LiquidâLiquid Extraction of Low-Concentration Aniline from Aqueous

Ind. Eng. Chem. Res. 2010, 49, 2581–2588

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Liquid-Liquid Extraction of Low-Concentration Aniline from Aqueous Solutions with Salts Xiaohua Wu, Zhigang Lei,* Qunsheng Li, Jiqin Zhu,* and Biaohua Chen State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Box 35, Beijing, 100029, China

The focus of this work was to concentrate aniline at low concentration from water samples using liquid-liquid extraction with salts. The salts included inorganic solid salts and ionic liquids and were added to the system of water + aniline + MTBE (methyl tert-butyl ether) to intensify the conventional liquid-liquid extraction process. The influence of such factors as extraction time, initial aqueous aniline concentration, phase volume ratio, and extraction temperature on the extraction process was investigated to determine the optimum extraction conditions. It was found that the salting effect of K2CO3 on aniline and water is the highest among all of the salts investigated, whereas the imidazolium-based ionic liquids do not bring about a good extraction efficiency as expected. The complex formation and interaction force of the systems containing salts were determined using FTIR (Fourier transform infrared) spectrometry and density functional theory (DFT). This work also tries to explain the separation mechanism by means of the Hofmeister series and quantum chemistry. 1. Introduction Amine compounds, such as aniline, morpholine, chloroanilines, and piperazine, are important organic chemical materials and also are generally harmful to human health because of their toxicity and carcinogenicity.1,2 Moreover, they can be translated into toxic N-nitroso compounds through reactions with nitrosylating agents.3 These pollutants are largely concentrated in the environment, as they are widely used in industry to make dyes, cosmetics, medicines, rubber, textiles, agrochemicals, and reagent intermediates in many chemical syntheses.4,5 Considering the importance and toxicity of these compounds, it is imperative to develop a rapid and sensitive method for preferentially monitoring aniline and its derivatives in environmental samples. On the other hand, in the field of sports science, stimulant detection starts with a urine sample. The stimulants prohibited by the WADA (World Anti-Doping Agency) in urine samples include adrafinil, amfepramone, amiphenazole, amphetamine, amphetaminil, benzphetamine, bromantan, carphedon, cathine, clobenzorex, cocaine, dimethylamphetamine, ephedrine, etilamphetamine, etilefrine, among others (see http:// www.wada-ama.org). They have chemical structures similar to that of aniline, with a benzene ring and a nitrogen atom. Therefore, in this work, aniline was selected as a representative amine compound in environmental and urine samples, and the common characteristic is that aniline in aqueous solutions is present at low concentration. Recently, several methods have been used for the determination of aniline and other aromatic amines: gas chromatography (GC),6 gas chromatography-mass spectrometry (GC-MS),7-9 high-performance liquid chromatography (HPLC),10,11 and capillary electrophoresis (CE).12-14 However, pretreatment processes are required to concentrate aniline from aqueous solutions prior to analysis of aniline at low concentration as the detection sensitivity of analytical apparatuses is limited and the samples cannot be detected directly. To date, several sample pretreatment techniques have also been used: liquid-liquid extraction (LLE),15-17 solid-phase extraction (SPE),18 solid* To whom correspondence should be addressed. Tel.: +86 10 64433695. Fax: +86 10 64419619. E-mail: [email protected] (Z.L.), [email protected] (J.Z.).

