H₂O₂ modification of cotton textile waste biochar for enhanced adsorption of basic red 46: performance, mechanism,...

The discharge of cationic azo dyes from textile industries poses significant environmental challenges due to their toxicity and resistance to conventional treatment methods. This study developed a sustainable adsorbent by modifying cotton textile waste biochar (CTWB) with hydrogen peroxide (H₂O₂) to enhance the removal of basic red 46 (BR46). Among the modified materials, biochar treated with 10% H₂O₂ (CTWB@HO10) exhibited the best performance. Under optimized conditions (pH 8, adsorbent dosage of 0.03 g/25 mL, and contact time of 90 min, initial BR46 of 52.82 mg L^-1), CTWB@HO10 achieved a maximum adsorption capacity of 38.34 mg g^-1 and a removal efficiency of 87.10%, which were substantially higher than those of pristine CTWB (31.69 mg g^-1 and 72.00%, respectively). The adsorption kinetics were satisfactorily described by both pseudo-first-order and pseudo-second-order models, with the pseudo-first-order model showing the closest agreement with the experimental data. Diffusion analyses based on the Weber-Morris and Boyd models suggested that BR46 adsorption involved both boundary-layer diffusion and intraparticle diffusion. The equilibrium data were best described by the Sips isotherm, suggesting adsorption on energetically heterogeneous surfaces. Mechanistic analyses suggest that electrostatic attraction, hydrogen bonding, and π-π interactions, and diffusion processes collectively contribute to BR46 adsorption, which may be facilitated by the increased density of oxygen-containing functional groups, and improved accessibility of adsorption sites after H₂O₂ modification. Furthermore, machine learning models were developed to predict adsorption capacity, with the support vector machine (SVM) model showing the highest predictive performance among the evaluated models (R ² = 0.956, RMSE = 2.263, and MAE = 1.662 within the available dataset). SHAP analysis provided additional insight into the relative influence of the operating variables, suggesting that adsorbent dosage and contact time were among the variables most strongly associated with the predicted adsorption capacity. These findings suggest that H₂O₂-modified cotton textile waste biochar may be a low-cost, sustainable adsorbent for removing cationic dyes, particularly in applications where material availability, waste valorization, and process simplicity are important considerations.