Complexation of tetravalent actinides (Th, U, Np, Pu) with 2,6-pyridinedicarboxylic acid in aqueous solutions

The interaction of An(IV) ions (An = Th, U, Np, Pu) with 2,6-pyridinedicarboxylic acid (2,6-PDCA) in solutions was studied by spectrophotometry. The electronic absorption spectra of the individual complex species An(PDC)2+, An(PDC)2, and An(PDC) 3 2− (PDC2− is 2,6-PDCA anion; An = U, Np, Pu) were obtained. At [2,6-PDCA] ≳ 3[An(IV)] + 0.01 M and [H+] ≲ 0.2 M, the prevalent An(IV) species are the complexes An(PDC) 3 2− . Their overall stability constant exceeds 1025 L3 mol−3 and increases in the series from Th(IV) to Pu(IV) by ∼8 orders of magnitude. Very high stability of An(IV) complexes with 2,6-PDCA anions leads to significant shifts of the redox potentials of couples involving An(IV). In particular, large difference in the stability of An(III) and An(IV) complexes is responsible for the fact that Pu(III) in the presence of 2,6-PDCA is readily oxidized with atmospheric oxygen to Pu(IV).