Adsorption of organic pollutants onto activated carbons, especially those based on plant waste, has received interest from scientists due to its low cost and energy consumption. This study deals with the development of activated carbon based on FCP powder by phosphoric acid (H₃PO₄) activation, named AC-FCP, and its application as an adsorbent of methylene blue dye. Due to its high toxicity, the dye content in wastewater must be within tolerable limits before its discharge into the aquatic environment. With a small quantity, these dyes can diffuse over a large area of aquatic environments, causing inhibition of photosynthesis by depriving light penetration into the water. The activated carbon obtained was characterized by physicochemical methods like Brunauer–Emmett–Teller (BET) theory, morphological studies (SEM), energy-dispersive X-ray (EDX), X-ray Diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FTIR), complementary analyses and thermodynamics. The effects of various parameters on the removal efficiency and adsorption capacity of methylene blue onto AC-FCP were studied by testing the effect of the mass, initial concentration, contact time, pH, and temperature. The characterization of AC-FCP shows that our activated carbon has a high surface area, a negative charge, a microporous and mesoporous structure, and an amorphous structure. The removal efficiency is at 99,28 % with an adsorption capacity of 72.58 mg.g^-1 in a contact time of 110 min. The adsorption mechanism is favorable and reversible, forming a monolayer with a chemosorption mechanism which is defined by the Langmuir isotherm and pseudo-second-order kinetics. Our activated carbon AC-FCP has the best characteristics for the high removal of toxic dyes like methylene blue in aqueous environments with low cost and less energy.