Engineering Biomass-Derived Activated Carbon for Caffeine Adsorption: Mass Transport and Interfacial Energetics

High Resolution Image Download MS PowerPoint Slide This study investigates the mass-transport processes controlling the adsorption of caffeine (CFN), used as a probe molecule, on engineered activated carbon derived from the valorization of biomass leaf residues. The resulting adsorbents exhibit a predominantly mesoporous architecture with a high specific surface area (2132 m 2 g –1 ). The highest experimental adsorption capacity (436 mgg –1 ) was achieved at pH 7.0–8.0. In this pH range, interfacial water layers may facilitate CFN diffusion within the pore network by providing additional transport pathways. In addition, hydrogen-bonding interactions between the heterocyclic nitrogen atoms of CFN and oxygen-containing surface functional groups (−COOH and −OH) may further contribute to the adsorption process. All samples exhibit negative entropy values (Δ S ads 0 ), indicating reduced interfacial randomness upon adsorption, likely due to the restricted mobility of CFN molecules at the solid–liquid interface. For the optimized sample, CFN adsorption proceeds spontaneously and is predominantly driven by a favorable enthalpic contribution (Δ H ads 0 < 0). The negative enthalpy change compensates for the unfavorable entropic contribution (Δ S ads 0 < 0), resulting in an overall negative Gibbs free energy change (Δ G ads 0 < 0), consistent with a thermodynamically spontaneous process. Furthermore, increasing temperature reduces the residence time of CFN at the interface, approaching values comparable to or exceeding the characteristic vibrational period of the adsorbate–surface system. Under these conditions, adsorption remains established, as the molecule persists at the interface long enough to reach near–thermal equilibrium with the surface. Overall, these findings reveal a correlation between solid-state structural evolution and surface energetics, providing deeper insight into the coupling between adsorption thermodynamics and interfacial transport phenomena.