Importance of weak interactions in the formulation of organic phases for efficient liquid/liquid extraction of metals
Introduction
Solvent extraction (also called liquid/liquid extraction, abbreviated into L/L extraction) is at the heart of most separation processes in various hydrometallurgical applications, especially in those dedicated to the recycling of metals [1, 2, 3, 4]. The aim of this technique is to separate target from nontarget ions which are solubilized in an aqueous phase, through liquid−liquid phase transfer of target ions into an organic phase. This is typically achieved by means of extracting molecules, named extractants, designed with a chelating polar group to ensure the metal coordination, and with an apolar part enabling solubility in the organic phase. Historically, industrial processes have been implemented mostly from empirical approaches. Effects of acids, diluents and competitive metals on extraction efficiency were simply tabulated for direct application. For many processes, it appears today that understanding the extraction properties is mandatory for their optimization, or ideally for their prediction. More knowledge and predictive modelling would moreover allow greener recycling process, for example, which generate reduced effluents and/or with better selectivity [5].
Traditional approaches based on macroscopic relationships established thanks to the so-called ‘graphical slope analysis’ (GSA, also called ‘slope method’) have regularly reached their limitations. Based on the analysis of extraction isotherms, GSA is used to determine the stoichiometry of the metallic species extracted in the organic phase. When the extraction process can be described using a simple set of equilibria involving a limited number of metallic species, log–log plots result in lines with a slope corresponding to stoichiometric coefficients, as long as the experimental conditions are chosen to simplify the extraction behaviour. The validity domain of the method has been widely described, and when taken into account, very satisfactory results are obtained in the case of simple systems, which can be related to well-defined coordination complexes [6]. However, when multiple equilibria need to be taken into account, slope analysis becomes cumbersome, when not impossible to apply, and computer modelling needs to be used in conjunction with graphical analysis [7]. It was earlier noticed that slope analysis cannot account for complex extraction mechanisms, especially in the case of mixtures of extractants and synergism, as deviation from expected slope and nonstraight lines are observed [8]. Furthermore, application of slope analysis can lead to results of questionable interpretation; mechanisms inconsistencies have been noticed, and bias has been reported. For instance, studies of the extraction mechanism of lanthanides (III) by malonamides lead to different metal species based on various sources, with 2–3 extracting molecules involved in the metallic cation extraction [9, 10, 11]. In the same system, inconsistencies with saturation experiments have been reported, with neodymium (III) [11] as well as palladium (II) [12]. Theoretically, the extractant-to-metal ratio in the organic phase at metal saturation corresponds to the ligand-to-metal ratio of the coordination complex responsible for metal stabilization in the organic phase. However, such assumption is valid only if extraction mechanism is same at low metal loading (used for GSA) and at high metal loading (for saturation experiments). Finally, diluent effects with the closely related diglycolamides cannot be interpreted by changes in the coordination sphere of the cation without further studies [13]. Recent results demonstrate that evolution of aggregation in the organic phase is mostly responsible for these observations (vide infra, section 3.1) and back the need for detailed analysis involving chemical as well as physicochemical equilibria: extraction cannot be summarized in a fixed set of molecular equilibria with complexes of well-defined stoichiometry.
Extractants are amphiphilic molecules and tend to behave similar to surfactants, albeit with peculiarities. Since the first hint from Osseo-Asare [14], the number of annual published articles considering aggregation of extractants has been multiplied by 10. This approach describes organic phases as organized solutions by taking into account long-range interactions as well as classical interactions between nearest neighbours. Indeed, self-assembling properties of surfactants leads to formation of reverse micelles occurring in the oil phase beyond a certain critical concentration. Weak aggregates formed by the extractants need to be understood on different length scales: the atomic scale for metal chelation, the nanoscale for micelle formation and the micron scale dealing with macroscopic-phase behaviour. The nanomolecular and supramolecular scales are perfectly illustrated by the scheme of Guilbaud et al. [15], showing that the same entity can be studied at molecular scale, as a complex, and at supramolecular scale, as a nanodroplet of polar entities solubilized in an organic phase, or as reverse-micelle–like aggregates (Figure 1). Complex, aggregate and micelle definitions are distinguished by considering the water which is extracted in their polar core and which gradually evolves from frozen to free water states. In parallel, the quantity of water in the organic phase increases from some molecules to mmol/L macroscopic concentrations.
High extractant concentrations can also bring the possibility of organic-phase separations and occurrence of different complex phases such as the ‘third-phase accident’ which leads to viscosity increase and stops mixer-settler operations. To avoid these and to understand interactions, one has to study phases, phase diagrams and phase transitions in a systematic way, knowing that the classical mass action laws cannot be used without introducing many parameters to describe all simultaneous chemical equilibria involved. Complete understanding of the extraction process requires the establishment of a relationship between distribution law, initially introduced by Nernst, and thermodynamic approach accounting for the different occurring chemical reactions (mass action law), and encompassing thermodynamics of self-assembly.
This article proposes an overview of the characterization techniques and methods developed in the last decades to describe the organic extraction phases. In a second part, we summarize the main experimental and theoretical approaches taking into account this double-scale description to unravel the extraction mechanisms, as well as the contributions of these recent approaches in the understanding of the solvent extraction problems.
Section snippets
Coordination at the molecular level
Metal ion coordination remains the basis used in the description of liquid–liquid extraction, and extracting molecules are usually classified based on their interaction mode with metallic cations. The coordination chemistry principles which underpin the sorting of extracting molecule polar heads have been well described in a recent review [16]. Extracting molecules usually interact with metallic species through bonding in their inner coordination sphere, forming what is generally named a
Recent examples of aggregation-driven extraction
We give here an overview of the studies exploiting such a supramolecular approach with a special focus on the thermodynamic interpretations of these mechanisms.
The effect of acid that is known to play a consequent role on extraction efficiency has been investigated with a supramolecular approach. Acid nature usually depends on the targeted application. Nitric media are used for the back end of nuclear cycle, while sulphuric or phosphoric acids are more often applied for the front end. Change of
Conclusion and perspectives for efficient formulation of extracting system
Recent results highlight the key role of weak interactions in the extraction of metallic ions into an organic phase. Along with metal coordination, supramolecular ordering of the organic phase has been demonstrated to be one of the key driving forces of the process. As a consequence, detailed understanding of extraction mechanism of a given metal is nowadays performed using tools dedicated to the analysis of supramolecular organization of the organic phase, especially SAXS and SANS. Such
Conflict of interest statement
Nothing declared.
Acknowledgements
The authors acknowledge the European Research Council under the European Union's Seventh Framework Programme (FP/2007–2013) / ERC Grant Agreement n.[320915] “REE-CYCLE”: Rare Earth Element reCYCling with Low harmful Emissions.
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