Complexation of Plutonium and Other Actinides in Different Oxidation States with Gluconate at Low pH Values─A...

Using a coupling between capillary electrophoresis and ICP-MS (CE-ICP-MS), the gluconate (GLU) complexation of plutonium in the major oxidation states (III)-(VI) as well as Am(III), Th(IV), Np(V), and U(VI) was investigated at pH ≤ 4. CE-ICP-MS enabled the determination of the Pu oxidation state by comparing its electrophoretic mobility to that of a redox-analogous actinide (An). For the Am(III)/Pu(III) pair, the complex formation constants of three successive binary [An(GLU)_x]^3-x (x = 1-3) complexes could be determined. For Np(V)/Pu(V), the complex formation constants of the first binary [AnO₂(GLU)]_(aq) complex were determined in accordance with previous literature for Np(V), and those of the second [AnO₂(GLU)₂]^- complex were estimated. For U(VI)/Pu(VI), the constants of the [AnO₂(GLU)]⁺, [AnO₂(GLU_-H)]_(aq), and [AnO₂(GLU_-H)(GLU)]^- complexes were also determined in accordance with previous literature for U(VI). Plutonium in the oxidation states (III), (V), and (VI) behaved very similarly to the redox analogues. This was not the case for Th(IV)/Pu(IV). Here, the first five binary [Th(GLU)_x]^4-x (x = 1-5) complexes were determined for Th(IV), whereas mixed Pu-OH-GLU complexes were proposed for Pu(IV). The comparison of the first complex formation constants of the binary An-GLU complexes suggests a different bonding motif between An^3+/4+ and AnO₂^+/2+, with AnO₂^+/2+ forming the weaker complexes.