Process-intensified synthesis of sulfonated peanut shell char through dilute acid hydrolysis and low-temperature sulfonation–carbonization for Pb^2 + removal

Lead (Pb²⁺) contamination in water poses persistent environmental and public health risks, driving the need for efficient and sustainable treatment technologies. In this study, a process-intensified and low-energy route is developed to valorize peanut shell waste into a high-performance sulfonated carbon adsorbent via an integrated dilute acid hydrolysis and simultaneous sulfonation–partial carbonization (DAH–SS-PC) strategy. The DAH step partially hydrolyzes the lignocellulosic matrix while generating a sugar-rich hydrolysate as a value-added co-product and retaining a reactive solid precursor. Subsequent low-temperature SS-PC treatment (80–120 °C) introduces sulfonic and oxygen-containing functional groups, yielding a chemically heterogeneous surface enriched with coordination-active sites. The optimized SPS-Char exhibits a high Pb²⁺ adsorption capacity of 3126.9 mg/g, while sigmoidal modeling yields an apparent asymptotic capacity of 3711.9 mg/g. The adsorption behavior deviates from classical monolayer models and follows a sigmoidal isotherm, indicating cooperative, multi-site adsorption driven by heterogeneous surface chemistry. Kinetic analysis reveals pseudo-second-order behavior, consistent with coordination-driven adsorption. Spectroscopic evidence (FTIR and XPS) confirms the involvement of sulfonic and oxygen-containing groups in Pb²⁺ binding through surface complexation. The adsorbent maintains high removal efficiency under competitive ionic conditions, highlighting its selectivity and robustness in complex aqueous environments. This work establishes a clear structure–function relationship linking low-temperature synthesis, surface chemical heterogeneity, and cooperative adsorption, offering a scalable and energy-efficient pathway for advanced heavy metal remediation. © 2026 Elsevier Ltd.