Does high hydrophilic complexing agent concentration enhance separation factors in “push-pull” extraction systems?

Abstract

General equations describing the separation factors applicable to “push–pull” extraction systems using neutral and acidic organophosphorus extractants were derived. Factors affecting the separation factors, such as complexation constants, extraction equilibrium constants, counter anion concentration, extractant content, and hydrophilic complexing agent concentration, were elucidated. Specifically, for the extraction systems where the two target metal ions share the identical stoichiometry with the hydrophilic complexing agents, the separation factors continuously increase with higher concentration of hydrophilic complexing agent. In contrast, when their stoichiometries differ, the separation factor reaches extreme values depending on the hydrophilic complexing agent concentration. Additionally, carefully designed “push–pull” extraction systems were experimentally tested, to verify the predicted extremum behavior of the separation factor. This study offers new insights into optimizing separation efficiency while minimizing the costs associated with hydrophilic complexing agents, providing practical implications for industrial extraction processes.