Activated carbons and MOF-based composite were prepared from the ashitaba biomass and they were applied and compared in the adsorption of methylene blue from aqueous solution. Surface chemistry of these adsorbents was characterized with different analytical techniques and results were used to analyze the corresponding adsorption interactions. Statistical physics calculations were performed to interpret the physicochemical parameters involved in the dye adsorption mechanism. Results showed that the dye adsorption with activated carbons was endothermic, while an exothermic dye removal was observed with the MOF-activated carbon composite. Dye adsorption mechanism was a multimolecular process mainly for ashitaba-based activated carbons where electrostatic interactions, van der Waals forces and hydrogen bonding were present. In particular, the activated carbon functionalized with a surfactant outperformed other adsorbents prepared with the ashitaba biomass thus showing adsorption capacities higher than 400 mg/g. This adsorbent is promising for its application in the treatment of wastewaters polluted by this and other dye molecules.