phase microextraction (SPME),19-21 liquid-phase microextraction (LPME),22 and dispersive liquid-liquid microextraction (DLLME).23-26 In this work, the liquid-liquid extraction method was selected to separate and concentrate anilines from water samples because it is simple and can deal with a large number of samples in a short time. It is evident that, for liquid-liquid extraction, selection of a suitable separating agent is fundamental to ensure an effective and economical separation. The separating agents reported for the separation of aromatic amines from aqueous solutions on an industrial scale are physical solvents (e.g., benzene, toluene, cyclohexane, etc.)27 and complexing solvents (e.g., mixture of tributyl phosphate and kerosene).28 The separating agents reported for sample pretreatment on the laboratory scale are the ion-pairing reagent (i.e., bis-2- ethylhexylphosphate dissolved in chloroform followed by a backextraction with 0.1 M HCl),15 MTBE (methyl tert-butyl ether),29 mixtures of 2-propanol and a solid salt,16 and ionic liquids.17,30 Among the latter, MTBE is currently being used in practice because of its low toxicity and high volatility, making it easy to separate from toxic compounds or stimulants with low volatility by air blowing in the extract phase. However, the separation performance of MTBE in the pretreatment process has not been reported. Therefore, the main objective of the work was to intensify the conventional liquid-liquid extraction process and find the optimum operating conditions for concentrating aniline from aqueous samples in the pretreatment process. To improve the extraction efficiency of MTBE, salts including both inorganic solid salts and ionic liquids were added to make mixture separating agents with MTBE. Room-temperature ionic liquids consisting of a large organic cation and an inorganic polyatomic anion have been very popular in recent years for their potential as “green solvents”.31-33 So it is interesting for us to compare the separation ability between solid salts and ionic liquids as additives in the pretreatment process. Most importantly, when solid salts and ionic liquids are applied to sample detection, there is no scaleup effect, which is often brought out by traditional chemical engineering.

10.1021/ie9012979  2010 American Chemical Society Published on Web 02/08/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010

Table 1. Chemicals, Stated Purities, and Suppliers chemical

purity (wt %)

aniline methyl tert-butyl ether (MTBE) sodium chloride (NaCl) potassium acetate (KAc) potassium carbonate (K2CO3) sodium bromide (NaBr) sodium iodide (NaI) 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]+[BF4]-)

>99.5 >95.0 >99.5 >92.0 >99.0 >99.0 >99.0 >98.0

1-ethyl-3-methylimidazolium acetate ([EMIM]+[Ac]-)

>98.0

1-butyl-3-methylimidazolium acetate ([BMIM]+[Ac]-)

>98.0

1-octyl-3-methylimidazolium acetate ([OMIM]+[Ac]-)

>98.0

2. Experimental Section 2.1. Materials. Table 1 lists the chemicals employed, their stated purities, and their suppliers. The purities of the ionic liquid (ILs) were observed by liquid chromatography. Before the experiments, all ILs were dried for 48 h at 323-343 K to remove volatile impurities. 2.2. Experimental Procedure. Aqueous solutions were prepared gravimetrically by dissolving aniline in deionized water using an electronic balance (FA2104B, uncertainty of 0.1 mg). The initial aniline concentration in aqueous solution was 0.1109 mol · L-1. A mixture with a known volume was held in a conical bottle (50 mL). The subsidiary separating agent (or extracting agent), namely, inorganic solid salt or ionic liquid, if used, was first added to the mixture. After the aqueous solution was completely mixed, a known volume of the main separating agent (i.e., MTBE) was added to form the organic and aqueous phases fulfilling the extraction process. The added subsidiary and main separating agents were weighed beforehand. The liquid-liquid extraction experiments were performed in a shaking air bath (type THZ-C) with a frequency of 200 rpm in which the sealed conical bottles filled with the water + aniline + MTBE or water + aniline + MTBE + salt mixtures were placed. Each sample system was maintained in the equilibrium state for about 0.5-1 h. The equilibrium temperature was measured by a resistance thermometer with an uncertainty of 0.1 K. 2.3. Analysis. The concentration of aniline in the organic phase was determined by gas chromatography. As the majority of components were organic substances in the organic phase and water was almost negligible, we could utilize a gas chromatograph (GC4000A Series) equipped with a flame ionization detector (FID). The chromatographic column (50 m × 0.20 mm) was an AT.PONA capillary. The carrier gas was nitrogen flowing at 40 mL min-1. The operating conditions were as follows: flow rate of hydrogen (H2), 30 mL min-1; flow rate of air, 400 mL min-1; detector and injector temperatures, 250 °C; and oven temperature, 130 °C. The injection volume was 0.4 µL. The gas chromatograph was calibrated using mixtures of known compositions of reagents. The maximum uncertainty in the mass fractions of the components was 0.001. Each analysis was done at least twice to ensure repeatability.

