Sustainably engineered chopstick–eggshell composite biochar: Integrated optimization and machine learning framework for heavy metal removal from acid mine drainage

• BC-CaCO 3 /TiO 2 removes Pb(II) well in synthetic wastewater (381.16 mg/g). • The biochar composite enhances adsorption predominantly via chemisorption. • RSM & MLP optimize and predict Pb(II) removal well at pH 5, 60 min, 0.25 g/L. • The composite treats real AMD, removing 97-99% of Cu, Fe, Mn, Pb, Zn at 2 g/L. • Endothermic, spontaneous adsorption confirmed by thermodynamic studies (ΔG° < 0). A novel composite biochar (BC-CaCO 3 /TiO 2 ) was developed by co-pyrolyzing bamboo chopsticks and eggshells, followed by a TiO 2 coating produced through a sol-gel method. This material showed strong potential for eliminating Pb(II) from wastewater. Various materials characterization techniques (e.g. FESEM-EDX, BET, FTIR, XPS, and XRD) were used to determine the structure and composition of the composite biochar. Adsorption parameters such as pH, contact time, and dosage—were optimized using response surface methodology (RSM), identifying optimal conditions being at pH 5, 60 minutes, and 0.25 g/L, with a model fit of R² = 0.988. The prediction accuracy was further improved to R 2 =0.997 by using a multilayer perceptron (MLP) in a machine learning framework, showing that MLP was more effective in prediction than RSM. At 25°C, the maximum Pb(II) adsorption capacity was 381.16 mg/g. Thermodynamic analysis further confirmed that the process was endothermic, spontaneous, and favoured chemisorption. The adsorption mechanisms involved in the study was predominantly chemisorption, where precipitation, ion exchange and surface complexation were involved in adsorption. BC-CaCO 3 /TiO 2 was also tested on real acid mine drainage (AMD), with the successful removal of heavy metals (Cu, Fe, Mn, Pb, Zn) at a higher dosage of 2 g/L. This indicates the potential of the composite to treat both synthetic wastewater and real AMD, making BC-CaCO 3 /TiO 2 a good prospect as a remediating adsorbent.