Separation of cobalt, nickel and copper with task-specific amido functionalized glycine-betaine-based ionic liquids
Graphical abstract
Introduction
In recent years, ionic liquids (ILs) have attracted significant interest in the field of metal extraction as an alternative medium in liquid-liquid extraction processes. ILs are salts composed of asymmetric organic cations, with melting point below than 100 °C. The proposal of ILs as environmentally friendly media to replace volatile organic solvents for liquid-liquid extraction, has been widely studied with the aim to develop “greener” extraction processes [1], [2]. Hydrophobic ionic liquids form biphasic systems with aqueous effluents and constitute an extracting phase for metals. However, conventional ionic liquids are constituted by weak chelating moieties, and have a low capability to dissolve metal salts. The use of chelatants [3], [4], [5], [6] or the design of Task-Specific Ionic Liquids (TSILs) [7], [8], [9], [10], [11], [12] by the introduction of chelating functions on the organic cation or the use of coordinating anions [13], [14], [15] can greatly increase the metals affinity for the IL phase, providing much greater extraction yields than those found with conventional molecular solvents. However, the development of green processes requires particular attention on metal extraction mechanisms, which must limit as much as possible the release of organic species into the aqueous effluent [16], [17], [18], [19], [20]. Indeed, the release of organic cations in the aqueous phase alters the quality of the effluents as well as the integrity of the ionic liquid, reducing its recyclability [21], [22]. In other words, the process must limit the cation exchange reactions, and exclusively favours ion-pairs extraction mechanisms. To reach this objective, extraction experiments in saline media were performed. The addition of an indifferent electrolyte favors the neutral extraction and increase the extraction yields of metals. The extraction in salt media is a credible path for the implementation of extraction process limiting the release of ionic liquids species in aqueous solutions, showing also a real environmental benefit. The use of saline environments has already been successfully proposed in combination with hydrophilic ionic liquids to generate aqueous biphasic systems (ABS) for the extraction and separation of strategic metals [23], [24], [25], [26], [27]. Previous studies have shown that saline media can be combined with hydrophobic ionic liquids to enhance metal extraction and to carry out selective metal separation [28], [29], [30]. The addition of a background electrolyte is also an effective way to erase the influence of the anion of the metal salt, in the extraction process. The nature of the electrolyte plays an important role in improving extraction efficiency. In a previous work [30], we highlighted the predominant role of the counter anion of the copper salt in the extraction process of Cu(II), by comparing the extraction yields of several copper salts with tetraalkylammonium dicyanamide salts. Copper sulfate is not extracted by ionic liquid phase whereas copper perchlorate is fully extracted. As it was already reported in our previous works [30] and those of Janssen et al. [31], the influence of the anion is closely correlated to its position in the Hofmeister series. The sulfate ion is located on the left side of the Hofmeister series and has a high hydration energy, estimated at −1080 kJ mol−1 [32], [33]. This anion interacts strongly with water molecules acting as a structuring agent, leading to “salting out” effect. For these reasons, sulfate ions are called kosmotropic, and will have a limited role in the extraction process. On the other hand, nitrate and perchlorate anions are positioned on the right side of the Hofmeister series [34]. These anions present a weak energy of solvation equal to −314 and −219 kJ mol−1 [32], respectively. Therefore, they are easily co-extracted with the metal cation in the ionic liquid phase which favors the extraction process by ion-pairing. These anions, which lead to “salting in” effect are called “chaotropic”, and allow the obtaining of high extraction yields. Here, we report the ability of task-specific ionic liquids, designed through association of cationic analogues of glycine-betaine (AGB-ILs) with coordinating anions (Fig. 1), to selectively extract Cu(II), Co(II) and Ni(II) in saline media. Therefore, the emphasis had been placed on the efficient separation of the mixture copper, nickel and cobalt.
The separation of Co(II) and Ni(II) is of industrial concern because of their presence in several electronic devices such as portable electronic products and electric vehicles. They are often present as nickel-metal hybride (Ni-MH) batteries, due to their high electrochemical performances. Considerable attention has been given to the recycling of used Ni-MH batteries because their random discharge can lead to serious environmental pollution [35], [36]. In this work, we propose to investigate the influence of chaotropic and kosmotropic electrolytes in extraction and stripping properties of ionic liquids to develop extraction and recovery processes of these metals.
The choice of the betaine derivatives is justified by their accessibility via simple synthetic route, the cost of starting materials, and its structural modularity, which allows the control of the hydrophobicity of cation by varying the alkyl chain length bound to either the ammonium group or carboxylate group [37], [38]. The choice of the anions is based on its hydrophobic and chelating nature. The saccharinate and chlorosalicylate anions [39], [40] are known for their complexing capacity towards heavy and first row transition metals. The dicyanamide anion, good chelator of metal ions [41], [42], [43], is among the cheapest anion commonly used to generate ILs.
Section snippets
Experimental section
Chemicals and reagents. All chemical and reagents used in this study were used as received without further purification. Sodium nitrate (99%) sodium chloride (99%), sodium saccharinate (98%), sodium dicyanamide (99%), and lithium bis(trifluoromethylsulfonyl)imide were obtained from Sigma-Aldrich. Sodium hydroxide (99%), n-butylamine (99%), n-octylamine (99%) and 4-chlorosalicylic acid (98%) were purchased from Acros Organics.
Physical measurements. Elemental analyses (C, H, and N) were carried
Results and discussion
Extraction of Cu(II), Ni(II) and Co(II). To examine the extraction properties of the synthesized ionic liquid towards Cu(II), Ni(II), Co(II) we determine the extraction yield (%E) from 0.05 mol L−1 solutions of the nitrate salts of these metals at 298 K. The extraction yields (%E) for each IL are depicted in Fig. 2.
The overall examination of Fig. 2 shows as expected that the influence of the anion on the extraction properties of ionic liquids follows the classical trends observed in our
Conclusion
Here, we have demonstrated the ability of task-specific ionic liquids, obtained by association of cationic analogue of glycine-betaine with coordinating anions, to remove Cu(II), Co(II) and Ni(II) from aqueous effluents and in saline media. The ILs are composed of inexpensive moieties and the extraction does not require the use of a chelating agent, which is a valuable advantage for process design.
The 4-chlorosalycilate based ionic liquids exhibit a high potential for the extraction of divalent
Author statements
Pape diaba Diabate: Formal analysis, Investigation, Methodology, Visualization. Stéphanie boudesocque: Funding acquisition, Supervision, Methodology, Conceptualization, Writing, Validation. Aminou Mohamadou: Conceptualization, Investigation, Methodology, Supervision, Validation, Visualization, Writing - review & editing. Laurent Dupont: Funding acquisition, Supervision, Conceptualization, Project administration, Original draft writing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors are grateful to the “Region Grand Est” and the FEDER for a Grant to P. D. Diabate and for its financial support (Grant Numbers: D201600637 and D201600639).
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