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Recycling and recovery of ammonium-based ionic liquids after extraction of metal cations from aqueous solutions
Accepted Manuscript Recycling and recovery of ammonium–based ionic liquids after extraction of metal cations from aqueous solutions M.A. Valdés Vergara, I.V. Lijanova, N.V. Likhanova, O. Olivares Xometl, D. Jaramillo Vigueras, A.J. Morales Ramirez PII: DOI: Reference:
S1383-5866(15)30015-0 http://dx.doi.org/10.1016/j.seppur.2015.05.031 SEPPUR 12360
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
20 August 2014 14 May 2015 22 May 2015
Please cite this article as: M.A. Valdés Vergara, I.V. Lijanova, N.V. Likhanova, O. Olivares Xometl, D. Jaramillo Vigueras, A.J. Morales Ramirez, Recycling and recovery of ammonium–based ionic liquids after extraction of metal cations from aqueous solutions, Separation and Purification Technology (2015), doi: http://dx.doi.org/10.1016/ j.seppur.2015.05.031
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Recycling and recovery of ammonium–based ionic liquids after extraction of metal cations from aqueous solutions. M.A. Valdés Vergara a, I.V. Lijanova a,*, N.V. Likhanova b , O. Olivares Xo metlc , D. Jaramillo Vigueras a, A.J. Morales Ramireza
a
Instituto Politécnico Nacional, CIITEC, Cerrada Cecati S/N, Colonia Santa Catarina de Azcapotzalco, CP
02250, México, D.F., México. Tel: 52 5557296000(68305); E-mail: [email protected] b.
Instituto Mexicano del Petróleo, Gerencia de Ingeniería de Recuperación Adicional , Eje Central Norte
Lázaro Cárdenas No. 152, Col. San Bartolo Atepehuacan, CP 07730, México, DF, México. Tel: 52 5591758382; E-mail: [email protected] c
Benemérita Universidad Autónoma de Puebla, Facultad de Ingeniería Química, Av. San Claudio y 18 Sur,
Col. San Manuel, Ciudad Universitaria, Puebla, 72570, México
ABSTRACT The recycling and recovery of ionic liquids (ILs) after the extraction of metal ions were investigated by using trioctylmethyl ammonium camphorate and trioctylmethyl ammonium dodecanedioate, which were synthesized from trioctylamine. The metal ions used in the aqueous solutions were: cadmium, copper and lead with initial concentrations of 50 and 100 ppm. In all the cases, the extraction percentage results were above 90%. After the extraction, the IL recovery was carried out by using solvents with medium polarity indexes. The present work features an extraction mechanism that goes through the use, recycling, recovery and reuse of ILs applied in the metal extraction process.
Keywords: ionic liquids; extraction; recycle; recovery; heavy metals; aqueous solutions.
1. Introduction In the last decades, ionic liquids (ILs) have generated rising interest because of their diversified range of applications. These solvents include a large number of molecular structures and consist entirely of cations and anions, featuring low melting points as low as 100 °C with low viscosities [1–4]. IL properties such as melting point, viscosity, density, and hydrophobicity can be designed and task-specifically adjusted, choosing either the structure of the cation or anion or both to meet the requirements of specific applications [5–7]. ILs are found increasingly in a number of fields such as catalysis, electrochemistry, separation processes and liquid- liquid extractions [8–17]. The liquid-liquid extraction is an important kind of separation method that is based on the distribution of chemicals between two different liquid phases [18]. In recent years, many researchers have studied the liquid-liquid extraction by using ILs as 'green' solvents in the partition of substituted-benzene derivatives between water and a hydrophobic IL [19], the removal of sulfides and nitrides from diesel and gasoline [20–24], and the removal of heavy metals from water [17,25]. In the first studies of heavy metal extractions that were performed with ILs assisted by crown ethers, the metals were removed from the aqueous phase, but in many cases, the ILs remained in the aqueous phase, and the whole extraction depended on the hydrophobicity of the extracting agent. The selectivity towards certain metal ions and extraction efficiency depend on the IL structure [26–28]. Other research works on metal ion extractions without extracting agents, only using IL/aqueous systems, have shown extremely high metal ion
partitionings to the IL phase [17,19,27,28], which is leading to the development of new water treatment methods. Some extractions without using an extracting agent have been done, like in the work published by Germani [29] and his group, where Cu cations were removed with 1-alkyl-3- methylimidazolium-based ILs, showing that the extraction can occur without a chelating agent. Another work was done by P. de Rios [30], where the extraction of Zn2+, Cd2+, and Fe3+ from aqueous hydrochloride solutions using ILs, in the absence of chelating agents, is shown. In this sense, our research group accomplished the extraction of five different heavy metals from aqueous solutions, in neutral pH, with 7 quaternary-ammonium-based ILs, obtaining a metal removal between 80 and 90 % [31].
As it can be seen from the facts stated above, ILs are very interesting compounds for performing extraction and separation processes, but actually, there are few methods for purifying them and less about ammonium–based ILs, which are the ones used in the present work. One of the proposed treatments is centrifugation, which recovers successfully hydrophobic ILs [32]. In another interesting report by Lee and Ha [33], a magnetic field was used to recover an IL with a strong magnetic response. Membrane filtration has been used to retain contaminants that ILs may have; the advantage of this method is that in comparison with the others, it does not require auxiliary substances and can be used with a wide variety of ILs [34,35]. Another treatment was carried out with the help of a rotary evaporator and then vacuum, resulting in a quite efficient tool to recover and purify ILs containing sulfur compounds. The operation was based on the IL vapor pressure, which is lower than that of the rest of the components [36]. The recovery of ILs containing sulfur compounds in their structure has also been achieved. In this case, the pH change was used to promote the
precipitation of pollutants, which are filtered in order to get the base IL; it is worth highlighting that for this treatment, it is necessary to regenerate the anionic part [37].
Finally, the addition of a phase has also been applied, where salting out is used in the recovery process, which consists of adding an electrolyte such as a solid or a saturated aqueous solution, which when put in contact removes some water, forming a second phase that can be removed by settling or evaporation. In some cases, a potassium phosphate electrolyte, carbon dioxide and kosmotropic salts (those that contribute to the stability of molecular interactions) have been used [38–40].
According to the aforementioned, in the present work, two goals were set. The first one was to propose a new methodology for recovering and purifying ammonium–based ILs applying the addition-phase method. The second one was to find a mechanism to explain the extraction of metallic cations from aqueous solutions. The ILs with trioctylmethyl ammonium camphorate (CTMA) and dodecanedioate (DTMA) structures used for running the experiments were synthesized free of halides, fluorides or cyanides [41,42] in order to comply with an environmentally- friendly-chemistry commitment. In order to do so, our ILs were considered similar to surfactants, which are less pollutant [43,44].
The extraction process was carried out with the two ILs mentioned above, selected from a previous work [31] that encouraged us to keep evaluating these promising compounds. Each IL was evaluated in five continuous cycles with three metals, where the corresponding aqueous solutions were prepared under neutral pH conditions at 30 °C. The recovery process was based on ionic liquid properties such as polarity and miscibility.
2. Experime ntal 2.1.
Materials
The reagents were: camphoric acid (99 %), dodecanedioic acid (99 %), cadmium (II) chloride (99 %), copper (II) sulfate (99 %), and lead (II) acetate trihydrate (99 %), which were purchased from Aldrich and used as received. Ethanol and acetone were reagent grade.
2.2.
IL Synthesis Procedure and Characterization.
The ILs (Table 1) were synthesized in one step, starting from methyltrioctylammonium methylcarbonate as feedstock. The synthesis procedure of this compound was described previously [31].
Table 1. Structures and names of the synthesized ILs. Molecular Weight (g/mol)
Yield (%)
Trioctylmethyl ammonium camphorate
567.93
81
Trioctylmethyl ammonium dodecanedioate
598
76
IL Number
Short name
Name
1
CTMA
2
DTMA
Cation
Anion
In this first step, anionic exchanges of the methyl carbonate ion by the corresponding carboxylate anions were performed as shown in Fig. 1.
Fig. 1. Synthesis of CTMA or DTMA.
