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.