Direct photolysis of oxytetracycline: Influence of initial concentration, pH and temperature

Oxytetracycline (OTC) commonly undergoes direct photolysis as the main elimination pathway in surface waters. The photolysis of OTC was studied as a function of initial concentration, pH and temperature. The experiments were performed in the absence of oxygen to avoid oxygen induced indirect photolysis. The photolysis rate decreased when increasing the initial OTC concentration, and was accelerated as raising the temperature. pH was the key factor controlling the photolysis rate. By measuring the UV-vis absorption spectra of OTC under various pH conditions and fitting them with global analysis, four OTC species H3L+, H2L +/-, HL-, and L2- (L represents OTC) were decomposed, meanwhile, the acid dissociation constant of OTC and the molar absorption coefficient of each species were resolved. Because apparent photolysis rate constants under varied light source conditions are often incomparable, therefore the direct photolysis quantum yield of OTC species were calculated by a mathematical approach which based on the molar absorption coefficients, the species composition and apparent kinetic rates of OTC under experimental pH conditions. The result shows the photolysis quantum yield of OTC followed the order L2- > (HL +/-)-L-2 > HL- > H3L+. The direct photolysis rates of OTC were tightly correlated to the energy dissipation processes at the excited states, e.g., the excited states inter-/intramolecular proton transfer (ESIPT), and the molecular conformation transfer induced internal energy transmission between ground state and excited state. Fluorescence emission was not an important energy dissipation process. The results of this study provide the useful parameters for predicting the environmental fate of OTC in surface waters. And the approach for calculating the photolysis quantum yield is also available for other antibiotics with multiple pKa values. (C) 2016 Published by Elsevier B.V.