Thermodynamics of the interaction between antibiotics and extracellular polymeric substances within activated sludge

The removal of antibiotics in biological wastewater treatment plants (WWTPs) has attracted an ever-increasing interest. Extracellular polymeric substances (EPS) play a crucial role in antibiotics' bio-adsorption using activated sludge, but the interaction mechanism between antibiotics and EPS remains unclear. In this study, the thermodynamics of interactions between EPS and antibiotics (acetaminophen (ACT) and sulfamethazine (SMZ)) were investigated via isothermal titration calorimetry (ITC). The results show that the extracellular proteins strongly combined with ACT and SMZ, and the binding process depended on entropy driven by the hydrophobic interaction as the main driving force. Environmental conditions have a significant impact on the adsorption performance. Therefore, binding of antibiotics and EPS at different pH and ionic strength were investigated to determine the optimal pH and ionic strength. At the near-neutral condition of pH 6.8, the binding reaction of EPS and antibiotics was the most favorable and the conformational change was the maximal. Ionic strength has an obvious effect on the interaction between EPS and antibiotics. The results of this study provided a better understanding of the interaction between antibiotics and EPS in the WWTPs.