Removal of methylene blue from aqueous solution by ryegrass straw

This study investigates the methylene blue adsorption by ryegrass by-products. Milled ryegrass straw (Lolium multiflorum Lam.), treated straw, and the biochar produced from the straw with chemical activation with sodium hydroxide solution were used as adsorbent materials. To obtain the biochar, the pyrolysis was performed in a tubular reactor with nitrogen flow at 623 K. The characterization of all the adsorbents was realized analyzing the real density, bulk density, particle diameter, proximate analysis, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. Kinetic studies and adsorption isotherms were evaluated with models fit predicted in the literature. The biochar showed a well-developed porous structure. In the treated straw, there occurred a degradation of compounds such as hemicelluloses and lignin, which gave the modification in the surface of the material. For the kinetic studies, the pseudo-first-order, pseudo-second-order, and Avrami models presented a good fit. For the adsorption isotherms, the Sips model provided the maximum adsorption capacity to the milled straw (28.7 mg/g) and treated straw (67.19 mg/g). The most promising results were obtained by the treated straw, with high efficiency in the removal of the methylene blue dye from the solution, reaching values higher than 99%.