Mechanism unravelling for ultrafast and selective ⁹⁹TcO₄⁻ uptake by a radiation-resistant cationic covalent organic framework: a combined radiological experiment and molecular dynamics simulation study

Abstract

⁹⁹Tc is one of the most problematic fission products in the nuclear fuel cycle owing to its large inventory in used nuclear fuel, long half-life, potential radiation hazard, high environmental mobility of its major species ⁹⁹TcO₄ ^-, and its redox-active nature. Ideally, ⁹⁹TcO₄ ^- should be removed at the first stage, when the used fuel rods are dissolved in highly concentrated nitric acid solution, which can substantially reduce its interference with the solvent extraction process through catalytic redox reactions with the key actinides and diminish the chance of discharge into the environment as the volatile species during the waste vitrification process. However, this task cannot be achieved by any of the reported anion-scavenging materials including traditional polymeric anion-exchange resins, inorganic cationic framework materials, and recently developed cationic metal-organic framework materials, because they either are not stable under the extreme conditions of the combined high acidity and strong radiation field or do not possess the required uptake selectivity towards ⁹⁹TcO₄ ^- in the presence of a huge excess of competing anions such as NO₃ ^- and SO₄ ^2-. Herein, we present the first study of ⁹⁹TcO₄ ^- removal under extreme conditions by a two-dimensional conjugated cationic covalent organic framework material, SCU-COF-1. This material exhibits ultrahigh acid stability, great resistance towards both large-dose β and γ irradiation and unprecedented ⁹⁹TcO₄ ^- uptake capabilities including extremely fast sorption kinetics (sorption equilibrium can be reached within 1 min), ultrahigh uptake capacity (702.4 mg g^-1 for the surrogate ReO₄ ^- at a slightly elevated temperature), and good anion-exchange selectivity towards ⁹⁹TcO₄ ^-. These excellent features endow SCU-COF-1 with the practical capabilities of separating ⁹⁹TcO₄ ^- from both simulant highly acidic fuel reprocessing solutions (3 M nitric acid) and low-activity waste streams at the US legacy nuclear site. The anion-exchange mechanism and the ⁹⁹TcO₄ ^- uptake selectivity are further demonstrated and clearly visualized by the molecular dynamics simulation investigations.