Binding Mechanism of the Amidoxime Functional Groupon Chelating Resins towardGallium(III) in Bayer Liquor.

Amidoxime is of broadinterest as a functional group for extractionand separation of different metals from aqueous solution due to itshigh affinity for a wide range of metal ions. Resins which containan amidoxime group have exhibited good extractive properties for galliumand are adopted as the most efficient adsorbent for the recovery ofgallium from Bayer liquor. However, the coordination mechanism betweenthe amidoxime functional group of the resin and Ga(III) in Bayer liquoris unclear so far. Here, to identify the preferred action and thebinding motif when amidoxime binds with Ga(III), we performed densityfunctional theory (DFT) calculations and subsequent FT-IR spectraanalysis for the amidoxime resin before and after adsorption of Ga(III)in Bayer liquor. Single-crystal X-ray diffraction was also used tovalidate the prediction. The fully relaxed structure and binding energycalculations of a series of complexes containing acetamidoximate (AO–) and gallium species, [Ga(OH)4-x(AO–)x]−, showed that, of the three general binding motifsof the amidoxime group toward metal ions, complexes with oxygen bindingmotif I are more stable than complexes with chelate binding motifII or η2binding motif III; and Ga(III) in Bayerliquor prefers to coordinate with only one amidoximate rather thantwo or more amidoximates. The FT-IR spectrum of the resin after adsorptionconfirmed the coordination between Ga and oximido by the shiftingof N–O and O–H stretching vibrations to higher wavenumberand the appearance of Ga–O stretching vibrations. No characteristicstretching band of Ga–N was found in the FT-IR spectra, indicatingbinding motifs II and III unlikely happened. Structural analysis ofthe single-crystal X-ray diffraction date revaled that the amidoximeligand is deprotonated at a single oxime oxygen atom and coordinatesto one gallium atom only through this oxygen, confirming the oxygenbinding motif. Accordingly oxygen binding mode I might be the mainbinding motifs of amidoxime toward Ga(III) in Bayer liquor, whichvalidated the prediction of DFT calculations. [ABSTRACT FROM AUTHOR]