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Antibiotics are categorized as pseudopersistent compounds because of their widespread use and continuous emission into the environment. Biological systems such as active sludge and biofilms are still the principal tools used to remove antibiotics in wastewater treatment plants (WWTPs). Consequently, it is important to determine the relationship between toxic effects in biological WWTPs and the structural characteristics of antibiotics. In the present study, toxic effects of 10 sulfonamides (SAs) on nitrification in an active sludge system were studied. The toxicity results (half-effective concentrations, EC 50 ) indicated that the toxicity of sulfadimethoxine (SDM) is approximately 4 times as large as that of sulfadiazine (SD). Based on the toxicity mechanism and the partial least squares regression (PLS) method, an optimum quantitative structure-activity relationship (QSAR) model was developed for the test system. The mechanistic model showed that the p K a , the binding energies between SAs with dihydropteroate synthetase ( E b i n d i n g S A − D H P S ) and the binding energies between SAs with ammonia monooxygenase ( E b i n d i n g S A − A M O ) are the key factors affecting the toxic effects of SAs on nitration process in active sludge system, following an order of importance of E b i n d i n g S A − D H P S > E b i n d i n g S A − A M O > p K a . [ABSTRACT FROM AUTHOR]