Effect of clay substrates on phosphate adsorption by zirconium-modified natural clays

Excessive release of phosphate to aquatic systems may cause eutrophication and associated environmental issues. Removal of phosphate by clay mineral adsorption has been drawing attention due to the low cost, natural abundance, non-toxic nature, chemical and mechanical stability of natural clays. Owing strong adsorption affinity, zirconium (Zr) oxides/hydroxides have been studied to modify clays in order to improve the phosphate removal performance. In the present work, kaolinite (1:1, no expansion), montmorillonite (2:1, max expansion), and vermiculite (2:1, some expansion) were selected as three representative natural clays for Zr modification to investigate the effect of clay structure and expansion property on phosphate adsorption. The intercalation of Zr species into the interlayers of montmorillonite and vermiculite was confirmed by X-ray diffraction (XRD) measurement, while modification occurred on the external surface of kaolinite. Particularly, Zr-modified montmorillonite had highest phosphate adsorption capacity among all Zr-modified clays. Furthermore, the phosphate adsorption kinetics by Zr-modified montmorillonite were 6 and 3 times faster than those by Zr-modified vermiculite and kaolinite, respectively. The improved performance of Zr-modified montmorillonite may be attributed to the good expansion property of montmorillonite that favored the uniform intercalation of Zr species, making the adsorption sites easily accessible by phosphate. Furthermore, all Zr-modified clays showed robust performance for phosphate adsorption under various water chemistry conditions, suggesting Zr-modified clays could be used as adsorbents for phosphate control in aquatic systems.