Ion-pairing extraction and their reaction modeling of anionic M-Cl species with cationic NTAamide(C6) extractant and comparison with density functional theory calculations

Abstract

Solvent extraction is conducted using a total of 20 di-, tri-, and tetravalent metals, revealing high stability constants with Cl− and the N,N,N′,N′,N″,N″-hexahexyl-nitrilotriacetamide (NTAamide(C6)) extractant. The metals used here may behave as anions at high Cl− concentrations, and NTAamide(C6), which contains a tertiary N atom, is protonated under acidic conditions. Most of the metal ions in this study display higher distribution ratios (D(M)) from HCl than those from HNO3, although NO3− hardly forms metal-complexes, and exhibit 1:1 stoichiometries with NTAamide, based on the slopes of acid and metal extraction. The high D(M) may be due to ion-pair extraction based on the combination of cationic extractants and anionic metal-chloride ions. Following the experimental results, the association constants and distribution coefficients of the group 12 elements are calculated via ion-pair extraction modeling using density functional theory calculations, and the simulations of D yield calculated values with the same trend as that of the measured values. In the future, it will be possible to calculate the distribution ratio precisely by considering the size effect in the exosphere of metal and chloride ions and the HSAB rule.