Influence of pH and pH adjustment conditions on photocatalytic oxidation of aqueous ammonia under airflow over Pt-loaded TiO2

The influence of pH and pH adjustment conditions on the photocatalytic oxidation of aqueous ammonia in the presence of Pt-TiO2 under airflow was investigated as a possible treatment for ammonia-containing wastewater. Ammonia oxidation proceeded only under alkaline conditions, and the maximum initial oxidation rate was obtained at pH similar to 10. The pH of the reaction suspension decreased with the formation of nitrite and nitrate species, stopping completely when the pH fell below 7 for reactions in which the initial pH (pH(i)) was adjusted within the range 8.7-11.3 using NaOH(aq). The rate of ammonia oxidation was higher when NaOH(aq) was used to adjust the pH(i) than that achieved using Na2CO3-NaHCO3 buffer solution at the same pH. The maximum ammonia adsorption on Pt-TiO2 (measured under dark conditions) was obtained at pH 10 and the amount of ammonia adsorbed when the pH(i) was adjusted with NaOH(aq) was higher than that achieved using the Na2CO3-NaHCO3 buffer solution at the same pH; thus, the amount of adsorbed ammonia strongly affected the oxidation rate. Although the selectivity for the sum of the undesired products, nitrite and nitrate was high when the ammonia concentration was low (e.g., 5 mM), the selectivity decreased as the initial ammonia concentration increased for each pH and pH adjustment condition. Suppression of the selectivity was evident when the pH(i) was adjusted to 10 with NaOH(aq). The selectivity was affected by the changes in the density of N-containing species on the photocatalyst surface as the pH and pH adjustment conditions were varied. (C) 2015 Elsevier B.V. All rights reserved.