(Deep) eutectic solvents for the separation of platinum group metals and rare earth elements: Characteristics, extraction mechanisms and state of the art

Critical metals, such as platinum group metals (PGMs) and rare earth elements (REEs), are strategically and economically significant due to their widespread use in electronics, clean energy, catalysis, and medical devices. With increasing demand and the continuous depletion of natural reserves, their recovery from metal-bearing waste is gaining attention, aligning with sustainability and circular economy principles. Although hydrometallurgy is a more controllable and energy efficient technique than pyrometallurgy, it employs strong acids with aggressive environmental, health, and safety (EHS) profiles. Therefore, solvometallurgy based on the use of (deep) eutectic solvents ((D)ESs) has gained prominence due to their tunable physicochemical properties, enabling safer and more selective metal separation processes. However, no review has comprehensively investigated the separation of PGMs and REEs utilising (D)ESs. Therefore, this critical review delves into the application of (D)ESs in PGM and REE recovery, focusing on the physicochemical characteristics of these solvents, alongside the phenomenology and mechanisms associated with solid–liquid extraction (SLX) and liquid–liquid extraction (LLX). Proton activity and complexation ability of (D)ESs are the most important factors influencing the dissolution of metal oxides. Furthermore, the LLX mechanism is predominantly governed by metal speciation and the ionic nature of (D)ESs, with ion exchange and neutral ion association being the most prevalent pathways. Additionally, the conducted in-depth evaluation of various operating conditions highlights that acidity and (D)ES composition are key contributors to achieving selective extraction. Finally, this study proposes a strategic framework for circular critical metal recovery, emphasising solvent regeneration and outlining future research directions, particularly concerning process intensification.