Task-specific ionic liquids as extractants for the solvent extraction of

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Task-specific ionic liquids as extractants for the solvent extraction of molybdenum (VI) from aqueous solution using different commercial ionic liquids as diluents Esteban Quijada-Maldonado, Felipe Sanchez, Barbara Perez, Ricardo Apati Tapia, and Julio Romero Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04147 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Industrial & Engineering Chemistry Research

Task-specific ionic liquids as extractants for the solvent extraction of molybdenum (VI) from aqueous solution using different commercial ionic liquids as diluents Esteban Quijada-Maldonado1*, Felipe Sánchez1, Bárbara Pérez1, Ricardo Tapia2, Julio Romero1 1

Laboratory of Membrane Separation Processes (LabProSeM), Department of Chemical Engineering, Universidad de Santiago de Chile.

2

Faculty of Chemistry, Pontificia Universidad Católica de Chile.

Abstract In this work, the recovery of molybdenum (VI) (Mo) from aqueous solutions was carried out by solvent extraction (SX) using ammonium and phosphonium-based task-specific ionic liquids (TSILs) as extractants diluted in kerosene and two other hydrophobic room temperature ionic liquids (RTILs). Four different TSILs were used as extractants: trioctyl methyl ammonium bis-(2-ethylhexyl) phosphate [TOMA][D2EHP]; trioctyl methyl ammonium benzoate [TOMA][BA]; trihexyltetradecylphosphonium bis-(2-ethylhexyl) phosphate [P6,6,6,14][D2EHP]; and trihexyltetradecylphosphonium benzoate [P6,6,6,14][BA]. These TSILs were synthetized to assess the effect of the cation and anion on the SX of Mo(VI). Experimental results indicated that high extraction percentages of Mo(VI) were 1 ACS Paragon Plus Environment

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obtained with all the synthetized TSILs. Besides that, the best cation was [TOMA]+ for all the diluents and [BA]- was the best anion when kerosene and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [omim][Tf2N] were the diluents. Furthermore, the SX process using these ionic liquids followed an anion exchange mechanism for kerosene and [omim][Tf2N] as diluents. Finally, the stripping was successfully performed with a solution of ammonium carbonate, and the extent of stripping depended on the selected diluent. Keywords: Molybdenum; solvent extraction; task-specific ionic liquids * Corresponding author: Prof. E. Quijada-Maldonado Department of Chemical Engineering, University of Santiago de Chile Alameda 3363, Estación Central, Santiago, Chile Phone: +56 22 7181838 / E-mail: [email protected]

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1.

Introduction

Molybdenum (VI), Mo(VI), is a transition metal that exhibits many industrial applications such as the forming of nickel-based super alloys, within the hydrodesulphurization of petroleum, engine lubrication and so on. Molybdenum is selectively recovered from acid leach solutions by means of solvent extraction (SX) using either alkylphosphonic acids 1-4, oximes

5-9

or alkylamines

10-12

. These extractants are normally dissolved in kerosene or

similar organic solvents which show several drawbacks such as high volatility, flammability and toxicity. Lately, room temperature ionic liquids (RTILs) have been proposed as an alternative to those organic solvents in SX because of their excellent properties, such as negligible vapor pressure, non-flammability and, in some cases, remarkable hydrophobicity. RTILs have already been used as diluents in previous works. For instance, the ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] and the liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf2N] have been applied as the diluent in the extraction of alkaline metals13,14, uranyl ions 15-17, rare earths elements 18,18-23, nickel, copper and lead 24-28, iron29 and gold 30 among others. The aforementioned ionic liquids provide high hydrophobicity, relatively low viscosity and poor coordinating ability 31 ; this latter advantage would not allow these ionic liquids to extract metal ions by themselves. However, in those previous works, the ionic liquids were used only as diluents in combination with organic extractants. Recently, the use of task-specific ionic liquids (TSILs) as extractants has gained attention due to the high-produced distribution coefficients, as well as potential selectivity in the SX process. Normally, in these works, the anion of an ionic liquid is functionalized with a specific group to extract metal species. For instance, the ionic liquids tetrabutylammonium bis(2ethylhexyl)-phosphate, [N4,4,4,4][D2EHP] and tetraoctylammonium bis(2-ethylhexyl)3 ACS Paragon Plus Environment


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phosphate, [N8,8,8,8][D2EHP] were used as the extractants to recover lithium

