This lesson is multidisciplinary in character and aims to show students one of the many processes to treat contaminated water with chemical substances (adsorption). It uses an alternative biomaterial, ground tamarind skin, as a filtering element in the adsorption process. The main objective is to provide a practical demonstration of one of the various mechanisms of adsorption in a given material. In this case it’s an adsorptive process mediated by electrostatic interactions (ion-ion interaction) between the positively charged dye and the negatively charged tamarind skin in an aqueous environment. For the best learning experience students should already understand intermolecular interaction, a topic taught in General Chemistry. If the adsorbent material is prepared in advance, a minimum of one hour will be required to introduce the lesson and complete the practical experiment, demonstrating adsorption in a column. Preparation of the biomaterial (the tamarind skin) demands 24 hours in total and includes the following procedures: skin collection, grinding, washing, drying and sieving. These preparative steps could be carried out in two distinct lessons of at least an hour each. The specific materials necessary to conduct the experiment are tamarind skin and the dyes methylene blue and indigo carmine. All other required items will be easily found in chemistry laboratories. Between lessons the teacher could explain the different types of intermolecular interaction and demonstrate other techniques used in the treatment of polluted water (flocculation, coagulation, ozonization, membrane filtration, photodecomposition etc.).