Effect of inorganic anions on the titanium dioxide-based photocatalytic oxidation of aqueous ammonia and nitrite

In this study, we investigated the effects of four inorganic anions (Cl−, SO42−, H2PO4−/HPO42−, and HCO3−/CO32−) on titanium dioxide (TiO2)-based photocatalytic oxidation of aqueous ammonia (NH4+/NH3) at pH ∼ 9 and ∼10 and nitrite (NO2−) over the pH range of 4–11. The initial rates of NH4+/NH3 and NO2− photocatalytic oxidation are dependent on both the pH and the anion species. Our results indicate that, except for CO32−, which decreased the homogeneous oxidation rate of NH4+/NH3 by UV-illuminated hydrogen peroxide, OH scavenging by anions and/or direct oxidation of NH4+/NH3 and NO2− by anion radicals did not affect rates of TiO2 photocatalytic oxidation. While HPO42− enhanced NH4+/NH3 photocatalytic oxidation at pH ∼ 9 and ∼10, H2PO4−/HPO42− inhibited NO2− oxidation at low to neutral pH values. The presence of Cl−, SO42−, and HCO3− had no effect on NH4+/NH3 and NO2− photocatalytic oxidation at pH ∼ 9 and ∼10, whereas CO32− slowed NH4+/NH3 but not NO2− photocatalytic oxidation at pH ∼ 11. Photocatalytic oxidation of NH4+/NH3 to NO2− is the rate-limiting step in the complete oxidation of NH4+/NH3 to NO3− in the presence of common wastewater anions. Therefore, in photocatalytic oxidation treatment, we should choose conditions such as alkaline pH that will maximize the NH4+/NH3 oxidation rate.