Speciation-Induced Modification in Extraction Mechanism for Tetra, Hexa, and Trivalent Actinides Using Tetra Alkyl Ammonium Hydrogen Phthalate-Based Ionic Liquid

The present investigation demonstrates the extraction of Pu, U, and Am in their tetra-, hexa-, and trivalent oxidation states, respectively, from aqueous hydrochloric acid feeds using an anion-functionalized tetraalkylammonium-based ionic liquid. Investigation on the extraction mechanism revealed that; the extractive mass transfer of the (R4N)2PuCl6 species occured via an “anion exchange” mechanism from 7 M HCl, while from 1 M HCl, there was the possibility of simultaneous existence of an “anion exchange” as well as “cation exchange” mechanism through R4N(PuCl5), (R4N)PuCl6, PuCl3(phthalate), and PuCl2(phthalate)2 species, respectively. In the case of U; from 7 M HCl, (R4N)UO2Cl3 and (R4N)2(UO2Cl4) were found to be predominated, whereas from 1 M HCl, it was UO2Cl(phthalate). For Am3+, a “cation exchange” mechanism was found to be predominantly operative from both the acid concentrations, though the metal–ligand stoichiometry differed. The extraction processes were exothermic in nature. The radiation-induced performance degradation was found to be moderate. The use of an aqueous phase complexing agent was found to be beneficial for back extraction of the metal ions. This solvent system achieved mutual separation between tetravalent Pu over hexavalent U with a maximum separation factor in the range of 19–22 over the 3 M HCl to 8 M HCl concentration range. However, in the case of the mutual separation of Pu(IV) over Am(III), an impressive separation factor of more than 100 was achieved at 8 M HCl.