Removal of Ammonia by OH Radical in Aqueous Phase

Many advanced oxidation technologies have been developed to remove ammonia in wastewater. All these technologies have one common characteristic, that is, the removal processes involve OH radical (•OH). In this research work, H₂O₂ was selected as •OH precursor. The removal of ammonia under 253.7 nm irradiation from low-pressure mercury lamp in the presence of H₂O₂ was studied to investigate the ammonia removal efficiency by •OH. Results show that the •OH, generated by H₂O₂ photolysis, could oxidize NH₃ to NO₂⁻ and further to NO₃⁻. Removal efficiencies of ammonia were low and were affected by initial pH value and ammonia concentration. Laser flash photolysis technique with transient absorption spectra of nanosecond was used to investigate the oxidation pathway and kinetics of ammonia oxidation by •OH. Results illustrate that •OH could oxidize NH₃ to form •NH₂ with a second-order rate constant of (1.0 ± 0.1) × 10⁸ M⁻¹ s⁻¹ (20 °C). •NH₂, the main product of •OH with NH₃, would further react with H₂O₂ to yield •NHOH. Since ^•NHOH could not stay stable in solution, it would rapidly convert to NH₂O₂⁻ and consequently NO₂⁻ and NO₃⁻. The rate constants for these elementary reactions were also given. The low removal efficiency of ammonia by •OH was mainly due to the slow reaction rate constant.