Solar photodegradation of oxytetracycline in brackish aquaculture water: New insights about effects of Ca2+ and Mg2

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Chemistry., 2019-03, Vol.372, p.218-225
• Main Authors: Leal, Joana F., Esteves, Valdemar I., Santos, Eduarda B.H.
• Format: Article
• Language: English
• Subjects: Antibiotic; Antibiotics; Aquaculture; Aqueous solutions; Binding sites; Brackish water; Calcium; Calcium ions; Cations; Complexes; Magnesium; Oxytetracycline; Photodegradation; Photolysis; Photoproducts; Quantum yields; Spectroscopy
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/j.jphotochem.2018.12.022

[Display omitted] •The kinetics of the OTC photo-degradation is enhanced by Ca2+ but not by Mg2+.•OTC: Ca2+ and OTC: Mg2+ complexes have the same binding pattern, at pH 7.3.•Photolysis rate and fluorescence emission of complexes are inversely related.•The sensitizing effect of Ca2+ is higher in synthetic than in aquaculture’s water.•Two new products are formed in the presence of Mg2+. Oxytetracycline (OTC) is an antibiotic used in aquaculture able to complex with the major cations Ca2+ and Mg2+, existing in brackish water. The effects of these cations on the solar photodegradation of OTC have been investigated. Calcium promotes a faster OTC photo-degradation in aqueous solution, while the same does not occur with Mg2+. For some authors the accelerating effect of Ca2+ is attributed to the occurrence of self-sensitized photodegradation of the complexes with Ca2+, which is not possible in the complexes with Mg2+, due to different binding patterns of these cations. In this work, the controversy concerning the binding sites of these cations in OTC is discussed. Based on spectroscopic evidence, similar binding patterns of OTC with Ca2+ and Mg2+ and a lower quantum yield of the direct photolysis of the complexes with Mg2+ are proposed. In addition, it was demonstrated that Mg2+ inhibits the formation of some OTC photoproducts observed in the presence of Ca2+, while at least two new OTC photoproducts are formed in the presence of Mg2+.