Spectroscopic characterisation and thermodynamics of the complexation of Np(V) with sulfate up to 200 °C.

Abstract

In the present work the complexation of Np(V) with sulfate in aqueous solution is studied in a temperature range up to 200°C by absorption spectroscopy. For this purpose, a new spectroscopic setup is implemented and tested for its suitability for Vis/NIR absorption spectroscopy at elevated temperatures. The complexation of Np(V) with sulfate is studied as a function of the total ligand concentration at various temperatures (T=25-200°C) and ionic strengths (Im(NaClO4)=1.0-4.0molkg-1 NaClO4). The exclusive formation of NpO2(SO4)- up to 200°C is confirmed by peak deconvolution and slope analyses. The thermodynamic stability constants log β01(T) are obtained from linear regressions according to the specific ion interaction theory (SIT). A systematic increase of the log β01(T) is observed with increasing temperature, resulting in a linear correlation of log β01(T) with T-1. The magnitude of the increase is 1.9 logarithmic units at 200°C in comparison to log β01(25°C)=1.05±0.16. Thus, the standard reaction enthalpy and entropy (ΔrH0m, ΔrS0m) are determined with the integrated Van't Hoff equation revealing ΔrH0m=31.0±1.0kJmol-1 and ΔrS0m=123±9Jmol-1K-1. In addition, the stoichiometric sum of the specific binary ion-ion interaction coefficient (Δε01(T)) is determined up to 200°C showing an insignificant temperature dependence. Thus, a temperature-independent ε(Na+, NpO2(SO4)-)=0.07±0.11 is calculated for the temperature range up to 200°C. Comparison of the present results with literature data confirms the excellent applicability of the new high-temperature absorption spectroscopic setup for complexation studies up to 200°C.