Interactions of Tetracycline with Ferrihydrite and HA As Affected by Solution pH and Ion Strength.

Antibiotics are commonly used as growth promoter in the animal or poultry feeding operations; nonetheless, most of the antibiotics administered to livestock are excreted as parent compounds. Land applications of the manures may result in the accumulations of antibiotics in soils, and the antibiotics would interact with soil components, influencing re-distributions and transportations of antibiotics in the media. In the current study, tetracycline(TCs) was selected as a target antibiotic due to its widely used and strong complexation with metals, e.g., Ca. The formations of Ca-TC complexes associated with pH changes may alter the electronic properties of TCs, leading to a different adsorptive behavior of TCs on adsorbents, such as ferrihydrite and humic acid. Results showed that TCs were favorably adsorbed on ferrihydrite at pH 6 (zwitterionic TC species) as compared with that at pH 8 (anionic TC species) because the electronic repulsions between ferrihydrite surfaces and TCs would be decreased at a lower pH. Unlike the results obtained at pH 6, the addition of Ca could enhance TCs adsorption on ferrihydrite at pH 8 due probably to the formation of a cationic bridge between these two reactants. At pH 2, humic acid exhibited an increase of TCs adsorption from 11.41 to 16.92 mg/g with decreasing NaCl concentrations from 0.1 to 0.01 N. The competitive effect of Na⁺ on TCs sorption agreed with the interaction of TCs with humic acid via cation exchange. That is, presence of a higher Na concentration may compete with cationic forms of TCs on the adsorption sites of humic acid, and thus, TCs adsorption was decreased.