Removal mechanism of trace oxytetracycline by aerobic sludge

To investigate the mechanism of removal of selected pharmaceuticals in activated sludge systems, laboratory-scale batch experiments were conducted to assess the adsorption and degradation behavior of trace oxytetracycline (OTC). The adsorption equilibrium of OTC was observed in 30 min and the adsorption process could be well described by a pseudo-second-order model with a rate of 0.362 L μg−1 min−1. The OTC adsorption rate decreased with increasing temperature and could be fitted by the Freundlich isotherm. The linear partition coefficients (Kd) were 1.19, 0.999, and 0.841 L g−1 at temperatures of 15, 20, and 25 °C, respectively. Thermodynamic analysis revealed that the adsorption of OTC onto the inactivated sludge was spontaneous (ΔG = −16.7 to −17.0 kJ mol−1), enthalpy-driven (ΔH = −24.9 kJ mol−1), entropy-retarded (ΔS = −27.4 J (mol K)−1), and predominantly a physical adsorption.

► We investigate mechanism of removal of trace oxytetracycline (OTC) by sludge.
► Adsorption of OTC by sludge is quick and fits the pseudo-second-order model.
► OTC sorption decreases with increase of temperature and could be fitted by the Freundlich isotherm.
► Adsorption of OTC onto the inactivated sludge was spontaneous and entropy-retarded.
► Most of the initial OTC was removed by adsorption during aerobic degradation.