Effects of pH and H2O2 on ammonia, nitrite, and nitrate transformations during UV254nm 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/H2O2) process, this study systematically investigated the rates of photo-oxidations of ammonia/ammonium (NH3/NH4+) and nitrite (NO2-) as well as the photo-reduction of nitrate (NO3-) at varying pH and H2O2 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 (N2) or other nitrogen-containing compound was formed. NH3 was more reactive than NH4+ with hydroxyl radical (OH), and by a stepwise H2O2 addition method NH3/NH4+ can be completely converted to NOx-; NO2- underwent rapid oxidation to form NO3- when H2O2 was present, suggesting that it is an intermediate compound linking NH3/NH4+ and NO3-; but once H2O2 was depleted, NO3- can be gradually photo-reduced back to NO2- at high pH conditions. Other than H2O2, the transformation kinetics of DINs were all dependent on pH, but to varying aspects and extents: the NH3 photo-oxidation favored a pH of 10.3, which fell within the pKa values of NH4+ (9.24) and H2O2 (11.6); the NO3- photo-reduction increased with increasing pH provided that it exceeds the pKa of peroxynitrous acid (6.8); while the NO2- photo-oxidation remained stable unless the pH neared the pKa of H2O2 (11.6). The study thereby demonstrates a picture of the evolutions of DIN species together during UV/H2O2 irradiation process, and for the first time presents a method to achieve complete conversion of NH4+ to NO3- with UV/H2O2 process.