Zinc and Cadmium Adsorption to Aluminum Oxide Nanoparticles Affected by Naturally Occurring Ligands

Nanoparticles of aluminum oxide (Al2O3) are efficient in removing Cd, Zn, and other heavy metals from wastewaters and soil solutions due to their high specific surface area and surface area to volume ratio. Naturally occurring ligands, such as phosphate (PO4), citrate, and humic acid (HA), may affect the efficiency of Al2O3 nanoparticles in adsorption of Cd and Zn. The objective of this study was to investigate Zn and Cd adsorption to Al2O3 nanoparticles as influenced by PO4, citrate, and HA. Adsorption of Zn and Cd was performed in mono-metal and binary-metal systems at pH 6.5 with initial metal concentration of 1.0 mmol L-1 and varying ligand concentration at a solid: solution ratio of 1:1000. Adsorption isotherms showed that Zn had higher affinity to the Al2O3 nanoparticle surface than Cd and that adsorption of Zn and Cd in the binary-metal system was lower than in the respective mono-metal systems. Phosphate and HA enhanced Zn and Cd adsorption in all systems, whereas citrate reduced Zn adsorption in the mono-metal system by 25% and increased adsorption in the other metal systems. Removal of Zn or Cd from the systems was generally accompanied by enhanced removal of PO4 and HA, which may indicate enhanced adsorption due to ternary complex formation or metal-ligand precipitation. Phosphate was the most effective among the three ligands in enhancing Zn and Cd adsorption. Overall, Al2O3 nanoparticles are suitably used for Zn and Cd adsorption, which can be significantly enhanced by the presence of PO4 or HA and to a lesser degree by citrate at low concentrations.