Accepted Manuscript Solvent extraction of Molybdenum (VI) from aqueous solution using ionic liquids as diluents E. Quijada-Maldonado, M.J. Torres, J. Romero PII: DOI: Reference:
S1383-5866(16)31365-X http://dx.doi.org/10.1016/j.seppur.2016.12.045 SEPPUR 13456
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
11 August 2016 24 November 2016 28 December 2016
Please cite this article as: E. Quijada-Maldonado, M.J. Torres, J. Romero, Solvent extraction of Molybdenum (VI) from aqueous solution using ionic liquids as diluents, Separation and Purification Technology (2016), doi: http:// dx.doi.org/10.1016/j.seppur.2016.12.045
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Solvent extraction of Molybdenum (VI) from aqueous solution using ionic liquids as diluents E. Quijada-Maldonado *, M. J. Torres, J. Romero
Laboratory of Membrane Separation Processes (LabProSeM), Department of Chemical Engineering, University of Santiago de Chile, Av. Lib. Bdo. O’Higgins 3363, Estación Central, Santiago – Chile. E-mail: [email protected]
Abstract This work deals with the recovery of Molybdenum (VI) (Mo) from aqueous solutions carried out by solvent extraction (SX) using the bis(2-ethylhexyl) phosphoric acid (D2EHPA) diluted in kerosene and compared with two hydrophobic room temperature ionic liquids (RTILs) as potential replacements of the kerosene. These RTILs were 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide, [bmim][Tf2N], and 1octyl-3-methyl-imidazolium
[omim][Tf2N], because of its excellent properties such as negligible vapor pressure, high hydrophobicity and better extraction capacity than conventional organic diluents. Experimental results indicate that the
extraction stoichiometry agrees with a cation exchange with formation of a neutral complex. The use of the selected RTILs as diluents involves higher extraction percentages for the whole measured range of extractant concentration and aqueous-organic ratios, reducing four times the required volume of diluent compared to the experiments carried out with kerosene. Furthermore, the proposed extraction medium offers excellent stripping capacity when a solution of ammonium carbonate is used as a stripping phase. Keywords: Molybdenum; solvent extraction; ionic liquids
Molybdenum (VI) is a high valuable metal, which is used in steel and iron alloys to improve the resistance to corrosion, high temperatures and bring about hardenability and weldability. Molybdenum is also used as an additive to automotive oils or to produce molybdate-based pigments. This metal specie is commonly recovered from leach liquors by means of the solvent extraction process. The extraction medium currently used in the hydrometallurgical industry consists of bis(2-ethylhexyl) phosphoric acid (D2EHPA) as the extractant and kerosene as the diluent at low pH values [1,2]. However, kerosene is volatile, flammable and toxic; characteristics that involve many environmental and health issues. Recently, ionic liquids have been proposed as a potential replacement of those volatile organic diluents due to their excellent properties such as hydrophobicity, nonflammability and negligible vapor pressure. This latter advantage could allow the recycling of the ionic liquid many times without decreasing the extraction performance of the process .
Ionic liquids have been already used as diluents in the solvent extraction of copper [4-7], vanadium , uranium [9-12] and other metal ions [13-17].
Besides that, some ionic liquids have been used as extractants in the extraction of rare earths with promising results. It has been demonstrated that the use of ionic liquids as diluent exhibits better extraction performance than common organic solvents, increasing the distribution coefficients [19,20]. This high extraction efficiency represents a very attractive advantage for future industrial applications, since it could allow reduction in the volume of diluent involved in the solvent extraction process, along with the possibility of recycling. To our knowledge, studies of solvent extraction of molybdenum using ionic liquids as diluents are not reported in the literature.
Therefore, the main goal of this work is to compare the commonly used organic diluent kerosene with the ionic liquids 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf2N] and 1-octyl3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [omim][Tf2N] as diluents in the solvent extraction of Molybdenum (VI) from aqueous solutions using the extractant bis(2-ethylhexyl) phosphoric acid (D2EHPA). These ionic liquids were selected because of their excellent immiscibility with water (1.6w/w% and 0.91w/w% respectively ) and their poor
ability to extract metal ions  by themselves. Furthermore, the solvent extraction stoichiometry has been determined and the operating variables such as extractant concentration and the aqueous-organic ratio (A/O) have been studied. Finally, several stripping agents were assessed for removal of the molybdenum from the loaded ionic liquid phase.
