Comparative adsorption of methylene blue dye using rice husk modified biochar through surface functionalization

Synthetic dye pollution remains a critical challenge in wastewater treatment, particularly due to phenothiazine dyes such as methylene blue (MB), which generate ecotoxic by-products harmful to aquatic ecosystems. This study investigates the adsorption of MB cations onto rice husk biochar modified by alkaline (NaOH, RHALK) and acid (RHACT) treatments. The modified biochar was characterized to elucidate enhanced physicochemical properties and their impact on MB removal. Batch adsorption experiments demonstrated that alkaline modification achieved a high removal efficiency of 89%, with maximum adsorption capacity (qmax) of 45.8 mg/g, compared to acid modification (qmax = 31.5 mg/g) in a reaction volume of 70 ml dye concentrate (20 mg/L), 30 mg biochar dosage, pH = 8, and incubation temperature of 25 °C. The superior performance of RHALK is attributed to hydroxyl (–OH) and carboxyl (–COOH) groups forming hydrogen bonds with nitrogen‑containing groups (–N(CH3)2) in MB, supported by its larger surface area (124.47 m2/g vs. 71.83 m2/g for RHACT). Kinetic modeling revealed that the pseudo‑first‑order model provided the best fit (R2 = 0.987, SSE = 5.18), indicating a predominantly physisorption mechanism, while isotherm analysis identified the Redlich–Peterson model as the most suitable, suggesting the presence of both homogeneous and heterogeneous adsorption sites. Overall, alkaline‑modified biochar significantly enhanced MB removal compared to acid modification, demonstrating its potential for reutilizing an agricultural residue, which may offer potential cost advantages and effective adsorbent for dye‑contaminated wastewater.