Mechanistic insights into the environmental fate of tetracycline affected by ferrihydrite: Adsorption versus degradation

Abstract

Tetracycline (TC), a widely used antibiotic, is frequently detected in soil environments. It has a strong tendency to form complexes with metals, including iron (oxyhydr)oxide. In this study, ferrihydrite (Fh), a representative iron oxyhydroxide of the iron plaques on the surface of plant roots, was chosen to study the contributions of iron oxyhydroxide on the environmental fate of TC in the rhizosphere environment. Fh adsorption isotherm of TC showed good fitting to the Freundlich model, and the Fh adsorption capacity of TC was found much larger than the other iron oxyhydroxide of high crystallinity. The adsorption mechanisms mainly included electrostatic interaction, H-bonding, and complexation. The results of FTIR and XPS spectra revealed that tricarbonylamide, dimethylamino, and the hydroxyl in the B ring of TC were mainly responsible for the complexation with Fh surface hydroxyl groups. Furthermore, it should be noted that the adsorbed TC on Fh could be degraded and the degradation kinetics of TC better fitted to the pseudo-second-order model. Fh could promote electron transfer from TC to Fe(III) on the Fh surface, which led to the degradation of TC and the formation of Fe(II) ions. The degradation pathways of TC mainly involved three reactions: hydroxylation, dealkylation, and deamination. This study provides mechanistic insights on TC-Fh interaction, which improves the understanding of TC fate in the rhizosphere environment.