Activated Carbons from Apricot Kernel Shells for Wastewater Treatment: Adsorption of Pb2+ and Rhodamine B with Equilibrium, Kinetics, Thermodynamics, and DFT Analysis

Apricot kernel shells were evaluated as a sustainable activated carbon precursor for wastewater treatment using experimental and theoretical methods. Two adsorbents were synthesized: physically activated with CO2 (AKS-CO2) and chemically activated with H3PO4 (AKS-H3PO4). Comprehensive materials characterization and adsorption tests using Pb2+ ions and Rhodamine B dye (RhB) as model pollutants revealed that AKS-H3PO4 significantly outperformed its physically activated counterpart. With an exceptionally high specific surface area (1159.4 m2/g) enriched with phosphorus-containing functional groups, the chemically activated carbon demonstrated outstanding removal efficiencies of 85.1% for Pb2+ and 80.3% for RhB. Kinetic studies showed Pb2+ adsorption followed pseudo-second-order kinetics, indicating chemisorption, while RhB adsorption fitted pseudo-first-order kinetics, suggesting intra-particle diffusion control. The thermodynamic analysis confirmed the spontaneity of both processes: Pb2+ adsorption was exothermic under standard conditions with positive isosteric heat at higher concentrations, reinforcing its chemisorption nature, whereas RhB adsorption was endothermic, consistent with physisorption. Density Functional Theory (DFT) calculations further elucidated the mechanisms, revealing that Pb2+ preferentially binds to oxygen-containing functional groups, while RhB interacts through hydrogen bonding and π–π stacking. These findings establish chemically activated apricot kernel shell carbon as a high-performance adsorbent, exhibiting exceptional removal capacity for both ionic and molecular contaminants through distinct adsorption mechanisms.