Complexation of Plutonium and Other Actinides in Different Oxidation States with Gluconate at Low pH Values─A CE-ICP-MS Study

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)x3-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)x4-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 An3+/4+ and AnO2+/2+, with AnO2+/2+ forming the weaker complexes.