A procedure of obtaining adsorbent from a waste material and its use are disclosed. This invention provides a natural waste material, specifically avocado seed, its preparation and use for the filling of columns used in the elimination of contaminants present in wastewater. The particles have an irregular shape and variable size for filling columns of different diameter and length. The adsorbent has sufficient hardness for avoiding a particle collapse. The material does not rot, is re-generable and ecological. In the present invention the preparation of the adsorbent, its pre-activation, carbonization and application to eliminate the color of textile wastewater is disclosed.The color elimination from textile wastewater, as well as decrease of salts, total suspended solids, chemical oxygen demand (COD) and total organic carbon (TOC), is carried out passing the textile wastewater through a column filled with the prepared adsorbent. In batch systems metals dissolved in water can be eliminated.