Introduction. A physicochemical analysis is performed on the basaltic rock (black slag) of El Salvador todetermine the amount necessary for the adsorption of chemicals present in a liter of wastewaterOrdinary of Ciudad Real, in San Sebastián Salitrillo, in the department of Santa Ana.Objective. Determine the kinetic adsorption model (Langmuir, Freundlich, Temkin and Henry isotherm that best fits the adsorption equilibrium between methylene blue and basaltic rock. The adsorption-desorption equilibrium constant formed between methylene blue is measured. and basaltic rock to determine the specific area of ​​basaltic rock and establish the number of active absorption sites per square nanometer of basaltic rock.Methodology. The technique used for the treatment of methylene blue was adsorption, since it is economical and competitive compared to other conventional methods. The adsorption effect was studied with 5, 10, 15, 20, 25, 30, 33 ppm of methylene blue in the presence of 1 gram of basaltic rock pulverized for 30 minutes, taking aliquots every 5 minutes; adsorption was measured with a Cary50 UV-VIS spectrophotometer. Results. Good adsorption is evidenced because after 20 minutes it became constant and has a good specific surface. The best applied isotherm is Langmuir, because that its correlation coefficient is closest to one.