Waste-Derived Porous Geopolymers for Pb(II) Removal: Kinetics, Thermodynamics, and Regeneration

Lead (Pb) is a highly toxic heavy metal frequently found in industrial wastewater, posing serious risks to both human health and the environment. In this study, a porous geopolymer synthesized from fly ash, metakaolin, and red mud was evaluated for Pb(II) removal via batch adsorption experiments under varying pH, dosage, contact time, temperature, and initial concentration. The synthesized material exhibited a favorable mesoporous structure, with a BET surface area of 42.05 m2 g−1 and an average pore width of 6.26 nm, making it suitable for heavy metal uptake. Adsorption kinetics followed the pseudo-second-order model (R2 = 0.9993), while the Langmuir isotherm (R2 ≈ 0.999) best described the equilibrium data, indicating monolayer chemical adsorption as the dominant mechanism, with a maximum capacity of 74.26 mg g−1 at 318 K. Thermodynamic analyses confirmed that the adsorption was spontaneous (ΔG° < 0), endothermic (ΔH° > 0), and accompanied by increased entropy (ΔS° > 0). Desorption and regeneration tests revealed EDTA to be a more effective agent than HNO3, maintaining a reuse efficiency of 81.35% after four cycles. These results highlight the potential of waste-derived porous geopolymers as regenerable, low-cost, and efficient adsorbents for lead removal.