32

trihexyl(tetradecyl)phosphonium benzoate [P6,6,6,14][BA] was used to extract rare earths

33-

;

35

; and trihexyltetradecylphosphonium 2-(propylthio)benzoate [P6,6,6,14][PTB] and 2-

(benzylthio)benzoate [P6,6,6,14][BTB] were reported to extract transition metals

36

and

diketanoate-anion-based ionic liquids for the extraction of Eu(III) and Am(III)37. Most of these studies have used neat TSILs without a diluent, and a few others have used TSILs in combination with molecular diluents producing high values of extraction percentage. In these cases, the stripping of the extracted metal from the ionic liquid phase was difficult because the metal ion solvated by the TSIL showed a high stability in the diluent, limiting the use of these ionic liquids for industrial applications. More recently, a new kind of extraction of metal ions, using TSILs diluted in ionic liquids as diluent, has attracted attention because this extraction system is free of organic solvents

38,39

. In our previous

work, it was demonstrated that the right selection of an ionic liquid as diluent could modify the distribution coefficient of the SX process facilitating the stripping of the metal

40

.

Therefore, the use of TSILs diluted in RTILs would create a tunable extracting solution. Therefore, the main objective of this work was to assess the effect of the cation and the anion of synthetized TSILs used as the extractants on the SX of Mo(VI) diluted in different diluents. For this purpose, two ammonium and two phosphonium-based TSILs were synthesized: trioctylmethylammonium bis(2-ethylhexyl) phosphate [TOMA][D2EHP]; trioctylmethyl ammonium benzoate [TOMA][BA]; trihexylthetradecylphosphonium bis(2ethylhexyl) phosphate [P6,6,6,14][D2EHP]; and trihexylthetradecylphosphonium benzoate [P6,6,6,14][BA]. These TSILs were selected because of their high hydrophobicity and because the anions [D2EHP]- and [BA]- provide a coordinating ability31. These ionic liquids were dissolved in three diluents: kerosene and two RTILs, i.e. 1-butyl-3-methylimidazolium 4 ACS Paragon Plus Environment

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bis(trifluoromethylsulfonyl)imide

[bmim][Tf2N]

and

1-octyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide [omim][Tf2N]. Additionally, the solvent extraction mechanism and the stripping of the Mo(VI) from the loaded phase were determined.



2.

Experimental procedure

2.1

Materials, apparatus and sample preparation

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For the synthesis of the TSILs, methyltrioctylammonium chloride (≥97% purity), trihexyltetradecylphosphonium chloride (≥95% purity) and bis(2-ethylhexyl) phosphoric acid (≥97% purity) were purchased at Sigma-Aldrich® and used without further purification. Benzoic acid (≥99.7% purity) was obtained from Winkler Chile©. The molybdenum solution was prepared from sodium molybdate dehydrate with a purity higher than 99% (Sigma-Aldrich®). The diluents used in this work were 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (≥99% purity) and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (≥99% purity) which were purchased at Iolitec GmbH. Kerosene (Shellsol®) was kindly provided by Minera Michilla Chile©.

An analytical balance with a precision of ±0.0001g (Denver Instruments Company, model AA200) was used to prepare the solutions. pH measurements were performed using a pH meter from Hanna Instruments® (Model HI 4212) with a HI 1083B microelectrode also from Hanna Instruments®. The equipment resolution is ±0.001. The concentration of molybdenum in the aqueous solutions before and after extraction was quantified by an Atomic Absorption Spectrophotometry (AAS) (GBC® Scientific Equipment model SensAA dual beam, equipped with a 4 mA lamp (single element) Photron International®). The equipment resolution is ± 0.1 mg/L. The water content in the ionic liquid before and after the extraction experiments was quantified using a Karl Fischer Titrator model 831 Coulometer. Finally, after the synthesis of the TSILs, 1H,

13

C NMR and 31P NMR spectra

were obtained on a Bruker AM-400 spectrometer. The chemical shifts were expressed in


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ppm (11.74 T, 400MHz to 1H and 126 MHz to 13C) downfield from TMS. A Raleigh Model UV 1800 UV-Vis device was used for the quantification of sulfate ions. The aqueous solution was prepared by dissolving molybdenum salt in MilliQ water (18.2 mΩ) in a volumetric flask. The pH of this solution was adjusted by adding diluted sulfuric acid. The organic phase was prepared by mixing the ionic liquids used as extractants in kerosene or [omim][Tf2N]. The concentration of the extractant in the diluent was measured using an analytical balance.

