In this work, we investigated the use of cotton scraps in natura and modified with iron nitrate, in the adsorption of one of the main water contaminants of the textile industries, the reactive black dye. Special attention was paid to the appropriate destination of the spent adsorbent, in compliance with the precepts of the circular economy. Cotton and polyester are excellent candidates for adsorbents and are produced on a large scale worldwide, but fabric wastes do not yet have a well-established method of application. We found that fabrics containing different types of fibers or colors maintain the ability to remove dye. The retention promoted by the cotton fabric in natura reached 18.8 mg g-1. After treatment with iron ions, there was an increase in the dye adsorption capacity to 31.0 mg g-1. In addition, the spent iron-containing adsorbent was pyrolyzed at 973 K, resulting in an activated magnetic carbon with a specific surface area ranging from 300 to 565 m2 g-1. Thus, it was possible to convert the used adsorbent into a new material with wide application possibilities.