Enhanced Adsorption of Bisphenol a on Lignin-Derived Biochars: Role of Thermal and Phosphoric Acid Activation in Surface Functionalization and Mechanism

This study investigated the adsorption behavior of bisphenol A (BPA) onto a series of thermally and acid-activated biochars to elucidate the relationship between the surface properties and adsorption performance. Characterization analyses (FTIR, SEM, BET, elemental composition, and PZC) revealed that phosphoric acid activation significantly increased the surface area, pore development, and oxygen/phosphate functionalization, lowering the point of zero charge (PZC = 1.3) and enhancing the surface acidity. The kinetic data fitted well to the pseudo-second-order model, indicating a chemisorption-controlled process, while the equilibrium data were best described by the Langmuir model, with a maximum adsorption capacity (<i>q_m</i> = 262.28 ± 14.3 mg·g^-1) for the acid-activated biochar (LB₄₅₀-H₃PO₄). Thermodynamic analysis confirmed that the adsorption process is spontaneous and endothermic (ΔH° > 0), with a highly favorable entropy contribution. The effects of solution pH, adsorbent dosage, initial BPA concentration, and temperature demonstrated optimal removal under acidic to neutral conditions and moderate dosage (0.2 g·L^-1). Overall, the findings highlight that phosphoric acid activation effectively enhances surface functionality and charge properties, transforming biochar into a highly efficient and sustainable adsorbent for the removal of phenolic contaminants from aqueous solutions.