Cementitious structures in geologic nuclear waste repositories act as barriers that limit the mobilization of radionuclides (RNs). As minor phases present in hydrated cement, AFm phases, belonging to layered double hydroxides (LDHs) family, are considered to be potential scavengers of soluble RN anions, via both surface adsorption and anion exchange processes[1]. In addition, AFm phases are also widely detected or predicted thermodynamically existing in coal ash [2, 3]. The objective of our work is to better understand RNs (e.g., Mo) sorption mechanisms on AFm phases and therefore the local geometry of different active adsorption sites on CaAl LDHs sites. Anionic exchange in the AFm interlayer was studied using wet chemical methods. Total aqueous concentrations of Mo indicate that the removal rates of Mo oxyanions by AFm-Cl2 from the solution are higher than for AFm-SO4. Compared to the averge Kd value of 0.03 m3 kg-1 for AFm-SO4, a value of 9.46 m3 kg-1 for AFm-Cl2 was obtained in [Mo]initial range of 0.001 to 0.02 mM, suggesting that the uptake capacity of AFm-Cl2 is larger than that of AFm-SO4. Mo K-edge XANES spectra showed different uptake mechanisms as [Mo]initial/[Cl2]intercalated ratio increased from 2×10-4 to 1 (in absence of CaMoO4 precipitate) in AFm-Cl2. Mo symmetry evolves from tetrahedral to octahedral and back to tetrahedral coordination with increasing surface coverage. Three sorption mechanisms are proposed: edge surface site complexation, structural co-precipitation/incorporation (confirmed by EXAFS with a second shell of Ca and O), and interlayer anion exchange (confirmed by in situ XRD with a shift of the (001) reflection to lower angle). In contrast, MoO42- interactions with AFm-SO4 at stoichiometric concentrations of total exchange lead to formation of a CaMoO4 precipitate. These results show different Ca-Al LDHs’ affinity for MoO42- and an interplay of multiple sorption modes.

Evidence of multiple sorption modes in AFm phases using Mo as structural probe

253rd National Meeting (2017)

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