Synthesis of Hydroxyapatite Porous Microspheres for Efficient Adsorption of Copper Ion from Water

ABSTRACT: Copper is a common heavy metal contamination source for water bodies, and achieving sustainable and cost-effective removal of Cu²⁺ from Cu-containing wastewater remains a challenge. In this study, an economical and eco-friendly adsorbent—hydroxyapatite (HA) porous microspheres—was synthesized via a simple one-step hydrothermal method. Adsorption experiments demonstrated that the maximum adsorption capacity of HA porous microspheres for Cu²⁺ is 116 mg/g, approximately 3.74 times that of reported HA nanosheet adsorbents. The adsorption process follows the pseudo-second-order kinetic model and the Sips isotherm model. The correlation coefficient R² = 0.9997. Linear fitting of the amounts of Cu²⁺ removed and Ca²⁺ leached at the same time revealed an R² value as high as 0.997, indicating that ion exchange is the dominant adsorption mechanism. Therefore, the excellent adsorption performance is attributed to the high specific surface area (207 m²/g) and mesoporous structure of the spherical HA adsorbent, which provides abundant active sites and promotes efficient ion diffusion. These structural advantages significantly enhanced the two primary adsorption mechanisms: ion exchange and surface complexation. Furthermore, the effects of adsorbent dosage, solution pH, reaction time, initial Cu²⁺ concentration, and temperature on adsorption performance were systematically investigated. Finally, the adsorption mechanism was investigated by characterizing the adsorbed material using XRD, FTIR, and XPS. It was determined that ion exchange, complexation, and electrostatic attraction are the main adsorption mechanisms. This study enhances the adsorption capacity of HA materials for Cu²⁺ by controlling morphology, offering new perspectives for developing high-performance, economical, eco-friendly, and sustainable adsorbents.