Efficient extraction and recovery of Co(II) and Ni(II) from spent ChCl-EG deep eutectic solvent via solvent extraction: A mechanism and performance study

Recycling secondary resources that are rich in valuable metals can help reduce dependence on primary metal ores, foster sustainable industrial development, and enhance the resilience of the metal resource supply chain. This study presents a facile solvent extraction strategy for the efficient recovery of cobalt and nickel from spent choline chloride-ethylene glycol (ChCl-EG, Ethaline) deep eutectic solvent (DES) using bis(2,4,4-trimethylpentyl) phosphonic acid (Cyanex272) as the extractant. By integrating a water-modification approach with pH regulation, the viscosity of the Ethaline system is significantly reduced, while the coordination structures of Co(II) and Ni(II) ions are altered from [CoCl 4 ] 2 ⁻ and [Ni(EG) 3 ] 2+ to [CoCl 2 (H 2 O) 2 (EG) 2 ] 0 and [NiCl(H 2 O) 2 (EG) 2 ] + . This dual modification enables selective extraction with recovery efficiencies exceeding 99 % for both metals, achieving a separation coefficient ( β Co²⁺/Ni²⁺ ) of 9550. Mechanistic insights from density functional theory (DFT) calculations reveal that the preferential extraction of Co(II) stems from its more negative Gibbs free energy change (ΔG Co(II) = -0.21 kJ/mol vs. ΔG Ni(II) = -0.04 kJ/mol) during complexation with saponified Cyanex272. Furthermore, the regenerated Ethaline and Cyanex272 exhibit exceptional stability, maintaining > 95 % extraction efficiency over 10 cycles. This work advances solvent extraction technology by demonstrating a synergistic water-pH strategy for DES-based systems, offering a facile and scalable pathway for recycling metal-laden DES-based electrolytes in industrial applications.