A revisit of the cation-cation interactions between NpO2(+) and UO2(2+) in nitric acid medium and their impact on separation processes: spectrophotometric and solvent extraction studies

There are contradicting reports on the thermodynamics of cation-cation interactions (CCIs; inner/outer sphere) involving NpO2(+) and UO2(2+). 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(+)-UO2(2+) 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(+)-UO2(2+) complexes are extractable leaving NpO2(+) in the aqueous phase.