The effect of pH, nitrate, iron (III) and bicarbonate on photodegradation of oxytetracycline in aqueous solution

Abstract Photodegradation is a very important elimination pathway of Oxytetracycline (OTC) in aquatic environments. The photochemical behavior of OTC in absence/presence of nitrate, iron (III) and bicarbonate, especially their combinations, was systematically studied in aqueous solution by employing a Suntest-CPS+ sunlight simulator in this work. Meanwhile, the effect of solution pH (4.8–9.1) and irradiation intensity (250–500 W/m 2 ) on the removal efficiency of OTC have also been assessed. The results showed that OTC degradation followed a pseudo-first-order kinetics in most conditions except for irradiation in neutral/alkaline solutions and in presence of NO 3 − , where a two-step pseudo-first-order kinetics was obeyed. OTC degradation was highly pH-dependent and increased significantly with increasing pH. The presence of NO 3 − and HCO 3 − promoted the photochemical loss of OTC in aqueous solution by generating hydroxyl radicals and adjusting solution pH, respectively; whereas iron (III) was found to play a negative role under the conditions studied. The influence of NO 3 − coupled with Fe 3+ was not a simple additive effect and the overall performance was to inhibit OTC degradation. In addition, combination of HCO 3 − and NO 3 − exhibited an antagonistic effect. Results from irradiation in co-presence of NO 3 − , Fe 3+ and HCO 3 − suggested that direct photolysis of OTC was much more efficient than its indirect photolysis. These results provide a meaningful reference for understanding the fate and transformation of OTC in natural water systems.