Valorization of secondary resources into silica-based adsorbents: Preparation, characterization and application in dye removal from wastewater

•Low-cost materials were used to prepare silica, magnetite and magnetic silica.•The location and preparation method affects the silica content of corn cob.•Heat treatment of extracted silica improves the adsorption capacity.•Heat-treated silica adsorbs RhB more than many other reported adsorbents.