Anion-exchanged and quaternary ammonium functionalized MIL-101-Cr metal-organic framework (MOF) for ReO₄^-/TcO₄^- sequestration from groundwater

There are few effective technologies for the sequestration of highly water-soluble pertechnetate (TcO₄^-) from contaminated water despite the urgency of environmental and public health concerns. In this work, anion exchanged and cetyltrimethylammonium bromide (CTAB) functionalized MIL-101-Cr-NO₃ were investigated for perrhenate (ReO₄^-), a surrogate of TcO₄^-, sequestration from artificial groundwater. Cl^-, I^-, and CF₃SO₃^- exchanged MIL-101-Cr proved more effective at ReO₄^- removal than the parent MIL-101-Cr-F. Compared to the parent framework, CTAB functionalized MIL-101-Cr-NO₃ increased ReO₄^- removal capacity from 39 to 139 mg/g, improved the reaction kinetics from ~30 to <10 min to reach full adsorption capacity and the selectivity for ReO₄^- over competing NO₃^-, CO₃^2-, SO₄^2-, and Cl^-. Spectroscopic data indicated that the chemical speciation of Re in the exchanged MIL-101-Cr remained ReO₄^-, indicating synergistic sequestration through both anion exchange and non-ion exchange binding with the positively charged ligand of CTAB. These studies foreshadow potential applications of MOFs for the remediation of ⁹⁹TcO₄^- from contaminated environments.