The antibiotic oxytetracycline is commonly used in aquaculture and can eventually lead to contamination of the surrounding environment such as surface water and soil. In nature, a major degradation mechanism of oxytetracycline is photolysis. Oxytetracycline is known to form complexes with cations. After complex formation, oxytetracycline changes from a folded to a planar conformation which is more sensitive to light. Thus, the oxytetracycline-cation complex can be used as a new means of accelerating oxytetracycline degradation in the environment. In this study, the complex formation of oxytetracycline with three different types of divalent cations, i.e. Mg2+,Ca2+, and Cu2+, was studied using a spectrofluorometer and the binding constants (K) of oxytetracycline with cations in either Tris-hydrochloride buffer pH 7.80 or phosphate buffer pH 7.80 were calculated. Tris-hydrochloride buffer was chosen for use as a solvent for further studies because of its higher buffer capacity. The binding constants of oxytetracycline with Mg2+, Ca2+, and Cu2+ ions in Tris-hydrochloride buffer pH 7.80 were 3,921.8 , 4,386.8 , and 1,149.9 M-1 respectively and the binding constants of oxytetracycline with Mg2+, Ca2+ ions in phosphate buffer pH 7.80 were 2,704.6 , 1,791.2 M-1 respectively. The binding constants were different in other buffer system. Photolysis degradation of oxytetracycline-Cu2+complex or of oxytetracycline-Ca2+complex followed first order reaction. Direct photolysis rate was found to be dependent on the initial oxytetracycline concentration, in the oxytetracycline concentration from 0.101 to 0.402 mM. While degradation of oxytetracycline-Mg2+complex was a zero order reaction, photolysis rate was not dependent on the initial oxytetracycline concentration. Ionic strength had influence on photolysis degradation of oxytetracycline-Cu2+complex. Increasing ionic strength from 0.07 to 0.1 M led to the increase of the degradation rate constant. But ionic stregnth above 0.1 M, the degradation rate constant decreased. While, photolysis degradation of oxytetracycline-Ca2+complex and oxytetracycline-Mg2+complex were increased in ionic strength concentration range 0.04 to 0.6 M, an indicative of ionic strength on photolysis. This study showed that photolysis could be accelerated by oxytetracycline-divalent cation complex. Thus, the oxytetracycline-divalent cation complex especially oxytetracycline-Cu2+ can be used as a new means of accelerating oxytetracycline degradation in the environment.
