Uptake of iodine by cement hydration phases: Implications for radioactive waste disposal

Iodine-129 ( 129 I) is an important radionuclide in the context of nuclear waste disposal owing to its long half-life and potentially high mobility in the environment. The uptake of iodide and iodate by cement hydration phases, including calcium silicate hydrates (CSH), AFm and ettringite, as well as hardened cement paste made from Ordinary Portland cement, has been studied in batch-type sorption experiments to enhance understanding of iodine retention mechanisms in engineered repositories. Uptake kinetics were generally fast, leading to steady state within 30 days. Strong uptake of iodine by AFm and ettringite was observed, the mechanism dependent on the iodine speciation. Iodide is retained in both AFm and ettringite by exchange for sulphate, whereas with iodate, iodate-substituted ettringite is formed by phase transformation or ion exchange in the case of AFm and ettringite, respectively. The contribution of CSH phases to iodine retention in cementitious systems depends on the Ca/Si-ratio of the CSH and the alkalinity of the solution, with stronger retention in young hyperalkaline cementitious materials. These findings have implications when selecting grouts for the immobilisation of radioactive waste streams containing 129 I or for choosing cementitious grouts and/or backfill materials in nuclear waste repositories. • Retention of iodine species by various cement hydration phases investigated. • Iodine uptake by AFm depends on speciation - anion exchange in interlayer (iodide) or neoformation of iodine-bearing AFt. • High content of AFm/AFt and CSH with high Ca/Si ratios beneficial for solidification of 129 I-bearing waste streams. • Data provided for component additive approach models and for evaluation of concrete clearance and disposal options.