supplier GuangFu Chemical Reagents Company GuangFu Chemical Reagents Company Beijing Chemical Plant Beijing Chemical Plant Beijing Chemical Plant FuChen Chemical Reagents Company JinKe Chemical Reagents Company Chemical Engineering Research Institute HeBei Chemical Engineering Research Institute HeBei Chemical Engineering Research Institute HeBei Chemical Engineering Research Institute HeBei

of the Normal University of of the Normal University of of the Normal University of of the Normal University of

much. In addition, the distribution coefficient D and extraction efficiency E are also important physical quantities in liquid-liquid extraction process and are defined as co ca

(1)

ms - coVo Va

(2)

coVo coVo ) ms coVo + caVa

(3)

D)

ca )

E)

where co and ca are the molar concentrations of aniline in the organic phase (determined by gas chromatography) and aqueous phase (determined by mass balance), respectively; Vo and Va are the corresponding phase volumes; and ms is the predetermined total amount of aniline. It is assumed that the two phase volumes remain constant before and after equilibration because the mutual solubility between MTBE and water is small and the minor amount of aniline has almost no influence on phase volumes. 3.1. Effect of Extraction Time on Phase Separation. Under the experimental conditions, transfer of aniline from the aqueous phase into the organic phase to reach the equilibrium state is very rapid. As shown in Figure 1, the rate of increase in the extraction efficiency E decreases after 30 min. Therefore, it is better to maintain the extraction equilibrium for about 1 h, and in this case, there is no substantial effect on phase separation.

3. Results and Discussion The aniline concentration in the organic phase (molar concentration co or mass fraction wo) provides a useful index for the selection of suitable salts in the pretreatment process because most of the task with respect to environmental and stimulant sample detection is to determine whether some prohibited chemicals are contained or not, regardless of how

Figure 1. Effect of extraction time on extraction efficiency E for the system of water + aniline + MTBE with the phase volume ratio Vo/Va ) 1 and MTBE as the separating agent at 293.2 K.


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K

RNH2(aq) 98 RNH2(o) The dissociation equilibrium of protonize organic amines in aqueous solution is Ka

RNH3+(aq) 98 RNH2(aq) + H+(aq) According to eq 1, the distribution coefficient D is D) Figure 2. Effect of the initial aniline concentration win on the aniline concentration in the organic phase wo with the phase volume ratio Va/Vo ) 6 and MTBE as the separating agent at 293.2 K:2, experimental data; 9, calculated results obtained with the UNIF-LL model.

3.2. Effect of Initial Aqueous Aniline Concentration on Phase Separation. The experiments were conducted with different initial concentrations of aniline in the aqueous phase, namely, win ) 0.899, 1.033, 1.261, 1.511, 1.749, and 1.948 wt %. As shown in Figure 2, there is a good linear relationship between the initial aqueous aniline concentration in the aqueous phase w in and the aniline concentration in the organic phase wo with MTBE as the separating agent. This indicates that the initial aqueous aniline concentration can be deduced from the concentration of aniline in the organic phase after extraction in sample analysis. The calculated results obtained using the UNIFLL model are also given in Figure 2. The UNIF-LL model is an extended version of the original UNIFAC model that is suitable for predicting liquid-liquid equilibrium. 34 It can be seen that the experimental data and the calculated results exhibit similar trends. The maximum absolute deviation is less than 0.02, and the average relative deviation (ARD) is 12.0%, thus verifying that the UNIF-LL model is reliable. In this work, we selected the initial aniline concentration 1.033 wt % in later experiments to compare the separation abilities of MTBE and mixtures of MTBE and salt at a low aniline concentration. 3.3. Effect of Phase Volume Ratio on Phase Separation. Although an increasing amount of separating agent can increase the extraction efficiency of aniline from the aqueous to the organic phase, the concentration of aniline in the organic phase can be diluted at the same time. Therefore, a suitable phase volume ratio should be determined for the extraction process. In this work, different phase volume ratios between the aqueous and organic phases (Va/Vo) were investigated. Figure 3 shows the change of the aniline concentration in the organic phase wo with the phase volume ratio. It is obvious that, as the phase volume ratio increases, the aniline concentration in the organic phase also increases. However, the phase volume in the organic phase decreases, so that it is difficult to remove the sample from the organic phase for composition analysis. Therefore, for the sake of convenient operation, a phase volume ratio equal to 6 (about 3 mL in the organic phase) was selected for later experiments. In addition, the calculated results obtained with the UNIF-LL model are also given in Figure 3, with a maximum absolute deviation of less than 0.014. 3.4. Effect of Extraction Temperature on Phase Separation. Figure 4 shows that both the aniline concentration in the organic phase and the extraction efficiency decrease slowly with increasing extraction temperature. Therefore, the optimum extraction temperature is room temperature. The extraction equilibrium of amine with the separating agent MTBE can be written as

