A Ca²⁺ MOF combining highly efficient sorption and capability for voltammetric determination of heavy metal ions in aqueous media

We recently discovered that a Ca²⁺ two-dimensional framework, namely [Ca(H₄L)(DMA)₂]·2DMA (Ca-MOF), was capable of exchanging the Ca²⁺ ions with Cu²⁺ almost quantitatively in a matter of seconds in aqueous solution. Herein, we report that Ca-MOF exhibits the capability of both removal and voltammetric determination of heavy metal ions in aqueous media. Ca-MOF shows one of the highest Pb²⁺ sorption capacities (∼522 mg g^-1) reported for MOFs. More importantly, a column filled with Ca-MOF (1% wt) particles dispersed in silica sand (99% wt) can quantitatively remove traces of Pb²⁺ (∼100 ppb) from a relatively large volume of a wastewater simulant solution (containing a large excess of competitive ions). Ca-MOF is also highly efficient for sorption of Cd²⁺, Ni²⁺ and Zn²⁺, even in the presence of several competitive cations. Actually, the Cd²⁺ sorption capacity (∼220 mg g^-1) of Ca-MOF is one of the largest reported for MOFs. Furthermore, detailed Ni²⁺ and Zn²⁺ sorption studies of MOFs have not been described prior to this work. The mechanism of the M²⁺ (M²⁺ = Pb²⁺, Cd²⁺, Ni²⁺, Zn²⁺) exchange process was elucidated based on a series of spectroscopic, analytical and X-ray diffraction methods. In addition, a simple ready-to-use electrochemical sensor based on modified graphite paste with Ca-MOF was fabricated and successfully utilized for the determination of Pb²⁺, Cd²⁺, Cu²⁺ and Zn²⁺ at μg L^-1 levels in aqueous solutions by anodic stripping voltammetry (ASV). Overall, this work demonstrates, for the first time, a dual function of a MOF as a sorbent and as an electrochemical sensor for heavy metal ions, thus opening a new window for materials with application in both environmental remediation and monitoring.