2.2

Synthesis of the ionic liquids

The synthesis of the TSILs was carried out by means of ion exchange reactions at 20°C according to previously reported methodologies 41. The ionic liquid [TOMA][D2EHP] was synthetized in two steps. Firstly, sodium hydroxide and trioctylmethylammonium chloride were separately dissolved in 2-propanol and then mixed in a round bottom flask. This solution was then stirred for 24 hours at 25°C. The obtained product of this reaction was [TOMA][OH]. Subsequently, this solution was filtered in a Celite Filter. Secondly, a solution of bis(2-ethylhexyl) phosphoric acid was equilibrated with 2-propanol and then mixed with the filtered [TOMA][OH] solution in a 1:1 mole ratio. The mixture was stirred for 24 hours in a round bottom flask. Finally, the solvent was evaporated in a rotary evaporator at 80°C and 100 mbar. The obtained product was [TOMA][D2EHP] as a yellowish viscous liquid.

The synthesis of [TOMA][BA] was conducted in a single reaction step. First trioctylmethylammonium chloride (1 eq) and sodium benzoate (1.03 eq) were mixed in a 7 ACS Paragon Plus Environment


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MilliQ water/2-propanol mixture (50/50 v/v). This mixture was stirred for 24 hours at 30°C. After this step, the solvent was evaporated in a rotary evaporator at 80°C and 60 mbar. Finally, the obtained ionic liquid was dissolved in chloroform and then refluxed five times with MilliQ water. The chloroform was evaporated in a rotary evaporator at 80°C and 200 mbar. On the other hand, the synthesis of [P6,6,6,14][BA] was conducted by dissolving tryhexyltetradecylphosphonioum chloride (1 eq) in a mixture of MilliQ water/2-propanol (70/30 %v/v). Also, sodium benzoate (1.03 eq) was dissolved in a mixture of MilliQ water/2-pronapol (50/50 %v/v). Once both salts had been dissolved, they were mixed in a round bottom flask and stirred for 24 hours at 30°C. After that, the solvent was evaporated in a rotary evaporator at 80°C and 60 mbar. Finally, the resulting ionic liquid was dissolved in chloroform and washed with 5 volumes of MilliQ water. The chloroform was evaporated at 80°C and 2400 mbar. The obtained ionic liquid was a transparent and viscous liquid. Finally,

to

prepare

[P6,6,6,14][D2EHP],

sodium

hydroxide

and

tryhexyltetradecylphosphonioum chloride were separately equilibrated in 2-propanol. Once dissolved, these solutions were mixed and stirred for 24 hours at 25°C. After this step, the resulting solution was filtered using a Celite filter. Along with this, a solution (1eq) of bis(2-ethylhexyl)phosphate acid in 2-propanol was also prepared. The filtered solution and the latter one were mixed and stirred for another 24 hours at 25°C. Finally, the solvent was evaporated in a rotary evaporator at 80°C and 100 mbar.

[TOMA] [D2EHP] 1

H NMR (400 MHz, CD3COCD3, ppm): 0.90-1.0 (m, 21H, 7CH3), 1.25-1.35 (m, 46H,

23CH2), 1.40-1.45 (m, 2H, 2CH), 1.50-1.65 (m, 6H, 3CH2), 3.29 (s, 3H, NCH3), 3.50-3.60 8 ACS Paragon Plus Environment

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(m, 6H, 3N-CH2), 3.80-3.85 (m, 4H, 2OCH2).

13

C NMR (100 MHz, CD3COCD3, ppm):

10.54, 13.51, 13.57, 22.44, 28.83, 28.99, 29.02, 29.21, 31.65, 40.22, 40.29, 47.98, 61.26, 62.56, 67.76. 31P NMR (160 MHz, CD3COCD3, ppm): 1.75 (s). Yield: 93.7% w/w, AgNO3 test: positive but with negligible quantities of AgCl.

[TOMA] [BA] 1

H NMR (400 MHz, CD3COCD3, ppm): 0.85-0.90 (m, 12H, 4CH3), 1.20-1.40 (m, 44H,

CH2), 1.60-1.75 (m, 6H, 3CH2), 3.34 (s, 3H, N-CH3), 3.54-3.62 (m, 6H, 3N-CH2), 7.207.25 (m, 3H), 8.00-8.10 (m, 2H).

