# Uptake of H14CO3−/14CO32− by calcite: impact of ISA and chloride

## TL;DR

This study examines how calcite retains radioactive 14C under conditions simulating cement degradation in waste repositories.

## Contribution

The study provides new insights into the mechanisms of 14C retention by calcite in the presence of ISA and chloride.

## Key findings

- 14C uptake by calcite increases over time, reaching distribution ratios of ≈10³ L kg⁻¹ after 60 days.
- ISA and chloride have a minor impact on 14C retention, possibly due to Ca-ISA complex formation and surface property changes.
- Recrystallization processes are key to long-term 14C retention in calcite.

## Abstract

The uptake of inorganic 14C (H14CO3−/14CO32−) 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.

Retention of inorganic 14C by calcite quantified and mechanistically assessed under advanced cement degradation conditions.