A revisit of the cation–cation interactions between NpO2+ and UO22+ in nitric acid medium and their impact on separation processes: spectrophotometric and solvent extraction studies

Abstract

There are contradicting reports on the thermodynamics of cation–cation interactions (CCIs; inner/outer sphere) involving NpO₂⁺ and UO₂²⁺. This paper revisits CCIs of NpO₂⁺ (2 × 10⁻⁴ M) under varying conditions such as reaction time, nitric acid (2 × 10⁻³–4 M HNO₃)/uranium (up to 1.2 M) concentrations, and temperature (283–343 K) by spectrophotometric measurements. This study reports for the first time the appearance of a signature peak of Np(IV) (∼964 nm) in addition to NpO₂⁺ (980 nm) and the NpO₂⁺–UO₂²⁺ complex (992 nm). For a pure NpO₂⁺ solution at 4 M HNO₃, there is a gradual increase in Np(IV) peak intensity with increasing temperature and correspondingly the Np(V) peak diminishes. The CCIs are more favored at higher uranium concentrations. However, the intensity of the 992 nm peak decreases steadily with increasing temperature suggesting the exothermic nature of the complexation process. The thermodynamic data and reported structural studies indicate the formation of an inner-sphere complex under the conditions of this study. In addition, the spectral changes also suggest the formation of Np(IV) even in the presence of uranium at elevated temperatures. Solvent extraction studies using 1.1 M TBP and 1.1 M DHOA solutions in n-dodecane show that NpO₂⁺–UO₂²⁺ complexes are extractable leaving NpO₂⁺ in the aqueous phase.