Phosphate affects adsorption and desorption of oxytetracycline in the seawater-sediment systems.

The fate and transport of antibiotics in the aquatic environment are usually influenced by the combined effects of environmental factors. In this study, batch-type experiments were conducted to investigate the combined effects of phosphate and salinity on oxytetracycline (OTC) adsorption on, and desorption from, two marine sediments. Pseudo-second-order kinetic model fitted the kinetic data better than pseudo-first-order model. The pseudo-second-order kinetic rate (k2) of OTC increased significantly with increasing phosphate concentrations. Sorption isotherms of OTC can be well described by both the Langmuir and Freundlich models. Compared with the control experiments (without phosphate addition), Freundlich distribution coefficients (KF) after addition of 50 mg L-1 phosphate decreased from 24.9 to 17.9 L kg-1 on sediments collected from Laizhou Bay, and from 52.2 to 31.3 L kg-1 on sediments collected from Bohai Bay, which indicated that phosphate could inhibit adsorption of OTC on marine sediments. Furthermore, phosphate had a stronger inhibitory effect on OTC adsorption at low salinity level than at high salinity level. The effects of phosphate on OTC adsorption can be explained by ion exchange, change of sediment surface charge, and electrostatic repulsion mechanisms. Desorption experiments showed that phosphate can enhance the total desorption percentage of OTC from marine sediments. Overall, the presence of phosphate in marine system may enhance OTC's long-term transport.