Photocatalytic Degradation of an Aromatic Pharmaceutical over TiO2: Experimental and Computational Insights into Inhibition Effects of Natural Organic Acids

The photocatalytic degradation of the pharmaceutical compound nadolol over TiO2 under UV-LED irradiation was investigated, with particular emphasis on the inhibitory effects of common low-molecular-weight organic acids. Due to its aromatic (tetralin-like) motif and multiple heteroatom-containing functional groups, nadolol serves as a representative model for aromatic micropollutants whose fate can be governed by surface competition and noncovalent interactions. While TiO2 showed high photocatalytic activity in ultrapure water, achieving complete nadolol degradation within 120 min, the presence of citric, oxalic, and acetic acids markedly reduced the degradation efficiency by approximately 72%, 62%, and 29%, respectively. Experimental results demonstrated that this inhibition could not be attributed solely to pH changes, indicating the contribution of additional molecular-level effects. To elucidate the underlying mechanism, molecular and periodic density functional theory (DFT) calculations were performed. The computational analysis revealed strong interactions between nadolol, organic acids, and the TiO2 surface, leading to competitive adsorption and partial blocking of photocatalytically active sites. These results provide mechanistic insight into the role of natural organic acids in TiO2-based photocatalytic systems and highlight the importance of considering real-water matrix components when designing efficient and sustainable photocatalytic water treatment processes.