Effect of temperature on the thermodynamic and spectroscopic properties of Np(v) complexes with picolinate

Abstract

Picolinate is a well-known chelating agent and is present in contaminated lands and nuclear wastes. Thermodynamic parameters of the complexation of picolinate with Np(V) at elevated temperatures are needed to help predict the migration behavior of Np(V) in nuclear waste repositories where the temperature could be high. In this work, the equilibrium constants of picolinate protonation and its complexation with Np(V) were determined at T = 283–343 K and I = 1.0 mol dm⁻³ NaClO₄ by potentiometry and spectrophotometry, respectively. Both the protonation and the complexation constants decrease with the increase of temperature, indicating the reactions are exothermic. The enthalpies of protonation and complexation were both found to be negative and moderately large at T = 298 K and I = 1.0 mol dm⁻³ NaClO₄ by titration calorimetry. The effect of temperature on the spectroscopic features of Np(V) is interpreted in terms of thermal expansion of the complex coordinates that results in blue shifts of the Np(V) absorption band at higher temperatures.