[RNH2(o)] [RNH2(aq)] + [RNH3+(aq)]

)

K 1 + 10pKa-pH

(4)

where K is the extraction equilibrium constant and Ka is the equilibrium dissociation constant of amine compounds. If the value of pH is given, D and K have a linear relationship D ) cK

(5)

where c ) 1/(1 + 10pKa-pH) and is a constant. The relationship between the extraction equilibrium constant K and the enthalpy change of the extraction process ∆H can be expressed as35

Figure 3. Effect of the phase volume ratio between the aqueous and organic phases (Va/Vo) on the aniline concentration in the organic phase wo with the initial aniline concentration win ) 0.01033 and MTBE as the separating agent at 293.2 K: 2, experimental data; 9, calculated results obtained with the UNIF-LL model.

Figure 4. Effect of extraction temperature T on the aniline concentration in the organic phase wo (2) and on the extraction efficiency E (9) with the phase volume ratio Va/Vo ) 6, the initial aniline concentration win ) 0.01033, and MTBE as the separating agent.

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ln K ) -

∆H + c' RT

(6)

where R is the molar gas constant (8.314 J mol-1 · K-1), T is the absolute temperature, and c′ is a constant. Therefore, the distribution coefficient D can be rewritten as ln D ) -

∆H ∆H + ln c + c' ) +C RT RT

(7)

where C is a constant. Figure 5 shows the linear change of the logarithmic distribution coefficient with inverse temperature with a correlation factor R2 ) 0.9468. In this case, the enthalpy change of the extraction process is deduced to be ∆H ) -12170.03 J mol-1, which means that the extraction process with MTBE as the separating agent is exothermic. 3.5. Effect of Inorganic Solid Salts on Phase Separation. To intensify the phase separation, a strategy is to add inorganic solid salt to the system of water + aniline + MTBE because the presence of a salt in the liquid phase has a substantial influence on the phase equilibria of the system by inducing salting-out or salting-in effects. In this work, four kinds of inorganic solid salts, namely, NaCl, NaBr, KAc, and K2CO3, were selected because they are commonly used in the distillation separation of aqueous organic solutions.36 Figure 6 shows the influence of NaCl, NaBr, KAc, and K2CO3 on the aniline concentration in the organic phase wo for salt concentrations in the aqueous phase of 0, 5, 10, and 15 wt %. It can be seen that the potassium salts are more effective than the sodium salts in increasing the aniline concentration in the organic phase. This exactly conforms to the well-known Hofmeister series,49,50 which originated from studying the solubility of proteins in water but can also be extended to include polar and nonpolar compounds with relatively low solubilities in water. According to the Hofmeister series, the salting-out effect of anions follows the order CO32> Ac- > Cl- > Br-, whereas the order of cations is K+ > Na+. Moreover, cation and anion effects are independent and additive, thus allowing for the successful prediction of the Hofmeister salting effect. Therefore, K2CO3 and KAc are potential additives. On the other hand, the effect of the amount of potassium salts (K2CO3 and KAc) added on the aniline concentration in the organic phase is illustrated in Figure 7. At low salt concentrations, the aniline concentration in the organic phase undergoes an irregular change with increasing salt concentration. This is due to the salting-out effect of potassium salts on both aniline and MTBE in the aqueous phase, which promotes the transfer of more MTBE dissolved in the aqueous phase into the organic phase and thus might dilute the aniline concentration in the organic phase. Although the salting-out effect is remarkable at 10 wt % for K2CO3 and KAc, the aniline concentration in the organic phase is unstable in this range. However, at high salt concentrations, K2CO3 and KAc can exhibit similar strong salting-out effects as at 10 wt %, and the aniline concentration in the organic phase remains almost constant. In this case, the salting-out effect of K2CO3 is higher than that of KAc. That is, the salting effect of K2CO3 on aniline and water is the highest among all of the salts investigated, and a suitable K2CO3 concentration in the aqueous phase should be above 25 wt %. In addition, the aniline concentration in the aqueous phase decreases as the amount of K2CO3 increases (see Figure 7b). Therefore, the salting-out effect of K2CO3 plays an important role in concentrating aniline, and the aniline concentration in the organic phase is much higher with a mixture of MTBE and

