Determination and fate of oxytetracycline and related compounds in oxytetracycline production wastewater and the receiving river

Abstract This study investigated the occurrence and fate of oxytetracycline (OTC) and its related substances, 4-epi-oxytetracycline (EOTC), α-apo-oxytetracycline (α-apo-OTC), and β-apo-oxytetracycline (β-apo-OTC), in a wastewater treatment plant (WWTP) treating OTC production wastewater and a river receiving the effluent from the WWTP using liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). The percent removal of OTC in the WWTP was 38.0 ± 10.5%, and the concentration of OTC was still up to 19.5 ± 2.9 mg/L in the treated outflow. The concentration slightly decreased along the river, from 641 ± 118 μg/L at site R2 (discharging point) to 377 ± 142 μg/L at site R4 (∼20 km from site R2), which was still higher than the minimal inhibition concentration of OTC reported (∼250 μg/L). On the other hand, the total amount of its related substances in the treated effluent was less than 5% of OTC. Concentrations of α-apo-OTC and β-apo-OTC increased along the river, from 5.76 ± 0.63 and 2.08 ± 0.30 μg/L at site R2 to 11.9 ± 4.9 and 12.0 ± 4.6 μg/L at R4, respectively, although EOTC decreased from 31.5 ± 3.8 to 12.9 ± 1.1 μg/L, respectively. The mean concentration of β-apo-OTC in river sediments was 20.8 ± 7.8 mg/kg, and its ratio to OTC was approximately 0.11, nearly twice the ratio of α-apo-OTC and EOTC to OTC (0.058 ± 0.014 and 0.061 ± 0.015, respectively).