Molecular dynamics study on absorption characteristics of anions (Br^- and I^-) in type-A zeolite in molten LiCl-KCl eutectic salt

When a type-A zeolite is immersed in molten LiCl-KCl eutectic salt, the alkali and halogen elements in the salt phase are absorbed in the vacancies of the zeolite in the form of ions. Recently, it was revealed that I^- in molten LiCl-KCl-KI salt exhibited greater absorption in the zeolite than Cl^-, although Br^- in molten LiCl-KCl-KBr salt did not show such absorption behavior. In the present study, molecular dynamics calculations were performed to understand the difference in the absorption behavior of I^- and Br^- in a type-A zeolite by calculating the dependences of the formation enthalpy of the zeolite containing several alkali halides on the composition of the halogen elements. From these calculations, it appeared that the enthalpy is negative in a Cl-I mixture system, indicating that when a few I^- ions are absorbed in a zeolite cage, more I^- is absorbed than that expected from the I^-/(I^-+Cl^-) ratio in the salt, i.e., I^- is preferentially absorbed. In contrast, no such negative potential appeared in the Cl-Br mixture system. Therefore, it was qualitatively indicated that I^- in molten LiCl-KCl salt is more preferentially absorbed in the type-A zeolite than Br^- in the salt.