Optimized adsorption of methylene blue dye onto bio-based citric acid-modified oleander (Nerium oleander) leaves/chitosan adsorbent using Box-Behnken design approach

This study introduces a novel adsorbent (hereinafter, CHT/OL-CIT) by integrating chitosan and citric acid-modified oleander leaves, demonstrating superior adsorption performance compared to conventional chitosan-based materials. The CHT/OL-CIT adsorbent was used to efficiently absorb organic dye (methylene blue, MB) from aqueous solutions. The Box-Behnken design (BBD) was employed to optimize the adsorption factors, which include A, CHT/OL-CIT dose (0.02–0.08 g); B, pH (4–10); and C, time (10–40 min). The BBD model’s results demonstrated that the optimal adsorption factors for maximal MB removal (98.52%) were as follows: The CHT/OL-CIT dosage is 0.073 g, with a pH of ~ 10 and a contact time of 10.8 min. The Freundlich and pseudo-first-order models were in accord with the experimental data of MB adsorption by CHT/OL-CIT. The developed biomaterial demonstrated a high capacity to adsorb MB, with an adsorption ability of 378.51 mg/g. The Gibbs free energy change (ΔG°) determines the spontaneity of a process, with negative values confirming favorable adsorption. The enthalpy change (ΔH°) indicates heat exchange, where a positive value (17.745 kJ/mol) signifies endothermic adsorption. The entropy change (ΔS°) measures disorder at the solid-solution interface, with a positive value (0.0804 kJ/mol·K) suggesting increased randomness. The adsorption mechanism of MB on the CHT/OL-CIT is characterized by a variety of interactions, including n-π stacking, Yoshida H-bonding, H-bonding, and electrostatic forces. This work advocates for the research and development of alternative adsorbent biomaterial that is capable of rapidly and efficiently treating wastewater-containing dyes. The current endeavor fulfills an assortment of Sustainable Development Goals (SDGs), including Clean Water and Sanitation (SDG 6), Responsible Consumption and Production (SDG 12), Climate Action (SDG 13), and Life Below Water (SDG 14).