Experimental and DFT studies on complexation of uranyl with N-(2-Hydroxyethyl)iminodiacetic acid in aqueous medium

Abstract

The nuclear decontaminating agents such as ethylenediamminetetraacetic acid (EDTA), picolinic acid, nitrilotriacetic acid (NTA) and citric acid are co-disposed with the nuclear waste at the storage sites. The thermal, catalytic and radiolytic degradation of this organic waste results in the formation of N-(2-Hydroxyethyl)iminodiacetic acid (HEIDA-H2) as one of the major degradation product. The knowledge on chemical speciation and aqueous thermodynamics of radionuclides in presence of organic chelates is a key input to the codes for framing safe and strategic disposal of radioactive waste. Present work aimed at the determination of thermodynamic parameters (log K, ΔG, ΔH and ΔS), speciation diagram, spectroscopic properties and structural analysis for UO22+-HEIDA complexes in aqueous solution. UO22+ forms ML and ML2 type of species with HEIDA as determined by potentiometry and further corroborated by absorption and luminescence spectroscopy. The Electrospray ionization mass spectrometry (ESI-MS) studies also identified the ML and ML2 complexes. In addition, the difference in the complexation behavior of UO22+ with HEIDA and IDA; which differ by hydroxyethyl substitution on imino nitrogen is explained on the basis of thermodynamic parameters. The 1:1 UO22+-HEIDA complex is exothermic formation while the 1:1 UO22+-IDA complexes are endothermic formations. The differences in enthalpy and entropy of formations are interpreted to reveal the role of hydroxyethyl group on UO22+- HEIDA complexation. NMR studies are carried to elucidate the structures of 1:1 and 1:2 UO22+-HEIDA complexes. The comparison of log K values with structurally similar ligands and NMR spectroscopic studies suggested the tridentate nature for HEIDA. The hydroxyethyl group was found to be uncoordinated with UO22+. The theoretical studies showed the indirect participation of hydroxyethyl group on complexation by charge polarization and by steric influence on coordinating groups.