Elucidating structure-activity relationships and mechanism on antibiotic degradation efficiency by visible-light-driven heterojunction

Tetracyclines, fluoroquinolones and sulfonamides frequently coexist in aquatic environments to form composite antibiotic contamination. Most existing investigations only explore the degradation of individual pollutants, while few comparative studies systematically analyze the degradation behaviors and distinct molecular mechanisms of multiple coexisting contaminants. Six representative antibiotic contaminants covering three categories were selected in this work, including tetracyclines (TC, CTC), fluoroquinolones (LOM, NOR), and sulfonamides (SMX, SMZ). Experimental characterizations combined with DFT simulations were adopted to compare their photocatalytic degradation performances and reveal the molecular mechanisms behind their divergent behaviors. Degradation tests reveal prominent discrepancies in elimination efficiency among the three antibiotic groups, with degradation efficiency following the order: tetracyclines > fluoroquinolones > sulfonamides. The elimination efficiencies of TC and CTC exceed 91%, while those of LOM and NOR stay above 85%. In contrast, SMX and SMX exhibit inferior removal performance. Besides, each contaminant requires a distinct pH condition to achieve optimal degradation. Theoretical calculations demonstrate that the interfacial adsorption energy of antibiotic molecules correlates positively with their degradation efficiency, and variations in functional groups constitute the primary cause for divergent degradation capacities. Electrostatic potential and HOMO orbital analyses verify that reactive oxygen species induce region-selective oxidation toward antibiotic molecules. The matching degree between pollutant adsorption configuration and HOMO orbital governs the subsequent degradation pathway. Solution pH modulates the protonation configuration and electronic distribution of antibiotics, thereby regulating their interfacial adsorption behaviors. Combining experimental measurements and theoretical simulations, this work preliminarily clarifies the micro-mechanisms underlying divergent antibiotic degradation, which could offer fundamental references for research on composite antibiotic pollution.