Effect of ISA and chloride on the uptake of niobium (V) by hardened cement paste and C-S-H phases: Quantitative description and mechanistic understanding

Abstract

The uptake of niobium by hardened cement paste (HCP) and calcium silicate hydrate (C-S-H) phases was investigated with 93Nb and 95Nb (t1/2 = 35.0 days). Structural materials used in nuclear reactors as well as cements contain the naturally occurring isotope 93Nb, while radioactive 94Nb with t1/2 = 2 x 104 years is relevant in the context of nuclear waste disposal. Strong uptake of Nb was observed for both materials, con-firming that C-S-H is the main sink of Nb in cement. Isotopic exchange with 93Nb in cement can play a role in the uptake of 94Nb under repository conditions. The formation of complexes with isosaccharinic acid (ISA) decreases the Nb uptake, although sorption remains strong up to [ISA]tot = 0.1 M. Chloride has a negligible effect on the uptake of Nb up to [NaCl] = 2 M. This work provides a sound basis for the quantitative description and mechanistic understanding of 94Nb retention in L/ILW repositories.