Speciation of plutonium and neptunium in α-FeOOH, α-Fe2O3 and γ-Fe2O3 suspensions

The types of surface complexes (mono- or bidentate, inner- or outer-sphere) and their stability constants were determined from dependencies of sorption upon pH, total metal concentration and ionic strength. The speciation of actinides in solution was studied by procedure that combines solvent extraction by TTA and HDEHP. The X-ray photoelectron spectroscopy (XPS) and solvent extraction procedure was used to determine the speciation of actinides on the mineral surfaces. It was found from FITEQL modeling that both plutonium and neptunium form inner-sphere monodentate complexes. The sorption of Np(V) is not accompanied by any redox reactions either in solution or on the surface. In case of Pu(V) no redox reactions occur in solution while the disproportionation (Pu total concentration is about 1.8·10-8 M) takes place upon sorption by α-FeOOH. This is possibly due to the high concentration gradient that occurs near the surface in double electrical layer in case of α-FeOOH for which the sorption capacity is sufficiently higher than for α-Fe2O3 and γ-Fe2O3. According to the XPS the binding of water molecule and/or CO32- group is observed in the coordination sphere of actinide atom. The strength of bonding (stability constant of surface complexes) of actinides to studied solids vary in the following consequence: α-FeOOH >> α-Fe2O3 ∼ γ-Fe2O3. (authors)