Materials and sample preparation
For the solvent extraction experiments, sodium molybdate dehydrate with a purity of 99% was supplied by Sigma-Aldrich®. The ionic liquids 1-butyl3-methylimidazolium bis(trifluoromethylsulphonyl)imide and 1-octyl-3methylimidazolium bis(trifluoromethylsulphonyl)imide were purchased at Iolitec GmbH, both with a purity higher than 98%. These ionic liquids were used without further purification. The extracting agent bis(2-ethylhexyl) phosphoric acid (D2EHPA) was supplied by Sigma-Aldrich® with a purity higher than 97% w/w and also used without further purification. The phase modifiers 1-dodecanol, with a purity higher than 98% w/w, and tributyl phosphate (TBP), with a purity higher than 99% w/w, were obtained from
Fluka Analytical®. Kerosene (Shellsol®) and were kindly provided by Minera Michilla Chile©.
An aqueous solution was prepared by dissolving an amount of the molybdenum salt in MilliQ water (18.2 mΩ). The pH of this solution was adjusted by adding small amounts of diluted sulfuric acid. The desired concentration of the salt was reached by using an analytical balance. The organic phase was prepared by mixing D2EHPA with the corresponding diluent. The concentration of the extractant in the diluent was measured using an analytical balance. Phase modifiers were required for extraction experiments with kerosene and [bmim][Tf2N], using 1-dodecanol and TBP, respectively. Finally, the striping experiments were carried out using three different stripping agents: Sulfuric acid with purity higher than 98.1% (J.T. Baker); an ammonia solution of 26% w/w from Merck; and ammonium carbonate salt with analytical grade from Merck. These agents were used to prepare three different aqueous stripping solutions.
Experimental procedure of solvent extraction assays
Solvent extraction experiments were carried out in an open flask where the aqueous and the different organic phases were mixed and stirred for 40 minutes to assure the appropriate equilibrium condition. After the solvent extraction was performed, the phase separation was carried out in a laboratory centrifuge for another 40 minutes in order to make sure of the phase separation. After this step, the concentration of the metal ions in the aqueous phase was quantified by Atomic Absorption Spectrophotometry (AAS) (GBC® Scientific Equipment model SensAA dual beam, equipped with a 4 mA lamp (single element) Photron International®). The concentration of the metal ions in the organic phase was estimated through the mass balance. Simultaneously, initial and equilibrium pH were measured during the solvent extraction tests. Therefore, the solvent extraction performance can be quantified through the extraction percentage and the distribution coefficient defined as follows:
where Vorg and Vaq represent the volume of the organic and aqueous phase, respectively. Finally, the stripping of molybdenum from the loaded organic phase was carried out by mixing and stirring the loaded organic phase with the stripping solution in an open flask for 40 minutes. The final concentration of Mo(VI) in the aqueous solution was also quantified by AAS.
2.3 σ-Profiles To assess the interaction between the diluent and the extractant, σ-Profiles were generated using the software COSMOThermX version C30_1602. These profiles were generated independently for each molecule and then the profiles compared. The BP_TZVP_C30_1402 parameterization was employed for the COSMO-RS calculations.
Preliminary results of extraction tests: third phase formation
Preliminary extraction assays allowed the observing of the formation of an undesirable third phase when the [bmim][Tf2N] phase was contacted with the aqueous solution containing Mo(VI). This third phase is observed as a supernatant volume in Figure 1a. This third liquid phase was not observed when [omim][Tf2N] was used as diluent (Figures 1b and c). This phenomenon can be explained by the low solubility of the formed organometallic complex during the extraction process in [bmim][Tf2N]. The low solubility value of the organometallic complex in the IL could be justified by the highly non-polar nature of the complex compared to a polar nature of this diluent. Therefore, the addition of a phase modifier was required for experiments using this IL. Previous works have pointed out that the tributyl phosphate (TBP) is good as a phase modifier, which facilitates the extraction of metals, particularly when D2EHPA is used as an extractant [23-25].
Figure 1. Third liquid phase formation for the solvent extraction of Mo(VI) for a) D2EHPA + [bmim][Tf2N], b) D2EHPA + TBP + [bmim][Tf2N] and c) D2EHPA + [omim][Tf2N].
The addition of TBP allowed the dissolution of the complex in the IL phase (see Figure 1.b). On the other hand, when [omim][Tf2N] was used as solvent, no third phase formation was observed during the extraction tests, and the addition of TBP was not required. The better solubility of the organometallic complex in [omim][Tf2N] can be explained by the high nonpolar character of the cation in this IL due to there being a longer alkyl chain than with [bmim][Tf2N].
