Molecular structure-dependent adsorption mechanisms of tetracycline antibiotics congeners on biochar

Tetracycline antibiotics (TCs) are ubiquitous aquatic contaminants that endanger human health by facilitating antibiotic resistance gene proliferation. Although biochar (BC)-based TCs adsorption has been widely studied, the effects of TCs' molecular structural heterogeneity remain underexplored. Herein, two-dimensional-Fourier transform infrared correlation spectroscopy (2D-FTIR-CoS) and quantum chemical computations were integrated to systematically investigate the structure-dependent adsorption behaviors and mechanisms of five TCs, including tetracycline (TC), oxytetracycline (OTC), minocycline (MNC), methacycline (MTC), and doxycycline (DC), on rice straw-derived BC pyrolyzed at 700 °C (BC700). Hydrogen bonding between TCs amide –NH₂ groups and BC700 surface C=O moieties was identified as the dominant mechanism across all pH conditions; at low TCs concentrations, −NH₂ preferentially binds carboxyl C=O over ketone/ester C=O, while this sequence becomes ambiguous at high concentrations. Kinetic studies confirmed that all TCs fitted the double-exponential model (DEM), with an adsorption rate order of DC > MNC > TC > MTC > OTC. Structure-kinetics correlation analysis revealed that electron-donating R1 (e.g., −N(CH₃)₂ of MNC) and non-electron-donating R3 (e.g., alicyclic –OH of DC) substituents of TCs enhanced adsorption by amplifying BC700-induced electronic polarization, elevating –NH₂ electron density, and strengthening hydrogen bonding, whereas electron-withdrawing R2 substituents (e.g., =CH₂ of MTC, −CH₃/−OH of TC and OTC) exerted inhibitory effects. Multiple linear regression (MLR) was further employed to construct reliable predictive models linking TCs' structural principal components to adsorption rate constants (k₁, k₂) derived by DEM. This study advances the fundamental understanding of structure-dependent TCs adsorption on BC, provides a quantitative basis for customizing high-efficiency BC adsorbents, and offers critical guidance for remediating TCs-contaminated aquatic environments.