Heavy metal contamination, particularly lead (Pb) pollution in aquatic environments, poses serious threats to ecosystems and human health, necessitating the development of efficient and cost-effective adsorbents. In this study, potassium permanganate-modified areca fiber biochar (PAFB) is successfully synthesized for Pb(II) removal from aqueous solutions. Characterization results confirm the successful loading of MnO₂ nanoparticles and the introduction of abundant –OH, –COOH, and Mn–OH active sites onto the biochar surface. Batch adsorption experiments reveal that Pb(II) removal by PAFB is highly pH- and temperature-dependent, with optimal performance at pH = 6.0 and 25 °C. The adsorption process follows the pseudo-second-order kinetic model (R² = 0.9997) and the Freundlich isotherm model (R² = 0.9363), indicating chemisorption-dominated adsorption on heterogeneous surfaces. Thermodynamic analysis confirmed the spontaneous and endothermic nature of the adsorption process. The maximum adsorption capacity reaches 45.62 mg/g at 25 °C, comparable to or higher than that of many similar biochar-based adsorbents. Mechanistic analysis revealed that Pb(II) removal by PAFB proceeds via a synergistic electrostatic attraction–surface complexation mechanism, wherein Pb(II) forms inner-sphere complexes with –OH, –COOH, and ≡Mn–OH groups, as evidenced by FTIR and XPS analyses. The Pb4f spectrum exclusively displayed Pb(II) complexation peaks without detectable Pb⁰ precipitates, confirming the immobilization of Pb(II) as amorphous surface complexes. Moreover, PAFB maintains an 87.00% removal efficiency after four regeneration cycles, demonstrating excellent reusability. This study provides an environmentally friendly adsorbent for Pb(II) removal, showing promising potential for heavy-metal-contaminated wastewater treatment.