The ILs 1-2 were synthesized by following a general procedure:
Camphoric or dodecanedioic acids (15 mmol) were dissolved in 50 mL of methanol and 30 mL of methanol solution containing 15 mmol of the IL feedstock (trioctylmethyl ammonium methyl carbonate) were added to the mixture. The reaction occurred with the release of gas. The reaction was kept under constant stirring for 30 min at 40 °C. After reaction completion, the solvents were removed by vacuum evaporation and the product was dried under vacuum. All the synthesized compounds were characterized by 1 H and
13
C NMR, Infrared (IR),
Ultraviolet–Visible (UV-Vis) spectroscopy. IR, UV-Vis spectra were recorded on a Thermoscientific Nicolet 8700 Spectrometer. 1 H (300 MHz) and
13
C NMR (75.40 MHz)
spectra were recorded on a JEOL Eclipse-300 equipment in CDCl3 , and chemical shifts were expressed in ppm with tetramethylsilane as the internal standard. Trioctylmethyl ammonium camphorate (CTMA): yellow viscous liquid; 81% yield. UV-vis CH2 Cl2 (nm): 231. FTIR (film): 3677, 3393, 2927, 2856, 2587, 1944, 1698, 1556, 1463, 1378, 1298, 1234, 1203, 1119, 1081, 1041, 995, 901, 868, 785, 766, 724cm-1 . 1 H NMR: δ 0.87 (t, 9H, CH3, J= 9.60 Hz), 1.06 (s, 3H, CH3 ), 1.08 (s,3H, CH3 ), 1.141 (s, 3H,
CH3 ), 1.32 (an, 30H, CH2 ), 1.67 (an, 6H, CH2 -N), 1.96- 2.32 (m, 4H, CH2 cycle), 2.70-2.82 (m, 1H, CH), 3.17 (s, 3H, CH3-N), 3.26- 3.34 (m 6H, CH2 -N), 7.38 (s, 1H, COOH) ppm. 13
C NMR: δ 13.80 (3CH3 ), 20.83 (2CH3 ), 21.75 (CH3 ) 22.13 (4CH2 ), 26.14 (6CH2 ), 28.85
(6CH2 ), 31.46 (4CH2 ), 48.58(C), 49.97 (CH), 52.01 (CH3 -N), 57.21 (C), 61.40 (3CH2 –N), 179.87 (C=O), 181.87 (2C=O) ppm. Trioctylmethyl ammonium dodecanedioate (DTMA): Amber viscous liquid; 76 % yield. UV-vis CH2 Cl2 (nm): 234 FTIR (film): 3558, 3400, 3205, 2925, 2855, 1710, 1664, 1619, 1559, 1466, 1411, 1377, 1343, 1274, 1204, 1110, 1066, 944, 921, 885, 807, 767 cm-1 . 1 H NMR: δ 0.88 (t, 9H, CH3, J= 6.60 Hz), 1.69 (t, 42H, CH2 , J= 10.8 Hz), 1.54-1.64 (m, 10H, CH2 ), 2.23 (t, 4H, CH2 , J= 10.8 Hz), 3.16 (s, 3H, CH3 -N), 3.25-3.31 (m 6H, CH2 -N), 8.71 (s, 1H, COOH) ppm.
13
C NMR: δ 13.80 (3CH3 ), 22.30 (3CH2 ), 23.90 (CH2 ), 25.80 (4CH2 ),
26.10 (3CH2 ), 28.80 (2CH2 ), 29.30 (10CH2 ), 31.40 (3CH2 ), 36.50 (2CH2 ), 49.60 (CH3 -N), 61.20 (3CH2 -N), 178.40 (2C=O) ppm.
2.3.
Test Solution
The test solution was prepared by using deionized water and the next reagent grade salts: CdCl2 , CuSO 4 and Pb (CH3 COO)2 (3H2O), separately. The concentrations of these aqueous neutral pH solutions were 50 and 100 mg/L for each solution. The atomic absorption technique was performed for measuring all the metal concentrations in each sample of the obtained solutions, where each reading was performed thrice by means of a Fast Sequential Atomic Absorption Spectrometer, model Varian AA240FS.
2.4.
Liquid-liquid Extraction Procedure of Metal Cations
The synthesized ILs were used as extracting agents. Both were immiscible with the aqueous phase. The synthesized ILs displayed low hydrophilicity, which is confirmed by the TGA spectrum (Fig. S1) of the CTMA sample, which was stored in an open vessel for one month, showing water concentration below 2 %. For the extraction, 100 mL of aqueous solution (tested solution) and 5 g of the corresponding IL were added to an Erlenmeyer flask (120 mL) containing a magnetic stirring bar. The flasks were sealed and the mixture stirred (600 rpm) for 30 min at 30 °C with atmospheric pressure; stirring was then stopped and centrifuged for 2 min to allow the phases to separate since o ne of the goals for this work was to recycle ILs and assess their behavior. The aqueous phase was removed and the loaded ionic liquid phase remains in the flask, allowing the reuse of the IL, which means that the IL is not removed at all from the beginning of the process until the end of the fifth cycle. The aqueous phase is fresh in each feed. The atomic absorption technique was used to determine the amount of metals present in the aqueous phase, after the extraction process, where samples of the aqueous phases were taken from the flasks with at least three sampling events. The removal efficiency was obtained by applying the following equation to each metal ion:
(1)
Where: E is the extraction percentage of the metal ion (%); Ci is the initial concentration of the metal ion in the solution (mg/L); and Cf is the final concentration of the metal ion in the solution (mg/L).
2.5.
Ionic Liquid Recovery after Liquid-liquid Extraction of Metal Ions
The recovery of ILs was carried out with some solvents with polarity indexes between 4.3 and 5.1, such as ethanol and acetone. A 1:1 mixture ratio (15 mL: 15 mL) of these solvents was added to the resultant loaded IL, which had to be stirred for 5 minutes and let stand for 5 minutes. After this operation, the metal ion precipitated and the upper phase that contained the IL was removed with a pipette and placed in a balloon flask; the solvents were removed by vacuum evaporation and the treated IL was dried under vacuum, the precipitated metal complex was dried. To corroborate the effectiveness of the treatment, the plasma emission technique was used; the equipment for this evaluation was an (ICP-AES) Varian Liberty Series II; it shows the metal concentration in the IL before and after the recovery treatment. In the case of the plasma analysis, the used solution concentration was 50 ppm; each measurement was repeated, three times. The weight was measured for all the evaluated ILs with 3 metals with both concentrations; the results represent the average.
After performing the recovery process and with the emission plasma results, it was necessary to prove that the ILs were still able to continue extracting metal ions and most importantly without lowering their efficiencies. According to the aforesaid, both recovered ILs were put back into the extraction process, using a cycle after treatment on three different occasions. The aqueous solutions were analyzed with atomic absorption and the extraction percentage was reported.
3. Results and discussion 3.1.
Synthesis and Characterization of ILs
The ILs employed in this work were synthesized in one step, through anionic exchanges of the IL methyl carbonate ion with the corresponding carboxylic acids, where the by-products of this exchange reaction are CO 2 and methanol; for this reason, a 30- min reaction time was established because the formation of bubbles caused by this gas in the reaction mixture finishes at this time, in accordance with a previously published analogous procedure [45]. These dicarboxylic acids were chosen as reagents because of their suitability to obtain the anion with the necessary characteristics to form the coordination bonds with the metal ions, and the chain length contributes to a lower miscibility in water. The ILs were characterized by NMR spectroscopy. Fig. 2 shows the
13
C NMR spectrum of
CTMA, where the signal at 13.89 ppm corresponds to the carbon from the methyl group of the aliphatic chain; the signals at 20.83 and 21.75 ppm are representative of the groups that are attached from the cycle to the quaternary carbons, respectively. The methyl signals assigned to carbon chains of cation, methylene, aliphatics and the methylene anionic part were 22.13, 26.14, 28.85, and 31.1 ppm. There is also a signal at 48.58 ppm representing one of the quaternary carbons of the cycle; at 49.97 ppm, there is a signal that corresponds to the methylenes near the methyl attached to oxygen groups; the methyl group attached to nitrogen shows a signal at 52.01 ppm; the quaternary carbon that is close to the methyl group attached to oxygen features a signal at 57.21 ppm, and the methylene groups attached directly to the amino group display a signal at 61.40 ppm. Finally, the carbon signals of the carbonyl group are shown at 179.87 and 181.87 ppm.
