Characterization and Solubility Determination of the Solid-Solution Between AFm-I₂ and AFm-SO₄

Abstract

¹²⁹I is expected to be released from radioactive waste in deep geological repositories as dissolved iodide (I⁻). Performance assessment calculations predict that ¹²⁹I is one of the dose-determining radioisotopes due to its long half-life and its weak retention by common minerals. Preliminary sorption experiments showed that LDH-type minerals could control iodide retention in the near field of a cement-based underground repository. AFm-SO₄ [Ca₂Al(OH)₆(SO₄)_0.5·nH₂O], also known as “monosulfate”, was found to be a potential candidate for iodine retention. Thus, replacing SO ²⁻₄ by I⁻ in the interlayer could potentially reduce the mobility and radiological doses of ¹²⁹I released from nuclear waste in deep geological cementitious repositories.

In this study, AFm phases were synthesized by co-precipitation of C₃A and various ratios of I⁻ and SO ²⁻₄ and analysed by X-ray diffraction (XRD). XRD measurements indicate the formation of a single AFm phase at every I⁻/SO ²⁻₄ ratio, whereby the interlayer distance increases with increasing the I⁻/SO ²⁻₄ ratio. This finding shows the existence of a continuous solid solution series between AFm-I₂ and AFm-SO₄ end members. The iodine uptake mechanism in AFm-SO₄ was also investigated using EXAFS spectroscopy at the iodine K-edge. The AFm-(I₂) _x (SO₄)_1−x samples show an additional shell at R + ΔR ~ 4Å in comparison to pure AFm-I₂. The intensity of the backscattering contribution increases in proportion to the amount of SO ²⁻₄ in the solid phase, thus supporting the assumption of a solid solution between AFm-I₂ and AFm-SO₄.

A thermodynamic model was developed for the AFm-(I₂, SO₄) solid solution with the purpose of predicting the fate of ¹²⁹I in the complex cement matrix. To this aim, a series of precipitation and dissolution experiments with AFm-(I₂) _x (SO₄)_1−x was carried out at 23 ± 2°C until equilibrium was reached. Thermodynamic properties of the pure AFm-I₂ and its solid solution series with AFm-SO₄ were estimated based on chemical analysis of the solids and the coexisting aqueous solutions. The data are consistent with the formation of an ideal solid solution with end members defined using the Vanselow convention. Overall, this study demonstrates and quantifies the potential of cement matrices to act as a sink for ¹²⁹I.