A mechanistic rationalization of the degradation of caffeine in homogeneous and heterogeneous media

The current study complements the experimental findings of Ziylan-Yavas et al. on the degradability of the emerging water contaminant caffeine through homogeneous and heterogeneous Advanced Oxidation Processes (AOPs). Density Functional Theory (DFT) and Plane-Wave Self-Consistent Field (PWSCF) were employed to propose plausible reaction mechanisms consistent with the experimental data. The study also involves modeling and discussion in detail of the radical adduct formation (RAF), hydrogen atom transfer (HAT), demethylation reactions (DEMET), byproduct formation, and the role of TiO2surface on the degradation process. The discussion is based on free energy barriers, reaction free energies, and the electronic energies of adsorption. The peak corresponding to the strongest binding to the TiO2surface (P1, MW = 228) aligns closely with the LC-MS signals observed for the byproducts with MW = 58 and 102. This correlation underscores the predictive power of the computational model and lays the groundwork for future mechanistic investigations.