Bio-inspired synthesis of hierarchical porous alginate-hydroxyapatite-carbonaceous composites for synergistic remediation of heavy metal and organic contaminants: Kinetics, thermodynamics, and long-term cyclic regeneration efficacy.

This study reports the fabrication of a hierarchically porous alginate-hydroxyapatite-carbonaceous (HPAHC) composite engineered for simultaneous removal of Pb2+ ions and Rhodamine B (RhB) from aqueous systems. The composite was synthesized through ionic crosslinking, in situ hydroxyapatite mineralization, and controlled carbonization, integrating ion-exchange functionality, phosphate-based coordination sites, and π-electron-rich carbon domains within a single framework. Structural characterization confirmed the formation of hexagonal hydroxyapatite and a well-developed interconnected pore network. Nitrogen adsorption-desorption analysis revealed a high specific surface area of 523.4 m2 g-1 with combined micro-mesoporous features that facilitate efficient mass transfer. Adsorption studies demonstrated high uptake capacities, with maximum equilibrium values reaching approximately 450 mg g-1 for Pb2+ and 340 mg g-1 for RhB at elevated concentrations. Isotherm modeling indicated strong agreement with the Langmuir model, suggesting dominant monolayer adsorption behavior within the investigated concentration range. Kinetic analysis followed a pseudo-second-order model, implying that surface interactions govern the rate-controlling step. Thermodynamic evaluation confirmed spontaneous and endothermic adsorption, with ΔG° values ranging from -18.5 to -25.1 kJ mol-1 and ΔH° values of approximately 80 kJ mol-1, accompanied by positive entropy changes associated with interfacial dehydration and structural rearrangement. The composite retained approximately 87.5% of its initial adsorption capacity after five regeneration cycles using an acid-ethanol eluent, indicating good structural stability. These results demonstrate that the HPAHC framework offers a structurally integrated platform for dual-contaminant remediation under mixed-pollutant conditions.