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

摘要

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 similar to 9 and similar to 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)-O-center dot 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 similar to 9 and similar to 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 similar to 9 and similar to 10, whereas CO32- slowed NH4+/NH3 but not NO2- photocatalytic oxidation at pH similar to 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. (c) 2006 Elsevier B.V. All rights reserved.