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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]

Photocatalysis is one of the most effective methods for treating antibiotic wastewater. Thus, it is of great significance to determine the relationship between degradation rates and structural characteristics of antibiotics in photocatalysis processes. In the present study, the photocatalytic degradation characteristics of 10 sulfonamides (SAs) were studied using two photocatalytic systems composed of nanophase titanium dioxide ( n TiO 2 ) plus ultraviolet (UV) and n TiO 2 /activated carbon fiber (ACF) plus UV. The results indicated that the largest apparent SA degradation rate constant ( K app ) is approximately 5 times as large as that of the smallest one. Based on the degradation mechanism and the partial least squares regression (PLS) method, optimum Quantitative Structure Activity Relationship (QSAR) models were developed for the two systems. Mechanistic models indicated that the degradation rule of SAs in the TiO 2 systems strongly relates to their highest occupied molecular orbital ( E homo ), the maximum values of nucleophilic attack ( f (+) x ), and the minimum values of the most negative partial charge on a main-chain atom ( q (C) min ), whereas the maximum values of OH radical attack ( f (0) x ) and the apparent adsorption rate constant values ( k ad ) are key factors affecting the degradation rule of SAs in the TiO 2 /ACF system. [ABSTRACT FROM AUTHOR]