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Influence of pH, Humic Acids, and Salts on the Dissipation of Amoxicillin and Azithromycin Under Simulated Sunlight

This work is focused on the study of the dissipation of the antibiotics amoxicillin (AMX) (an amino penicillin) and azithromycin (AZT) (belonging to the macrolide group), performed at a laboratory scale, under simulated sunlight and in the dark, at pH values 4.0, 5.5, and 7.2, and in the presence of humic acids and different inorganic salts. The results indicate that AMX is more affected than AZT by simulated sunlight, with half-lives ranging 7.7–9.9 h for AMX and 250–456 h for AZT. The lowest half-life values were obtained at pH 7.2 for AMX (7.7 h) and at pH 4.0 for AZT (250 h). Regarding the effect of various salts, the presence of NaNO3 causes that C/C0 decreases from 0.6 to 0 under simulated sunlight, having no effect on the dissipation of AMX in the dark. However, in the presence of FeCl3 at concentrations of 500 mg L−1, AMX suffered total dissipation, both under simulated sunlight and in the dark. For AZT the influence was lower, and the salts that caused a higher increase in its dissipation were NaCl (with C/C0 decreasing from 0.5 to 0.2) and CaCl2 (C/C0 decreasing from 0.5 to 0.3). The presence of humic acids caused a slight increase in the dissipation of AMX, both under simulated sunlight and in the dark, a fact that was attributed to the adsorption of the antibiotic onto these organic substances, which, however, caused a more marked increase in the dissipation of AZT (reaching 68%) under simulated sunlight. The overall results of this research can be considered clearly relevant, mainly to determine the fate of AMX and AZT when these antibiotics reach the environment as pollutants, either as regards their exposure to natural sunlight, or in relation to the use of inactivation/photo-degradation systems in decontamination procedures focused on environmental compartments.