13

C NMR (100 MHz, CD3COCD3, ppm): 13.49, 22.41,

28.61, 28.80, 28.96, 29.00, 29.19, 29.38, 31.61, 47.82, 61.06, 126.62, 127.86, 129.45, 142.05, 169.31. Yield: 84,7% w/w, AgNO3 test: negative.

[P6,6,6,14] [BA] 1

H NMR (400 MHz, CD3COCD3, ppm): 0.85-0.95 (m, 12H, 4CH3), 1.25-1.55 (m, 48H),

2.20-2.30 (m, 8H, 4PCH2), 7.15-7.20 (m, 3H), 7.95-8.00 (m, 2H).

13

C NMR (100 MHz,

CDCl3, ppm): 13.72, 13.80, 13.91, 18.49, 18.96, 21.52, 21.57, 22.123, 22.21, 22.48, 28.75, 29.10, 29.15, 29.33, 29.44, 29.47, 30.10, 30.25, 30.43, 30.57, 30.86, 31.11, 31.71, 127.08, 128.91, 129.25, 139.50, 171.48. 31P NMR (160 MHz, CD3COCD3, ppm): 33.26 (s). Yield: 84,3% w/w, AgNO3 test: negative.

[P6,6,6,14] [D2EHP] 1

H NMR (400 MHz, CD3COCD3, ppm): 0.85-0.95 (m, 24H, 8CH3), 1.25-1.55 (m, 66H),

2.50-2.60 (m, 8H, 4PCH2), 3.80-3.85 (m, 4H, 2OCH2). 13C NMR (100 MHz, CD3COCD3, ppm): 10.58, 13.45, 13.59, 21.37, 22.23, 22.95, 23.33, 28.60, 28.79, 28.98, 29.18, 29.37, 9 ACS Paragon Plus Environment


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29.49, 29.55, 30.13, 30.46, 30.96, 40.35, 40.43, 67.19, 67.25.

31

P NMR (160 MHz,

CD3COCD3, ppm): 1.89 (s), 33.29 (s). Yield: 90.7% w/w, AgNO3 test: positive but with negligible quantities of AgCl.

Figure 1 shows the structure of the synthetized TSILs which exhibit two important characteristics. Firstly, the cations [TOMA]+ y [P6,6,6,14]+ (see Figures 1a and 1b) are aliphatic as they contained long alkyl chains providing good hydrophobicity. Secondly, the anion [D2EHP]- was synthetized from the organic extractant bis(2-ethylhexyl) phosphoric acid (D2EHPA) which represented a suitable extractant for Mo(VI)3. This last structure, described in Figure 1c, showed two parts: the two aliphatic chains and a coordinating group corresponding to the phosphate. On the other hand, the anion [BA]- (see Figure 1d) was selected because this molecule also showed a hydrophobic part given by the benzene group and the coordinating part provided by the carboxylate group.

(a)

(c)


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(b) (d)

Figure 1. Molecular structure of the corresponding cations and anions of the TSILs used in this work: (a) trioctylmethylammonium [TOMA]+; (b) trihexyltetradecyl [P6,6,6,14]+; (c) bis(2-ethylhexyl) phosphate [D2EHP]- and (d) benzoate [BA]-.



2.3

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Solvent extraction experiments

Solvent extraction experiments were carried out at 293K, measured with a thermocouple with a resolution of 0.1K, in an open flask where 2 mL of aqueous solution containing Mo(VI) and 2mL of the organic phase were mixed and stirred for 40 minutes in order to reach the equilibrium condition. After that, another 40 minutes of centrifugation were required to achieve a complete phase splitting. All the SX experiments were carried out in duplicate. The concentration of the remaining Mo(VI) in the aqueous phase was quantified by AAS. The concentration of molybdenum in the organic phase was determined through a mass balance. Simultaneously, the initial and the equilibrium pH were measured during each of the SX tests. The solvent extraction performance can be quantified through the extraction percentage and the distribution coefficient as follows:

% =

+ /

=

where and

(1)

(2)

are the volume of the organic and aqueous phases, respectively.

Finally, after the extraction assays, the stripping of molybdenum from the loaded phase was carried out by stirring the organic phase and the stripping solution of ammonium carbonate in an open flask for 40 minutes. The final concentration of Mo(VI) in the stripping solution was determined by AAS.


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3.

