A sustainable bio-adsorbent from cork oak leaves for methylene blue removal: RSM-BBD optimization, regeneration, DFT/MD insights, and contributions to the SDGs

This research presents the valorization of Quercus suber L. (cork oak) leaves into an economical and sustainable bio-adsorbent for the efficient removal of methylene blue (MB) from wastewater. Through simple chemical activation with sulfuric acid, this abundant agroforestry waste is transformed into a regenerable activated carbon with remarkable performance. The optimization of adsorption parameters (adsorbent dose, initial dye concentration, pH, and contact time) using Response Surface Methodology (RSM) based on a Box-Behnken Design (BBD) achieved a removal rate of 98.94% under optimal conditions: adsorbent dose of 0.0926 g/100 mL, MB concentration of 0.0039 g/L, pH 6, and contact time of 297 min. Experimental validation confirmed the robustness of the predictive model, with a deviation of only 1.06%. Physicochemical characterizations (FTIR, SEM, pHPZC) revealed an amorphous and microporous structure, rich in acidic surface functional groups (notably carboxyl, hydroxyl, sulfonate), giving the adsorbent a particular affinity for cationic pollutants under basic conditions (pH > pHPZC = 4.43). To elucidate the adsorption mechanisms at the molecular scale, a theoretical study combining Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations was conducted. DFT calculations confirmed the high reactivity of MB, with a low HOMO-LUMO gap (1.863 eV) and high electrophilicity, favoring electron transfer. Fukui function and Mulliken charge analyses identified nitrogen and sulfur atoms of MB as key reactive sites, likely to interact with oxygenated groups on the activated carbon surface. MD simulations validated the stability of the MB-adsorbent complex in an aqueous environment, with a favorable adsorption energy of -427.19 kcal/mol, highlighting the role of electrostatic interactions, hydrogen bonding, and π-π effects. Finally, the adsorbent demonstrated excellent regeneration capacity, particularly with a 0.1 M HCl solution, retaining over 81% of its efficiency after six adsorption-desorption cycles. This work highlights the potential of cork oak leaves as a sustainable, high-performance adsorbent. Through integrated experimental, theoretical, and circular economy approaches, the study aligns with multiple UN Sustainable Development Goals: promoting health (SDG 3) by removing toxic pollutants, ensuring clean water (SDG 6), fostering innovation (SDG 9), encouraging responsible production (SDG 12), reducing energy use (SDG 13), and supporting ecosystem management (SDG 15). It demonstrates how scientific innovation can advance environmental preservation, local resource valorization, and global health.