The complexation of neptunium(V) with fluoride at elevated temperatures: Speciation and thermodynamics

The complexation of NpO 2 + with fluoride is studied in aqueous solution using Vis/NIR absorption spectroscopy. The total ligand concentration (NaF), ionic strength (NaClO 4 ), and temperature (T = 20–85 °C) is varied, yielding detailed information on the thermodynamics of the complexation reaction. Two distinct complex species (NpO 2 F n 1−n , n = 1, 2) are identified by peak deconvolution of the absorption spectra. At 20 °C and I m  = 1.0 the absorption band of NpO 2 F is located at 983.8 ± 0.2 nm. The absorption band of the NpO 2 F 2 − complex is located at 988.4 ± 0.2 nm. With increasing temperature and ionic strength the chemical equilibrium of the complexation reaction shifts towards the complexed Np(V) species. The conditional stability constants log β‘ n (T) are calculated according to the law of mass action and are extrapolated to I m  = 0 for each temperature with the specific ion interaction theory (SIT). The determined log β 0 n (T) values increase by 0.2–0.6 in the studied temperature range. Furthermore, the log β 0 n (T) values correlate linearly with the reciprocal temperature and fitting the data according to the integrated Vanu0027t Hoff equation yields the standard reaction enthalpies (Δ r H 0 m ) and entropies (Δ r S 0 m ). The reactions are endothermic and are solely driven by the high gain of entropy. In addition, the binary SIT ion-ion interaction coefficient e j,k of the respective complex species are determined.