Cationic Amorphous Metal–Organic Cage-Based Materials for the Removal of Oxo-Anions from Water

Oxo-anion pollutants pose a serious threat to natural water systems. It is highly necessary to develop cationic porous materials capable of removing oxo-anion from sewage. However, they are relatively rarely reported compared with anionic and neutral materials. Herein, two cationic amorphous metal–organic cage-based solids containing cages of different sizes, namely, aMOC-1 and aMOC-2, were constructed and use for removing oxo-anions from water. Because the cages in these materials have large cavities (with about 3.5 and 4.5 nm in diameter for aMOC-1 and aMOC-2, respectively) and high positive charges (positive +24 for both), both aMOC-1 and aMOC-2 exhibit rapid kinetics in carcinogenic chromate (CrO42– and Cr2O72–) and ReO4– (a surrogate anion of radioactive isotope TcO4–) adsorption. The adsorption equilibrium can be reached within only a few minutes. The adsorption capacities of aMOC-1 for CrO42–, Cr2O72–, and ReO4– are 157.4, 203.8, and 350 mg/g, respectively. Because aMOC-2 contains larger metal–organic cage, the corresponding adsorption capacities are 242.0, 407.0, and 583.9 mg/g, respectively. The capacity of aMOC-2 for Cr(VI) oxo-anion adsorption ranks the highest among all related materials ever reported. In addition, the oxo-anions can be released rapidly within several minutes from the oxo-anion-loaded aMOCs in 2 M NaNO3 solution, which allows these materials to exhibit good reusability. Finally, the aMOCs have a potential application in the removal of Cr(VI) from electroplating bath wastewater, in which the concentration changes from 10 to 0.17 ppm after treatment with aMOCs.