Study on the Behaviors and Mechanism of Ni(II) Adsorption at the Hydroxyapatite-Water Interface: Effect of Particle Size

Hydroxyapatite (HAP) was a highly efficient decontamination material for its strong adsorption capacity used in the immobilization of heavy metals, while the particle-size effect was insufficiently investigated during the sorption process. In the present study, the mechanisms of nickel (Ni(II)) adsorption on HAPs with two different particle sizes were investigated by combing batch experiments, desorption, and XRD analysis. The results showed that the adsorption capacity of 20 nm HAP (nano-HAP) was much higher than that of 12 μm HAP (micro-HAP). It was noticed that the results of the present study also clarified the distinct mechanisms in each adsorption process. As for micro-HAP, Ni2+ adsorbed through slow diffusion and replacement with Ca2+ and then incorporated in the lattice at pH between 6.5 and 9.0, which was confirmed by the results of kinetics, thermodynamics, and desorption. And a more compact crystalline structure and irreversible desorption behavior of micro-HAP after Ni(II) adsorption was confirmed by results of XRD and desorption isotherms, respectively. At pH > 9:0, lattice incorporation and precipitation controlled together. However, for nano-HAP, the sharp increase of Ni(II) adsorption and ionic strength dependent at pH 6.5 to 9.0 revealed that the dominant mechanisms were ionic exchange and inner-sphere complexation. XRD results showed that characteristic peaks of cassidyite appeared in Ni(II)-loading nano-HAP. At pH > 9:0, a precipitate of Ni(II) was the dominant mechanism. The experimental finds demonstrated that nanoscale HAP was a more fast, efficient, and desorbable adsorbent than micro-HAP for Ni(II) removal. These findings would be favorable for investigating the removal mechanisms of heavy metals on the HAP materials and designing the synthesis methods.