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Molecular dynamics simulation of hydrocalumite as adsorbent for anionic radionuclides
Hydrocalumite, is a hydration product of aluminum-rich cements, and is known in cement chemistry as an AFm phase. Structurally, it belongs to the family of layered double hydroxides, or “anionic clays”, where positively charged crystal layers require the presence of negatively charged ions in the interlayer space. Therefore, AFm phases can serve as potential adsorbents for anionic radionuclides (e.g., 35Cl−, 125I−, 129I−, 131I−) from aqueous solutions. Here we use classical molecular dynamic simulations to analyze the structure and properties of AFm phases containing Cl− and I−. The classical ClayFF force field is used to quantitatively study the structure, energetics and mobility of anions and H2O molecules in the interlayers of these phases and at their interfaces with CsCl and CsI aqueous solutions. In this study we report that the basal (001) surfaces of AFm phases can strongly adsorb hydrated Cl− and I− anions due to the donated hydrogen bonds from the interfacial hydroxyls, but primarily due to their strong attraction to the structural Ca cations exposed at the surface. However, our simulations show that the adsorption of I− is weaker than that of Cl−, leading to the higher surface mobility of I− due to its stronger chaotropic effect. The interlayer diffusional mobility of the Cl− and I− anions in the AFm phases is also investigated by using the Eyring-Vineyard approach and is shown to be significantly lower than in larger nanopores. Hence, the most likely transport of such anionic radionuclides takes place through the nano- and micro-pores of hardened cement.