Functional Carbon Capsules Supporting Ruthenium Nanoclusters for Efficient Electrocatalytic 99TcO4−/ReO4− Removal from Acidic and Alkaline Nuclear Wastes

The selective removal of the β-emitting pertechnetate ion (99TcO4−) from nuclear waste streams is technically challenging. Herein, a practical approach is proposed for the selective removal of 99TcO4− (or its surrogate ReO4−) under extreme conditions of high acidity, alkalinity, ionic strength, and radiation field. Hollow porous N-doped carbon capsules loaded with ruthenium clusters (Ru@HNCC) are first prepared, then modified with a cationic polymeric network (R) containing imidazolium-N+ units (Ru@HNCC-R) for selective 99TcO4− and ReO4− binding. The Ru@HNCC-R capsules offer high binding affinities for 99TcO4−/ReO4− under wide-ranging conditions. An electrochemical redox process then transforms adsorbed ReO4− to bulk ReO3, delivering record-high removal capacities, fast kinetics, and excellent long-term durability for removing ReO4− (as a proxy for 99TcO4−) in a 3 m HNO3, simulated nuclear waste-Hanford melter recycle stream and an alkaline high-level waste stream (HLW) at the U.S. Savannah River Site (SRS). In situ Raman and X-ray absorption spectroscopy (XAS) analyses showed that adsorbed Re(VII) is electrocatalytically reduced on Ru sites to a Re(IV)O2 intermediate, which can then be re-oxidized to insoluble Re(VI)O3 for facile collection. This approach overcomes many of the challenges associated with the selective separation and removal of 99TcO4−/ReO4− under extreme conditions, offering new vistas for nuclear waste management and environmental remediation.