Results and discussion

3.1

Effect of the initial pH on the extraction percentage

A leaching of the molybdenite generates an acid solution of Mo (VI) with pH values between 0 and 0.5 2 where, mostly, the cation is present in aqueous solution

42-44

,

and it can be extracted to different extents depending on the pH value. Figure 2 shows the extraction percentage of Mo(VI) as a function of the initial pH when the TSILs [TOMA][D2EHP] and [TOMA][BA] were used as extractants and kerosene used as the diluent.

It can be observed that both TSILs allowed the obtaining of high extraction percentages in the whole range of initial pHs. Nevertheless, the extractions improved when the initial pH was nearly at a value of 1 for both tested extractants. However, the difference between the extraction percentages obtained with both extractants can be considered negligible in the studied pH range. In spite of this, [TOMA][BA] showed better extractions than [TOMA][D2EHP].



100.0

99.9

(%)E

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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99.8

99.7 [TOMA][D2EHP] [TOMA][BA] 99.6 0.6

0.9

1.2

Initial pH

Figure 2. Extraction percentage of Mo(VI) as a function of the initial pH using two different TSILs as the extractants and kerosene as the diluent. Initial concentration of Mo(VI) in the aqueous phase was equal to 1.0 g/L. Maximum uncertainties are ±0.7% for [TOMA][D2EHP] and ±0.2% for [TOMA][BA] with 95% confidence level.

3.2

Effect of the cation and the anion on the extraction percentage

In the previous results, it was observed that the anion [BA]- showed the best values of extraction percentages. In Figure 3 the effect of the anion and the cation of the extractant on the extraction percentage can be analyzed. Furthermore, the effect of the three different diluents on the extraction percentage can be assessed in the same figure.


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kerosene

100

[omim][Tf2N]

[bmim][Tf2N]

80

(%)E

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0

[TOMA][D2EHP]

[TOMA][BA]

[P66614][D2EHP]

[P66614][BA]

Figure 3. Effect of the cation, the anion and the diluent on the extraction percentage. Initial concentration of Mo(VI) = 1g/L, initial pH = 1 and the extractant concentration was 1%v/v. Maximum uncertainty ±11% with a confidence level of 95%.

The results reported in Figure 3 show that high extraction performances were observed for all the synthesized ionic liquids showing high ability to extract molybdenum. Therefore, these TSILs could be promising extractants for future industrial applications. On the other hand, when kerosene was used as the diluent, all the extractants showed the highest extraction percentages with negligible differences between them. Next, when replacing kerosene with the ionic liquid [omim][Tf2N], the extraction percentages decreased and the differences between the performance of each extractant became more notorious. As observed from the entire results reported in Figure 3, the best extractant was [TOMA][BA] followed by [TOMA][D2EHP]. Therefore, the more favorable cation was [TOMA]+. On the other hand, slightly lower extraction percentages were obtained with phosphonium ionic liquids when compared to ammonium ones, where the anion [BA]-



Page 16 of 38

showed the highest extraction percentages. Finally, when using [bmim][Tf2N] as the diluent, more marked differences between the TSILs could be observed where the best extraction percentages corresponded to the extractant [TOMA][D2EHP] and no extraction was obtained with [TOMA][BA]. The same trend was observed for the cation [P6,6,6,14]+ where [D2EHP]- showed better results than [BA]-. Thus, in order of extraction percentages, [TOMA]+ > [P6,6,6,14]+ and [BA]- > [D2EHP]-, except for the diluent [bmim][Tf2N] where [D2EHP]- > [BA]-. Although, the ammonium based TSILs showed the best extraction performances, when selecting an extractant for a hydrometallurgical process, other technical considerations should be taken into account. For instance, viscosity and interfacial tension.

3.3

Effect of the diluent on the extraction percentage

A second examination of the results reported in Figure 3 indicates that when replacing the diluent from kerosene to the ionic liquids, a marked decrease in the extraction percentage was observed. Hence, the diluent exhibited a strong effect on the extraction percentage due to the interaction between the extractant and the diluent. In the first case, the diluent was kerosene which is mainly a mixture of several aliphatic compounds. This diluent could exhibit a weak interaction with a more polar compound such as a TSIL. In the second case, the diluent [omim][Tf2N] shows more polar behavior than kerosene. Then, there would have been a stronger interaction between the extractant and the diluent decreasing the extraction percentage. Finally, the diluent [bmim][Tf2N] was even more polar than [omim][Tf2N] due to the short aliphatic chain of the [bmim]+ cation. This resulted in a very strong affinity between the extractant and the diluent that could be observed through the significant decrease in the extraction percentage in Figure 3. 16 ACS Paragon Plus Environment

