Development of nano-hydroxyapatite@biochar composite from low-cost precursors for enhanced immobilization of lead and cadmium.

The composite of nano-hydroxyapatite (nHAP) and biochar offers dual advantages: enhancing biochar's heavy metal adsorption capacity while improving the dispersion of nHAP, positioning nHAP@biochar as a promising candidate for heavy metal immobilization. In this study, ammonia-soda residue (ASR) leachate and Doudan skin waste (DDS) were explored for the first time as low-cost precursors to synthesize nHAP@biochar for efficient adsorption of lead (Pb(II)) and cadmium (Cd(II)). The optimal mass ratio of nHAP to biochar, 2:1 (HMD2), was determined. The results showed that the uptake of both metal ions by HMD2 was favorable and spontaneous, with Pb(II) exhibiting a faster adsorption rate than Cd(II). Molecular dynamics (MD) simulations were applied to model the adsorption kinetics of metal ions on the pure HAP surface, offering a molecular-level interpretation for the experimental results. Furthermore, HMD2 demonstrated exceptionally high theoretical adsorption capacities of 1250 and 172.4 mg/g for Pb(II) and Cd(II), respectively, outperforming many other similar adsorbents. Mechanistic analysis revealed multiple processes involved in the adsorption of metal ions on HMD2, including pore filling, ion exchange, cation-π interaction, precipitation, complexation, and electrostatic interactions. Density functional theory (DFT) calculations further supported that surface oxygen atoms in HAP serve as the primary active sites for adsorption. This study not only provides new insights into the valorization of ASR and DDS but also lays a theoretical foundation for the application of nHAP@biochar composites in the immobilization of heavy metals in contaminated soils.