Retention of niobium(V) by calcite and carbonated cement paste: quantitative description and impact of isosaccharinic acid and chloride

Sorption experiments with niobium(V) in calcite and carbonated cement were conducted simulating cement degradation stage IV. 93 Nb and 95 Nb ( t 1/2 = 35.0 days) were used as probes of 94 Nb ( t 1/2 = 2·10 4 years), expected in nuclear waste. In calcite systems, an initial uptake (R d ≈ 10 3 L·kg −1 at t = 3 days) was followed by a steady increase of distribution ratios with time (R d > 2·10 4 L·kg −1 at t = 89 days). This was explained as fast adsorption, followed by slow incorporation into the calcite structure. The stronger uptake observed for carbonated cement is attributed to the presence of amorphous phases with larger surface area. The formation of complexes with isosaccharinic acid (ISA) slightly decreases Nb(V) sorption, whereas chloride has a minor effect on the uptake of Nb(V) up to 2.0 M NaCl. This work provides an improved quantitative description and mechanistic understanding of 94 Nb retention in repositories for nuclear waste.