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To provide a better understanding about these interaction effects between extractant and diluents, COSMO-RS calculations (Conductor Like Screening Model for Real Solvents) were performed to obtain the density charge distribution around these molecules in order to quantify how polar they are by means of the σ-profiles 45. The calculations were performed using COSMOThermX® software version C30_1601. The peaks presented in the σ-profiles plots allowed the identification of whether a chemical species was either polar or non-polar. If the peak was tall, narrow and located in the center of the chromatogram then the compound was non-polar because the same density charge distribution was found along the molecule. However, if a molecule showed two peaks far from the center of the diagram then it was a polar molecule 46. Figure 4 depicts the σ-profiles for the system [TOMA][BA] – kerosene and Figure 5 shows the σ-profiles for the system [TOMA][BA] – [omim][Tf2N] to describe the interaction between the extractant and the diluent.

80

60 +

P(σ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[TOMA] [BA] n-decane

40

20

0 -0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

σ (e/A^2)

Figure 4. σ-profiles for the system [TOMA][BA] – kerosene.



In Figure 4 the σ-profiles of n-decane were taken as a representative molecule for kerosene which is a mixture of aliphatic compounds (C6 – C16). This profile exhibited a tall peak near the center of the diagram (-0.001 e/A2) showing the highly non-polar nature of this molecule. The ionic liquid [TOMA][BA], though, showed two peaks: a tall peak around 0.002 e/A2, corresponding to the positive charge of the cation; and a little peak near to 0.006 e/A2, corresponding to the benzoate anion. To have a high interaction and thus a decrease in the extraction performance, two molecules should exhibit the same peaks but with opposite signs, and this was not the case. Therefore, the slight interaction between [TOMA][BA] and n-decane can explain these high extraction percentages.

80 +

[TOMA] [BA] + [omim] [Tf2N]

60

P(σ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

20

0 -0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

σ (e/A^2)

Figure 5. σ-profiles for the system [TOMA][BA] – [omim][Tf2N].

In Figure 5 it can be observed that the ionic liquid [omim][Tf2N] shows two distinct peaks: one at -0.002 e/A2, corresponding to the [omim]+; and the other at 0.011 e/A2 of [Tf2N]-. In


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this case, a stronger interaction could be predicted than the previous case because the peaks of the [Tf2N]- and the peak of [TOMA]+ are of similar value. However, the peak of [omim]+ was not similar to [BA]-, allowing for high extraction percentages. Finally, when the ionic liquid [omim][Tf2N] was replaced with [bmim][Tf2N] as the diluent, more interaction between this and [TOMA][BA] was expected because the [bmim]+ provided more polarity than [omim]+ resulting in a strong affinity with the [BA]and thus a decrease in the extraction percentage.

3.4

Solubility of water in the synthetized ionic liquids

The hydrophobic character of the synthetized ionic liquids was tested by means of additional solvent extraction experiments. Table 1 reports these results, which summarize the water content quantified in each organic phase before and after extraction. Table 1. Water content of the extractant solutions measured before and after the extraction process for 1.4% (v/v) of extractant. Uncertainties are expressed with a confidence level of 95%. Extractant solution

Sample

Water content (%V/V)

before

0.02 ± 0.0008

after

0.16 ± 0.003

before

0.06 ± 0.001

after

1.28 ± 0.03

kerosene – [TOMA][D2EHP]

[bmim][Tf2N] – [TOMA][D2EHP]

From the results shown in Table 1, it can be observed that the initial water content of both samples was quite low and this percentage slightly increased after the solvent extraction



Page 20 of 38

experiments. The hydrophobic nature of kerosene was noticed in Table 1 with the lowest water content before and after the extractions. In this extractant solution, kerosene was the major component explaining the marginally higher water content after the extraction. In the second solution, a slightly higher water content was measured after the extraction. Nevertheless, the water absorption in the ionic liquid phase was low; the system can be considered hydrophobic and the water co-extraction should not affect the extraction performance of Mo(VI).

3.5

Solvent extraction mechanism

Several studies have proposed the extraction stoichiometry when ammonium based ionic liquids were used as extractant in metal extraction processes. In those studies, a solvation mechanism was reported when the ionic liquids were diluted in molecular diluents

32,35,38

.

