Separation of cobalt, nickel and copper with task-specific amido functionalized glycine-betaine-based ionic liquids

Highlights

• Amido functionalized analogue of glycine-betaine based ionic liquids have been synthesized.

• The extractive capacities of these ionic liquids were improved by the saline media.

• Ionic liquids combined with NaCl medium ensure an efficient separation of Co(II) and Ni(II).

• Chaotropic electrolyte improves the extraction capacities of these ionic liquids.

• Dicyanamide based ionic liquids and the saline medium are the most effective system.

• Quantitative recovery of metals was achieved with a kosmotropic electrolyte.

Abstract

Task Specific Ionic Liquids (TSILs) were generated by association between a cationic amido functionalized analogues of glycine-betaine (AGB) and coordinating anions such as dicyanamide (Dca⁻), 4-chlorosalicylate (ClSal⁻), saccharinate (Sac⁻) and weak coordinating one such as bis(trifluorosulfonyl)imide (Tf₂N⁻). Extraction of Cu(II), Ni(II), Co(II), from pure water and salt media was performed with these AGB-ILs at room temperature. The obtained results show that in pure water Clsal based AGB-ILs have a high extraction efficiency towards these metals, whereas dicyanamide based AGB-ILs may extract efficiently Cu(II). In some cases, extraction can be improved by using simple chaotropic salts or intermediate anion such as NO₃⁻ or Cl⁻. Co(II) may be quantitatively extracted by Dca based AGB-ILs in NaCl 4 mol L⁻¹. The rational use of background salts can improve the selectivity of the extraction process and allow the separation and the recovery of the three metals separately from a mixture containing two or three divalent metals. The metal could be back-extracted from the ionic liquid phase with aqueous solution of Na₂SO₄, limiting the release of ionic liquid cation in aqueous media, thus preserving its integrity. The metal extractability by the Dca based ionic liquids after the stripping is quasi-equivalent to that of the fresh AGB-IL showing that it can be reused for three or four extraction and stripping cycles. A regeneration of Dca based ionic liquids after several extraction cycles is made possible by adding a solution of NaDca which restore their extractive capacity.

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.