Pb ²⁺ and Cd ²⁺ removal from aqueous solutions onto calcium-phosphate hydroxyl surface: kinetics, isotherms and thermodynamic investigations

This study evaluates the adsorption efficiency of calcium-phosphate hydroxyl (CPH) for removing Pb2+ and Cd2+ from water. Characterisation via XRD, FT-IR, TEM and BET confirmed a mesoporous structure (2.15 nm pore size, 44.33 m2/g surface area) and functional groups essential for adsorption. Optimal conditions (pH 5.0, 45°C, 0.4 g dose) yielded maximum capacities of 15.084 mg/g (Pb2+) and 20.240 mg/g (Cd2+). Pb2+ followed pseudo-second-order kinetics (chemisorption) and Cd2+ followed pseudo-first-order (physisorption). Freundlich and Langmuir's isotherms best described Pb2+ and Cd2+ adsorption, respectively. Thermodynamics indicated endothermic, spontaneous processes with increased randomness. Desorption using 0.1 M HCl showed initial efficiencies of 79.7% (Cd2+) and 68.32% (Pb2+), with stable reuse over four cycles. Selectivity tests confirmed CPH's preference for Pb2+ and Cd2+ over other metals. Overall, CPH is a cost-effective, selective and reusable adsorbent, making it a promising material for heavy metal removal in wastewater treatment.