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Abstract Introduction: Nowadays, due to the reduction of fresh water resources and the increase in demand in the world, the removal of various pollutants, such as dyes, from industrial wastewaters to reuse them has received much attention. Methods: In this research, activated carbon (AC) was synthesized by the chemical activation of apricot kernel shell, and then, this adsorbent was modified by iron-containing functional groups (FeOOH-AC) and used as an adsorbent in the removal of methyl orange from the aqueous solution. The properties of these adsorbents were characterized using BET, SEM, XRD, and FT-IR analyses. Findings: The matching of the equilibrium data with the Langmuir model showed that the adsorption process is single-layer and the maximum adsorption capacity of AC and FeOOH–AC absorbents equal 174.6 mg/g and 249 mg/g, respectively. The increase in adsorption capacity after modification is due to the positive adsorption sites of iron (Fe-O-H2+) on the surface of the modified adsorbent, which adsorb the methyl orange by electrostatic mechanism. Also, the physical adsorption due to the porous structure of the adsorbent and π-π interaction are the effective mechanisms in the adsorption of methyl orange on the FeOOH-AC adsorbent. The analysis of kinetic data with different models showed that the pseudo-second-order kinetic model is consistent with the experimental result. The spontaneity and exothermic nature of the adsorption process were determined by determining the thermodynamic parameters. Conclusion: The successive cycles of adsorption and desorption indicate the ability to regenerate and reuse the synthesized adsorbent, which can be a suitable option for use on the industrial scale.