Reusable biochar adsorbent from forestry waste for aqueous dye remediation: isotherms, kinetics, and regeneration

Biochar-based adsorbents provide an efficient, low-cost approach for dye removal. This study evaluates biochar derived from waste Eucalyptus globulus bark (EGB) for methylene blue (MB) removal from aqueous solutions. FESEM revealed a porous carbonaceous matrix with dispersed mineral phases, FTIR identified oxygen-containing functional groups (hydroxyl and carbonyl), and the point of zero charge (pHPZC) was ∼8.0. Batch adsorption experiments showed that under optimised conditions (pH 8.0, adsorbent dosage 4 g/L, MB concentration 20 mg/L, contact time 90 min), MB removal efficiency reached 97–98%. Equilibrium data were best described by the Langmuir isotherm model with a maximum adsorption capacity of 24.39 mg/g and high partition coefficient of 121.95 L/g. The adsorption kinetics were best described by a pseudo-first-order model. Regeneration studies showed desorption efficiencies up to 72.8% using organic solvents, and the biochar retained approximately 70% of its adsorption capacity after three successive cycles. Mechanistic interpretation based on pH-dependent study, pHPZC, FTIR shifts, and desorption results indicated that MB adsorption was governed by a combination of electrostatic attraction, π–π interactions, hydrogen bonding, and pore-filling effects. Overall, EGB biochar shows potential as a sustainable, low-cost, and reusable adsorbent for dye-laden wastewater treatment, supporting waste valorization and circular economy strategies.