Photolytic degradation of tetracycline in the presence of Ca(II) and/or humic acid

Abstract

Photolytic degradation of tetracycline (TC) was investigated in mono-and binary solute systems of Ca(II) and humic acid (HA) under UVA light emitting diode (UVA-LED) light irradiation. TC photolysis proceeded via pseudo-first-order reaction kinetics. The presence of Ca(II) significantly accelerated the degradation rate constants of TC, with the highest value at 0.0314 ± 0.0019 min-1 when the Ca(II) concentration was 5.0 mM. The promoted degradation was attributed to complexation of TC with Ca(II), which increased the light absorption. Absorbance and fluorescence measurements revealed that the strong complexation between TC and Ca(II) likely occurred via the C11 and C12 oxygen groups in the phenolic-diketone moiety of TC in nearly neutral solutions. The formation of HA-Ca(II) complex was found in the binary solute system of HA and Ca(II). Thus, the promotional effect of Ca(II) on photolysis was diminished by HA addition. The largest reduction of 32.5% in rate constants was observed with the highest Ca(II) concentration. Scavenger studies revealed that TC could undergo direct photolysis and self-sensitization by 1O2. These results suggest that the coexistence of HA and Ca(II) greatly influences the fate of TC in natural waters, which has important implications for understanding the behavior of antibiotics coexisting with other metal species and ligands. © 2020 by the authors.