Adsorption behavior and mechanism of tetracycline onto hematite: Effects of low-molecular-weight organic acids

Tetracycline (TC) is known to be one of the most widely used antibiotics and is released into the natural envi-ronment, where amounts of iron oxides exist. Low-molecular-weight organic acids (LMWOAs), widespread water-soluble organic substances in the eco-environment systems, may affect the interactions between iron oxides and TC molecules. Nevertheless, our knowledge about the impacts of LMWOAs on the adsorption process of TC onto iron oxides still remains incomplete. In this study, we investigated the adsorption characteristics of TC onto hematite (as a model iron oxide) with various LMWOAs (i.e., acetic acid, glycolic acid, oxalic acid, malic acid, and citric acid). The results clearly showed that LMWOAs suppressed TC adsorption onto hematite particles at pH 7.0 (e.g., the maximum adsorption amounts (q(max)) decreased from 9.05 mg/g (without LMWOAs) to 5.90 mg/g (with 0.5 mM citric acid)), which was mainly ascribed to the competitive adsorption between TC and LMWOAs on the hematite surface, the increased electrostatic repulsion between negatively charged iron oxide particles and TC- species, and the steric hindrance. Additionally, divalent cations (Ca2+ and Cu2+) enhanced the adsorption-inhibition effects by forming cation-bridging with deprotonated LMWOAs. Interestingly, different mechanisms were involved in the adsorption process with increasing pH from 3.0 to 10.0. At pH < 5.0, electrostatic repulsion, adsorption site competition, and steric effect co-controlled the inhibitory effects. Differently, at pH > 5.0, electrostatic repulsion solely dominated the adsorption-inhibition effects. The findings of this study reveal that ubiquitous LMWOAs are critical factors controlling the fate of antibiotics in the natural environment.