Macroscopic and Extended X-ray Absorption Fine Structure Spectroscopic Investigation of Ligand Effect on Zinc Adsorption to Kaolinite as a Function of pH

Adsorption of zinc (Zn) to mineral surfaces is affected by various factors including naturally occurring ligands in soil environment. This study was conducted to elucidate the influence of citrate, humic acid (HA), siderophore desferrioxamine B (DFO-B) and phosphate (PO4-3) on Zn adsorption to kaolinite at varying pH using macroscopic adsorption isotherm and extended X-ray absorption fine structure spectroscopy. Zinc was adsorbed in kaolinite suspensions at Zn: ligand ratio of 1: 1 and pH 4.5 to 7.5. Humic acid and PO4-3 enhanced Zn adsorption throughout the pH range, whereas citrate and DFO-B slightly increased Zn adsorption at approximately pH 5.7 or less but greatly suppressed it at greater than pH 5.7. Extended X-ray absorption fine structure analysis revealed that, at pH 5.5, Zn was adsorbed to kaolinite forming innersphere edge-sharing bidentate linkages in the absence (control) and presence of citrate, HA, and DFO-B. With pH increase to 7.5, the same adsorption geometry was evident in the presence of HA and DFO-B, whereas Zn was incorporated into a Zn-aluminum LDH surface precipitate in the control and formed outer-sphere complexes in the citrate presence. Phosphate precipitated Zn across the pH range studied. Overall, Zn adsorption mechanism of kaolinite varied with specific ligand and pH.