Complexation of U(VI) with picolinic acid in aqueous solution at variable temperatures: Potentiometric, spectrophotometric and calorimetric studies

[Display omitted] •Three stepwise U(VI)/picolinate complexes were identified in aqueous solutions.•Thermodynamic parameters of the complexation were determined.•Coordination modes of picolinate to U(VI) in the complexes are discussed. Thermodynamics of the U(VI) complexation with picolinic acid (HL) was investigated under constant ionic strength (1.05mol·kg−1 NaClO4) with multiple techniques, including potentiometry, spectrophotometry and calorimetry. Three stepwise complexes, UO2L+, UO2L2(aq) and UO2L3−, were identified, and their formation constants were determined by potentiometric titrations at 283–313K. The spectrophotometric work at 298K further demonstrated the formation of these three complexes. By calorimetric titrations, stepwise enthalpies of the complexation at 298K were determined to be ΔH11=−(3.43±0.24)kJ·mol−1, ΔH12=−(3.3±0.6)kJ·mol−1, and ΔH13=9.4±0.6kJ·mol−1. A formation of the 1:1 and 1:2 complexes is driven by both enthalpy and entropy, but the formation of the 1:3 complex by entropy only. The thermodynamic parameters also suggest that in the 1:1 and 1:2 complexes, picolinate coordinates to U(VI) through the chelation mode, i.e., one carboxylate oxygen and an aromatic nitrogen bind to the U(VI) atom.