Sequestering uranium from UO₂(CO₃)₃⁴⁻in seawater with amine ligands: density functional theory calculations

Abstract

The polystyrene-supported primary amine -CH2NH2 has shown an at least 3-fold increase in uranyl capacity compared to a diamidoxime ligand on a polystyrene support. This study aims to understand the coordination of substitution complexes from UO2(CO3)3(4-) and amines using density functional theory calculations. Four kinds of amines (diethylamine (DEA), ethylenediamine (EDA), diethylenetriamine (DETA) and triethylenetetramine (TETA)) were selected because they belong to different classes and have different chain lengths. The geometrical structures, electronic structures and the thermodynamic stabilities of various substitution complexes, as well as the trends in their calculated properties were investigated at equilibrium. In these optimized complexes, DEA groups bind to uranyl as monodentate ligands; EDA groups serve as monodentate and bidentate ligands; DETA groups act as monodentate and tridentate ligands; while TETA groups serve as monodentate, bidentate and tridentate ligands. The thermodynamic analysis confirmed that the primary amines coordinate to uranyl more strongly than does the secondary amine. The stabilities of substitution complexes with primary amines were calculated to decrease with increasing chain length of the amine, except for UO2(L2)(2+). Of the complexes analyzed, only UO2L(CO3)2(2-) (L = EDA and DETA) and UO2L2CO3 (L = EDA) were predicted to form from the substitution reactions with UO2(CO3)3(4-) and protonated amines as reactants in aqueous solution. Amines were calculated to be comparable to, or sometimes weaker than, amidoximate in replacing CO3(2-) in UO2(CO3)3(4-) to coordinate to uranium. Therefore, the coordination mechanism, in which amines replace carbonates to bind to uranyl, is not primarily responsible for the experimentally observed 3-fold or greater increase in uranyl capacity of primary amines compared to a diamidoxime ligand. Based on the results of our calculations, we believe that the cation exchange mechanism, in which the protonated amines bind to uranyl tricarbonate directly, plays a leading role in the uranium recovery from seawater by amines.

Preparation of Amidoximated Ultrahigh Molecular Weight Polyethylene Fiber by Radiation Grafting and Uranium Adsorption Test

“…Although a number of other uranium extraction methods have been investigated, − amidoxime (AO)-based adsorbents are recognized by many groups as the most promising material. − Under the memorandum of understanding between the Chinese Academy of Science (CAS) and the U.S. Department of Energy (DOE), the research groups in SINAP − and ORNL − parallel-developed and compared several adsorbents based on AO groups. Nevertheless, real ocean adsorption experiments must still be further promoted.…”

The coordination of amidoxime ligands with uranyl in the gas phase: a mass spectrometry and DFT study

“…45 Although the most stable binding modes (η 2 for NAO and GDO, tridentate for GIO) may be disrupted upon dehydration, the residual amine groups may account for the binding with uranyl. 46 For the [UO 2 (L − H)] + complexes, unique fragmentation pathways were observed, depending on the structures of the ligands themselves. For the NAO ligand, two fragment ions at m/z of 453 and 427 were observed (Fig.…”

Theoretical Insights on Improving Amidoxime Selectivity for Potential Uranium Extraction from Seawater

“…At present, theoretical calculations are essential for designing efficient adsorption groups for extracting uranium from seawater. − Previous studies , showed that the alkyl-substituted amidoximes (Figure a) increase the selectivity for uranyl ions, and the ortho position amino to the amidoxime group can enhance the extraction ability of uranyl. These results raise the question of whether introduction of an ortho position amino group of the alkyl-substituted amidoxime can enhance its selectivity to uranyl.…”