Sorption and Surface Complexation of Plutonium

Doctoral thesis, 2005

Plutonium is a constituent of spent nuclear fuel. As such, its behavior in contact with the surfaces of solids is of interest. This behavior will affect the migration of plutonium in case it is released into the environment. The chemistry of plutonium is rather complex. There are four oxidation states that can co-exist in solution, and limitations apply to what can be done experimentally with each of them. Some of these limitations have been mapped in this work. The distribution of plutonium in different oxidation states, between a solid and an aqueous phase, has been studied individually in experiments. This has been done for Pu(III), Pu(IV) and Pu(VI) onto TiO2, and Pu(III) onto UO2, at a plutonium concentration of about 10–6 M. The solid phase that has primarily been used is TiO2, but due to the reducing property of UO2 — which has been shown in experiments — it was also used in the case of Pu(III). Furthermore, it has been shown that TiO2 and another stable oxide, ThO2, do not appear to reduce or oxidize plutonium that is originally in its tetravalent state. MnO2 has been shown to oxidize Pu(IV) which is in agreement with previous studies. It was found that while Pu(III) and Pu(VI) yielded stable and repeatable sorption data, the sorption of Pu(IV) varied with time. This is consistent with disproportionation or reduction of Pu(IV) in the experiment. Sorption was interpreted as a result of surface complexation. The surface complexation model was used to describe the sorption of Pu(VI) onto TiO2. This could be done without the inclusion of electrostatic effects from charge accumulated on the surface of the solid.