Adsorption and transformation of tetracycline antibiotics with aluminum oxide

Tetracycline antibiotics (TCs) including tetracycline (TTC), chlorotetracycline (CTC) and oxytetracycline (OTC) adsorb strongly to aluminum oxide (Al₂O₃), and the surface interaction promotes structural transformation of TCs. The latter phenomenon was not widely recognized previously. Typically, rapid adsorption of TCs to Al₂O₃ occurs in the first 3h ([TC]=40μM, [Al₂O₃]=1.78gL^-1, pH=5, and T=22°C), followed by continuous first-order decay of the parent compound (k_obs=15±1.0, 18±1.0 and 6.2±0.9×10^-3h^-1 for TTC, CTC and OTC, respectively) and product formation. The transformation reaction rate of TCs strongly correlates with adsorption to Al₂O₃ surfaces. Both adsorption and transformation occur at the highest rate at around neutral pH conditions. Product evaluation indicates that Al₂O₃ promotes dehydration of TTC to yield anhydrotetracycline (AHTTC), epimerization of TTC, and formation of Al-TTC complexes. Al₂O₃ promotes predominantly the transformation of CTC to iso-CTC. The surface-bound Al(+III) acts as a Lewis acid site to promote the above transformation of TCs. Formation of AHTTC is of special concern because of its higher cytotoxicity. Results of this study indicate that aluminum oxide will likely affect the fate of TC antibiotics in the aquatic environment via both adsorption and transformation.