Adsorption was conducted in 50ml centrifuge tubes at a 1:1 metal to ligand molar ratio and pH range of 4.5 to 7.5. Results showed that the adsorption of Zn on minerals was influenced by the presence of soluble ligands. The presence of phosphate in the gibbsite system had the largest impact among the ligands on Zn adsorption, especially at pH 6.5. For goethite, there were only minor differences between the ligands in terms of Zn adsorption. In the case of ferrihydrite, the differences were observed at pH levels < 7.5 where citrate had the largest impact on adsorption. Results from X-Ray Absorption Fine Structure Spectroscopy (XAFS) indicated that Zn formed a bidentate inner sphere complex with ferrihydrite in both the control and in the presence of citrate.
This study demonstrated that ligands influence Zn adsorption depending on the type of mineral and pH. The possible formation of inner sphere complexes in some of the systems studied suggests limited mobility of Zn under similar environmental situations.
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