Mechanistic and Kinetic Insights into Hydroxyl Radical-Mediated Tetracycline Transformation in Photocatalytic Oxidation Processes

Antibiotic pollution, particularly via tetracycline (TC), poses significant environmental risks due to its recalcitrance and potential to induce antibiotic resistance. This study employed density functional theory (DFT) and transition state theory (TST) to investigate TC degradation by hydroxyl radicals (·OH), focusing on hydrogen atom transfer (HAT) and radical adduct formation (RAF) pathways. Geometry optimizations and vibrational analysis validated stationary points, while intrinsic reaction coordinate (IRC) calculations confirmed transition states. Key findings reveal that RAF pathways exhibit lower activation barriers (1.23–30.33 kJ/mol) and greater exothermicity (−164.42 kJ/mol) compared to HAT pathways (3.51–42.04 kJ/mol, −109.58 kJ/mol), making them kinetically and thermodynamically dominant. Frontier molecular orbital (FMO) analysis links HAT to TC’s HOMO (π-orbital character on aromatic rings) and RAF to its LUMO (electrophilic sites). Rate constants calculated at 298 K (TST with Wigner correction) confirm RAF’s kinetic superiority (up to 7.0 × 1011 s−1), surpassing HAT’s fastest pathway (6.2 × 1011 s−1). These insights advance the understanding of TC degradation mechanisms and help with the design of efficient photocatalytic oxidation processes for antibiotic removal.