Antibiotics in the environment: Occurrence in Italian STPs, fate, and preliminary assessment on algal toxicity of amoxicillin

Amoxicillin is a widely used penicillin-like antibiotic, and due to its presence in several effluents of ItalianSTPs, its environmental fate along with its toxicity toward simple organisms have been investigated in model conditions. The present study shows that under abiotic conditions both hydrolysis and direct photolysis could be responsible for the transformation and removal of amoxicillin in aquatic environment, especially in slightly basic media. Quantum yields for the solar direct photolysis have been calculated along with kinetic constants and half-life times. Indirect photolysis experiments in the presence of natural photosensitizers such as nitrate ions and humic acids indicate that nitrate ions have no influence on the photodegradation rate of amoxicillin, while humic acids are able to enhance it. Standard batch experiments have been also performed under biotic conditions. The results indicated that also biodegradation on activated sludge is an effective pathway through which amoxicillin can be removed from the aquatic environment. Rate constants for biodegradation and adsorption have been calculated by applying simple pseudo-first-order kinetic models. Algal bioassays indicate that, in the range of concentrations from 50 ng/L to 50 mg/L, amoxicillin is not toxic toward eucariotic organisms such as the Chlorophyceae Pseudokirkneriella subcapitata and Closterium ehrenbergii and the Bacillariophyceae Cyclotella meneghiniana, but it shows a marked toxicity toward the Cyanophyta Synechococcus leopolensis.