Fig. 2. 13 C NMR spectrum of CTMA. The 1 H NMR spectrum of DTMA is shown in Fig. 3. A triplet at 0.88 ppm with a constant mesh J = 6.6 Hz is observed, which was assigned to the protons of the aliphatic chain methyl group; another triplet at 1.69 ppm with a coupling constant (J) = 10.8 Hz can be observed, which belongs to the protons of the cationic aliphatic chain methylenes and the methylenes of the anionic part. A multiplete at 1.54-1.64 ppm can also be seen, which corresponds to the protons close to the carboxylic group and amino group methylenes; there is another hat-trick at 2.23 ppm with a J = 7.65 Hz, which represents the methylene protons joined to the carboxyl group of the dodecanedioic acid; at 3.16 ppm, there is a singlet that corresponds to the protons of the methyl that is attached to nitrogen; there is a multiplete at 3.25 – 3.31 ppm, which corresponds to the methylene groups that are attached directly to the cationic
part of nitrogen, and finally, a signal at 8.71 ppm corresponds to the protons of the carbonyl group of the anionic part.
Fig. 3. 1 H NMR spectrum of DTMA. 3.2.
Removal Efficiency of ILs
The present work evaluated two halogen-free IL solvents that were synthesized with specific characteristics to achieve a complete removal of metal ions according to their structure and assess the behavior of both ILs in the extraction process. In this sense, it is important to mention that our ILs can be considered as solvents because their viscosity (Table S1 and Fig. S2) is close to that displayed by glycols.
The high concentration of the aqueous solutions prepared in this work opens the possibility of using this process in more areas. The extractions were carried out in neutral pH and the corresponding results are shown in Table 2, where the metals were evaluated with different concentrations.
Table 2. pH of aqueous test solutions pH 25 ppm
30 ppm
35 ppm
40 ppm
45 ppm
50 ppm
Cadmium
7.31
-
-
-
-
7.04
Copper
6.64
6.68
6.62
6.67
6.63
6.44
Lead
6.58
6.82
6.09
6.11
6.11
6.16
These ILs were used in a 1:20 weight ratio of IL to aqueous solution. The results derived from this study are summarized in Tables 3 and 4, where all the ionic liquids presented encouraging results for extracting metal cations such as cadmium, copper, and lead for 50 and 100 ppm concentrations.
Table 3. Extraction of metal ions from a 50-ppm aqueous solution in cycles CYCLE –Extraction (%)
IL –METAL 1
2
3
4
5
CTMA-Cd(1)
95.96
95.18
95.3
97.22
94.50
DTMA-Cd(1)
93.82
91.98
93.04
94.68
96.02
CTMA-Cu(1)
93.98
93.94
93.42
91.60
86.44
DTMA-Cu(1)
99.76
99.74
99.72
99.70
98.18
CTMA-Pb(1)
94.68
95.72
94.18
93.50
95.18
DTMA-Pb(1)
99.70
99.74
99.68
99.74
99.83
Table 4. Extraction of metal ions from a 100-ppm aqueous solution in cycles CYCLE –Extraction (%)
IL –METAL 1
2
3
4
5
CTMA-Cd(2)
93.22
92.79
92.26
92.21
92.64
DTMA-Cd(2)
92.82
92.48
91.87
92.95
94.12
CTMA-Cu(2)
89.44
84.22
79.64
81.96
79.48
DTMA-Cu(2)
99.68
99.58
99.48
99.62
99.32
CTMA-Pb(2)
94.78
95.60
95.37
95.26
94.64
DTMA-Pb(2)
98.50
98.50
98.50
97.78
98.50
As it can be seen in the previous tables, the results concerning CTMA for Cd are not quite different for the 50- and 100-ppm solutions (95 and 92 %, respectively). For Cu, it is a curious case because in this extraction, the percentage was lowered slightly for the 50- and 100-ppm solutions (86 and 79 %, respectively) in comparison with the other metal results. As for Pb, all the results are above 94 and 95 % for both concentrations.
As shown before in Tables 3 and 4, the most efficient IL for removing the metal cations is (DTMA), which could extract more than 98-99 % for Cu and Pb, and for Cd, the percentage is above 92-95 % in both concentration cases. As it can be seen, the anionic part exerts a direct influence on the extraction rate. The results regarding the two ILs tested in this study are compared to each other. For CTMA, which has a ring in the anionic part, the extraction results are less efficient, and when the anion of the IL is larger, which is the case of DTMA, the extraction percentage is increased. This fact reveals a direct anion - extraction relationship.
The good results obtained with the two solution concentrations encourage the use of these ILs in aqueous solutions with higher metal ion concentrations, and, if necessary, the increase in the life cycles of different ILs because in these tests, maximum saturation was not reached. Two more evaluations were carried out: one in batch mode (500 ppm of Cu2+) and the other in continuous mode (100 ppm of Cu2+). For the batch evaluation, the same liquid – liquid extraction procedure mentioned before was used. In this case, 5 g of DTMA were fed and 100 mL of 500 ppm of Cu2+ were added 5 times. The results are shown in Fig. 4. It is observed that the extraction percentage drops to half the percentage obtained with lower concentrations (50 and 100 ppm).
DTMA-Cu 500 ppm Trend
56
Metal extraction (%)
54
52
50
48
46
44 1
2
3
Number of extraction
4
5
Fig. 4. Batch extraction evaluation with DTMA and 500-ppm solution of Cu2+ at 30 °C, 600 rpm.
The results show that DTMA is capable of continuing the extraction despite the increase in concentration. By feeding 5g (8.36 mmol) of DTMA, it is possible to extract 19.39 mmol of Cu2+.
For the continuous system evaluation, the procedure was carried out in a glass column (10 mL) with pyrex glass packaging (30/40) in which a total of 5 g of DTMA were added to the column and 5 liters of Cu2+ 100 ppm solution were fed to the system with a feed rate of 5 mL/min. The results of this assessment are shown in Fig. 5.
100.2 DTMA-Cu 100 ppm Tendency
100.0 99.8
Extraction (%)
99.6 99.4 99.2 99.0 98.8 98.6 98.4 0
1000
2000
3000
4000
5000
Volume (mL)
Fig. 5. Continuous extraction evaluation with DTMA and 100- ppm of Cu2+ solution at 30 °C.
The removal percentages indicate that saturation occurs slowly, where 8.36 mmol of DTMA are capable of extracting 12.48 mmol of Cu2+. It should be noted that these results were obtained at the end of the experiment, where 5 L of Cu2+ solution were injected to the column, no less than 97% of extraction efficiency was obtained.
From the results described above, it could be said that DTMA is a better extracting agent than CTMA, because the design of the ionic liquids is based on their poor miscibility with water, but also the extraction process depends on both the number of carbons and their arrangement, which exerts a direct influence. The ionic liquid with twelve carbon atoms and a linear arrangement favors the extraction of a higher quantity of metal, which establishes the difference with respect to CTMA, which has a ring in its structure that exerts a steric hindrance which affects the extraction process. The other parameter that explains the extraction preference is the ionic radius and nature of each metal.
3.3.
Recovery of ILs
In order to recover the ILs, a mixture of acetone and ethylic alcohol was used. The selection of these solvents was done by taking into account their refractive indexes. This property, and not the polarity index, was used because, sometimes, the dipole moment of ILs is very complex to be measured or simulated; thus, based on the study performed by Shiro et al. [46], where the refractive index was related to the polarizability of the ILs, CTMA refractive index was measured according to the ASTM D1218 method, which was equal to 1.4761 and close to those indexes shown by acetone, ethylic alcohol, chloroform and dichloromethane [47]. In addition to the aforesaid, acetone and ethanol were selected to carry out the purification of the ILs due to their low cost,
low toxicity (in comparison with
dichloromethane), and the capacity of removing the metal complex, which is insoluble in this solvent mixture.
