## 1 Introduction  
The presence of pharmaceutical compounds in natural water bodies, even at low concentrations, raises health concerns. Pharmaceutical substances, used to prevent and fight diseases, are produced to guarantee their maximum effectiveness and, at the same time, ensure their resistance to inactivation until they perform their intended functions. Thus, these compounds can be excreted through feces and urine in the form of metabolite mixtures and their unchanged product, which flow into wastewater treatment plants (WWTPs). The recent widespread detection of these compounds in such environments [1, 2, 3, 4, 5, 6] have led to their designation as emerging contaminants, as they are still unregulated. The environmental persistence and high biological activity that characterizes them make these substances harmful, even at low concentrations. These contaminants can, in fact, cause alterations to the endocrine system [7] and an increase in microbial resistance to drugs [8, 9, 10, 11]. They can also be adsorbed by plants [12] and bioaccumulated [13] in the food chain. Additional risks are associated with biodiversity loss [7], infertility, and cancer [14, 15]. One of the categories of drugs on which the attention of the scientific community is most focused is that of antibiotics present in the aquatic environment and in foods, which have the possibility of inducing the formation of antibiotic-resistant bacteria and the health risks that may derive from them [16; 17]. Amoxicillin (AMO) is among the most prescribed antibiotics for human use in Italy [18] and other European countries [19], which is a drug synthesized in large amounts and used in aquaculture farms to cope with the most common fish diseases. In Italy, it is estimated that more than 210 t are used annually, of which 86 \(\pm\) 8% [20] are excreted in parental form with a theoretical environmental load estimated at around 190 t/year. Risk assessment studies for aquatic species and humans are under development, but despite the small amount of ecotoxicological data, some studies found it possible to establish compounds, such as AMO, in surface waters at non-negligible risk levels for aquatic species organisms [21; 22]. AMO has been detected at \(\mu\)g/L concentrations in the influent and effluent of WWTPs and surface water [23; 24], while its levels in pharmaceutical industry effluents may reach mg/L concentrations [25]. Although the treatment processes used in the plants shows high AMO removals [26; 27; 28], even if the percentage of removal depends on the duration of the treatment [29], at the same time, they have the disadvantage of increasing effluent toxicity and producing its transformation compounds, which may be more toxic than the product from which they derive [30]. As a consequence, WWTP effluents and the practice of reusing sewage sludge in agriculture to recover nitrogen compounds useful for soil fertilization can contribute to its introduction into water bodies and its diffusion in the terrestrial environment of the degradation byproducts (DPs) of the drug [31; 32; 33; 34]. Humans can be exposed to DPs through the consumption of aquatic organisms, agricultural products, or drinking water. In this paper, DPs of AMO were investigated under the same conditions as the chlorination process normally used in WWTPs to reduce similar emerging pollutants [35; 36] at three different pH values. In particular, a chromatographic profile of the possible DPs was obtained with an experiment at low concentrations of AMO (about \(10^{-5}\) M), which were then isolated and structurally determined by repeating the chlorination experiment at concentrations levels at least 100 times higher. The structures of 16 isolated DPs, one of which was isolated for the first time, were determined by combining mass spectrometry, using as source a matrix-assisted laser desorption/ionization and as mass analyzer a time-of-flight analyzer (MALDI-MS/TOF) and nuclear magnetic resonance (NMR) data. They were then justified by a proposed formation mechanism.