Dye removal from wastewater using ferrihydrite-modified sugarcane bagasse and biochar: Adsorption performance and modeling analysis

• Modification of sugarcane bagasse produces sustainable adsorbent for dye removal. • Ferrihydrite–biochar composite exhibits enhanced dual dye adsorption efficacy. • 2Fh-SBB showed Langmuir capacities of 141.75 mg g⁻¹ for MB and 138.25 mg g⁻¹ for MO. • Binary dye systems demonstrate synergistic adsorption through charge heterogeneity. • Adsorption kinetics match the pseudo-second-order model, supporting chemisorption. Cationic and anionic dyes in wastewater pose significant environmental risks due to their toxicity, persistence, and mobility. To address this issue, sugarcane bagasse (SB) and its biochar (SBB) were treated with ferrihydrite (Fh) to produce cost-effective and durable adsorbents for the simultaneous removal of methylene blue (MB, cationic) and methyl orange (MO, anionic). In single-dye systems, the ferrihydrite-biochar composite (2Fh-SBB) achieved peak Langmuir adsorption capacities of 141.75 mg g⁻¹ for MB and 138.25 mg g⁻¹ for MO, outperforming SB, SBB, Fh, and 2Fh-SB. The Freundlich model further suggested heterogeneous adsorption behavior compatible with the composite's mixed carbon-ferrihydrite surface. The kinetic data were well characterized by the pseudo-second-order model (R² = 0.97–0.99), suggesting the participation of surface interactions; yet, kinetics alone cannot differentiate between physisorption and chemisorption. Temperature-dependent thermodynamic analyses (ΔG°, ΔH°, ΔS°) are necessary for conclusive mechanistic validation. Statistical diagnostics validated the reliability of the isotherm and kinetic parameters. MB adsorption favored alkaline conditions, while MO adsorption was higher at acidic pH, reflecting electrostatic influences relative to surface charge and pHPZC. In binary systems, MB adsorption was enhanced by MO, indicating partial synergistic effects associated with charge heterogeneity and improved Fh dispersion. Characterization tests (BET, zeta potential, FTIR, SEM-EDS) suggested that MB interacted with both carbon and Fh domains via electrostatic and π-π interactions, while MO adsorption involves Fe-O surface interactions. The substantial and largely stable adsorption fractions highlight the effectiveness of ferrihydrite modification in pollutant stabilization. Overall, 2Fh-SBB demonstrates strong potential as a low-cost, robust adsorbent for multi-dye wastewater treatment while promoting the valorization of agricultural waste.