As it was mentioned, the emission plasma technique was employed for detecting the concentration of ion metals in the ILs, before and after treatment. Table 5 shows the concentrations. Table 5. Metal ion concentrations in the ILs before and after recovery. Metal concentration (ppm)
IL – Metal
Before IL recovery
After IL recovery
35.49 34.10 40.39 54.30 52.05 38.33
10.85 13.94 14.10 5.96 10.10 4.21
CTMA-Cd DTMA-Cd CTMA-Cu DTMA-Cu CTMA-Pb DTMA-Pb
The results in Table 5 show that the recovery of the ionic liquid is possible by adding medium-polarity solvents. In some cases, like those regarding DTMA-Cu (1) and DTMA – Pb (1), the IL recovery was almost complete. As for the results regarding 5 and 4 ppm obtained after treatment, and from the vantage point of the relevance of recovering ionic liquids, these results represent the feasibility of applying this process to the reuse of ILs. An example phase separation can be seen in Fig. S3.
Initial and final weight data of ILs are shown in Table 6.
Table 6. Initial and final IL weights after treatment
Metal ion
Concentration (ppm)
IL
Initial weight (g)
Final Weight (g)
Metallic salt precipitate Weight (g)
50 Cd 100 50 Cu 100 50 Pb 100
3.4.
CTMA
5
4.66
0.33
DTMA CTMA
5 5
4.58 4.54
0.37 0.35
DTMA
5
4.30
0.25
CTMA
5
4.35
0.37
DTMA CTMA
5 5
4.79 4.03
0.31 0.33
DTMA
5
4.18
0.48
CTMA DTMA
5 5
4.47 4.15
0.12 0.27
CTMA
5
4.09
0.16
DTMA
5
4.38
0.29
Extraction after recovery
Along with the analysis of plasma emission, the corroboration of the treatment was carried out by using the IL in an extraction process after its recovery. This procedure was performed with both ILs with a metal concentration of 50 ppm for the three metals. The ILs were recovered thrice and evaluated in the extraction process. The results are shown in Table 7. Table 7. Extraction after recovery
Metal ion
Concentration (ppm)
Cd
50
Cu
50
Pb
50
IL CTMA DTMA CTMA DTMA CTMA DTMA
Treatment – Extraction (%) 1 2 3 94.42 94.24 94.08 98.24 98.68 98.78 99.67 99.24 99.01 99.81 99.72 99.70 99.78 98.46 98.42 96.60 96.40 96.20
The extraction and treatment were repeated three times.
The Cu extraction by DTMA showed a good extraction behavior, which was still good after the purification process; it is likely that these ILs have affinity to metal cations with minor ionic radius such as copper, forming a stable metal complex. The carboxylic group present in the anionic part of ILs is responsible for forming a coordinate bond with metallic cations. However, during the purification process of the IL, the alkyl-ammonium is the easiest exiting group of this complex.
3.5.
Extraction mechanism
The extraction mechanism was proposed according to the assessments carried out with both ILs, Fig. 6. As a reference, the mechanism proposed by Castillo et al., 2014 [48] was considered, where the ion exchange process takes place with acid pH.
Fig. 6. Extraction mechanism by Castillo et al.
Where: R4 N + cation (quaternary amine) CnHnOO - anion, (dicarboxylic acids) Me2+ metal (Cd, Cu, and Pb) Y-
counterpart of the metal salt (Cl-,
, CH3 COO-)
It is noteworthy to mention that the anions used in the present work were dicarboxylic acids, which vary in the size of their aliphatic chains, which makes these compounds insoluble in water, and the extraction process depends directly on this fact. As it can be seen, the anionic
part of the IL attracts the metal ion, which was demonstrated by retro-synthesis. Afterwards, dodecanedioate sodium (acid salt) was used to perform the ionic exchange. The dodecanedioate sodium (DS) was synthesized according this procedure: dodecanedioic acid (2:1) and sodium methoxide were put to react. In order to do so, 13 mmol of dodecanedioic acid and 6 mmol of sodium methoxide were dissolved in 40 mL of ethanol. The solution was stirred for 2 hours under reflux. After DS precipitated, it was filtered and dried. The confirmation of the product formation was done by using the melting point (200 – 222 °C) and infrared analysis: 3435, 2938, 2917, 2848, 1723, 1631, 1600, 1467, 1401, 1337, 1283, 1233, 1185, 1035, 907, 805, 792, 722, 680, 581, 474 cm-1 .
In order to perform the ionic exchange with copper, 0.85 mmol of the DS were dissolved in ethanol and put in contact with a copper solution (50 mg/L), stirred for 2 hours, then cooled down to allow the metal salt to precipitate. The product was filtered, washed with water and acetone and finally dried under vacuum. The resulting solution was analyzed by atomic absorption, establishing the initial and final concentrations of the copper ion in the solution. From the initial and final concentrations (48.13 and 1.06 ppm, respectively) it can be said that DS almost extracted all the copper ions from the solution. It is evident that copper displaces sodium, forming a salt with dicarboxylic acid. In this way, the ionic exchange is possible during the extraction process with ILs.
The retro-synthesis results and the assessments of the IL maximum saturation reflect extraction ratios of 1:2.3 and 1:1.5, confirming the ion exchange, however, this gives rise to different extraction mechanism proposals since the ratios are not 1:1 as it would be if only ion exchange occurred. The proposed mechanism is shown in Fig. 7.
Fig. 7. Proposed extraction mechanism.
As it can be seen, in this mechanism, besides an ion exchange, the functional group of the IL anion is capable of forming bonds or ligands with the metal ion. In this way, the mechanism is much more complex than the one previously presented, and it can explain the recovered metal amount during the liquid- liquid extraction.
To prevent the ILs from being worn out during the extraction process, the synthesized DS salt was used; 0.85 mmol of DS were solubilized in approx. 7.41 mmol of DTMA. This mixture was evaluated and the resulting weights at the end of the extraction and recovery o f DTMA are shown in Table 8.
Table 8. IL weights after recovery following the extraction process using a DS mixture
Metal ion
Concentration (ppm)
Cd Cu Pb
50 50 50
IL (DTMA) Initial weight (g) Final weight (g) 4.58 4.79 4.15
4.56 4.69 4.14
The IL weight losses are not that significant since they are close to 2%. On the other hand, this procedure prevents the IL from being worn out, making the extraction recovery a feasible process. The resulting salt with heavy metal could be treated with a strong acid such as HCl. In this way, the organic acid and heavy metal can be recovered separately, whereas the IL can be reused.
Conclusions
In this work, new water- immiscible-halogen-free ILs derived from trioctylmethylammonium were synthesized with good yields (76-81%). The extraction results regarding Cd 2+, Cu2+ and Pb2+ from neutral aqueous solutions at room temperature and atmospheric pressure by using the synthesized ILs are promising. For many metal ions, the extraction results with 50 and 100 ppm, after five continuous cycles, are above 90%. The extraction mechanism proposed in this work is the ion exchange of the anionic part of the IL, but the use of the same organic acid sodium salt, mixed with the IL, reduces IL weight losses.
The ionic liquids were successfully recovered by using solvents with polarity indexes between 5.1 and 4.3, making without losing extraction efficiency, making their recovery and reuse a feasible process.
All this has been done in order to promote advances in green chemistry, as well as to reduce costs in the extraction processes. In the case of the removal of heavy metal cations from water, it goes from the use, recovery and reuse of ILs, forming a closed cycle. Despite the fact that conventional solvents were used, the ratios needed for the recovery were minimal, and by comparing this process with the traditional way to treat water containing metals, which usually is carried out by adding acid or basic compounds, it can be seen that there is truly an important advance in green chemistry. The water treatment was carried out under ambient conditions without pH, pressure or temperature changes. Associated content
Supporting information. TGA spectrum of the CTMA sample (Fig. S1); Viscosity data of CTMA sample (Fig. S2); Metal complex precipitation (Fig. S3); Viscosity data of CTMA at 25 °C (Table S1).
