Deep eutectic solvent reuse limitations driven by proton depletion and hydration-induced chemical drift

Abstract

Deep eutectic solvents (DESs) are promising sustainable media for metal dissolution and recovery, yet their long-term stability during reuse remains poorly understood. Here, the reusability of a choline chloride : p-toluenesulfonic acid (ChCl : pTSA) DES was evaluated over multiple dissolution cycles for Ni and Co removal. After each cycle, dissolved metals were removed by ion exchange prior to solvent drying and reuse, enabling isolation of intrinsic solvent evolution. Dissolution efficiency decreased from ∼100% to <70% over seven cycles. Acid–base titration showed a reduction in total acidity from 3.94 to 2.85 mmol g⁻¹ (∼27%), while Karl Fischer analysis revealed increased water retention from <1 to ∼13 wt%. Ion chromatography indicated changes in measurable chloride content during repeated cycling, consistent with chemical evolution of the DES. Performance correlated strongly with total acidity, demonstrating that proton depletion is a key driver of efficiency loss. Combined with water accumulation, this reduces effective proton activity through hydrogen-bond network restructuring. A mechanistic model is proposed in which repeated hydration–dehydration cycling induces chemical drift via coupled proton depletion, water retention, and evolving ionic speciation, ultimately limiting DES reuse. These findings highlight the importance of monitoring proton inventory and solvent speciation in recycled DES systems and provide mechanistic insight into performance limitations and potential strategies for sustainable hydrometallurgical processing.