Rapid and highly selective Sr2+ uptake by 3D microporous rare earth oxalates with the facile synthesis, high water stability and radiation resistance

Radiostrontium remediation is of vital significance due to the hazard of radiostrontium to ecological environments and human health. However, it remains a challenge to effectively remove Sr2+ ion from complex water environments due to its solubility, mobility and influence of interfering ions. Herein, we report a series of rare earth oxalates with three-dimensional (3D) anionic microporous frameworks, namely [Me2NH2][Ln(C2O4)2(H2O)]·3H2O (Ln-ox; Ln = Y, Nd, Sm, Eu, Gd, Er, Tm, Yb). Ln-ox can be easily synthesized in large scale and possess high water stability and radiation resistance. The rapid and highly selective uptake of Sr2+ ion is achieved by Eu-ox with fast kinetics (3 min) and KdSr value up to 2.61 × 105 mL/g. Under high concentrations of interfering metal ions (M = Na+, Cs+, Mg2+ and Ca2+), Eu-ox exhibits high selectivity for Sr2+ with high separation coefficient SFSr/M. The Sr2+ removal mechanism has been revealed and attributed to the ion exchange between Sr2+ and [Me2NH2]+ cations. Further, by comparing a series of rare earth oxalate frameworks, it is found that the occurrence of ion exchange in 3D microporous rare earth oxalate materials is related to the channel size and the interaction of exchangeable cations with anionic framework. The strong complexation ability of oxalate groups to Sr2+, the large channel size of Eu-ox and the exchangeable [Me2NH2]+ cations are crucial factors for Sr2+ ion exchange. Rare earth oxalate frameworks as ion exchange materials are firstly used for the Sr2+ removal, confirming the potential of rare earth oxalates in radionuclide remediation.