Acknowledge ments
The authors would like to acknowledge the support provided by SIP IPN (20151065)
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Graphical abstract
Highlights
Liquid/liquid extraction of Cd(II), Cu(II), Pb(II) ions. Extraction using ammonium–based ionic liquids. Recycling and recovery of ammonium–based ionic liquids after treatment. IL recovery was carried out by using solvents with medium polarity index.
S1383-5866(15)30015-0 http://dx.doi.org/10.1016/j.seppur.2015.05.031 SEPPUR 12360
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
20 August 2014 14 May 2015 22 May 2015
Please cite this article as: M.A. Valdés Vergara, I.V. Lijanova, N.V. Likhanova, O. Olivares Xometl, D. Jaramillo Vigueras, A.J. Morales Ramirez, Recycling and recovery of ammonium–based ionic liquids after extraction of metal cations from aqueous solutions, Separation and Purification Technology (2015), doi: http://dx.doi.org/10.1016/ j.seppur.2015.05.031
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Recycling and recovery of ammonium–based ionic liquids after extraction of metal cations from aqueous solutions. M.A. Valdés Vergara a, I.V. Lijanova a,*, N.V. Likhanova b , O. Olivares Xo metlc , D. Jaramillo Vigueras a, A.J. Morales Ramireza
a
Instituto Politécnico Nacional, CIITEC, Cerrada Cecati S/N, Colonia Santa Catarina de Azcapotzalco, CP
02250, México, D.F., México. Tel: 52 5557296000(68305); E-mail: [email protected] b.
Instituto Mexicano del Petróleo, Gerencia de Ingeniería de Recuperación Adicional , Eje Central Norte
Lázaro Cárdenas No. 152, Col. San Bartolo Atepehuacan, CP 07730, México, DF, México. Tel: 52 5591758382; E-mail: [email protected] c
Benemérita Universidad Autónoma de Puebla, Facultad de Ingeniería Química, Av. San Claudio y 18 Sur,
Col. San Manuel, Ciudad Universitaria, Puebla, 72570, México
ABSTRACT The recycling and recovery of ionic liquids (ILs) after the extraction of metal ions were investigated by using trioctylmethyl ammonium camphorate and trioctylmethyl ammonium dodecanedioate, which were synthesized from trioctylamine. The metal ions used in the aqueous solutions were: cadmium, copper and lead with initial concentrations of 50 and 100 ppm. In all the cases, the extraction percentage results were above 90%. After the extraction, the IL recovery was carried out by using solvents with medium polarity indexes. The present work features an extraction mechanism that goes through the use, recycling, recovery and reuse of ILs applied in the metal extraction process.
Keywords: ionic liquids; extraction; recycle; recovery; heavy metals; aqueous solutions.
1. Introduction In the last decades, ionic liquids (ILs) have generated rising interest because of their diversified range of applications. These solvents include a large number of molecular structures and consist entirely of cations and anions, featuring low melting points as low as 100 °C with low viscosities [1–4]. IL properties such as melting point, viscosity, density, and hydrophobicity can be designed and task-specifically adjusted, choosing either the structure of the cation or anion or both to meet the requirements of specific applications [5–7]. ILs are found increasingly in a number of fields such as catalysis, electrochemistry, separation processes and liquid- liquid extractions [8–17]. The liquid-liquid extraction is an important kind of separation method that is based on the distribution of chemicals between two different liquid phases [18]. In recent years, many researchers have studied the liquid-liquid extraction by using ILs as 'green' solvents in the partition of substituted-benzene derivatives between water and a hydrophobic IL [19], the removal of sulfides and nitrides from diesel and gasoline [20–24], and the removal of heavy metals from water [17,25]. In the first studies of heavy metal extractions that were performed with ILs assisted by crown ethers, the metals were removed from the aqueous phase, but in many cases, the ILs remained in the aqueous phase, and the whole extraction depended on the hydrophobicity of the extracting agent. The selectivity towards certain metal ions and extraction efficiency depend on the IL structure [26–28]. Other research works on metal ion extractions without extracting agents, only using IL/aqueous systems, have shown extremely high metal ion
partitionings to the IL phase [17,19,27,28], which is leading to the development of new water treatment methods. Some extractions without using an extracting agent have been done, like in the work published by Germani [29] and his group, where Cu cations were removed with 1-alkyl-3- methylimidazolium-based ILs, showing that the extraction can occur without a chelating agent. Another work was done by P. de Rios [30], where the extraction of Zn2+, Cd2+, and Fe3+ from aqueous hydrochloride solutions using ILs, in the absence of chelating agents, is shown. In this sense, our research group accomplished the extraction of five different heavy metals from aqueous solutions, in neutral pH, with 7 quaternary-ammonium-based ILs, obtaining a metal removal between 80 and 90 % [31].
As it can be seen from the facts stated above, ILs are very interesting compounds for performing extraction and separation processes, but actually, there are few methods for purifying them and less about ammonium–based ILs, which are the ones used in the present work. One of the proposed treatments is centrifugation, which recovers successfully hydrophobic ILs [32]. In another interesting report by Lee and Ha [33], a magnetic field was used to recover an IL with a strong magnetic response. Membrane filtration has been used to retain contaminants that ILs may have; the advantage of this method is that in comparison with the others, it does not require auxiliary substances and can be used with a wide variety of ILs [34,35]. Another treatment was carried out with the help of a rotary evaporator and then vacuum, resulting in a quite efficient tool to recover and purify ILs containing sulfur compounds. The operation was based on the IL vapor pressure, which is lower than that of the rest of the components [36]. The recovery of ILs containing sulfur compounds in their structure has also been achieved. In this case, the pH change was used to promote the
precipitation of pollutants, which are filtered in order to get the base IL; it is worth highlighting that for this treatment, it is necessary to regenerate the anionic part [37].
Finally, the addition of a phase has also been applied, where salting out is used in the recovery process, which consists of adding an electrolyte such as a solid or a saturated aqueous solution, which when put in contact removes some water, forming a second phase that can be removed by settling or evaporation. In some cases, a potassium phosphate electrolyte, carbon dioxide and kosmotropic salts (those that contribute to the stability of molecular interactions) have been used [38–40].
According to the aforementioned, in the present work, two goals were set. The first one was to propose a new methodology for recovering and purifying ammonium–based ILs applying the addition-phase method. The second one was to find a mechanism to explain the extraction of metallic cations from aqueous solutions. The ILs with trioctylmethyl ammonium camphorate (CTMA) and dodecanedioate (DTMA) structures used for running the experiments were synthesized free of halides, fluorides or cyanides [41,42] in order to comply with an environmentally- friendly-chemistry commitment. In order to do so, our ILs were considered similar to surfactants, which are less pollutant [43,44].
The extraction process was carried out with the two ILs mentioned above, selected from a previous work [31] that encouraged us to keep evaluating these promising compounds. Each IL was evaluated in five continuous cycles with three metals, where the corresponding aqueous solutions were prepared under neutral pH conditions at 30 °C. The recovery process was based on ionic liquid properties such as polarity and miscibility.
2. Experime ntal 2.1.
Materials
The reagents were: camphoric acid (99 %), dodecanedioic acid (99 %), cadmium (II) chloride (99 %), copper (II) sulfate (99 %), and lead (II) acetate trihydrate (99 %), which were purchased from Aldrich and used as received. Ethanol and acetone were reagent grade.
2.2.
IL Synthesis Procedure and Characterization.
The ILs (Table 1) were synthesized in one step, starting from methyltrioctylammonium methylcarbonate as feedstock. The synthesis procedure of this compound was described previously [31].
Table 1. Structures and names of the synthesized ILs. Molecular Weight (g/mol)
Yield (%)
Trioctylmethyl ammonium camphorate
567.93
81
Trioctylmethyl ammonium dodecanedioate
598
76
IL Number
Short name
Name
1
CTMA
2
DTMA
Cation
Anion
In this first step, anionic exchanges of the methyl carbonate ion by the corresponding carboxylate anions were performed as shown in Fig. 1.
Fig. 1. Synthesis of CTMA or DTMA.
