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Hormesis is an intriguing phenomenon that is characterized by low dose stimulation and high dose inhibition. Several traditional parameters, such as the concentration of the zero equivalent point ( ZEP ) and the maximal stimulatory effect ( Y max ), have been used to characterize the zone of hormesis or the extent of the stimulatory effect. However, the characteristics of hormesis for chemicals cannot be quantified completely by one parameter, which is important to accurately compare the hormetic effects of chemicals and to describe the combined effects of chemical mixtures at low doses. In the present study, a novel parameter, termed the relative standard area of hormetic zone ( H o r A r e a R ), was developed and proposed to quantify the hormetic effects (24 h exposure) of nine metal ions (Cr 3+ , Cu 2+ , Mg 2+ , Cd 2+ , Fe 3+ , Ni 2+ , Zn 2+ , Co 2+ , Cs + ) towards Vibrio fischeri , both individually and as binary mixtures. The results indicate that H o r A r e a R can be used not only to accurately assess the hormetic effects and its relationship with structural characteristics but also to conveniently describe the combined effects of interactive mixtures at low dose. Thus, the H o r A r e a R parameter can quantitatively assess the hormetic effects and can offer a useful approach to perform environmental risk assessments of chemicals at low doses. [ABSTRACT FROM AUTHOR]

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]