Mechanism of oxytetracycline hydrothermal degradation process in pig manure based on molecular dynamics simulation

Antibiotics have been widely used to enhance disease resistance and growth rate in the livestock and poultry breeding industry. However, residual antibiotics can enter the water and soil environment via manure, easily leading to an increase in bacterial resistance to the environment. Thus, it is often required to remove residual antibiotics from the ecological environment. Conventional fecal treatments cannot fully meet the large-scale production in recent years, such as aerobic composting and anaerobic fermentation, due to the long degradation cycles and limited effects on antibiotics. In contrast, an efficient thermochemical conversion, hydrothermal treatment, can be expected to effectively promote the rapid and complete degradation of antibiotics. This study aims to simulate oxytetracycline hydrothermal degradation in pig manure using molecular dynamics. Oxytetracycline, which has the highest proportion of antibiotic residues in pig manure, was selected as the research object. The oxytetracycline-water model was established and then optimized using Materials Studio software. The degradation of oxytetracycline was conducted under hydrothermal conditions using molecular dynamics simulation with LAMMPS software and the ReaxxFF reaction force field. The hydrothermal degradation of oxytetracycline was explored in the hydrothermal system and the reaction path. A systematic analysis was implemented to explore the trends of water and hydroxyl radicals and their reaction mechanisms. Results indicate that hydrothermal temperature was the critical factor controlling oxytetracycline degradation. As the temperature increased, long-chain oxytetracycline molecules progressively fragmented into short-chain small-molecule fragments: The proportion of C_1-5 fragments rose from 45.45% at 1 800 K to 80.68% at 3 000 K, while the proportion of complex long-chain molecules with C>15 decreased from 51.52% to 6.82%. The hydrothermal degradation of oxytetracycline was characterized by rapid and slow increases in fragment formation, and then molecular decomposition intensified with rising simulation temperature. In the temperature range of 1 800 to 2 000 K, there was a low increase in the total number of molecular fragments produced by the degradation of oxytetracycline. In contrast, there was a significant increase in temperature over 2 200 K. The number of H₂O molecules was closely related to the dominant reaction types, lower than the initial value in the initial stage of the reaction at a lower hydrothermal temperature (1 800-2 200 K). Oxytetracycline also shared the hydrolysis reaction. The number of H₂O molecules gradually increased as the hydrothermal temperature rose, and a dehydration reaction also occurred. In addition, the number of hydroxyl radicals gradually increased and fluctuated within a higher range as the hydrothermal temperature increased. These hydroxyl radicals preferentially attacked high-electron-density active sites on the oxytetracycline molecule, thus promoting a series of complex reactions, such as demethylation, deamination, dehydration, hydroxylation, and ring-opening of oxytetracycline. Ultimately, there was the degradation of oxytetracycline into small-molecule substances. Molecular dynamics can be expected to explain the hydrothermal degradation of oxytetracycline at the molecular level. This finding can also provide theoretical guidance and perspective for the efficient removal of antibiotics from swine manure.