water THF using an atypical IL, 1-(-hydroxyethyl)-3-methylimidazolium chloride [9]. Recently, we reported preliminary research on the possible use of more popular hydrophilic IL, 1-butyl-3-methylimidazolium chloride () as the salting-out phase separation agent for the water THF system as a rapid communication [1]. Although pure is solid at room temperature, it liquefies with a small amount of water and is miscible to water at arbitrary ratio. In other words, high concentration aqueous solutions of can be easily prepared. In this paper, we report on the possible use of together with as salting-out phase separation agents. Concretely, phase separation behavior in the water THF/ system and that in the water THF/ mixed salting-out agent system were investigated. Furthermore, the possible application of the latter system for the extraction separation of metals as their chloro-complexes was investigated.. Experimental.1 Apparatus The IL,, was synthesized from 1-methylimidazole and 1-chlorobutane according to reported procedure [11]. Other chemicals and solvents were of reagent-grade and were used without further purification. Distilled deionized water was used throughout. The absorption spectra of the solutions were recorded on a JASCO U-best 3 UV-Visible spectrophotometer with a 1. cm quartz cell. The concentrations of metals, iron and zinc, in solution were determined using a Hitachi Z-61 polarized Zeeman atomic absorption spectrometer.. Evaluation of salting-out phase separation behavior The salting-out phase separation behavior was investigated at 5ºC as follows. Into a stoppered graduated cylinder,. cm 3 (. g,.11 mol) of water, 3.4 cm 3 (3. g,.4 mol) of THF (or other hydrophilic organic solvent) and weighed amount of the salting-out agents ( and/or ) were added. After mixing using a Vortex mixer to dissolve the agents completely, the cylinder was left standing for several minutes until completion of the phase separation, and the volume of each phase (V upper and V lower ) was measured. The polarity of each phase was evaluated using the E N T value [1,13]. The value was determined from λ max for Reichardt s Dye (,6-diphenyl-4-[(,4,6-triphenylpyridinium)-1-yl]phenolate) as follows: E N T {8591/ ( / nm)} 3.7 max (1) 3.4 where E N T (water) and E N T (tetramethylsilane) are defined as 1 and, respectively..3 Distribution of metals The distribution behavior of Fe(III) and Zn(II) was investigated at 5ºC as follows. Into a stoppered centrifuge tube,. cm 3 of. mol dm 3 HNO 3 containing 1. mg cm 3 of metal, 3.4 cm 3 of THF and

3 mmol in total of the salting-out agents ( and/or ) were added. The tube was shaken for 1 min using a mechanical shaker. After the two phases were completely separated by centrifugation, their volumes were measured and the concentration of the metal in each phase (C upper and C lower ) was determined using FAAS after suitable dilution. The distribution ratios (D, upper phase/lower phase) and extractabilities (%E, into the upper phase) for the metals were calculated using the following equations: D C upper C lower () C %E 1 upper V upper (3) C upper V upper C lower V lower 3. Results and Discussion 3.1 Preliminary research At first, we examined whether the addition of results in salting-out phase separation for several water hydrophilic organic solvent systems. The hydrophilic solvents used were methanol (E N T =.76 [13]), ethanol (.654), -propanol (.546), acetonitrile (.46), acetone (.355), pyridine (.3) and THF (.7). When was used as the salting-out agent, all the solvents except for ethanol showed phase separation. In contrast, when using, only acetone and THF showed phase separation. Although is solid at room temperature, many ILs have similar polarities to hydrophilic alcohols. (The E N T values for general C 4 mim + -type ILs range from.64 to.68 [14].) The above-mentioned results seem to be consistent with the nature of ILs. Since THF showed a higher phase-separation ability, we selected the water THF system. Dissolution of Reichardt s Dye is necessary to obtain the E N T value for the solvent. In the water THF mixture containing more than 7 mol % THF, Reichardt s Dye was soluble. For the 7 mol % THF mixture, the E N T value was determined as.783. From these results, it was found that the E N T value for the Reichardt s Dye-insoluble mixture can be estimated as >ca Phase separation for the water THF/ system The phase separation behavior for a mixture of. cm 3 of water and 3.4 cm 3 of THF on adding different amounts of or is shown in Figure 1, with the obtained E N T value for each of the separated phases [1] (1 mmol =.59 g for,.175 g for ). The minimum amounts of and for the phase separation were ca. 3 mmol and ca..8 mmol, respectively. In the conventional system, the upper (THF-rich) phase volume (V upper ) was similar to the volume of THF used and the lower (water-rich) phase volume (V lower ) was similar to that of water. In the system, in contrast, V upper was very small and V lower was relatively large. Although the two systems showed similar behavior in upper phase polarity, the lower phase in the system showed a lower polarity than that in the system. Namely, the difference in the polarity between the two phases in the system was smaller than that in the system. From these results, it was suggested that the low polarity of the lower phase in the system arises because of THF and remained in the phase. In other words, acts in the biphasic system not only as a salting-out

