Hydroxyapatite-loaded coal gasification slag for synergistic heavy metal adsorption.

Lead-zinc mining wastewater contains Pb(II), Cu(II), and Cd(II) that are difficult to remove efficiently. A coal gasification slag-supported nano-hydroxyapatite composite (nHAP-CGS) was prepared via chemical precipitation. The composite achieved removal efficiencies of 97.6%, 94.9%, and 79.8% for Pb(II), Cu(II), and Cd(II), respectively, under optimized conditions (2.5 g/L, pH 4, 120 min, 100 mg/L), outperforming unmodified CGS with a selectivity order of Pb>Cu>Cd. Adsorption followed Langmuir and pseudo-second-order models, indicating chemisorption with intra-particle diffusion. Thermodynamic parameters (ΔG < 0, ΔH > 0, ΔS > 0) confirmed spontaneous endothermic adsorption. BET analysis showed a surface area of 57.65 m2/g for nHAP-CGS, significantly higher than raw CGS. Coexisting-ion tests and leaching experiments confirmed selectivity and environmental safety. This work provides a basis for coal gasification slag valorization and heavy metal remediation.