Figure 5. Logarithmic distribution coefficient as a linear function of inverse temperature with the phase volume ratio Va/Vo ) 6, the initial aniline concentration win ) 0.01033, and MTBE as the separating agent.

Figure 6. Effects of inorganic solid salts NaCl, K2CO3, KAc, and NaBr at 0, 5, 10, and 15 wt % on the aniline concentration in the organic phase wo with the phase volume ratio Va/Vo ) 6, the initial aniline concentration win ) 0.01033, the extraction equilibrium time of 1 h, and a mixture of MTBE and solid salt as the separating agent at 293.2 K.

K2CO3 than with pure MTBE as the separating agent. However, formerly, only pure MTBE was recommended as the separating agent. 3.6. Effect of Ionic Liquids on Phase Separation. Ionic liquids, as a new class of solvents, are often fluid at room temperature and consist entirely of positive and negatively charged ions.37-39 Because of their extremely low vapor pressures and designable physical and chemical properties, ionic liquids have been used as green solvents to replace traditional volatile organic solvents in organic synthesis,40 solvent extraction,41-46 and electrochemistry.47 In this work, four kinds of ionic liquids, namely, [EMIM]+[BF4]-, [EMIM]+[Ac]-, [BMIM]+[Ac]-, and [OMIM]+[Ac](see Table 1), were selected to add to the system of water + aniline + MTBE because the ionic liquids based on the imidazolium cations with anions BF4- and Ac- are miscible with water and might bring out salting-out effect on aniline in the aqueous phase. In addition, they are easily obtained from chemical markets at a low price. Figure 8 shows the effects of [EMIM]+[BF4]-, [EMIM]+[Ac]-, [BMIM]+[Ac]-, and [OMIM]+[Ac]- on the aniline concentration in the organic phase at different concentrations of the IL. It can be seen that, as the concentration of the ionic liquid increases, the salting effect of the ionic liquids eventually is in the order [EMIM]+[BF4]- ≈ [EMIM]+[Ac]- > [BMIM]+[Ac]- > [OMIM]+[Ac]-. Therefore, a short alkyl chain length on the cations is favorable for extracting aniline from the aqueous phase because, with an increase in the alkyl chain


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Figure 9. FTIR spectra in the aqueous phase after extraction with the phase volume ratio Va/Vo ) 6, the initial aniline concentration win ) 0.01033, and a mixture of MTBE and K2CO3 as the separating agent at 293.2 K: (1) salt-free, (2) 10 wt % K2CO3, and (3) 30 wt % K2CO3.

Figure 7. Effects of the amount of solid salts added on the aniline concentration (a) in the organic phase wo and (b) in the aqueous phase wa with the phase volume ratio Va/Vo ) 6, the initial aniline concentration win ) 0.01033, and a mixture of MTBE and solid salts as the separating agent at 293.2 K: 9, K2CO3; 2, KAc.

