Iron-based nanoparticles were formed in the pores of a micro- and mesoporous activated carbon made from banana spike by the impregnation of iron sulfate at various ratios and further pyrolysis, in order to prepare three catalysts AC@Fe/1, AC@Fe/2, AC@Fe/3 having iron mass contents of 1.6%, 2.2% and 3.3%, respectively. The pore size distributions, transmission electron microscope observations and X-ray photoelectron spectroscopy analyses have revealed that iron-based nanoparticles of 1–50 nm diameter, containing O and P, are located mainly in the supermicropores and mesopores of the activated carbon. Catalysts have been used to remove Rhodamine B in an aqueous solution by the heterogeneous Fenton process. AC@Fe/3 catalyst has allowed achieving 93% of solution discoloration compared to 87.4% for AC@Fe/2 and 78.5% for AC@Fe/1 after 180 min in batch reaction. The catalytic efficiency of AC@Fe/3 is attributed to the highest dispersion of the iron-based nanoparticles in the activated carbon porosity. The effects of hydrogen peroxide and initial dye concentration, pH, catalyst amount and temperature on the Rhodamine B removal kinetics catalyzed by AC@Fe/3 were studied. This catalyst showed remarkable performances of the Rhodamine B mineralization and possibility of recycling.