K2FeO4-modified rice husk biochar for the efficient adsorptive removal of pyridine from water: Behavior, mechanism, and DFT study

In this study, nanostructured Fe-loaded biochar (Fe-BC) was prepared from rice husk via pyrolysis using K 2 FeO 4 as an activator, and applied for the adsorption of pyridine. The structure of Fe-BC was systematically characterized by BET, SEM, XRD, FTIR, and XPS. The results revealed a high specific surface area of 434.2183 m 2 /g. The activating role of K 2 FeO 4 not only enabled the abundant loading of Fe 3 O 4 and Fe 2 O 3 on the biochar surface but also increased the oxygen-containing functional groups, thereby providing active sites for pyridine adsorption. The maximum adsorption capacity of Fe-BC toward Pyridine was 79.796 mg/g. The pseudo-second-order kinetic and Langmuir isotherm models were well suited to describe the Pyridine adsorption process onto Fe-BC. Thermodynamic analysis revealed that after K 2 FeO 4 modification, the negative Gibbs free energy change (ΔG) indicated a spontaneous adsorption process, while the positive enthalpy change (ΔH) and entropy change (ΔS) confirmed the endothermic nature of the adsorption and an increase in randomness at the solid-liquid interface. DFT calculations demonstrated a reduction in the energy gap (ΔE) upon the loading of OH, COOH, and Fe. Among all modifications, Fe loading resulted in the most substantial reduction in ΔE, reaching a minimum value of 0.066 eV. This ultimately demonstrates a significant enhancement in the surface reactivity of the Fe-modified biochar. Integrated DFT simulations and experimental characterization elucidated that the adsorption of pyridine by Fe-BC is governed by a combination of mechanisms, including electrostatic interactions, pore filling, π-π interactions, hydrogen bonding, and surface complexation.