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The fate of seven steroids: estrone (E1), estradiol (E2), estriol (E3), ethinylestradiol (EE2), testosterone (TEST), androstenedione (AND), and progesterone (PROG), in the presence of synthetic wastewater was studied in order to establish the role abiotic processes play in the elimination of these chemicals from the environment. Comprehension of these mechanisms will foster the optimization of the existing wastewater treatment technologies and the development of sustainable alternatives.Distinctive behavior was encountered for the target compounds in accordance with their chemical structure, hence, different physico-chemical properties and reactivity. Estrogenic compounds, comprising E1, E2, E3 and EE2, were found to undergo a catalytic transformation when contacted with a model vegetable material present in the synthetic wastewater. This transformation occurred in the absence of biological and enzymatic activity. On the other hand, the concentration of TEST, AND, and PROG stayed constant and in agreement with the spiked amount. The fastest transformation rate corresponded to E3, the least hydrophobic compound in the study. This may indicate that the catalytic reaction occurred in the aqueous phase. The contribution of steric and electronic factors, such as critical oxidation potential, in the reaction rate cannot be discarded; consequently, the hypothesis of a surface catalyzed reaction cannot be rejected.The use of 14C4-estradiol (14C-E2) as model estrogenic compound corroborated the occurrence of a catalytic reaction, most likely through an oxidative coupling mechanism. Under oxic conditions, the mass balance for radioactivity was closed after extended experimental periods (72 h), while the concentration of 14C-E2 measured by Liquid Chromatography coupled with a Triple Quadrupole Mass Spectrometer (LC/MS/MS) did not match the spiked one when analyzed independently in liquid and solid phases. Furthermore, radioactivity was found to distribute in the aqueous phase as well as extractable and non-extractable solids, suggesting that phenoxy radicals formed on the phenolic ring of 14C-E2 could react among themselves (to form dimers), with the functional groups present on the surface of the lignin-type vegetable model material (resulting in covalent bonded matter), and with other chemicals species in the solution. 14C4-estrone was also monitored in this study, but it was not detected in any sample. Behavior encountered under anoxic conditions emphasized the role of molecular oxygen in the catalytic process. In the absence of oxygen, the reaction was completely halted; this was confirmed by the closure of the mass balance of 14C-E2, which was performed by radioactivity and LC/MS/MS analyses. This indicated that estrogens were transformed in an oxidation reaction catalyzed by some vegetable matter component and in which the dissolved oxygen acted as oxidant. Preliminary investigations suggested that manganese oxides could be acting as catalyst in this scenario. |
