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 (center dot OH). In this research work, H2O2 was selected as center dot 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 center dot OH. Results show that the center dot OH, generated by H2O2 photolysis, could oxidize NH3 to NO and further to NO(3) over bar. 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 center dot OH. Results illustrate that center dot OH could oxidize NH3 to form center dot NH2 with a second-order rate constant of (1.0 +/- 0.1) x 10(8) M-1 s(-1) (20 degrees C). center dot NH2, the main product of center dot OH with NH3, would further react with H2O2 to yield center dot NHOH. Since center dot NHOH could not stay stable in soluition, it would rapidly convert to NH2O(2) over bar and consequently NO(2) over bar and NO(3) over bar. The rate constants for these elementary reactions were also given. The low removal efficiency of ammonia by center dot OH was mainly due to the slow reaction rate constant