Complexation determines the removal of multiple tetracyclines by ferrate

Complexation of tetracyclines with Fe species can be expected during oxidation by ferrate and be part of the complicated reactions that lead to the transformation of tetracyclines. Herein, we focused on the role of complexation in four tetracyclines removals by ferrate. The quenching experiment, desorption experiment, total organic carbon detection, and UV absorbance characterization demonstrated that, for the removal of tetracyclines, the complexation of tetracyclines with Fe3+ (derived from the reduction and decomposition of ferrate) contributed ∼79 %, followed by ∼11 % oxidation and ∼10 % adsorption/flocculation. Density functional theory calculations rationalized the formation of Fe-tetracyclines complexes and reactions of ferrate and hydroxyl radicals with each tetracycline. Thermodynamic and kinetic calculations indicated that complexation reactions between Fe3+ and tetracyclines were more likely to occur than oxidation reactions between ferrate and hydroxyl radicals. The complexation rate was highest for tetracycline and lowest for chlortetracycline, and the order of complexation rates was consistent with the experimental removal ratios of four tetracyclines. Complexation rates of tetracyclines with Fe3+ determine the removal of multiple tetracyclines by ferrate rather than oxidation rates. Insights based on forming Fe-tetracyclines complexes differed from the reported oxidation mechanisms, and complexation cannot be negligible in removing pollutants due to the potential secondary release.