Strontium-doped hydroxyapatite as an efficient adsorbent for Cd(II) removal from wastewater: Performance, kinetics, and mechanism

Abstract

A strontium-doped hydroxyapatite (Sr-HAP) adsorbent was synthesized by a sol–gel method and used to remove Cd(II) from wastewater. The adsorption performance of the as-prepared Sr-HAP and the kinetics and mechanism of Cd(II) removal were investigated. The experimental results showed that the Sr-HAP adsorbent achieved the maximum Cd(II) adsorption capacity (158.30 mg/g) at a temperature of 298 K, an adsorbent dosage of 0.08 g/50 mL, and an initial solution pH of 6. The adsorption of Cd(II) onto the Sr-HAP adsorbent was well described by the Langmuir isothermal adsorption model and pseudo-second-order kinetic model, and was found to follow a monolayer chemisorption process. In addition, the internal diffusion model was also used to fit the experimental data. The results indicated that the Cd(II) adsorption process was controlled by both internal diffusion and surface adsorption. Thermodynamic analysis revealed that the Cd(II) adsorption process was a spontaneous endothermic process. Moreover, acidic reagents favored the desorption of Cd(II) from the Sr-HAP adsorbent, and the recycled Sr-HAP adsorbent still maintained a high adsorption capacity for Cd(II). Mechanism analysis revealed that the removal process of Cd(II) ions by the Sr-HAP adsorbent possibly involved ion exchange, co-precipitation, and surface complexation. Overall, the synthesized Sr-HAP exhibited an excellent adsorption capacity and good stability, thus acting as a promising candidate for removing Cd(II) from wastewater. • An efficient strontium-doped hydroxyapatite was synthesized for Cd(II) removal. • The Sr-HAP adsorbent had a high Cd(II) adsorption capacity. • The adsorption of Cd(II) by Sr-HAP was proved to be a monolayer chemiadsorption process. • The Sr-HAP adsorbent could maintain its high adsorption ability after regeneration. • The mechanism of Cd(II) uptake by Sr-HAP was comprehensively elucidated.