Selective Adsorption of Hg(II) Ions Using Functionalized Zr Mixed-Linker MOFs

The functionalization of MOFs (metal–organic frameworks) is a challenging issue in developing superior porous networks with a high surface area. To do so, one of the most powerful tools is using the mixed-linker strategy, in which different linkers are used in the synthesis of the MOF structure. In this study, we synthesized a mixed-linker MOF by applying a mixture of fumaric acid and 3,5-pyrazoledicarbixylic acid as linkers under solvothermal conditions to enhance the both porosity and functionality of Zr-MOF. Mixed-linker zirconium MOF is denoted as "[Zr(PZDC)x(FUM)1–x]n" (PZDC = 3,5-pyrazoledicarboxylic acid and FUM = fumaric acid). Furthermore, the incorporation of N-rich linkers into the mixed-linker MOF allowed for the control of the efficient and high adsorption capacity of the mercury ions in an aquatic medium compared with the parent MOF. The results showed that the Zr mixed-linker MOF had a remarkable adsorption capacity of 700 mg/g under 60 min for Hg(II) at pH 7 and 25 °C, which was much higher than that of the parent MOF. The adsorption kinetics followed the pseudo-second-order model, and the adsorption isotherm fitted well with the Langmuir model, indicating a monolayer and homogeneous adsorption process. Moreover, Zr-MOF showed excellent selectivity for Hg(II) over other common metal ions, including Zn(II), Co(II), Cd(II), Ni(II), Hg(II), and Pb(II), and good stability and recyclability after five cycles of adsorption–desorption. Therefore, applying the mixed-linker strategy can significantly improve the MOF porosity, functionality, and adsorption capacity of MOFs for heavy metal removal.