However, the equilibrium pH of those works was around a value of 4. In this study, the initial pH was around a value of 0.5 to assure the extraction of the cation. At this low pH value, the ionic liquid extractant could undergo splitting forming the and species where the organic acid extracts Mo(VI). Therefore, a reasonable extraction mechanism for the ionic liquids used in this work, when diluted in kerosene, is: + 2 + ⇌ ∙ +

(3)

The experimental determination of the proposed stoichiometry involves the slope analysis where the distribution coefficient and the extraction equilibrium constant can be estimated by means of equations 4 and 5, respectively:


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=

$%& =

2 ∙ 2 "#

(4)

2 4"# ∙ ()

(5)

*# = 2 log + log + *# $%& − log

(6)

Therefore, to demonstrate this extraction mechanism, a slope with a value of 2 should be obtained when plotting the distribution coefficient as a function of the concentration of the TSIL at equilibrium. Figure 6 shows the slope analysis when the extractant solution was [TOMA][BA] – kerosene.



0.8

Log D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

0.4

Slope = 2.2 0.0

-2.6

-2.4

-2.2

Log [TOMA][BA]

Figure 6. Linear representation of eq. 4 to verify the extraction stoichiometry of Mo(VI) as a function of the concentration of [TOMA][BA] at equilibrium. Initial [Mo(VI)] = 0.83 g/L; initial pH = 0.65; average equilibrium pH = 0.74. Maximum uncertainty of [Mo(VI)] at equilibrium ±0.07 g/L with a confidence level of 95%. It can be observed in Figure 6 that the slope of the linear regression was close to 2.0 demonstrating the proposed extraction mechanism. In addition to this, the equilibrium pH is higher than the initial pH; a demonstration of the fact that [TOMA][BA] extracts acid prior to the extraction of Mo(VI). The next step considers the use of the ionic liquid [omim][Tf2N] as the diluent. Recent studies38,47 have demonstrated that the extraction with ammonium based ionic liquids of acidic extractants diluted in another ionic liquid as diluent, at low pH values, occurs in two steps. First, the cation extracts the mineral acid into the extractant phase and, second, the anion reacts with protons to produce the organic acid which, in turn, is a cationic extractant. 22 ACS Paragon Plus Environment

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Therefore, considering this last mechanism, the extraction of Mo(VI) in sulfuric acid media could occur as follows:

2 + 2 + ⇌ + 2

(7)

+ 2 ⇌ ∙ + 2

(8)

These two equilibrium reaction steps result in the following overall equilibrium reaction for the extraction of Mo(VI):

+ 2 + ⇌ ∙ +

(9)

Therefore, the slope analysis for this stoichiometry results in:

*# = 2 log + log + *# $%& − log

(10)

Equation 10 suggests that when plotting *# as a function of log , with an increasing extractant concentration, the slope of the experimental points should be 2. On the other hand, [omim][Tf2N] should not appear in the equilibrium equations since this ionic liquid does not have the ability to coordinate metal ions 26,31. Figure 7 shows the slope analysis when the ionic liquid is the extractant.



1.2

0.8

Log D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

Slope = 2.2

0.4

0.0

-0.4 -2.0

-1.8

-1.6

-1.4

log [TOMA][BA]

Figure 7. Linear representation of eq. 9 to verify the extraction stoichiometry of Mo(VI) as a function of the concentration of [TOMA][BA] at equilibrium when [omim][Tf2N] was the diluent. Initial [Mo] = 1.0 g/L; initial pH = 0.7. Maximum uncertainty of [Mo(VI)] at equilibrium ±0.02 g/L with a confidence level of 95%.

In Figure 7, it is noted that the slope of the linear regression was slightly higher than the expected value of 2. Therefore, with these results, the proposed extraction mechanism given by equation 9 was corroborated. However, this slightly higher value in the slope proposes that the organic acid, HBA, formed in the first step of the extraction mechanism, could be also extracting Mo(VI) to some extent. To corroborate this affirmation, additional solvent extraction experiments, using HBA solely as the extractant diluted in [omim][Tf2N],


Page 25 of 38

were carried out. Figure 8 shows the *# as a function of log for the extraction of Mo(VI).

