Trapping of cadmium(II) and nickel(II) from aqueous solutions using functionalized Albizia lebbeck seed pods: Isotherm, kinetic, and thermodynamic studies

Heavy metal contamination poses significant risks to human health and ecosystems due to its persistence and high toxicity. Consequently, the urgent removal of heavy metals from the environment is imperative. This study evaluated the adsorptive performance of unmodified (UAPS) and citric acid modified (MAPS) Albizia lebbeck seed pods as low-cost adsorbents for removing Cd²⁺ and Ni²⁺ from aqueous solutions. The physicochemical properties of the adsorbents were characterized using Fourier transform infrared spectrometry and scanning electron microscopy to examine surface chemistry and morphology. Batch adsorption experiments were conducted to assess the effects of pH, initial metal ion concentration, adsorbent dosage, and contact time. Optimal adsorption occurred at pH of 2, with an equilibrium time of 15 min for both UAPS and MAPS. The Temkin model best described the experimental data, with UAPS exhibiting higher maximum adsorption capacities (2.587 mg/g for Cd²⁺ adsorption and 25.900 mg/g for Ni²⁺ adsorption) than MAPS (1.488 mg/g for Cd²⁺ adsorption and 1.400 mg/g for Ni²⁺ adsorption). Thermodynamic and kinetic analyses revealed that the adsorption process was spontaneous, endothermic, and well explained by the pseudo-second-order model. Adsorption–desorption experiments demonstrated strong adsorbent reusability over four cycles (with removal efficiencies greater than 60%). Overall, these findings indicate that UAPS is more effective in removing Cd²⁺ and Ni²⁺ and exhibits a stronger affinity for Ni²⁺.