Sustainable Plant-Based Biochar as Effective Methylene Blue Adsorbents: The Case of Alfalfa and Corn

A comprehensive study was conducted to determine the suitability of biochar produced from agricultural waste in the form of alfalfa (BL500) and corn (BC500) for methylene blue (MB) adsorption. As part of the research, biochar was produced at 500 °C by pyrolysis using a CO2 atmosphere. BL500 and BC500 biochar were characterised in terms of their physicochemical and structural properties using FTIR spectroscopy, Raman spectroscopy, and N2 adsorption–desorption. The produced biochars are characterised by a significant ash content and high carbon content. They have a specific surface area of 4.12 m2/g (BL500) and 19.84 m2/g (BC500), a micro-mesoporous structure and are rich in functional groups (including OH, COOH, CO). BL500 biochar showed greater effectiveness in removing methylene blue (MB) than BC500, with maximum sorption capacities of 39.94 mg/g and 19.47 mg/g, respectively. Furthermore, kinetic model fitting indicated that the adsorption process follows a pseudo-second-order model and a Langmuir monolayer model. However, the intramolecular diffusion model (IPD) and Bangham models confirmed that the adsorption process does not occur in a single stage. The produced biochar can be used as a sustainable adsorbent for MB from aqueous solutions.