Effects of pH and H₂O₂ on ammonia, nitrite, and nitrate transformations during UV_254nm irradiation: Implications to...

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 NO_x^-; 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 pK_a values of NH₄⁺ (9.24) and H₂O₂ (11.6); the NO₃^- photo-reduction increased with increasing pH provided that it exceeds the pK_a of peroxynitrous acid (6.8); while the NO₂^- photo-oxidation remained stable unless the pH neared the pK_a 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.