Highly selective crystallization of uranium complexes for actinide partitioning in nuclear waste streams

Partitioning actinides from lanthanides is essential for the safe management of nuclear waste and the long-term sustainability of nuclear energy. Here, we report a crystallization-based strategy that leverages the metal-dependent hydrolysis of 3,6-bis-2-pyridyl-1,2,4,5-tetrazine (L1) to generate distinct coordination environments. Specifically, U(VI) hydrolysis produces ( E )-N′-(pyridin-2-ylmethylene) picolinohydrazide (L2), directing the formation of a robust tetranuclear uranyl complex with pentagonal bipyramidal geometry. In contrast, Ln(III) fails to crystallize under the same conditions, and Pd(II) forms crystalline products only under specific solvothermal conditions via L1 hydrolysis, which generates 2-pyridinecarboxylicacid, 2-(2-pyridinylcarbonyl)hydrazide (L3). Systematic evaluation of acidity, ligand stoichiometry, and crystallization kinetics revealed conditions under which U(VI) selectively crystallizes from simulated spent nuclear fuel streams containing 22 competing elements, affording crystalline products with purities up to 99.8 %. High separation factors (SFs) were achieved, with values of 985, 2813, 2536, 1786, 1612, and 6571 for U/Y, U/Ce, U/Pr, U/Eu, U/Gd, and U/Dy, respectively, while ReO₄^–, used as a surrogate for ⁹⁹TcO₄^–, exhibited an exceptional SF of 303905. This work demonstrates that crystallization can provide a simple, highly selective, and waste-minimizing pathway for uranium recovery, offering a promising new paradigm for actinide partitioning in spent nuclear fuel reprocessing.