Binding Mechanism of the Amidoxime Functional Group on Chelating Resins toward Gallium(III) in Bayer Liquor

Abstract

Amidoxime is of broad interest as a functional group for extraction and separation of different metals from aqueous solution due to its high affinity for a wide range of metal ions. Resins which contain an amidoxime group have exhibited good extractive properties for gallium and are adopted as the most efficient adsorbent for the recovery of gallium from Bayer liquor. However, the coordination mechanism between the amidoxime functional group of the resin and Ga(III) in Bayer liquor is unclear so far. Here, to identify the preferred action and the binding motif when amidoxime binds with Ga(III), we performed density functional theory (DFT) calculations and subsequent FT-IR spectra analysis for the amidoxime resin before and after adsorption of Ga(III) in Bayer liquor. Single-crystal X-ray diffraction was also used to validate the prediction. The fully relaxed structure and binding energy calculations of a series of complexes containing acetamidoximate (AO − ) and gallium species, [Ga(OH) 4-x (AO − ) x ] − , showed that, of the three general binding motifs of the amidoxime group toward metal ions, complexes with oxygen binding motif I are more stable than complexes with chelate binding motif II or η 2 binding motif III; and Ga(III) in Bayer liquor prefers to coordinate with only one amidoximate rather than two or more amidoximates. The FT-IR spectrum of the resin after adsorption confirmed the coordination between Ga and oximido by the shifting of N−O and O−H stretching vibrations to higher wavenumber and the appearance of Ga−O stretching vibrations. No characteristic stretching band of Ga−N was found in the FT-IR spectra, indicating binding motifs II and III unlikely happened. Structural analysis of the single-crystal Xray diffraction date revaled that the amidoxime ligand is deprotonated at a single oxime oxygen atom and coordinates to one gallium atom only through this oxygen, confirming the oxygen binding motif. Accordingly oxygen binding mode I might be the main binding motifs of amidoxime toward Ga(III) in Bayer liquor, which validated the prediction of DFT calculations.

Synthesis of Sawdust‐based Poly(amidoxime) Ligand for Heavy Metals Removal from Wastewater

“…The oxime is a chemical compound with general structure R 1 R 2 C=OH that belonging to the imines, where R 1 is an organic side chain and R 2 can be hydrogen (aldoxime) or other organic groups (ketoxime) . Amidoximes are the chemical compound of amide oxime with general structure RC(=NOH)(NRR’).…”

“…The use of biomass waste as economical adsorbents for heavy metal removal was highly focused by some research groups ,. Many research has been put effort to design and produce better polymeric ligands which are not only on removal of heavy metal ions but also produce stable precipitates which can easily separate from the wastewater.…”

Uniform macroporous amidoximated polyacrylonitrile monoliths for gallium recovery from Bayer liquor

“…After adsorption, the peak of NH stretching vibration at 3100–3500 cm −1 increased and shifted toward lower wavenumbers, while bending vibration at 1541 cm −1 was increased, showing that the amidoxime group had undergone alkaline hydrolysis in simulated Bayer liquor to form an amide group 42 . Also, a shift from 930 to 914 cm −1 was observed, which reflects the chelation between amidoxime groups and Ga 3+ 22 …”

“…42 Also, a shift from 930 to 914 cm −1 was observed, which reflects the chelation between amidoxime groups and Ga 3+ . 22…”

“…20 Kataoka et al 21 first used amidoxime ion exchange resin to separate pure gallium from Bayer liquor, and the recovery efficiency was over 96%, while the aluminum adsorption amount was only 0.1%, showing efficient selectivity. Long et al 22 considered the binding mechanism of the amidoxime group toward Ga 3+ in Bayer liquor is that Ga 3+ is bound to the oxygen atom after the deprotonation of amidoxime. Besides, Ga 3+ prefers to coordinate with only one amidoximate rather than two or more amidoximates.…”