Abstract In this work, coal gangue from the department of Cesar, Colombia, was used in the synthesis of a geopolymer by alkaline activation. The effect of the variation of the geopolymerization temperature and of four parameters on the composition of the activating solution was evaluated: SiO2/Al2O3, SiO2/Na2O, NaOH concentration and amount of H2O2. The geopolymer was characterized structurally by X-Ray diffraction and Fourier transform infrared spectroscopy. The sorption properties of methylene blue in aqueous solutions of geopolymer were studied through ultraviolet-visible spectrophotometry. Effects of various parameters such as pH value, geopolymer dose and initial concentration on the sorption capacity were studied. The adsorption kinetics and adsorption isotherm fitted well with the pseudo-second order model and Langmuir model, respectively. In the kinetic study it was evidenced that the adsorption equilibrium time was reached quickly, therefore this material constitutes a viable alternative for water decontamination.