Table 1 shows the minimum TBP concentration required to avoid formation of the third phase during extraction with [bmim][Tf2N]. This table shows that the required concentration of TBP increases for higher initial concentration of Mo(VI).
Table 1. Volumetric ratio between phase modifier TBP and the extractant D2EHPA in [bmim][Tf2N]. Mo (VI) [M] 0.001 0.01 0.05
TBP/D2EHPA ratio (v/v) 1/1 1.5/1 2.5/1
Effect of the initial pH value on the extraction performance
Extraction tests with kerosene and both imidazolium-based ILs were carried out in order to assess the effect of the initial pH in the aqueous solution on the extraction efficiency. Figure 2 shows the extraction percentage value as a function of the initial pH measured in the aqueous phase containing Mo(VI). From this figure, it is possible to see that the optimum initial pH value for both diluents ranged between 1 and 1.2. This fact confirms that the extractant D2EHPA shows the best performance at this pH range.
Figure 2. Effect of pH on the extraction percentage of Mo(VI) using kerosene as diluent (closed circles) and [bmim][Tf2N] as diluent (open circles). Initial concentration [Mo] = 0.013M.
Mo (VI) exhibits many species in the aqueous phase, depending on its pH value . At very acid conditions, with pH values varying from 0 to 1, the predominant specie is the molybdenile cation (MoO2+2), and with a slight increase in pH, the molybdic acid (H2MoO2) is formed. The selected extractant in this work is able to extract both species at pH = 1 .
Solvent extraction stoichiometry
Figure 3 shows the solvent extraction stoichiometry of Mo (VI) when using kerosene and [bmim][Tf2N] as diluents. The extraction stoichiometry using [omim][Tf2N] was not considered because this IL should exhibit the same stoichiometry as when kerosene was used as diluent.
0.8 0.4 0.0 -0.4
log D = 2 Log([HR]2+pH) + 1.8
Log D = 1.9Log[HR]2 + 3.3 0.6
-1.5 Log [HR]2
Log D = 2.1pH -0.68
Figure 3. Calculations of Log D (equation 2) for the determination of the solvent extraction stoichiometry using two different diluents: a) kerosene; b) and c) [bmim][Tf2N]. Initial concentration of Mo(VI) was a) 0.05M and b) 0.06 M.
Three possible extraction stoichiometries for solvent extraction of Mo(VI) have been explained in the literature . Since, the extractant D2EHPA is a weak organic acid, the extraction stoichiometry of cation exchange with a formation of a neutral complex could be expected. Figure 3a shows that the slope of the regression line is close to a value equal to 2 when keresone is the diluent, which corresponds to the expected stoichiometry. This result is in agreement with previous works where acid alkyl phosphonic acids as
extractants were used [27,28]. Therefore, Equation 3 represents the extraction stoichiometry of Mo(VI) with kerosene as the diluent:
For the second diluent, [bmim][Tf2N], the modifier TBP was required to avoid a third phase formation and the concentration of this compound depends on the initial concentration of Mo(VI). Then TBP could take part in the solvent extraction of Mo(VI). This coupled action is called ‘synergistic extraction’ and it has been reported in previous works for solvent extraction of several metal ions from aqueous solutions [25,29-31], identifying TBP as a part of the formed organometallic complex . Thus, the stoichiometry for the solvent extraction of Mo(VI) could be represented by equation 4, as follows:
From the linearization of this stoichiometry, equation 5 can be proposed:
where Kex represents the extraction equilibrium constant. Therefore, to demonstrate this stoichiometry, the independent variable can be either the extractant concentration at equilibrium, the equilibrium TBP concentration or the equilibrium pH. The slope of the line should be 2 for the three alternatives. In this case, the concentration of the extractant was varied at constant TBP concentration. In Figure 3b, it is observed that the slope of the linear regression is close to a value equal to 2. This demonstrates that the extraction mechanism is the same as when kerosene was used as the diluent. If this extraction mechanism is correct, also the equilibrium pH in the liquid phase should change when increasing the concentration of the extractant. In Figure 3c, a third set of experiments is shown. The plot of Log D as a function of the equilibrium pH at the liquid phase shows that the slope of the linear regression is also close to a value equal to 2 confirming the cation exchange with the formation of a neutral complex. It is worth noting that, the ionic liquid could take part in the extraction stoichiometry as it has been mentioned in a previous work . However, since the ionic
liquid is the diluent in the solvent extraction experiments, the concentration is much higher than the concentration of the extractant and the phase modifier. Therefore, any possible effect of the ionic liquid in the extraction stoichiometry cannot be identify by the slope analysis presented in this work and additional experiments would be necessary.
