Adsorption and cosorption of Cu(II) and tetracycline on two soils with different characteristics

Elevated concentrations of heavy metals and antibiotics often coexist in soil due to land application of large amounts of animal wastes and other sources of inputs. This study aims at evaluating the interaction of Cu(II) and a widely-used veterinary antibiotic tetracycline (TC; H3L) regarding to their adsorption and cosorption on a Red soil (RS, Udic Ferrosols) and a Wushan soil (WS, Anthrosol) using batch experiments. Cosorption of Cu(II) and TC strongly depended on soil characteristics and solution pH, and the adsorption isotherms of Cu(II) and TC on the two soils were well fitted with the Freundlich equation. The presence of TC enhanced Cu(II) adsorption on the RS at pH < 5.0 and the WS at pH < 6.0 in comparison with that not received TC, which resulted from the formation of the positive charged complexes of Cu(II) and TC with greater sorption affinity to soil surface than Cu2+ itself; while the presence of TC suppressed Cu(II) adsorption on the RS at pH > 7.0 and the WS at pH > 7.4, which is ascribed to the formation of water-soluble complexes of Cu(II) and TC that had lower affinity to the soil surface. However, the presence of Cu(II) in the soil solution suppressed the adsorption of TC on the RS at pH < 4.7 and the WS at pH < 3.4, due to the competition of Cu2+ with TC and TC-Cu complexes and to the increased positive surface charge of soil by Cu(II) adsorption. On the other hand, the presence of Cu(II) in the soil solution increased the adsorption of TC at pH > 4.7 and the WS at pH > 3.4 by Cu2+ bridging, similar to that of Ca2+ and Mg2+. It is important to consider the interactions of Cu(II) and TC when modeling the fate of Cu(II) and TC.