Study of electronic and steric effects of organic substituents on complexation between U(VI) and aromatic carboxylates in aqueous solution

Abstract

In order to accurately predict and control the chemical behaviors of actinides in aqueous environments, the complexation of actinide ions with relevant inorganic or organic ligands is required to be comprehensively investigated because the stable complexation reactions dominate the aquatic chemical properties of actinides, such as the oxidation state, solubility, species distribution, and chemisorption. Among the diverse chemical functionalities, aromatic carboxyl groups are ubiquitous in various natural organic matters and significantly govern the migration of actinide ions in natural water. Additionally, due to the excellent stability of the benzene moiety, the aromatic carboxyl groups have a potential to be further utilized as the basic structure of the chemical complexants for separation and recovery of actinides dissolved in the aqueous radioactive wastes. In this study, for the purpose of improving the current understanding of complexation behaviors between actinyl(VI) ions and aromatic carboxylates, methoxy and methyl groups were selected as major organic substituents, and 1:1 aqueous complexation of uranyl(VI) with the substituted benzoates was scrutinized. The spectroscopic properties and stability constants of the uranyl(VI) complexes were evaluated by means of UV-Vis absorption and Raman spectroscopy for the first time, and correlation between the resulting thermodynamic constants related to the substituents’ electronic and steric effects was analyzed based on the Hammett equation and the reported structural characteristics of several uranyl(VI) compounds. Furthermore, in the case of 2-methoxybenzoate, formation of crystalline 1:1:3 sodium uranyl(VI) 2-methoxybenzoate dodecahydrate was observed in the solutions. The spectroscopic properties, molecular structure, and solubility product of the U(VI) precipitate were identified using UV-Vis absorption, photoluminescence, Raman and IR spectroscopy, X-ray diffraction, and thermogravimetric analysis for the first time. The solid U(VI) complex has distinctive one-dimensional channels in the crystal structure, and a driving force of the precipitation reaction was discussed in relation to the steric effect of the ortho-substituent.