Effects of pH and HO on ammonia, nitrite, and nitrate transformations during UV irradiation: Implications to nitrogen removal and analysis.

In order to achieve better removal and analyses of three dissolved inorganic nitrogen (DIN) species via ultraviolet-activated hydrogen peroxide (UV/HO) process, this study systematically investigated the rates of photo-oxidations of ammonia/ammonium (NH/NH) and nitrite (NO) as well as the photo-reduction of nitrate (NO) at varying pH and HO conditions. The results showed that the mass balances of nitrogen were maintained along irradiation despite of interconversions of DIN species, suggesting that no nitrogen gas (N) or other nitrogen-containing compound was formed. NH was more reactive than NH with hydroxyl radical (OH), and by a stepwise HO addition method NH/NH can be completely converted to NO; NO underwent rapid oxidation to form NO when HO was present, suggesting that it is an intermediate compound linking NH/NH and NO; but once HO was depleted, NO can be gradually photo-reduced back to NO at high pH conditions. Other than HO, the transformation kinetics of DINs were all dependent on pH, but to varying aspects and extents: the NH photo-oxidation favored a pH of 10.3, which fell within the pK values of NH (9.24) and HO (11.6); the NO photo-reduction increased with increasing pH provided that it exceeds the pK of peroxynitrous acid (6.8); while the NO photo-oxidation remained stable unless the pH neared the pK of HO (11.6). The study thereby demonstrates a picture of the evolutions of DIN species together during UV/HO irradiation process, and for the first time presents a method to achieve complete conversion of NH to NO with UV/HO process.