Generations of process engineers have been educated that distillation is applied for separations, unless it is not possible. Although there are many accounts of industrial applications where liquid–liquid extraction (LLX) outperforms distillation economically, for example, in separating aromatics from aliphatics1, there are reservations about the use of LLX owing to fear of contamination with the solvent. Indeed, solvent regeneration is inseparable from LLX and raffinate treatment is often indispensable to recover leached solvent. Solvent regeneration and recovery are sometimes overlooked in scientific contributions focusing on achieving the highest distribution coefficient2, but they have a substantial impact on the overall process. Solvent regeneration should be a core concern during process design, especially when distillation is technically not viable. In any process that requires back-extraction owing to infeasibility of direct thermal regeneration, in the absence of any stimulus that boosts the concentration, the concentration in the back-extract is lower than that in the feed. The theoretical maximum concentration under reversible conditions equals the feed concentration, but in a real process (Fig. 1), a driving force is needed, which lowers the concentration. A concentration-increasing stimulus to induce such a driving force is mostly obtained using pH jumps; however, pH-jumping extraction is accompanied by proton transfer from wash to raffinate, and net salt production is the result.

Fig. 1: Liquid–liquid extraction with solvent regeneration through back-extraction.
figure 1

The coupled process at constant KD implies that Xout,BE < Xin. Solute-to-carrier ratios are given with X for aqueous carrier streams and with Y for the organic loop. Ef, extraction factor; KD, distribution constant; N, number of stages; BE, back-extract stream; E, extract stream; F, feed stream; R, raffinate stream; S, solvent stream; W, wash stream.

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Now, writing in Nature Chemical Engineering, Xiao Su and co-workers3 report a process in which 1,1'-didodecylferrocene (ddFc) is applied as redox-responsive extractant for critical metal-ion extractions. They demonstrate convincingly that gold, platinum and iridium can be extracted selectively with the extractant in the oxidized state (ddFc+), while the back-extraction is carried out with the extractant in the reduced state (ddFc). The redox responsiveness of the system results in a substantial impact on the extraction performance, and gold concentrations after back-extraction reach 16 times the feed concentration without generating salt. Su and co-workers used aqueous-phase oxidizing and reducing agents because the organic phases they applied were not conductive enough, but essentially, the electron transfer did drive the LLX process. In their approach, they studied multiple ferrocene-based extractants and multiple diluents (solvent to dilute the extractant). The best combination was ddFc in dichloromethane, although it is recognized that potentially butyl acetate may also work.

This approach offers opportunities to develop extraction systems that are by-product(s) free and that can run on green electricity. This is exactly what we need to improve the sustainability of our industries, especially in recycling applications as studied by Su and co-workers, otherwise more salt will be produced than metals recycled. The authors have shown with brief calculations that the economic aspects are appealing.

Seeing this work in perspective, it should be realized that coupling redox chemistry to LLX is far from new. It was reported decades ago4, and with the current hype around electrification, many activities are being reported5, including the Aqua Metals battery recycling pilot plant based on patented6 technology. These recent examples5,6 also consider green electricity as a driving force to avoid large quantities of salts as by-products, but what makes the work of Su and co-workers interesting is that it is not the metal itself that is electrochemically reduced; it is the solvent that changes oxidation state. This also offers opportunities for a much wider range of LLX-based separations to be driven by an electrochemical driving force that can yield back-extract concentrations that are much higher than feed concentrations. Specifically for carboxylic acid extractions, electrochemical concentration pumping has been shown7 through electrochemical pH-swinging of both the aqueous feed stream and the aqueous wash liquor, thus avoiding salt by-product formation. The strength of the work by Su and co-workers is that use of an organic-phase redox switch may also be applied to systems where the solute is not a metal, nor an acid or base. As long as there is a difference in the distribution coefficient KD of the solute for the solvent in the oxidized state and the reduced state, the electrochemical concentration pump can be applied.

Although this work showing electrochemical concentration pumping in extraction–back-extraction cycles with an organic-phase redox-responsive solvent is exciting, Su and co-authors found that the ddFc system is far from generic, as demonstrated for copper and silver, for which the system was not applicable. This means that for other metals, the chemistry needs to be tailored to tune the affinity between the extractant and metal — not too strong binding and not too weak. But this is not a specific drawback of this extractant. Future research should thus aim to widen the library of redox-responsive extractants, not only for metals but also for other types of solute.

Of concern is the choice of a chlorinated solvent, which should be limited as much as possible. For future research, it is advised to consider the ‘safe and sustainable by design’ framework, a recommendation published by the European Commission8. Also, considering the need for aqueous oxidizing and reducing agents, there may be other directives to reduce the number of chemicals in the aqueous phases. For example, the use of ionic liquids for electrochemical CO2 absorption/desorption could be considered9, as the use of ionic liquids makes the organic phase conductive. Taken together, this study by Su and co-workers opens a new field of organic-phase redox-switchable solvents in electrochemical LLX that offers many directions for future work.