First Water-to-Ligand Substitution Dictates Co2+/Ni2+ Extraction Selectivity

Solvent extraction is a crucial technology in the hydrometallurgical recovery of critical metals. A molecular-level understanding of how extractant ligands mediate metal transfer at the aqueous–organic interface, and how selectivity arises, is essential for elucidating the separation process. In this work, we focus on the interfacial solvation exchange during the extraction of Co2+ and Ni2+, using two representative organophosphorus extractants, Cyanex 272 and D2EHPA. Interfacial ligand substitution in the primary solvation shell, in which coordinated water molecules are replaced by extractant ligands, is the key mechanistic event governing the extraction process. Using well-tempered metadynamics simulations, we mapped the free-energy landscape of the stepwise ligand exchange at the aqueous–organic interface. The substitution of water molecules by extractant ligands is thermodynamically favorable, with the maximum free- energy differences arising in the initial step of breaking the metal hydration shell. We show that the free-energy cost (∆F ) of the first ligand-substitution step, and the corresponding energy-gap differences (∆∆F ), dictate the extraction selectivity, with Co2+ being more easily extracted than Ni2+, and Cyanex 272 exhibiting higher selectivity than D2EHPA. These results provide molecular-level thermodynamic insights into solvent extraction and selectivity.