Sustainable Pb (II) Removal via Pine-Bark Biochar Valorization: Removal Mechanisms and Response-Surface Modeling

Lead (Pb (II)) is a persistent contaminant in industrial wastewater. This study evaluated pine bark (PB)-derived biochars produced at different pyrolysis temperatures for Pb (II) removal. Biochar produced at 500°C (PB5) achieved the highest removal efficiency and was selected for subsequent experiments. The adsorption equilibrium for PB5 was best described using the Sips isotherm, indicating energetic heterogeneity, whereas its kinetics followed the pseudo-second-order model. The maximum monolayer adsorption capacity estimated using the Langmuir model was 29.754 mg/g, with combined spectroscopic, elemental, and textural analyses suggesting that Pb-containing surface precipitation and pore blocking/filling played major roles in Pb removal. Response surface methodology was used to optimize the pH, adsorbent dosage, temperature, and ionic strength, with the PB5 dosage found to have the strongest influence on adsorption, followed by the pH, ionic strength, and temperature. Among the investigated experimental conditions, the highest removal efficiency of 91.43% was achieved at pH 7, an adsorbent dosage of 5 g/L, a temperature of 25°C, and an ionic strength of 5. The developed RSM model showed satisfactory agreement with the experimental data, with a root mean square error (RMSE) of 3.75 percentage points.