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 mu g(-1) min(-1). The OTC adsorption rate decreased with increasing temperature and could be fitted by the Freundlich isotherm. The linear partition coefficients (K-d) were 1.19, 0.999, and 0.841 Lg(-1) at temperatures of 15, 20, and 25 degrees C, respectively. Thermodynamic analysis revealed that the adsorption of OTC onto the inactivated sludge was spontaneous (Delta G = -16.7 to -17.0 kJ mol(-1)), enthalpy-driven (Delta H = -24.9 kJ mol(-1)), entropy-retarded (Delta S = -27.4J (mol K)(-1)), and predominantly a physical adsorption. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.