Affiliation:
1. Paul Scherrer Institut, Laboratory for Waste Management, CH-5232 Villigen PSI, Switzerland
2. Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal P.O. Box 3640, D-76021 Karlsruhe, Germany
Abstract
Abstract
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 (TiO2) and by calcium silicate hydrates
(C-S-H) under alkaline conditions. TiO2 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 TiO2
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 TiO2 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.
The similar R
d values for Np(IV,V,VI) sorption at pH = 10 can be explained qualitatively by invoking inter-ligand electrostatic repulsion between OH groups in the coordination sphere of Np(V) and Np(VI). This mechanism was proposed earlier in the literature for the prediction of actinide complexation constants with inorganic ligands. A limiting coordination number for each Np redox state, resulting from the inter-ligand electrostatic repulsion, allows the weaker sorption of the highest hydrolysed Np(V,VI) species to be explained.
Subject
Physical and Theoretical Chemistry
Cited by
16 articles.
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