The ILs 1-2 were synthesized by following a general procedure:
Camphoric or dodecanedioic acids (15 mmol) were dissolved in 50 mL of methanol and 30 mL of methanol solution containing 15 mmol of the IL feedstock (trioctylmethyl ammonium methyl carbonate) were added to the mixture. The reaction occurred with the release of gas. The reaction was kept under constant stirring for 30 min at 40 °C. After reaction completion, the solvents were removed by vacuum evaporation and the product was dried under vacuum. All the synthesized compounds were characterized by 1 H and
13
C NMR, Infrared (IR),
Ultraviolet–Visible (UV-Vis) spectroscopy. IR, UV-Vis spectra were recorded on a Thermoscientific Nicolet 8700 Spectrometer. 1 H (300 MHz) and
13
C NMR (75.40 MHz)
spectra were recorded on a JEOL Eclipse-300 equipment in CDCl3 , and chemical shifts were expressed in ppm with tetramethylsilane as the internal standard. Trioctylmethyl ammonium camphorate (CTMA): yellow viscous liquid; 81% yield. UV-vis CH2 Cl2 (nm): 231. FTIR (film): 3677, 3393, 2927, 2856, 2587, 1944, 1698, 1556, 1463, 1378, 1298, 1234, 1203, 1119, 1081, 1041, 995, 901, 868, 785, 766, 724cm-1 . 1 H NMR: δ 0.87 (t, 9H, CH3, J= 9.60 Hz), 1.06 (s, 3H, CH3 ), 1.08 (s,3H, CH3 ), 1.141 (s, 3H,
CH3 ), 1.32 (an, 30H, CH2 ), 1.67 (an, 6H, CH2 -N), 1.96- 2.32 (m, 4H, CH2 cycle), 2.70-2.82 (m, 1H, CH), 3.17 (s, 3H, CH3-N), 3.26- 3.34 (m 6H, CH2 -N), 7.38 (s, 1H, COOH) ppm. 13
C NMR: δ 13.80 (3CH3 ), 20.83 (2CH3 ), 21.75 (CH3 ) 22.13 (4CH2 ), 26.14 (6CH2 ), 28.85
(6CH2 ), 31.46 (4CH2 ), 48.58(C), 49.97 (CH), 52.01 (CH3 -N), 57.21 (C), 61.40 (3CH2 –N), 179.87 (C=O), 181.87 (2C=O) ppm. Trioctylmethyl ammonium dodecanedioate (DTMA): Amber viscous liquid; 76 % yield. UV-vis CH2 Cl2 (nm): 234 FTIR (film): 3558, 3400, 3205, 2925, 2855, 1710, 1664, 1619, 1559, 1466, 1411, 1377, 1343, 1274, 1204, 1110, 1066, 944, 921, 885, 807, 767 cm-1 . 1 H NMR: δ 0.88 (t, 9H, CH3, J= 6.60 Hz), 1.69 (t, 42H, CH2 , J= 10.8 Hz), 1.54-1.64 (m, 10H, CH2 ), 2.23 (t, 4H, CH2 , J= 10.8 Hz), 3.16 (s, 3H, CH3 -N), 3.25-3.31 (m 6H, CH2 -N), 8.71 (s, 1H, COOH) ppm.
13
C NMR: δ 13.80 (3CH3 ), 22.30 (3CH2 ), 23.90 (CH2 ), 25.80 (4CH2 ),
26.10 (3CH2 ), 28.80 (2CH2 ), 29.30 (10CH2 ), 31.40 (3CH2 ), 36.50 (2CH2 ), 49.60 (CH3 -N), 61.20 (3CH2 -N), 178.40 (2C=O) ppm.
2.3.
Test Solution
The test solution was prepared by using deionized water and the next reagent grade salts: CdCl2 , CuSO 4 and Pb (CH3 COO)2 (3H2O), separately. The concentrations of these aqueous neutral pH solutions were 50 and 100 mg/L for each solution. The atomic absorption technique was performed for measuring all the metal concentrations in each sample of the obtained solutions, where each reading was performed thrice by means of a Fast Sequential Atomic Absorption Spectrometer, model Varian AA240FS.
2.4.
Liquid-liquid Extraction Procedure of Metal Cations
The synthesized ILs were used as extracting agents. Both were immiscible with the aqueous phase. The synthesized ILs displayed low hydrophilicity, which is confirmed by the TGA spectrum (Fig. S1) of the CTMA sample, which was stored in an open vessel for one month, showing water concentration below 2 %. For the extraction, 100 mL of aqueous solution (tested solution) and 5 g of the corresponding IL were added to an Erlenmeyer flask (120 mL) containing a magnetic stirring bar. The flasks were sealed and the mixture stirred (600 rpm) for 30 min at 30 °C with atmospheric pressure; stirring was then stopped and centrifuged for 2 min to allow the phases to separate since o ne of the goals for this work was to recycle ILs and assess their behavior. The aqueous phase was removed and the loaded ionic liquid phase remains in the flask, allowing the reuse of the IL, which means that the IL is not removed at all from the beginning of the process until the end of the fifth cycle. The aqueous phase is fresh in each feed. The atomic absorption technique was used to determine the amount of metals present in the aqueous phase, after the extraction process, where samples of the aqueous phases were taken from the flasks with at least three sampling events. The removal efficiency was obtained by applying the following equation to each metal ion:
(1)
Where: E is the extraction percentage of the metal ion (%); Ci is the initial concentration of the metal ion in the solution (mg/L); and Cf is the final concentration of the metal ion in the solution (mg/L).
2.5.
Ionic Liquid Recovery after Liquid-liquid Extraction of Metal Ions
The recovery of ILs was carried out with some solvents with polarity indexes between 4.3 and 5.1, such as ethanol and acetone. A 1:1 mixture ratio (15 mL: 15 mL) of these solvents was added to the resultant loaded IL, which had to be stirred for 5 minutes and let stand for 5 minutes. After this operation, the metal ion precipitated and the upper phase that contained the IL was removed with a pipette and placed in a balloon flask; the solvents were removed by vacuum evaporation and the treated IL was dried under vacuum, the precipitated metal complex was dried. To corroborate the effectiveness of the treatment, the plasma emission technique was used; the equipment for this evaluation was an (ICP-AES) Varian Liberty Series II; it shows the metal concentration in the IL before and after the recovery treatment. In the case of the plasma analysis, the used solution concentration was 50 ppm; each measurement was repeated, three times. The weight was measured for all the evaluated ILs with 3 metals with both concentrations; the results represent the average.
After performing the recovery process and with the emission plasma results, it was necessary to prove that the ILs were still able to continue extracting metal ions and most importantly without lowering their efficiencies. According to the aforesaid, both recovered ILs were put back into the extraction process, using a cycle after treatment on three different occasions. The aqueous solutions were analyzed with atomic absorption and the extraction percentage was reported.
3. Results and discussion 3.1.
Synthesis and Characterization of ILs
The ILs employed in this work were synthesized in one step, through anionic exchanges of the IL methyl carbonate ion with the corresponding carboxylic acids, where the by-products of this exchange reaction are CO 2 and methanol; for this reason, a 30- min reaction time was established because the formation of bubbles caused by this gas in the reaction mixture finishes at this time, in accordance with a previously published analogous procedure [45]. These dicarboxylic acids were chosen as reagents because of their suitability to obtain the anion with the necessary characteristics to form the coordination bonds with the metal ions, and the chain length contributes to a lower miscibility in water. The ILs were characterized by NMR spectroscopy. Fig. 2 shows the
13
C NMR spectrum of
CTMA, where the signal at 13.89 ppm corresponds to the carbon from the methyl group of the aliphatic chain; the signals at 20.83 and 21.75 ppm are representative of the groups that are attached from the cycle to the quaternary carbons, respectively. The methyl signals assigned to carbon chains of cation, methylene, aliphatics and the methylene anionic part were 22.13, 26.14, 28.85, and 31.1 ppm. There is also a signal at 48.58 ppm representing one of the quaternary carbons of the cycle; at 49.97 ppm, there is a signal that corresponds to the methylenes near the methyl attached to oxygen groups; the methyl group attached to nitrogen shows a signal at 52.01 ppm; the quaternary carbon that is close to the methyl group attached to oxygen features a signal at 57.21 ppm, and the methylene groups attached directly to the amino group display a signal at 61.40 ppm. Finally, the carbon signals of the carbonyl group are shown at 179.87 and 181.87 ppm.
