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Wet chemistry experiments were carried out to investigate the effect of the redox state and aqueous speciation on the uptake of neptunium by titanium dioxide (TiO 2 ) and by calcium silicate hydrates (C-S-H) under alkaline conditions. TiO 2  was chosen as a reference sorbent to determine the surface complexation behaviour of neptunium under alkaline conditions. C-S-H phases are important constituents of cement and concrete. They may contribute significantly to radionuclide retention due to their high recrystallization rates making incorporation the dominating sorption mechanism for many radionuclides ( e.g.  the actinides) on these materials. The sorption of neptunium on both solids was found to depend strongly on the degree of hydrolysis. On TiO 2  R  d  values for Np(IV), Np(V) and Np(VI) are identical at pH = 10 and decrease with progressing hydrolysis in case of Np(V) and Np(VI). On C-S-H phases,  R  d  values for the three redox states are also identical at pH = 10. While the  R  d  values for Np(VI) sorption on C-S-H phases decrease with progressing hydrolysis, the  R  d  values for Np(IV) and Np(V) sorption are not affected by the pH. In addition to the effect of hydrolysis, the presence of Ca is found to promote Np(V) and Np(VI) sorption on TiO 2  whereas on C-S-H phases, the present wet chemistry data do not give unambiguous evidence. Thus, the aqueous speciation appears to have a similar influence on the sorption of the actinides on both types of solids despite the different sorption mechanism.

radiochimica acta

Influence of the redox state on the neptunium sorption under alkaline conditions: Batch sorption studies on titanium dioxide and calcium silicate hydrates