Highly effective lead removal by novel alkaline biochar prepared by pyrolysis of woody biomass impregnated with low-level NaOH

The remediation of heavy metal-contaminated environments, particularly those polluted with lead (Pb), remains a critical challenge due to the metal's toxicity and persistence. This study developed a novel alkaline biochar for enhanced Pb adsorption, prepared from pine wood through low-level NaOH (0–2 wt%) dry impregnation followed by pyrolysis at temperatures ranging from 350 to 600 °C. Characterization using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) elucidated the alkaline biochar's surface modifications and adsorption mechanisms. Adsorption studies showed that 2 % NaOH-modified biochar (2 % NaOH-BC) achieved a maximum adsorption capacity of 230 mg g⁻¹, representing a 14-fold improvement over non-alkaline treated biochar (0 % NaOH-BC, 16.1 mg g⁻¹). Kinetic studies confirmed chemisorption as the dominant mechanism, described by the pseudo-second-order model, while Langmuir isotherm analysis (R² = 0.933–0.970) indicated monolayer adsorption. XPS analysis revealed the emergence of Pb²⁺ peaks after adsorption, indicating successful Pb²⁺ uptake. The analysis provided insights into the adsorption mechanism, suggesting ion exchange and coordination interactions involving oxygen-containing functional groups. Electrostatic interactions also played a role, as increasing pH (3.0–11.0) enhanced Pb²⁺ binding due to surface deprotonation, with optimal adsorption at pH 11.0 (266 mg g⁻¹, 100 % efficiency). Additionally, desorption studies demonstrated effective recyclability, with the 2 % NaOH-BC retaining 60.59 % of its adsorption capacity after four cycles. These findings highlight the potential of low-cost NaOH-treated biochar for effective and sustainable Pb remediation.