-0,8 Slope = 0.6 -1,0 log D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1,2

-1,4 -2,2

-2,0

-1,8

-1,6

-1,4

Log [HBA]

Figure 8. Linear representation of eq. 8 to verify the extraction stoichiometry of Mo(VI) as a function of the concentration of [omim][Tf2N] at equilibrium. Initial [Mo] = 1.0 g/L; initial pH = 0.85. Maximum uncertainty of [Mo(VI)] at equilibrium ±0.04 g/L with a confidence level of 95%.

It can be observed that, the slope of the linear regression resulted in a value around 0.6 indicating that there is a synergistic extraction between the ionic liquid extractant [TOMA][BA] and the formed benzoic acid in the second step of the equilibrium reaction. This is beneficial for the solvent extraction of Mo(VI) from aqueous solutions. Finally, it is 25 ACS Paragon Plus Environment


Page 26 of 38

important to notice that, when using only 2 % v/v of [TOMA][BA] in [omim][Tf2N] it is possible to achieve an extraction percentage of 93%; this is very promising for the use of these kind of ionic liquids in future industrial applications. Another important point in this extraction mechanism is the consumption of sulfate ions. To corroborate this, measurements of sulfate concentration at equilibrium were performed using UV-Vis analysis. Results are shown in Table 2 and reported as extraction percentage (%E). Also in this table, the measured equilibrium pH values for the overall extraction reaction (equation 9) are reported. Table 2. Extraction percentages (% E) of sulfate ions corresponding to equation 9 and the equilibrium pH (7% ) as a function of the extractant concentration. Initial pH = 0.7. Uncertainties are expressed with a level of confidence of 95%. [TOMA][BA] %v/v

%(89:;?. Maximum uncertainty ± 28.7% with a confidence level of 95%.

Figure 9 shows the stripping percentages of Mo(VI) from the loaded extracting phase. In general, these organometallic complexes are stable in acidic conditions. Therefore, the application of a basic solution would make the complex unstable to finally release the Mo(VI) to the stripping solution. Moreover, this figure shows a very low molybdenum recovery when kerosene was used as the diluent. As shown above, the selection of the diluent plays an important role on the extraction percentage of Mo(VI). When kerosene was used as the diluent, very high extractions were obtained for all the extractants due to the low interaction kerosene-ionic liquid. Furthermore, it was experimentally demonstrated that Mo(VI) was extracted via 28 ACS Paragon Plus Environment

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solvation by the ionic liquids. Therefore, due to low interaction kerosene-extractant, the solvation is favorable and a difficult stripping was observed. On the other hand, better stripping percentages were observed when the diluents were the ionic liquids. With the ionic [omim][Tf2N] the extractions were lower than when kerosene was the diluent, also via solvation. However, in this case, the interaction between ionic liquid–ionic liquid is stronger than the previous case. Therefore, the solvation is less favorable and then the stripping was easier. As a conclusion, high extraction percentages in the SX stage result in poor stripping performances. This conclusion could provide new insights on the design of ionic liquids as extractants and as diluents for an optimal metal extraction-stripping by only knowing the extraction mechanism and the interaction extractant-diluent.



Page 30 of 38

4. Conclusions Solvent extraction of Mo(VI) was investigated using synthetized ammonium and phosphonium-based TSILs as extractants and commercially available imidazolium-based ionic liquids as diluents. In general, the TSILs produced high extraction percentages when these were diluted in kerosene and [omim][Tf2N]. Then, when comparing the extraction capacities of the tested TSILs, the best performances were observed for the cation [TOMA]+ and the anion [BA]when kerosene and [omim][Tf2N] were the diluents, and for the anion [D2EHP]- when [bmim][Tf2N] was the diluent. Furthermore, the selection of the diluent showed a significant influence in the solvent extraction performance. From the experiments carried out to establish the extraction stoichiometry, it was found that, when having an ionic liquid as extractant diluted in kerosene or [omim][Tf2N], the extraction process followed ion exchange mechanism. On the other hand, the stripping was carried out with a solution of ammonium carbonate 1 [M] and the extent of recovery of Mo (VI) in the stripping process also depended on the diluent selected in the solvent extraction step. Finally, this work provides new insights on the use and selection of ionic liquids as extractants and diluents for the recovery of metal ions in hydrometallurgy.


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Acknowledgements Financial support from Project PAI 79140047 (CONICYT Chile) and Project RC-130006CILIS, granted by Fondo de Innovación para la Competitividad del Ministerio de Economía, Fomento y Turismo, Chile, is kindly acknowledged.



Page 32 of 38

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Task-specific ionic liquids as extractants for the solvent extraction of

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