Valorization of biochars derived from corn straw waste for the adsorption of methylene blue and methyl orange dyes: influence of acid and alkaline activation.

The increasing utilization of agro-industrial waste underscores the necessity for sustainable valorization strategies consistent with circular economy principles. In this study, corn husk residue was investigated as a precursor to produce chemically activated biochars with tunable surface chemistry and porosity, enabling targeted adsorption of the cationic dye methylene blue (MB) and the anionic dye methyl orange (MO). Two activation strategies, acidic (H₃PO₄) and alkaline (NaOH), were applied to tailor the physicochemical properties of the resulting materials, designated as acid corn biochar (ACB) and alkaline corn biochar (AKCB). ACB exhibited uniform mesoporosity and polar surface groups, which enhanced interactions with both dyes, while AKCB demonstrated hierarchical porosity and neutral or basic surface groups that promoted π-π interactions and affinity for anionic molecules. Both biochars effectively removed dyes from aqueous solutions; however, ACB achieved higher adsorption capacities, reaching 60.2 mg g⁻1 for MB and 54.4 mg g⁻1 for MO, compared to 33.7 mg g⁻1 and 40.8 mg g⁻1 for AKCB, respectively. These findings indicate that chemical activation of corn-based biochar is a promising, cost-effective approach for developing efficient adsorbents for water treatment. This study advances process intensification and circular economy objectives by converting agricultural waste into functional materials for the remediation of industrial effluents.