4 6.7 V (cm 3 (cm 3 ) V ) lower upper mim]cl or (mmol) mim]cl or (mmol) Figure 1. Effect of added amount of salting-out agent on volume (left) and E N T (right) of each phase in the water THF/ (circle) and water THF/ (diamond) aqueous biphasic systems [1]. Initial mixture;. cm 3 of water and 3.4 cm 3 of THF. Dotted lines (left) show the initial THF volume (upper) and the initial water volume (lower). agent but also as a component of the mixed-solvent. (Actually, the V upper + V lower value increased with adding.) N N Lower phase E Upper phase E T T Phase separation for the water THF/ mixed system Although the upper phase polarity depends on the amount of the salting-out agent to some extent, the lower phase polarity shows a quite low dependency, as seen in Figure 1. Therefore, simultaneous use of a pair of salting-out agents, and, was investigated. Figure (a) shows the phase separation behavior for a mixture of. cm 3 of water and 3.4 cm 3 of THF on adding 8.5 mmol of mixed salting-out agent at different mole fractions. The volumes of both phases varied linearly with the increase in the mole fraction. Figure (b) shows the obtained E N T value for each of the phases. This suggests that the lower phase polarity changes appreciably between 5 mol % and 75 mol % of fraction. (At a fraction < Volume (cm 3 ) N E T (a) (b) Lower phase Upper phase Lower phase Upper phase mim]cl fraction (mol %) Figure. Effect of the salting-out agent composition on the volume (a) and E T N (b) of each phase in the water THF/ aqueous biphasic system. Broken lines shown in (a) were obtained by the linear least-square fitting. Initial mixture;. cm 3 of water and 3.4 cm 3 of THF. Total amount of the salting-out agents; 8.5 mmol

5 5 %, E T N value was estimated as >ca..8 as mentioned in Section 3.1.) In the mixed salting-out agent system, the difference in the two phase polarities varied as the composition of the agents was varied. 3.4 Distribution behavior of chloro-complexes in the water THF/ mixed system In the recent communication [1], we reported that the distribution of a neutral molecule in the mixed salting-out agent system changes with the composition, when using Thymol Blue as a probe. Furthermore, in the water THF/ mixed system, the polarity of both phases can be controlled without changing the added amount of Cl. Therefore, the distribution behavior of chlorocomplexes in the biphasic system was investigated with using Fe(III) and Zn(II) as probes. (To avoid their hydrolysis, HNO 3 -acidic condition was selected.) As is well known, many metal cations form stable chloro-complexes in the presence of high amounts of Cl. In the present experimental conditions, for example, it is estimated that + Fe(III) and Zn(II) forms mainly cationic FeCl or nonionic FeCl 3 species, and anionic ZnCl 3 or 3 1 Fe(III) ZnCl 4 species, respectively. (In aqueous -1 Zn(II) solution, log β 1 = 1.48, log β =.13, log β 3 = 1.99 and log β 4 =.1 for Fe 3+, and log β 1 =.43, - 1 (a) log β =.61, log β 3 =.53 and log β 4 =. for Fe(III) Zn + [15].) Although the distribution behavior of Cl was not investigated in detail, it is considered 5 Zn(II) that these metals not only exist as the (b) chloro-complexes in the lower phase but also are extracted as the same forms into the upper phase. mim]cl fraction (mol %) Figure 3 shows log D and %E for Fe(III) and Figure 3. The logarithmic distribution ratio (log D, upper phase/lower phase (a)) and extractability Zn(II) as a function of the mole fraction of (%E, into the upper phase, (b)) for Fe(III) for a fixed total amount of the (triangle) and Zn(II) (square) in the water THF/ salting-out agents in the water THF/ aqueous biphasic system. Initial mixture;. cm 3 of. mol dm 3 HNO 3 and 3.4 aqueous biphasic system. In the water cm 3 of THF. Total amount of the salting-out THF/ and water THF/ single agents; 8.5 mmol. salting-out agent systems, in addition, the amount of the salt hardly affected the metal distribution. (For example, the log D value for Fe(III) in the water THF/ system ranged from.55 to.6 on adding mmol of, and that for Zn(II) in the water THF/ system ranged from.48 to.51 on adding mmol of.) Namely, the distribution was controlled by changing the : ratio under a fixed salt amount, and the change in the lower phase polarity seems to contribute mainly to this behavior. From these results, it was concluded that the mixed salting-out phase separation system using a hydrophilic IL can be a useful tool in extraction separation chemistry. log D %E

6 Acknowledgements This study was financially supported in part by a Grant-in-Aid for Scientific Research (No. 5571) from the Japan Society for the Promotion of Science, MEXT Supported Program for the Strategic Research Foundation at Private Universities (1 16) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the Basic Research Fund from Faculty of Science, Toho University. References 1) C. E. Matkovich, G. D. Christian, Anal. Chem., 46, 1 16 (1974). ) M. Tabata, M. Kumamoto, J. Nishimoto, Anal. Chem., 68, (1996). 3) M. H. Chung, M. Tabata, Talanta, 58, (). 4) S. S. Samaratunga, J. Nishimoto, M. Tabata, Environ. Sci. Pollut. Res., 15, 7 3 (8). 5) C. F. Poole, S. K. Poole, J. Chromatogr. A, 117, (1). 6) E. M. Martinis, P. Berton, R. P. Monasterio. R. G. Wuilloud, Trends Anal. Chem., 9, (1). 7) N. Hirayama, Solvent Extr. Res. Dev., Jpn., 18, 1 14 (11). 8) K. E. Gutowski, G. A. Broker, H. D. Willauer, J. G. Huddleston, R. P. Swatloski, J. D. Holbrey, R. D. Rogers, J. Am. Chem. Soc., 15, (3). 9) X. Hu, J. Yu, H. Liu, Water Sci. Technol., 53(11), (6). 1) N. Hirayama, T. Higo, H. Imura, Anal. Sci., 8, (1). 11) J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser, R. D. Rogers, Chem. Commun., (1998). 1) C. Reichardt, E. Harbusch-Görnert, Liebig Ann. Chem., (1983). 13) C. Reichardt, Chem. Rev., 94, (1994). 14) M. J. Muldoon, C. M. Gordon, I. R. Dunkin, J. Chem., Soc., Perkin Trans., (1). 15) J. G. Speight (ed.), Lange s Handbook of Chemistry, 16th ed., 5, McGraw-Hill, New York, pp