The Role of Surface Chemistry and pH Shifts in the Sorption of Common NSAIDs on Coconut and Orange Waste-Derived Carbon Materials

This study investigated the sorption of three widely detected non-steroidal anti-inflammatory drugs, naproxen, diclofenac, and ibuprofen, onto biochars and activated carbons derived from coconut shell and orange peel. Activated carbons were prepared under chemical activation with ZnCl2 and H3PO4. The sorbent materials were characterized in terms of surface functional groups, acid–base properties, and pzc, while the pH was monitored along the sorption experiments. Results suggest that the activating agent played an important role in controlling surface chemistry and acid–base properties; ZnCl2-activated carbons exhibited near-neutral pzc values and a balanced distribution of functional groups, while H3PO4-activated materials showed highly acidic surfaces with lower pzc. The solution pH evolved during sorption instead of remaining at its initial value. The sorption performance showed a clear dependance on surface chemistry, ZnCl2-activated carbons exhibited a superior sorption capacity as well as faster kinetics under neutral conditions, this behavior may be associated to reduced electrostatic repulsion toward anionic model molecules. Under acidic conditions, some H3PO4-activated materials showed comparatively favorable sorption behavior, consistent with possible contribution from hydrogen bonding and hydrophobic interactions. Kinetic analysis showed time-dependent uptake profiles consistent with more than one mass-transfer contribution; however, no single rate-controlling mechanism could be established from the available data. The results indicate that NSAID sorption under the investigated conditions was associated with the surface acid–base properties of the carbon materials, the initial-to-final pH shifts, and the pH-dependent speciation of the pharmaceuticals. These findings provide an empirical basis for evaluating the performance of agroindustrial waste-derived carbon materials toward ionizable pharmaceuticals and highlight the importance of considering both surface chemistry and the measured solution pH when interpreting sorption behavior.