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Uranium(VI) Uptake by Synthetic Calcium Silicate Hydrates

Published online by Cambridge University Press:  01 February 2011

J. Tits ,
T. Fujita ,
M. Tsukamoto  and
E. Wieland
J. Tits
Affiliation:
Paul Scherrer Institute, Villigen PSI, Switzerland
T. Fujita
Affiliation:
Central Research Institute of the Electric Power Industry (CRIEPI), Tokyo, Japan
M. Tsukamoto
Affiliation:
Central Research Institute of the Electric Power Industry (CRIEPI), Tokyo, Japan
E. Wieland
Affiliation:
Paul Scherrer Institute, Villigen PSI, Switzerland
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Abstract

The immobilization of U(VI) by C-S-H phases under conditions relevant for the cementitious near field of a repository for radioactive waste has been investigated. C-S-H phases have been synthesized using two different procedures: the “direct reaction” method and the “solution reaction” method.

The stabilities of alkaline solutions of U(VI) (presence of precipitates or colloidal material) were studied prior to sorption and co-precipitation tests in order to determine the experimental U(VI) solubility limits. These U(VI) solubility limits were compared with the U(VI) solubilities obtained from thermodynamic speciation calculations assuming the presence of combinations of different solid U(VI) phases. The solid phase controlling U(VI) solubility in the present experiments was found to be CaUO4(s).

The U(VI) uptake kinetics and sorption isotherms on C-S-H phases with different C:S ratios were determined under various chemical conditions; e.g., sorption and co-precipitation experiments and different pH’s. U(VI) was found to sorb fast and very strongly on C-S-H phases with distribution ratios (Rd values) ranging in value between 103 L kg-1 and 106 L kg-1. Both sorption and co-precipitation experiments resulted in Rd values which were very similar, thus indicating that no additional sorption sites for U(VI) were generated in the co-precipitation process. Furthermore, C-S-H synthesis procedures did not have a significant influence on U(VI) uptake. The U(VI) sorption isotherms were found to be non-linear, and further, increasing Ca concentrations resulted in increasing U(VI) uptake. The latter observation suggests that U(VI) uptake is controlled by a solubility-limiting process, while the former observation further indicates that pure Ca-uranate is not the solubility-limiting phase. It is proposed that a solid solution containing Ca and could control U(VI) uptake by C-S-H phases.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1107: Scientific Basis for Nuclear Waste Management XXXI , 2008 , 467
Copyright
Copyright © Materials Research Society 2008

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