Subangstrom tuning of pore size for selective separation of radioactive strontium over environmental calcium by lanthanide-oxalate frameworks

Nankawa, Takuya ; Sekine, Yurina   ; Yamada, Teppei*

Selective separation of radioactive ions is essential for reducing or cleaning radioactive wastes. Among the radioisotopes to be removed, Sr poses a major threat to human health and the environment. However, removal of Sr from environmental wastewater is still challenging due to the difficulty of separating Sr from Ca. Here, we developed a series of isostructural lanthanide oxalate frameworks (LOFs) comprising oxalate and eight kinds of lanthanide (Ln) ions, i.e., from Samarium (Sm) to Thulium (Tm) for application to selective removal of Sr from wastewater using its tuned porous structure. The LOFs had ion exchangeable anionic pores, in which the size of the pores changed in a stepwise manner depending on the host Ln species. When Tb was the host Ln of the LOF, the LOF showed extremely high Sr selectivity and was able to distinguish the subtle difference in ionic radius (0.2 ) between Sr and Ca. The Sr selectivity was higher than that of conventional adsorbents. The pore size tuning of the LOFs by selecting the constituent Ln species yielded a highly ion-selective adsorbent material. This novel strategy will be useful in developing custom porous materials that are easy to prepare and applicable across various fields.