Ferula orientalis L. stalks were used as an agricultural solid biomass waste for preparation of activated carbon with zinc chloride activation using slow pyrolysis in a fixed-bed reactor. The chemical characteristics of the activated carbon obtained at 550 °C were identified by elemental, fourier transform infrared spectroscopy, Brunauer–Emmett–Teller, scanning electron microscopy analyses and found to have a surface area of 1,476 m2/g. The ability of the activated carbon to adsorb methylene blue and crystal violet from aqueous solutions has been investigated. Adsorption studies were performed at different initial dye concentrations (200–800 ppm), contact time (0.5–90 min) and temperature (25–45 °C). Adsorption data were modeled with Langmuir, Freundlich and Temkin adsorption isotherms. Equilibrium data of the adsorption processes showed that adsorption of methylene blue is fitted to the Langmuir and crystal violet to the Freundlich isotherm. Pseudo-first-order, pseudo-second-order and Elovich kinetic models were used to analyze the kinetic data obtained at different initial dye concentrations. The adsorption kinetic data were very well described by the pseudo-second-order model. The calculated thermodynamic parameters such as change in free energy, enthalpy and entropy showed that adsorptions of both dyes onto activated carbon were spontaneous and endothermic under the experimental conditions.