3.4 Effect of the concentration of D2EHPA
Figure 4 shows the effect of the extractant concentration on the extraction performance of Mo(VI). This figure reports the results of extraction percentage of Mo(VI) as a function of the concentration (%v/v) of D2EHPA in three different diluents.
Extraction percentage (%)
kerosene [bmim][Tf2N] [omim][Tf2N]
Figure 4. Effect of the extractant concentration on the extraction performance of Mo(VI) using the three different diluents. The initial concentration of Mo(VI) was 0.06[M] and the initial pH was 1.0.
From these results, it is observed that by only replacing the organic diluent kerosene by either [bmim][Tf2N] or [omim][Tf2N], the extraction percentage increases even at low extractant concentrations. It can be observed that higher values of extractions percentages are obtained with [bmim][Tf2N] as diluent rather than when using kerosene. This could be explained by the fact that the extractant D2EHPA is used along with TBP 19
when these ionic liquids are the diluent. TBP is also an extractant for Mo(VI) [33,34] and when it is used with D2EHPA there is a synergistic extraction of metal ions [30,35,36]. On the other hand, when [omim][Tf2N] is used as diluent, the extraction percentage increases to around 40% with regard to kerosene. This improvement in the extraction performance could be explained by the chemical structure and charges shown by ionic liquids, which exhibit higher dielectric constants than kerosene [37,38]. Table 2 shows the values of dielectric constants of kerosene and the ionic liquids used in this work.
Table 2. Dielectric constant of the diluents used in this work Diluent
*Predicted value from known values of internal pressure and cohesive energy
In previous works, it has been pointed out that higher dielectric constant results in higher distribution coefficients [39,40]. Nevertheless, other works 20
 have concluded that not only the dielectric constant could explain the increase in extraction percentage but also the interaction between the diluent and extractant would be an important effect as well. Thus, a high interaction between extractant and diluent would decrease the distribution coefficient. The interaction between the compounds in the organic phase can be described by means of the Conductor Like Screening Model for Real Solvents COSMO-RS developed by Klamt , specifically through the σprofiles. Figure 5 shows the σ-profiles for D2EHPA and the ionic liquid [bmim][Tf2N]. In the case of kerosene, the molecule n-decane was taken to perform the analysis due to kerosene being a mixture of the C6 – C16 aliphatic compound.
n -decane D2EHPA [bmim] [Tf2N]
Figure 5. σ-profiles of the different diluents and the extractant D2EHPA.
The non-polar molecules exhibit a narrow and high peak in the center of the diagram; meanwhile, polar molecules show two distinct peaks lying in the range of ≥ ± 0.01 e/A2 . The n-decane molecule displays a peak around -0.001 e/A2 near by the center of the diagram having a non-polar nature. The extractant D2EHPA also exhibits a non-polar behavior with a peak similar to the one obtained for n-decane (-0.002 e/A2); this is because of the two long alkyl chains attached to a phosphate group of the extractant (see
the chemical structure in Figure 6), giving the indication that D2EHPA presents a high affinity for n-decane and, therefore, for kerosene. On the other hand, the ionic liquid [omim][Tf2N] shows two peaks, the first one in -0.002 e/A2 corresponding to the [omim]+ cation, and the second one in 0.003 e/A2 corresponding to the [Tf2N]- anion. Thus, this IL exhibits a more polar nature than the extractant D2EHPA. Therefore, when the ionic liquid is used as diluent, there would be a weaker interaction with D2EHPA and higher capacity to extract metal ions than when kerosene is used as diluent. This provides the insight that these type of ionic liquids can be better diluents than the commonly used organic solvents
Figure 6. Chemical structure of bis(ethylhexyl) phosphoric acid (D2EHPA)
3.5 Effect of the volumetric A/O ratio The effect aqueous-organic ratio (A/O) on the extraction percentage is reported through the results shown in Figure 7. In terms of the extraction capacity, represented by the distribution coefficient, D, the best diluent is by far the ionic liquid [omim][Tf2N] followed by [bmim][Tf2N] for all the A/O ratios. The results of Figure 7 could be important for industrial applications, since for a required distribution coefficient, for example D = 0.8, the required volume of the [omim][Tf2N] is four times lower than the conventional diluent kerosene, decreasing the operational cost. It is important to notice that [bmim][Tf2N] is also a good alternative for replacing kerosene because this ionic liquid reduces in about two times the volume of diluent compared to the organic diluent to reach the same distribution coefficient. Besides that, this ionic liquid is less viscous than [omim][Tf2N]. However, an additional organic compound is needed to dissolve organometallic complex and the main idea is to reduce the amount of organic substances in the solvent extraction process.
