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

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 η2 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 X-ray 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.