Figure 8. Effects of the amount of ionic liquids [EMIM]+[BF4]-, [EMIM]+[Ac]-, [BMIM]+[Ac]-, and [OMIM]+[Ac]- on the aniline concentration in the organic phase wo with the phase volume ratio Va/Vo ) 6, the initial aniline concentration win ) 0.01033, and a mixture of MTBE and ionic liquid as the separating agent at 293.2 K. (, [EMIM]+[BF4]-; 0, [EMIM]+[Ac]-; 2, [BMIM]+[Ac]-; ×, [OMIM]+[Ac]-.

length on the imidazolium ring from C2 to C8, the polarity of the ionic liquid decreases. However, it was found that the addition of ionic liquids does not intensify the extraction process, but rather decreases the aniline concentration in the organic phase. This can be explained by the Hofmeister series, which demonstrates that large monovalent ions are poor salting-out agents.51 Therefore, the salting-out effect of ionic liquids is weaker than that of solid salts, even though the former are normally regarded as excellent green separating agents in separation processes.

However, when a mixture of MTBE and solid salt is used for intensifying the conventional liquid-liquid extraction process on an industrial scale, dissolution, reuse, and transport of solid salts are serious problems. Tub-jam and erosion problems can arise because solid salts, unlike ionic liquids, are in the solid state at room temperature. Therefore, ionic liquids seem to be more promising in industrial applications than solid salts, despite their high price in chemical markets. 3.7. Theoretical Analysis of the Salting Effect. The mechanism of the Hofmeister series was explored by previous researchers based on surface spectroscopic measurements and computational studies.52-54 In this work, we try to interpret the salting effect by means of the combination of FTIR (Fourier transform infrared) spectrometry and density functional theory. The salts added to the system of water + aniline + MTBE will change the complex formation and molecular interactions in the aqueous phase. The samples in the aqueous phase after extraction were determined by FTIR spectrometry (Nicolet Nexus 670) to qualitatively analyze the interaction between the salt and the component to be separated. As observed from Figure 9, the absorption peaks near 3500 and 1600 cm-1 correspond to the stretching vibrations of OH and NH groups, respectively. The addition of K2CO3 results in a significant broadening of the OH stretching vibration and a shift from low to high wave numbers. Moreover, the higher the concentration of K2CO3 in the aqueous phase is, the more obvious this trend becomes. This situation arises from the formation of hydrogen bonds between K2CO3 and water, which reinforces their interaction and thus reduces the free water molecules in the aqueous phase. However, the NH stretching vibration remains almost unchanged near 1600 cm-1, which means that the interaction between K2CO3 and aniline is not as strong. Therefore, the addition of K2CO3 can fulfill the separation of aniline from the aqueous to the organic phase. A theoretical study was also carried out by an ab initio method to quantitatively analyze the interaction between the salt and the component to be separated. Density functional theory (DFT) was used for energy calculation and geometry optimization of the molecules concerned. The B3LYP function was used to describe electron exchange and correlation, and the 6-31G** basis set was used to locate optimized ground-state and transition-state structures. All geometry optimizations were performed using Gaussian 03 software.48 The results calculated by quantum chemistry are shown in Figure 10. It can be seen from Figure 10a,b that, after the addition of K2CO3, the bond length of the OH group increase from (0.96597, 0.96616) Å to

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Figure 10. Optimized geometric structures of the systems (a) water + aniline, (b) water + aniline + K2CO3, (c) water + aniline + [EMIM]+[BF4]-, and (d) water + aniline + [OMIM]+[Ac]-.

(1.04050, 0.96426) Å, whereas the bond length of the NH group apparently does not change, being (1.01003, 1.01642) Å and (1.02480, 1.01224) Å, respectively. This is consistent with the FTIR analysis. In addition, the binding energy between K2CO3 and water is -158.98584 kJ mol-1, the absolute value of which is much higher than that between aniline and water (-20.62514 kJ mol-1). Therefore, more water molecules are restrained in the aqueous phase and thus promote the transfer of aniline from the aqueous to the organic phase. The FTIR spectra for the addition of [EMIM]+[BF4]- and [OMIM]+[Ac]- at different concentrations are shown in Figures 11 and 12, respectively. It can be seen that, after the addition of the ionic liquids, the shifting of the wavenumber of OH group near 3500 cm-1 is not apparent, and the NH stretching vibration peak still remains almost unchanged near 1600 cm-1. This confirms the experimental result that the salting-out effect of ionic liquids is weaker than that of solid salts. Similar theoretical

