Coordination of Np(V) with Carboxylate/Phenolate Ligands: Implications for Environmental Migration and Separation Material Design.

Neptunium (Np), a key actinide element in the nuclear fuel cycle, presents critical scientific challenges regarding its environmental migration behavior and efficient separation technologies, which are essential for nuclear waste safe disposal and resource recovery. This study systematically investigates the aqueous coordination chemistry of Np(V) with three representative small-molecule ligands, benzoic acid, phenol, and salicylic acid, that model the coordination environment of natural organic matter. Utilizing variable-temperature spectrophotometric titration coupled with density functional theory (DFT) calculations, we elucidated the composition, stability constants, and thermodynamic parameters of the formed complexes. The coordination processes were found to be endothermic and entropy-driven, with ligand affinity following the order: salicylic acid ≫ phenol > benzoic acid. Salicylic acid exhibited exceptional entropy gain and coordination stability due to the chelating effect of its ortho-carboxylate-phenolate groups. This work provides molecular-level insights into the coordination mechanisms between Np(V) and oxygen-containing functional groups. The obtained quantitative thermodynamic and structural data establish a molecular-level reference framework for understanding the intrinsic coordination behavior of Np(V) with representative oxygen-donor functional groups. These fundamental insights may inform future assessments of Np speciation in complex environmental media and provide a thermodynamic basis for the rational design of functional materials for Np separation.