“…27,28 Crown ethers are cyclic compounds made of bonded ether. 29,30 They can react with metal ions through dipole-charge electrostatic interaction to form stable complexes, which are often used to selectively extract metal ions. Cheng et al grafted 2-(hydroxymethyl)-12-crown-4-ether onto the surface of chitosan using a low-temperature phase separation approach to obtain a crown ether-modified chitosan nanofiber membrane, which demonstrated high selectivity for Li + with separation factors of 1.40, 34.05, 1.33 and 3.46 for Ca 2+ , K + , Mg 2+ and Na + , respectively.…”