First water-to-ligand substitution dictates Co²⁺/Ni²⁺ extraction selectivity.

Area of Science:

• Chemical Engineering
• Materials Science
• Physical Chemistry

Background:

• Solvent extraction is vital for recovering critical metals like cobalt and nickel.
• Achieving high ionic selectivity, especially for similar ions, remains a significant challenge.
• Manipulating ionic solvation offers a promising strategy to enhance selectivity.

Purpose of the Study:

• To investigate the thermodynamics of organophosphorus ligand binding with Co²⁺ and Ni²⁺.
• To understand how ligand-induced solvation changes affect metal ion selectivity.
• To elucidate the mechanisms of water-ligand substitution during solvent extraction.

Main Methods:

• Combined experimental techniques (UV-vis and IR spectroscopy) with computational methods (well-tempered metadynamics).
• Analyzed the thermodynamics of ligand binding at the aqueous-organic interface.
• Mapped free-energy landscapes to identify water-ligand substitution pathways.

Main Results:

• Extraction confirmed solvation structure changes for Ni²⁺ and Co²⁺.
• Identified two competing routes for water-ligand substitution: direct substitution and formation of an oversaturated state.
• The substitution of the first water molecule is critical for determining Co²⁺/Ni²⁺ selectivity.

Conclusions:

• The ligand-binding process is a competition between different water-ligand substitution routes.
• Rational design of ligands should focus on engineering the free energy of the first water molecule substitution.
• This approach can lead to improved Co²⁺/Ni²⁺ separation technologies.