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.