Experimental investigation on decomposition characteristics of doxycycline in wastewater using hydrothermal treatment: Exploring the way to complete decomposition

Doxycycline is widely found in medical and aquaculture wastewater, posing a great threat to the ecological environment. In this study, the doxycycline was decomposed by hydrothermal method, and its decomposition properties and paths were analyzed. By using univariate experimental method, the influence of hydrothermal temperature (100–340 °C), hydrothermal time (30–120 min) and mass concentration (1–4 wt%) on the decomposition rate of doxycycline, chemical oxygen demand and nitrogen distribution in the product were studied, and the Pearson correlation coefficient of chemical oxygen demand, Total nitrogen and doxycycline removal rate was obtained. The results showed that the removal rate of doxycycline increased with the hydrothermal temperature, decreased with the reactant concentration, and increased first and then decreased with the hydrothermal time. The quantitative relation between the removal rate of doxycycline and operation conditions (temperature, time, and concentration) was obtained by using the response surface method, and the optimization conditions of the hydrothermal decomposition of doxycycline were gotten. The averaged removal rate of doxycycline reached 99.5% at temperature of 260 °C, reaction time of 102.2 min, and mass concentration of 1.4 wt%. Finally, the liquid chromatography and mass spectrometry were used for quantitative and qualitative analysis of liquid products, and three potential hydrothermal decomposition routes of doxycycline were proposed. This study will lay the foundation for the hydrothermal treatment of medical and aquaculture wastewater containing antibiotics.