Cu²⁺ sorption from aqueous media by a recyclable Ca²⁺ framework

A new Ca²⁺ two-dimensional framework, namely [Ca(H₄L)(DMA)₂]·2DMA (Ca-MOF) was obtained from the initial use of N,N′-bis(2,4-dicarboxyphenyl)-oxalamide (H₆L). We discovered that this Ca-MOF is capable of exchanging the Ca²⁺ ions by Cu²⁺ almost quantitatively in a matter of seconds in aqueous solution. The highly efficient Cu²⁺ sorption properties exhibited by the Ca-MOF were investigated in detail via batch ion-exchange studies. In addition, the Ca-MOF was utilized as a stationary phase in an ion-exchange column for Cu²⁺ removal from aqueous media. Furthermore, we were able to recycle the most expensive part of the framework, i.e. the H₆L, by treating the column with dilute NaOH followed by HCl. The Ca-MOF represents the first example of any MOF capable of exchanging its constituent metal ions that is shown to be a highly effective ion-exchange material, thus opening a new window in the exploitation of MOFs for ion-exchange applications.