Abstract
Both, experimental and modelling evidence is presented in this study showing that interlayer anion exchange is the dominant sorption mechanism for iodide (I-) on AFm phases. AFm phases are Ca-Al(Fe) based layered double hydroxides (LDH) known for their large potential for the immobilization of anionic radionuclides, such as dose-relevant iodine-129, emanating from low- and intermediate-level radioactive waste (L/ILW) repositories. Monosulfate, sulfide-AFm, hemicarbonate and monocarbonate are safety-relevant AFm phases, expected to be present in the cementitious near-field of such repositories. Their ability to bind I- was investigated in a series of sorption and co-precipitation experiments. The sorption of I- on different AFm phases was found to depend on the type of the interlayer anion. Sorption Rd values are very similar for monosulfate, sulfide-AFm and hemicarbonate. A slightly higher uptake occurs by AFm phases with a singly charged anion in the interlayer (HS-AFm) as compared to AFm with divalent ions (monosulfate), whereas uptake by hemicarbonate is intermediate. No significant sorption occurs onto monocarbonate. Our derived thermodynamic solid solution models reproduce the experimentally obtained sorption isotherms on HS-AFm, hemicarbonate and monosulfate, indicating that anion exchange in the interlayer is the dominant mechanism and that the contribution of I- electrostatic surface sorption to the overall uptake is negligible.
Highlights
Iodine, 129I, is of major concern for the safety case of geological repositories for low- and intermediate-level radioactive waste (L/ ILW), due to its long half-life (1.6 Â 107 y) and high mobility [1]
The main objective of this study is to investigate the sorption behavior of I- on different AFm phases, which are expected to be present in the cementitious near-field of a low- and intermediate-level radioactive waste (L/ILW) repository, i.e. monosulfate (SO4-AFm), sulfide-AFm, hemicarbonate ((OH/CO3)AFm) and monocarbonate (CO3-AFm)
The developed thermodynamic solid solution models allow I- uptake to be estimated in the cases where interlayer anion exchange is the predominant sorption mechanism
Summary
129I, is of major concern for the safety case of geological repositories for low- and intermediate-level radioactive waste (L/ ILW), due to its long half-life (1.6 Â 107 y) and high mobility [1]. In Switzerland, L/ILW originates from the operation and decommissioning of nuclear power plants, as well as from industry, research and medicine [2]. Based on four different scenarios, depending on the operating lifetime of the current nuclear power plants, the total expected volume of L/ILW to be deposited of in Switzerland amounts to $ 36 000–42 000 m3 [2]. The exact chemical form of 129I in the waste is unknown, the thermodynamically stable iodine species in aqueous solutions under the alkaline (10 < pH < 13.5) and reducing conditions (-750 mV < Eh < -230 mV (standard hydrogen electrode, SHE)) expected in the near-field of a cement-based L/ ILW repository [4], is iodide, I-. Several types of materials have shown high sorption potential for iodide - layered bismuth hydroxides (LBHs), Ag-impregnated carbon (AgC), synthetic or natural argentite (Ag2S) and Ag-zeolites (AgZ) [5,6,7]
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