Polysubstituted guanidine-based receptors for the selective extraction of pertechnetate anions from high-level waste

In this work, we report the design and synthesis of a novel series of guanidine-based extractants tailored for the selective and efficient extraction of the pertechnetate anion (TcO₄^-). A library of 24 structurally diverse ligands was prepared and systematically evaluated for the extraction of technetium from nitric acid solutions and high-level waste (HLW) simulants. Several compounds exhibited outstanding performance, with distribution ratios (D_Tc) exceeding 2000 and selectivity factors of up to 400 over competing anions. The extraction mechanism was investigated via back-extraction experiments, revealing the influence of nitric acid concentration, temperature, and alkaline stripping agents. The composition of the extracted species was confirmed by single-crystal X-ray diffraction (SC-XRD) and X-ray absorption spectroscopy (XAS), supporting the formation of discrete ion pairs between the guanidinium cation and pertechnetate anion. These results highlight the potential of guanidine-based receptors for targeted separation of Tc(VII) from complex nuclear waste matrices.