Fig. 2. 13 C NMR spectrum of CTMA. The 1 H NMR spectrum of DTMA is shown in Fig. 3. A triplet at 0.88 ppm with a constant mesh J = 6.6 Hz is observed, which was assigned to the protons of the aliphatic chain methyl group; another triplet at 1.69 ppm with a coupling constant (J) = 10.8 Hz can be observed, which belongs to the protons of the cationic aliphatic chain methylenes and the methylenes of the anionic part. A multiplete at 1.54-1.64 ppm can also be seen, which corresponds to the protons close to the carboxylic group and amino group methylenes; there is another hat-trick at 2.23 ppm with a J = 7.65 Hz, which represents the methylene protons joined to the carboxyl group of the dodecanedioic acid; at 3.16 ppm, there is a singlet that corresponds to the protons of the methyl that is attached to nitrogen; there is a multiplete at 3.25 – 3.31 ppm, which corresponds to the methylene groups that are attached directly to the cationic
part of nitrogen, and finally, a signal at 8.71 ppm corresponds to the protons of the carbonyl group of the anionic part.
Fig. 3. 1 H NMR spectrum of DTMA. 3.2.
Removal Efficiency of ILs
The present work evaluated two halogen-free IL solvents that were synthesized with specific characteristics to achieve a complete removal of metal ions according to their structure and assess the behavior of both ILs in the extraction process. In this sense, it is important to mention that our ILs can be considered as solvents because their viscosity (Table S1 and Fig. S2) is close to that displayed by glycols.
The high concentration of the aqueous solutions prepared in this work opens the possibility of using this process in more areas. The extractions were carried out in neutral pH and the corresponding results are shown in Table 2, where the metals were evaluated with different concentrations.
Table 2. pH of aqueous test solutions pH 25 ppm
30 ppm
35 ppm
40 ppm
45 ppm
50 ppm
Cadmium
7.31
-
-
-
-
7.04
Copper
6.64
6.68
6.62
6.67
6.63
6.44
Lead
6.58
6.82
6.09
6.11
6.11
6.16
These ILs were used in a 1:20 weight ratio of IL to aqueous solution. The results derived from this study are summarized in Tables 3 and 4, where all the ionic liquids presented encouraging results for extracting metal cations such as cadmium, copper, and lead for 50 and 100 ppm concentrations.
Table 3. Extraction of metal ions from a 50-ppm aqueous solution in cycles CYCLE –Extraction (%)
IL –METAL 1
2
3
4
5
CTMA-Cd(1)
95.96
95.18
95.3
97.22
94.50
DTMA-Cd(1)
93.82
91.98
93.04
94.68
96.02
CTMA-Cu(1)
93.98
93.94
93.42
91.60
86.44
DTMA-Cu(1)
99.76
99.74
99.72
99.70
98.18
CTMA-Pb(1)
94.68
95.72
94.18
93.50
95.18
DTMA-Pb(1)
99.70
99.74
99.68
99.74
99.83
Table 4. Extraction of metal ions from a 100-ppm aqueous solution in cycles CYCLE –Extraction (%)
IL –METAL 1
2
3
4
5
CTMA-Cd(2)
93.22
92.79
92.26
92.21
92.64
DTMA-Cd(2)
92.82
92.48
91.87
92.95
94.12
CTMA-Cu(2)
89.44
84.22
79.64
81.96
79.48
DTMA-Cu(2)
99.68
99.58
99.48
99.62
99.32
CTMA-Pb(2)
94.78
95.60
95.37
95.26
94.64
DTMA-Pb(2)
98.50
98.50
98.50
97.78
98.50
As it can be seen in the previous tables, the results concerning CTMA for Cd are not quite different for the 50- and 100-ppm solutions (95 and 92 %, respectively). For Cu, it is a curious case because in this extraction, the percentage was lowered slightly for the 50- and 100-ppm solutions (86 and 79 %, respectively) in comparison with the other metal results. As for Pb, all the results are above 94 and 95 % for both concentrations.
As shown before in Tables 3 and 4, the most efficient IL for removing the metal cations is (DTMA), which could extract more than 98-99 % for Cu and Pb, and for Cd, the percentage is above 92-95 % in both concentration cases. As it can be seen, the anionic part exerts a direct influence on the extraction rate. The results regarding the two ILs tested in this study are compared to each other. For CTMA, which has a ring in the anionic part, the extraction results are less efficient, and when the anion of the IL is larger, which is the case of DTMA, the extraction percentage is increased. This fact reveals a direct anion - extraction relationship.
The good results obtained with the two solution concentrations encourage the use of these ILs in aqueous solutions with higher metal ion concentrations, and, if necessary, the increase in the life cycles of different ILs because in these tests, maximum saturation was not reached. Two more evaluations were carried out: one in batch mode (500 ppm of Cu2+) and the other in continuous mode (100 ppm of Cu2+). For the batch evaluation, the same liquid – liquid extraction procedure mentioned before was used. In this case, 5 g of DTMA were fed and 100 mL of 500 ppm of Cu2+ were added 5 times. The results are shown in Fig. 4. It is observed that the extraction percentage drops to half the percentage obtained with lower concentrations (50 and 100 ppm).
DTMA-Cu 500 ppm Trend
56
Metal extraction (%)
54
52
50
48
46
44 1
2
3
Number of extraction
4
5
Fig. 4. Batch extraction evaluation with DTMA and 500-ppm solution of Cu2+ at 30 °C, 600 rpm.
The results show that DTMA is capable of continuing the extraction despite the increase in concentration. By feeding 5g (8.36 mmol) of DTMA, it is possible to extract 19.39 mmol of Cu2+.
For the continuous system evaluation, the procedure was carried out in a glass column (10 mL) with pyrex glass packaging (30/40) in which a total of 5 g of DTMA were added to the column and 5 liters of Cu2+ 100 ppm solution were fed to the system with a feed rate of 5 mL/min. The results of this assessment are shown in Fig. 5.
100.2 DTMA-Cu 100 ppm Tendency
100.0 99.8
Extraction (%)
99.6 99.4 99.2 99.0 98.8 98.6 98.4 0
1000
2000
3000
4000
5000
Volume (mL)
Fig. 5. Continuous extraction evaluation with DTMA and 100- ppm of Cu2+ solution at 30 °C.
The removal percentages indicate that saturation occurs slowly, where 8.36 mmol of DTMA are capable of extracting 12.48 mmol of Cu2+. It should be noted that these results were obtained at the end of the experiment, where 5 L of Cu2+ solution were injected to the column, no less than 97% of extraction efficiency was obtained.
From the results described above, it could be said that DTMA is a better extracting agent than CTMA, because the design of the ionic liquids is based on their poor miscibility with water, but also the extraction process depends on both the number of carbons and their arrangement, which exerts a direct influence. The ionic liquid with twelve carbon atoms and a linear arrangement favors the extraction of a higher quantity of metal, which establishes the difference with respect to CTMA, which has a ring in its structure that exerts a steric hindrance which affects the extraction process. The other parameter that explains the extraction preference is the ionic radius and nature of each metal.
3.3.
Recovery of ILs
In order to recover the ILs, a mixture of acetone and ethylic alcohol was used. The selection of these solvents was done by taking into account their refractive indexes. This property, and not the polarity index, was used because, sometimes, the dipole moment of ILs is very complex to be measured or simulated; thus, based on the study performed by Shiro et al. [46], where the refractive index was related to the polarizability of the ILs, CTMA refractive index was measured according to the ASTM D1218 method, which was equal to 1.4761 and close to those indexes shown by acetone, ethylic alcohol, chloroform and dichloromethane [47]. In addition to the aforesaid, acetone and ethanol were selected to carry out the purification of the ILs due to their low cost,
low toxicity (in comparison with
dichloromethane), and the capacity of removing the metal complex, which is insoluble in this solvent mixture.
