Sustainable Management of Metal-Laden Deep Eutectic Solvents Composed of Choline Chloride and Urea: Selective Nickel and Cobalt Separation with Integrated Solvent Regeneration

The efficient recovery and utilization of postextraction solutions generated in hydrometallurgical circuits based on deep eutectic solvents (DESs) are crucial for enhancing resource efficiency and promoting environmental sustainability. However, such solutions, particularly those based on choline chloride-urea (Reline) DESs, pose significant challenges in the selective separation of chemically similar metal ions and in solvent regeneration, due to their high viscosity, strong metal coordination bonds, and the tendency of urea to decompose. Here, we report a strategy that simultaneously modulates hydrogen-bond (H-bond) strength and carboxylate coordination ability to achieve selective nickel/cobalt separation from Ni(II)/Co(II)-laden Reline. Ethanol (EtOH) is introduced to reconstruct the DES H-bond network, lower viscosity, and attenuate metal coordination. Selectivity is then fine-tuned by varying the alkyl-chain length of the precipitating carboxylate: succinate quantitatively precipitates nickel (99.3%), whereas oxalate recovers cobalt (99.6%). Subsequent vacuum evaporation of EtOH returns 98% of the DES, whose structure and physicochemical properties remain intact. Impressively, the proposed approach readily separates other binary couples─Ni(II)/Cu(II), Ni(II)/Zn(II), and Zn(II)/Cu(II)─in spent Reline, offering a highly efficient, universal, and environmentally benign pathway for the recovery of resources from waste metal-laden DESs.