Preparation and Characterization of Hydroxyapatite-Modified Natural Zeolite: Application as Adsorbent for Ni2+ and Cr3+ Ion Removal from Aqueous Solutions

Natural zeolites (NatZ) are widely available, porous, crystalline aluminosilicate minerals that are commonly used as cost-effective adsorbents in water treatment processes. Despite their efficiency in removing various heavy metal ions from wastewater, NatZ show relatively low affinity toward Ni2+ and Cr3+ ions. This study aimed to develop composite adsorbents based on NatZ and hydroxyapatite using two methods, hydrothermal and mechanochemical, and their adsorption properties for the removal of Ni2+ and Cr3+ ions from aqueous solutions were investigated. X-ray powder diffraction and scanning electron microscopy analyses confirmed that under hydrothermal conditions, needle-like hydroxyapatite crystals were formed on the surface of NatZ, while the zeolite structure remained unchanged. Compared to the mechanochemically prepared sample, this adsorbent showed higher efficiency, binding 6.91 mg Ni2+/g and 16.95 mg Cr3+/g. Adsorption kinetics of the tested cations in both cases can be described by a pseudo-second-order model (R2 is higher than 0.95 for all adsorbents). It is concluded that the presence of hydroxyapatite on the zeolite surface significantly improves the adsorption performance of NatZ, demonstrating its potential for the removal of heavy metal ions in wastewater treatment.