Comparative Study of Methylene Blue and Congo Red Adsorption Using Raw and ZnCl2-Activated Artichoke Leaves: Kinetic, Isotherm, and Thermodynamic Insights

In this study, artichoke leaves were valorised into bioadsorbents via ZnCl2 chemical activation (AC-AL) and compared to raw artichoke leaves (Raw-AL) for the removal of Methylene Blue (MB) and Congo Red (CR) dyes from aqueous solutions. Characterization techniques (SEM, EDX, XRD, FTIR, and pHpzc) demonstrated that ZnCl2 chemical activation induced profound morphological, structural, and surface chemical modifications. These changes were further supported by BET–BJH analyses, which confirmed the successful development of a highly porous and well-structured carbon material. The maximum adsorption capacities reached 336 mg/g for MB on AC-AL and 219 mg/g on Raw-AL, both higher than the values obtained for CR, indicating a stronger affinity of both adsorbents, particularly AC-AL, toward cationic dyes. Kinetic, isothermal, and thermodynamic analyses showed that the adsorption of both dyes onto AC-AL follows pseudo-second-order kinetics and fits the Langmuir isotherm, indicating a chemisorption process with monolayer coverage. Thermodynamic parameters suggested spontaneous and exothermic adsorption for MB, while CR adsorption was spontaneous and endothermic. Diffusion studies indicated that the adsorption mechanism involves both external mass transfer and intraparticle diffusion. Overall, ZnCl2 activation significantly enhanced the adsorption capacity of artichoke leaves, highlighting their potential as cost-effective, sustainable adsorbents for dye removal.