calculations based on density functional theory have been performed on ionic liquids. The results calculated by quantum chemistry are shown in Figure 10c,d. It can be seen that, after the addition of the ionic liquids, there is no apparent change in the bond lengths of the OH and NH groups. The bond length of OH group changes from (0.96597, 0.96616) Å to (0.97491, 0.97114) Å and (0.97166, 0.98689) Å for [EMIM]+[BF4]- and [OMIM]+[Ac]-, respectively. The bond length of the NH group changes from (1.01003, 1.01642) Å to (1.01759, 1.01041) Å and (1.02180, 1.01214) Å for [EMIM]+[BF4]- and [OMIM]+[Ac]-, respectively. The binding energy is -52.45014 kJ mol-1 between [EMIM]+[BF4]- and water and -53.56020 kJ mol-1 between [OMIM]+[Ac]- and water. Both absolute values are much smaller than that between solid salt and water. Therefore, the hydrophilic imidazolium-based ionic liquids are unsuitable for intensifying the extraction process discussed in this work.


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It is evident that the work present also can be extended to the separation of trace amounts of organic substances from aqueous solutions for other sources (e.g., biological samples, toxic substances in food, soil and pesticide residues), because their separation mechanism is consistent and there is no scaleup effect. Acknowledgment This work was financially supported by the Fok Ying Tong Education Foundation (No. 111074), the Program for New Century Excellent Talents in University, and the National Nature Science Foundation of China under Grants 20821004 and 20706005. Figure 11. FTIR spectra in the aqueous phase after extraction with the phase volume ratio Va/Vo ) 6, the initial aniline concentration win ) 0.01033, and a mixture of MTBE and [EMIM]+[BF4]- as the separating agent at 293.2 K: (1) salt-free, (2) 5 wt % [EMIM]+[BF4]-, and (3) 10 wt % [EMIM]+[BF4]-.

Figure 12. FTIR spectra in the aqueous phase after extraction with the phase volume ratio Va/Vo ) 6, the initial aniline concentration win ) 0.01033, and a mixture of MTBE and [OMIM]+[Ac]- as the separating agent at 293.2 K: (1) salt-free, (2) 5 wt % [OMIM]+[Ac]-, and (3) 10 wt % [OMIM]+[Ac]-.

4. Conclusions This work relates the separation principle of chemical engineering to organic detection for environmental and stimulant samples in the pretreatment process, although liquid-liquid extraction itself is a simple and mature technology in chemical engineering. The separation performance for the system of water + aniline + MTBE was first investigated, and then the traditional liquid-liquid extraction process was intensified to concentrate aniline in the organic phase. The strategy was to add the salts, including inorganic solid salts and ionic liquids, to make mixture separating agents. Inorganic solid salts (i.e., NaCl, NaBr, KAc, and K2CO3) and the ionic liquids (i.e., [EMIM]+[BF4]-, [EMIM]+[Ac]-, [BMIM]+[Ac]-, and [OMIM]+[Ac]-) were selected, and it was found that the saltingout effect of solid salts was more prominent than that of ionic liquids in increasing the concentration of aniline in the organic phase. Among the additives tested, the best salt is K2CO3. However, solid salts as separating agents are difficult to use on an industrial scale. Therefore, this work promotes the conventional separation process to find new applications that are no longer limited to the field of chemical engineering. The experimental phenomenon was interpreted by means of FTIR spectrometry and density functional theory, which corresponds to the mechanism of the Hofmeister series reported by previous researchers. The results from experiment, theory, and calculation are consistent. From this viewpoint, this work also combines the separation process in chemical engineering with the quantum calculation in chemistry.

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ReceiVed for reView August 18, 2009 ReVised manuscript receiVed January 18, 2010 Accepted January 26, 2010 IE9012979

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