0.6 0.4 kerosene [bmim][Tf2N [omim][Tf2N]
Figure 7. Effect of the volumetric A/O ratio on the distribution coefficient, D, for the three diluents used in this work. Initial pH = 1.0, initial concentration of Mo(VI) was 0.05[M].
3.6 Stripping of the Mo(VI)
A final experimental step was carried out in this work to test various stripping agents and to select the most efficient one. Table 3 summarizes the re-extraction percentages obtained for the different alternatives. The volumetric A/O ratio was 1.0 for all the stripping experiments.
Table 3. Re-extraction percentages obtained with the different stripping agents. The initial Mo(VI) concentration in the loaded organic phase was 0.9 g/L for [bmim][Tf2N], 1.0 g/L for [omim][Tf2N] and 0.83 g/L for kerosene. Stripping agents H2SO4, 1M H2SO4, 3M (NH4) CO3, 1M NH3, 0.5M
[bmim][Tf2N] 8.20 16.54 Third liquid phase White precipitate
Kerosene 10.22 23.70
From the results summarized in Table 3, two kinds of stripping agents were used: An acid solution of sulfuric acid; and a basic solution of ammonium carbonate or an aqueous solution of ammonia. The first solution was applied with the purpose of displacing the equilibrium to the reactants in equation 3 and dissociating the organometallic complex. However, the results in Table 3 show poor stripping efficiencies, which are due to the formed complex in the ionic liquid phase which seems to be very stable and with it being difficult to release the Mo(VI) to the aqueous phase. Nevertheless, with basic solutions of ammonium carbonate, good reextraction percentages were obtained. For instance, when [omim][Tf2N] was used as diluent, the best stripping agent is ammonium carbonate 1M, where 90% of the metal in ionic liquid phase was extracted back to the
aqueous phase in only one equilibrium stage. Previous works have already used solutions of ammonium carbonate to remove the metal ion from the organic phase with good results [2,44]. This is possible, basically, because at high pH values, the formed organometallic complex is not stable and the molybdate ions (MoO4-2) are released to the aqueous phase. On the other hand, diluted ammonia solutions were also evaluated as stripping agent. However, a white precipitate was formed being an unsuitable solution for stripping. Finally, after these stripping assays, it was observed that the aqueous phases were clear, no emulsions were formed and the phase separation was fast. For tests with [bmim][Tf2N], no results were shown in Table 3 because three liquid phases were formed after the stripping.
In this work the solvent extraction of Mo(VI) using D2EHPA as extractant agent was carried out with the goal of comparing the conventional diluent kerosene with two imidazolium-based ionic liquids: [bmim][Tf2N] and [omim][Tf2N]. The optimum initial pH was found to be 1.0 for all the diluents. Furthermore, the use of [bmim][Tf2N] required the addition of phase modifier TBP to avoid a third liquid phase formation. [omim][Tf2N] did not require a phase modifier to carry out the same tests. The solvent extraction stoichiometry was verified in agreement with a cation exchange mechanism with formation of a neutral complex for both kerosene and [bmim][Tf2N]. On the other hand, when the diluents were compared as a function of the operating variables, such as extractant concentration and A/O ratio, the best performance was obtained by [omim][Tf2N], which allows to reduce four times the amount of diluent required to achieve a certain Mo(VI) extraction. A diluted solution of ammonium carbonate allowed the stripping of 90% of the Mo(VI) from the loaded ionic liquid phase in one equilibrium stage when [omim][Tf2N] was the diluent. With these results, the ionic liquid
[omim][Tf2N] becomes a promising alternative in replacing the organic diluents in the SX process of Mo(VI).
Acknowledgements Financial support from Project PAI 79140047 (CONICYT Chile) and Project RC-130006-CILIS, granted by Fondo de Innovación para la Competitividad, del Ministerio de Economía, Fomento y Turismo, Chile, are kindly acknowledged.
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Effect of the ionic liquids as diluents on the solvent extraction of molybdenum.
Ionic liquids showed better extractions than the conventional diluent kerosene.
The re-extraction is possible using a basic solution of ammonium carbonate.