Characterization of Zn Adsorption by Alumosilicates and Oxides in the Presence of Citrate, Phosphate and Siderophore Using X-Ray Fine Structure Spectroscopy.

Adsorption is an important process that controls the mobility and bioavailability of metals in soils. The presence of soluble ligands in soils may significantly influence metal adsorption. They may either promote or hinder heavy metal adsorption depending on factors like the type and concentration of organic ligand and on pH. This study investigated the effect of organic and inorganic ligands (citrate, humic acid, siderophore and phosphate) on Zn adsorption by various mineral systems such as kaolinite, montmorillonite, goethite, ferrihydrite, and gibbsite as a function of pH. In addition, X-Ray Absorption Fine Structure Spectroscopy (XAFS) was employed to characterize the adsorption mechanisms of Zn onto mineral surfaces.

 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.