Probing the absence of third phase formation during the extraction of trivalent metal ions in an ionic liquid medium

Abstract

The polar nature of the room temperature ionic liquid, N,N,N′,N′-tri-n-octylmethylammonium nitrate [N₁₈₈₈][NO₃], has been exploited to prevent undesirable third phase formation during the extraction of trivalent metal ions from solutions of bis(2-ethylhexyl)diglycolamide (T2EHDGA) or carbamoylmethylphosphine oxide (CMPO) in [N₁₈₈₈][NO₃]. The extraction behaviors of Am(III) and Nd(III) were studied in 0.1 M T2EHDGA or CMPO in [N₁₈₈₈][NO₃] from a nitric acid medium over a concentration range from 0.01 M to 8 M. The extraction of nitric acid and trivalent metal ions in the ionic liquid phase increased with an increase in the concentration of nitric acid and trivalent metal ions, without leading to the third phase formation that otherwise occurs in molecular diluent systems, such as n-dodecane (n-DD), even at low concentrations of nitric acid and with trace trivalent metal loading. To unravel the role of the ionic liquid in preventing third phase formation, dynamic light scattering studies were performed in the ionic liquid phase obtained after extraction. The results revealed that the ionic liquid minimized the aggregate size of polar metal–ligand complexes and dispersed the aggregates efficiently in the ionic liquid phase. As a result, the concept of third phase formation in the present ionic liquid did not arise and perhaps this is also applicable to other ionic liquids.