Photodegradation of Oxytetracycline in the Presence of Dissolved Organic Matter and Chloride Ions: Importance of Reactive Chlorine Species

This paper investigated the photodegradation of oxytetracycline (OTC) in the presence of dissolved organic matter (DOM) and chloride ions, which is relevant to the estuary environment. The separate effects of chloride ions and DOM on the photodegradation of OTC were first studied, and then, the combined effects were studied. The photodegradation of OTC showed a tendency to decrease with increasing DOM levels: a low concentration of DOM (< 2 mg/L) enhanced the degradation of OTC, and a high concentration of DOM (> 5 mg/L) inhibited it. The addition of chloride ions (10–500 mmol/L) to DOM solutions (20 mg/L) significantly increased the degradation rate of OTC. The observed promotion effects may be a consequence of the participation of reactive chlorine species. Quenching experiments verified that the main active species in the presence of chloride ions and DOM are radicals including Cl^•/Cl₂^•− and HO^•. These results indicate a promotion of OTC degradation in saline water compared with fresh water, and this finding is important to better understand the environmental fate of OTC in estuarine and coastal waters.