Tetracycline antibiotics are excreted by humans and animals, eventually reaching the aquatic environment where they promote antimicrobial resistance and adversely affect aquatic organisms. Ultraviolet light at 254 nm is often used to disinfect wastewater effluent; however, this process can also partially transform tetracyclines. Divalent calcium (Ca2+) and magnesium (Mg2+) ions influence the photochemical transformation of tetracyclines via formation of metal-ligand complexes. Dissolved organic matter (DOM) also interacts with divalent metals and potentially outcompetes tetracyclines for complexation. Furthermore, DOM can affect photolysis via photosensitization, light screening, and scavenging of reactive species. Previous studies have individually evaluated the influences of Ca2+, Mg2+, and DOM on tetracycline photolysis, but the interactions of these three water quality parameters have not been co-examined. In this work, the photodegradation kinetics of doxycycline, minocycline, and oxytetracycline were investigated under 14 unique conditions. The experimental solutions initially contained 10 µM tetracycline, 10 mM tris (pH 6.5), 0-70 mM NaClO4 (70 mM ionic strength for all solutions), 0-10 mM Ca(ClO4)2, 0-10 mM Mg(ClO4)2, and 0-10 mgC L-1 of Suwannee River Natural Organic Matter. The solutions were equilibrated overnight in the dark before irradiation with light at 254 nm. All observed photodegradation rates for the tetracyclines adhered to pseudo-first-order kinetics (R2 ≥ 0.98), and the rate constants ranged from (6.02 ± 0.38) × 10-5 to (2.05 ± 0.21) × 10-4 cm2 mJ-1. The observed rate constants were modeled as a function of the Ca2+, Mg2+, and DOM concentrations using multi-linear regression, and the significant terms (α &lt; 0.05) were identified for each tetracycline. For doxycycline and oxytetracycline, the model indicates that photodegradation was enhanced by Ca2+ and inhibited by DOM. For minocycline, both Ca2+ and Mg2+ decreased the photolysis rate, and DOM had no measurable effect. The additional tertiary amine group in minocycline likely caused the observed differences with respect to doxycycline and oxytetracycline. In all cases, the metal-DOM interaction terms were insignificant, suggesting the negligible influence of the Ca2+-DOM and Mg2+-DOM complexes on tetracycline photodegradation rates. These findings demonstrate how water composition and minor structural variations affect the fate of tetracycline antibiotics in UV treatment processes.

Multi-variate, kinetic analysis of tetracycline photolysis at 254 nm in the presence of calcium, magnesium, and dissolved organic matter

ACS Spring Meeting 2021

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