Complexation of plutonium and other actinides in different oxidation states with gluconate at low pH values : a CE-ICP-MS study

Abstract

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 An3+/4+ and AnO2+/2+, with AnO2+/2+ forming the weaker complexes.