Adsorption behaviour of cationic and anionic dyes on new chitosan-activated carbon@metal oxide hydrogels beads: Effect of the metal nature and comparative study

The present work concerns the preparation of hydrogel composite beads based on activated carbon@metal oxide (M: CaO, Al₂O₃, CaO/Al₂O₃) from the calcination of olive stones biomass encapsulated by chitosan as a cross-linking matrix. The biocomposites were characterized by SEM, BET, Zeta potential, TGA, XRD and FTIR in order to investigate the morphological, thermal, textural and structural properties of these solids. The encapsulation of activated carbon-based nanocomposites leaded to obtain a porous structure. The nanocomposite beads were used as biosorbents for the MB and OG dyes removal. Several parameters affecting the adsorption process were investigated such as effect of the pH, dyes concentration, temperature and time. The adsorption process follows the Langmuir isotherm with R² ≈ 1 and the pseudo-second order model confirming the uniform, monolayer, endothermic and chemisorption nature of the dye's adsorption. All biosorbents showed a higher affinity to anionic dye “Orange G (OG)” due to their positive surface charge. The CSAC@Ca showed the best adsorption capacity Q_max = 323.625 mg/g against OG while the CSAC@Al showed the best adsorption capacity Q_max = 134.589 mg/g against methylene blue. Finally, the hydrogel beads were reused in five successive cycles and showed very good efficiency even at the last cycle contrary to the parent chitosan.