A thiol-functionalized zirconium metal-organic cage for the effective removal of Hg²⁺ from aqueous solution

The mercury ions in waste water have threatened public health and environmental protection. In this sense, novel materials with outstanding performances for removal of Hg²⁺ are imperative. Herein, we demonstrate a thiol-functionalized zirconium metal-organic cage (MOC-(SH)₂) with excellent dispersion displays ideal properties for Hg²⁺ capture. MOC-(SH)₂ exhibits the ability of removing Hg²⁺ in aqueous solutions with a capacity of 335.9 mg_Hg2+/g_MOC-(SH)2, which surpasses that of classical Zr-based metal-organic framework Uio-66-(SH)₂ by 1.89 folds. The higher loading capacity of MOC-(SH)₂ is probably owing to the excellent dispersion of the discrete cage, which makes the accessibility of binding sites (thiol) easier. Additionally, 99.6% of Hg²⁺ can be effectively captured by MOC-(SH)₂ with the concentration decreased from 5 to 0.02 ppm reaching the permissible limit for Hg²⁺, outperforming the performance of Uio-66-(SH)₂. The excellent absorption property of MOC-(SH)₂ is also achieved in terms of superior selectivity under the presence of competitive metal ions. Meanwhile, the regenerated MOC-(SH)₂ can be reused without apparent loss of Hg²⁺ loading capacity. UV-vis absorption spectra, IR spectra and emission spectra further verified the strong chemical affinity between Hg²⁺ and the thiol of MOC-(SH)₂. The study lays the groundwork for using Zr-MOCs in the removal of toxic metal ions and environmental sustainability.