Adsorption on activated carbon is an efficient method for the removal of toxic dyes. However, since commercially available charcoal is quite expensive, activated carbon obtained from agricultural by‐products may serve as a good replacement. In this study, activated carbon was prepared from pepper peduncle, an agricultural waste product, by microwave activation. The synthesized carbon was characterized by X‐ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and thermal gravimetric analysis techniques. It was then used for the adsorption of methylene blue dye from an aqueous solution, which was studied as a function of the dye concentration, contact time, and temperature. The adsorption data were fitted to Freundlich and Langmuir isotherm models. The adsorption kinetics was studied by employing first‐ and second‐order kinetic models, and it was found that the adsorption of methylene blue on the synthesized activated carbon follows a second‐order kinetic model. Effect of temperature on the adsorption process was studied, and the thermodynamic parameters such as activation energy, change in enthalpy, entropy, and free energy of adsorption were calculated on the basis of the absolute theory of reaction rate expressions. About 99.5–91.8% of the dye was removed for an initial dye concentration in the range 20–100 mg/g in 1 h. Thus the synthesized activated carbon was found to be very efficient in adsorbing the dye.

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