Capture of radioactive TcO4 – from nuclear wastes is extremely desirable for waste disposal and environmental restoration. Here, we report the synthesis of hydrolytically stable cationic covalent organic nanosheets (iCON) for efficient uptake of ReO4 –, a nonradioactive surrogate of TcO4 –. The iCON combines cationic guanidine-based knots with hydroxyl anchored neutral edge units and chloride ions loosely bonded in the pores, rendering extremely fast exchange kinetics toward ReO4 – with high uptake capacity of 437 mg g–1 and prominent distribution coefficient of 5.0 × 105. The removal efficiency remains stable over a pH range of 3–12 and allows selective capture of ReO4 – in the presence of excessive competing anions such as NO3 –, CO3 2–, PO4 3– and SO4 2– with good removal efficiency for ReO4 – in a simulated Hanford LAW Melter Recycle Stream. Anion exchange between the ReO4 – in solution and the chloride ion in iCON plays dominant role in the adsorption of ReO4 –. The iCON shows promise for effective removal of radioactive 99Tc from nuclear waste.

A knot-linker planarity control strategy was developed to construct highly crystalline cationic covalent organic frameworks with C3-symmetric guanidine units for fast removal of 2,4-dichlorophenol with high adsorption capacity.