Chelate extraction of metal ions in aqueous/chloroform system based on molecular crowding environment

Abstract

In this study, we propose a novel concept for the solvent extraction of metal ions (Co, Zn, and Pb) by mimicking a molecular crowding environment using dextran (Dex). The metal ions were extracted from the aqueous phase into the organic phase (chloroform) in the presence of 8-hydroxyquinoline (HQ). The extraction constant of the metal complex (Kex) increased with increasing Dex concentration (CDex) for all metal ions. When examining the dependence of CDex on the four equilibrium constants (distribution coefficient of HQ, acid dissociation of HQ, complexation constant of metal complex (β), and distribution coefficient of the metal complex) that contribute to Kex, only β increased with CDex. This suggests that an increase in, β, a parameter reflecting the molecular crowding effect, results in an increase in Kex. The increase in β was analyzed based on volume exclusion and osmotic pressure effects. The analytical model effectively explained the enhanced the complexation due to the increase in β and volume exclusion, whereas the osmotic pressure suppressed β. Consequently, we unveiled the effect of molecular crowding on the solvent extraction of metal ions for the first time.