Nanocellulose (NC) has recently emerged as of high interest in water purification because of its high surface area, aspect ratio, and versatile surface chemistry. In the present study, nanocellulose has been isolated from cellulose pulp, extracted from barley straw residues via a mechanical pulping treatment, through a combination of chemical and mechanical refining treatments. The obtained cellulose pulp and (ligno)cellulose nanofibrils (L)CNFs, in the form of papers and nanopapers, respectively, were characterized in terms of mechanical properties. Finally, (L)CNFs-based aerogels were developed, and adsorption tests at 25 °C using 0.1 g of aerogels in 50 mL of methylene blue (MB) dye solution at 10 mg/L were studied. The results revealed that CNF-based aerogels can be exploited as an effective eco-adsorbent for the removal of methylene blue dye and provide a new platform for dye removal.