Uptake of H14CO3 -/14CO3 2- by calcite: impact of ISA and chloride.

Abstract

The uptake of inorganic ¹⁴C (H¹⁴CO₃ ^-/¹⁴CO₃ ^2-) by calcite was investigated at pH ≈ 8.3 in the absence and presence of isosaccharinate ([ISA]_tot = 10^-5-0.2 M) and chloride ([NaCl]_tot = 10^-4-2.0 M). Calcite is representative of the last degradation stage of Portland cement, whereas ISA and chloride are components expected in low- and intermediate level short-lived waste (L/ILW-SL). A moderate ¹⁴C uptake is observed at short contact times, followed by a steady increase of the retention with time resulting in distribution ratios ≈ 10³ L kg^-1 at t = 60 days. This is explained by fast adsorption, followed by incorporation into the calcite structure through recrystallization phenomena. The minor impact of ISA and chloride on ¹⁴C uptake is possibly related to the formation of Ca-ISA complexes and the alteration of the surface properties of calcite. The modelling of the recrystallization process allows long-term predictions of the ¹⁴C uptake in the context of repositories for L/ILW-SL.