Efficient Capture of Perrhenate and Pertechnetate by a Mesoporous Zr Metal-Organic Framework and Examination of Anion Binding Motifs

At the Hanford Site in southeastern Washington state, the U.S. Department of Energy intends to treat 56 million gallons of legacy nuclear waste by encasing it in borosilicate glass via vitrification. This process ineffectively captures radioactive pertechnetate (TcO4-) because of the ion's volatility, thereby requiring a different remediation method for this long-lived (t(1/2) = 2.1 x 10(5) years), environmentally mobile species. Currently available sorbents lack the desired combination of high uptake capacity, fast kinetics, and selectivity. Here, we evaluate the ability of the chemically and thermally robust Zr-6-based metal organic framework (MOF), NU-1000, to capture perrhenate (ReO4-), a pertechnetate simulant, and pertechnetate. Our material exhibits an excellent perrhenate uptake capacity of 210 mg/g, reaches saturation within 5 min, and maintains perrhenate uptake in the presence of competing anions. Additionally, experiments with pertechnetate confirm perrhenate is a suitable surrogate. Single-crystal X-ray diffraction indicates both chelating and nonchelating perrhenate binding motifs are present in both the small pore and the mesopore of NU-1000. Postadsorption diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) further elucidates the uptake mechanism and powder X-ray diffraction (PXRD) and Brunauer-Emmett-Teller (BET) surface area analysis confirm the retention of crystallinity and porosity of NU-1000 throughout adsorption.