Removal of rhodamine B dye from aqueous solution via adsorption using alkaline-treated human hair: optimization, isotherm, and kinetic studies

Rhodamine B dye contamination poses serious environmental and health risks due to its carcinogenic and neurotoxic properties. This study explores the use of alkaline-treated human hair as a low-cost adsorbent for dye removal from water. The hair was treated with 0.1 M sodium hydroxide, rinsed, and dried to enhance its adsorption capacity. Fourier transform infrared spectroscopy (FTIR) confirmed the presence of key functional groups such as hydroxyl, carboxyl, and amine groups, which facilitate dye binding. Scanning electron microscopy (SEM) revealed surface modifications after treatment. The effects of contact time, initial dye concentration, and adsorbent dose were analyzed using a central composite design under response surface methodology. Optimal conditions—34.66 mg/L initial concentration, 3.5 g adsorbent dose, and 20.59 min contact time—achieved a maximum removal efficiency of 85.71 ± 0.61%. The adsorption followed the Langmuir isotherm (R² = 0.99426) and pseudo-second-order kinetics (R² = 0.98211), indicating a monolayer and chemisorption mechanism. The maximum adsorption capacity based on optimal conditions was 4.95 mg/g. The findings suggest that human hair could be a viable adsorbent in removing dye from contaminated water.