Résumé
Fundamental knowledge of the intrinsic colloids of plutonium is a major issue in predictingtheir behavior in the environment.[1,2] It has been demonstrated that these species arespherical nanoparticles measuring ca. 2 nm with a local structure similar to the one of bulkPuO2.[3,4] Recent studies have questioned the contribution of small Pu clusters in theformation mechanism of Pu(IV) intrinsic colloids.[5] The current work draws attention to thisissue by investigating an unprecedented kinetic isotopic effect (KIE) observed during thepreparation of Pu(IV) colloids in light (H2O) or heavy (D2O) water.Plutonium colloids have been synthetized by the dilution of acidic Pu(IV) solutions in H2Oand D2O.Time dependent measurements carried out with UV-Vis absorption spectroscopydemonstrated a much lower formation rate of the Pu(IV) colloids in D2O compared to H2O(Figure 1a). The observed KIE was attributed to the difference in zero-point energies of OH and O-D bonds. This finding has confirmed that the formation of Pu(IV) colloids is limitedby the rupture of O-H bonds during hydrolysis and polymerization of Pu(IV) species. Inaddition, formation of some unstable intermediate Pu(IV) species was observed duringcolloid formation. A thorough investigation of this intermediate using synchrotron SAXS andXAS techniques revealed the presence of an hexanuclear Pu(IV) cluster of quasi-sphericalshape measuring ca. 0.8 nm and agreeing with the literature (Figure 1b).[6,7]