title: Effects of pH and catalyst concentration on photocatalytic oxidation of aqueous ammonia and nitrite in titanium dioxide suspensions.
abstract: Batch experiments were conducted to study the effects of titanium dioxide (TiO2) concentration and pH on the initial rates of photocatalytic oxidation of aqueous ammonium/ ammonia (NH4+/NH3) and nitrite (NO2-) in UV-illuminated TiO2 suspensions. While no simple kinetic model could fit the data at lower TiO2 concentrations, at TiO2 concentrations > or = 1 g/L, the experimental data were consistent with a model assuming consecutive first-order transformation of NH4+/NH3 to NO2- and NO2- to nitrate (NO3-). For TiO2 concentrations > or = 1 g/L, the rate constants for NO2 photocatalytic oxidation to NO3 were far more dependent on TiO2 concentration than were those for NH4+/NH3 oxidation to NO2-, suggesting that, without sufficient TiO2, complete oxidation of NH4+/NH3 to NO3- will not occur. Initial NH4+/NH3 photocatalytic oxidation rates were proportional to the initial concentrations of neutral NH3 and not total NH3(i.e., [NH4+] + [NH3]). Thus, the pH-dependent equilibrium between NH4+ and NH3, and not the pH-dependent electrostatic attraction between NH4+ and the TiO2 surface, is responsible for the increase in rates of NH4+/NH3 photocatalytic oxidation with increasing pH. Electrostatic adsorption, however, can partly explain the pH dependence of the initial rates of NO2- photocatalytic oxidation. Initial rates of NO2- photocatalytic oxidation were 1 order of magnitude higher for NO2- versus NH4+/NH3, indicating thatthe rate of NH4+/NH3 photocatalytic oxidation to NO3- was limited by NH4+/NH3 oxidation to NO2- under our experimental conditions.
authors.affiliation: School of Civil Engineering and Environmental Science, University of Oklahoma, Norman, Oklahoma 73019, USA.
authors.forename: Xingdong
authors.initials: X
authors.lastname: Zhu
authors.forename: Sunny R
authors.initials: SR
authors.lastname: Castleberry
authors.forename: Mark A
authors.initials: MA
authors.lastname: Nanny
authors.forename: Elizabeth C
authors.initials: EC
authors.lastname: Butler
mesh_terms: D000641:Ammonia / Q000737:chemistry / Q000528:radiation effects*; D002384:Catalysis; D006863:Hydrogen-Ion Concentration; D009573:Nitrites / Q000737:chemistry / Q000528:radiation effects*; D010084:Oxidation-Reduction; D010782:Photolysis; D014025:Titanium / Q000737:chemistry*; D014466:Ultraviolet Rays; D014874:Water Pollutants, Chemical / Q000528:radiation effects*; D014876:Water Pollution / Q000517:prevention & control; D018508:Water Purification / Q000379:methods*
chemical_list: D009573:Nitrites; D014874:Water Pollutants, Chemical; C009495:titanium dioxide; D000641:Ammonia; D014025:Titanium