As it was mentioned, the emission plasma technique was employed for detecting the concentration of ion metals in the ILs, before and after treatment. Table 5 shows the concentrations. Table 5. Metal ion concentrations in the ILs before and after recovery. Metal concentration (ppm)
IL – Metal
Before IL recovery
After IL recovery
35.49 34.10 40.39 54.30 52.05 38.33
10.85 13.94 14.10 5.96 10.10 4.21
CTMA-Cd DTMA-Cd CTMA-Cu DTMA-Cu CTMA-Pb DTMA-Pb
The results in Table 5 show that the recovery of the ionic liquid is possible by adding medium-polarity solvents. In some cases, like those regarding DTMA-Cu (1) and DTMA – Pb (1), the IL recovery was almost complete. As for the results regarding 5 and 4 ppm obtained after treatment, and from the vantage point of the relevance of recovering ionic liquids, these results represent the feasibility of applying this process to the reuse of ILs. An example phase separation can be seen in Fig. S3.
Initial and final weight data of ILs are shown in Table 6.
Table 6. Initial and final IL weights after treatment
Metal ion
Concentration (ppm)
IL
Initial weight (g)
Final Weight (g)
Metallic salt precipitate Weight (g)
50 Cd 100 50 Cu 100 50 Pb 100
3.4.
CTMA
5
4.66
0.33
DTMA CTMA
5 5
4.58 4.54
0.37 0.35
DTMA
5
4.30
0.25
CTMA
5
4.35
0.37
DTMA CTMA
5 5
4.79 4.03
0.31 0.33
DTMA
5
4.18
0.48
CTMA DTMA
5 5
4.47 4.15
0.12 0.27
CTMA
5
4.09
0.16
DTMA
5
4.38
0.29
Extraction after recovery
Along with the analysis of plasma emission, the corroboration of the treatment was carried out by using the IL in an extraction process after its recovery. This procedure was performed with both ILs with a metal concentration of 50 ppm for the three metals. The ILs were recovered thrice and evaluated in the extraction process. The results are shown in Table 7. Table 7. Extraction after recovery
Metal ion
Concentration (ppm)
Cd
50
Cu
50
Pb
50
IL CTMA DTMA CTMA DTMA CTMA DTMA
Treatment – Extraction (%) 1 2 3 94.42 94.24 94.08 98.24 98.68 98.78 99.67 99.24 99.01 99.81 99.72 99.70 99.78 98.46 98.42 96.60 96.40 96.20
The extraction and treatment were repeated three times.
The Cu extraction by DTMA showed a good extraction behavior, which was still good after the purification process; it is likely that these ILs have affinity to metal cations with minor ionic radius such as copper, forming a stable metal complex. The carboxylic group present in the anionic part of ILs is responsible for forming a coordinate bond with metallic cations. However, during the purification process of the IL, the alkyl-ammonium is the easiest exiting group of this complex.
3.5.
Extraction mechanism
The extraction mechanism was proposed according to the assessments carried out with both ILs, Fig. 6. As a reference, the mechanism proposed by Castillo et al., 2014 [48] was considered, where the ion exchange process takes place with acid pH.
Fig. 6. Extraction mechanism by Castillo et al.
Where: R4 N + cation (quaternary amine) CnHnOO - anion, (dicarboxylic acids) Me2+ metal (Cd, Cu, and Pb) Y-
counterpart of the metal salt (Cl-,
, CH3 COO-)
It is noteworthy to mention that the anions used in the present work were dicarboxylic acids, which vary in the size of their aliphatic chains, which makes these compounds insoluble in water, and the extraction process depends directly on this fact. As it can be seen, the anionic
part of the IL attracts the metal ion, which was demonstrated by retro-synthesis. Afterwards, dodecanedioate sodium (acid salt) was used to perform the ionic exchange. The dodecanedioate sodium (DS) was synthesized according this procedure: dodecanedioic acid (2:1) and sodium methoxide were put to react. In order to do so, 13 mmol of dodecanedioic acid and 6 mmol of sodium methoxide were dissolved in 40 mL of ethanol. The solution was stirred for 2 hours under reflux. After DS precipitated, it was filtered and dried. The confirmation of the product formation was done by using the melting point (200 – 222 °C) and infrared analysis: 3435, 2938, 2917, 2848, 1723, 1631, 1600, 1467, 1401, 1337, 1283, 1233, 1185, 1035, 907, 805, 792, 722, 680, 581, 474 cm-1 .
In order to perform the ionic exchange with copper, 0.85 mmol of the DS were dissolved in ethanol and put in contact with a copper solution (50 mg/L), stirred for 2 hours, then cooled down to allow the metal salt to precipitate. The product was filtered, washed with water and acetone and finally dried under vacuum. The resulting solution was analyzed by atomic absorption, establishing the initial and final concentrations of the copper ion in the solution. From the initial and final concentrations (48.13 and 1.06 ppm, respectively) it can be said that DS almost extracted all the copper ions from the solution. It is evident that copper displaces sodium, forming a salt with dicarboxylic acid. In this way, the ionic exchange is possible during the extraction process with ILs.
The retro-synthesis results and the assessments of the IL maximum saturation reflect extraction ratios of 1:2.3 and 1:1.5, confirming the ion exchange, however, this gives rise to different extraction mechanism proposals since the ratios are not 1:1 as it would be if only ion exchange occurred. The proposed mechanism is shown in Fig. 7.
Fig. 7. Proposed extraction mechanism.
As it can be seen, in this mechanism, besides an ion exchange, the functional group of the IL anion is capable of forming bonds or ligands with the metal ion. In this way, the mechanism is much more complex than the one previously presented, and it can explain the recovered metal amount during the liquid- liquid extraction.
To prevent the ILs from being worn out during the extraction process, the synthesized DS salt was used; 0.85 mmol of DS were solubilized in approx. 7.41 mmol of DTMA. This mixture was evaluated and the resulting weights at the end of the extraction and recovery o f DTMA are shown in Table 8.
Table 8. IL weights after recovery following the extraction process using a DS mixture
Metal ion
Concentration (ppm)
Cd Cu Pb
50 50 50
IL (DTMA) Initial weight (g) Final weight (g) 4.58 4.79 4.15
4.56 4.69 4.14
The IL weight losses are not that significant since they are close to 2%. On the other hand, this procedure prevents the IL from being worn out, making the extraction recovery a feasible process. The resulting salt with heavy metal could be treated with a strong acid such as HCl. In this way, the organic acid and heavy metal can be recovered separately, whereas the IL can be reused.
Conclusions
In this work, new water- immiscible-halogen-free ILs derived from trioctylmethylammonium were synthesized with good yields (76-81%). The extraction results regarding Cd 2+, Cu2+ and Pb2+ from neutral aqueous solutions at room temperature and atmospheric pressure by using the synthesized ILs are promising. For many metal ions, the extraction results with 50 and 100 ppm, after five continuous cycles, are above 90%. The extraction mechanism proposed in this work is the ion exchange of the anionic part of the IL, but the use of the same organic acid sodium salt, mixed with the IL, reduces IL weight losses.
The ionic liquids were successfully recovered by using solvents with polarity indexes between 5.1 and 4.3, making without losing extraction efficiency, making their recovery and reuse a feasible process.
All this has been done in order to promote advances in green chemistry, as well as to reduce costs in the extraction processes. In the case of the removal of heavy metal cations from water, it goes from the use, recovery and reuse of ILs, forming a closed cycle. Despite the fact that conventional solvents were used, the ratios needed for the recovery were minimal, and by comparing this process with the traditional way to treat water containing metals, which usually is carried out by adding acid or basic compounds, it can be seen that there is truly an important advance in green chemistry. The water treatment was carried out under ambient conditions without pH, pressure or temperature changes. Associated content
Supporting information. TGA spectrum of the CTMA sample (Fig. S1); Viscosity data of CTMA sample (Fig. S2); Metal complex precipitation (Fig. S3); Viscosity data of CTMA at 25 °C (Table S1).
Acknowledge ments
The authors would like to acknowledge the support provided by SIP IPN (20151065)
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Graphical abstract
Highlights
Liquid/liquid extraction of Cd(II), Cu(II), Pb(II) ions. Extraction using ammonium–based ionic liquids. Recycling and recovery of ammonium–based ionic liquids after treatment. IL recovery was carried out by using solvents with medium polarity index.