Synergistic removal of ammonium by monochloramine photolysis.

The presence of ammonium (NH) in drinking water treatment results in inhibition of disinfection efficiency and formation of nitrogenous disinfection by-products. Our previous study found monochloramine (NHCl) photolysis under 254 nm UV irradiation can be effective for removal of NH; however, the mechanisms of NH degradation in this process were unknown. The kinetics and fundamental radical chemistry responsible for NH removal in the UV/NHCl process were investigated in this study. The results showed that the pseudo first-order rate constant for NH degradation in the UV/NHCl process ranged between 3.6 × 10 to 1.8 × 10 s. Solution pH affected radical conversion and a higher NH degradation efficiency was achieved under acidic conditions. The effects of chloride were limited; however, the presence of either bicarbonate or natural organic matter scavenged radicals and inhibited NH removal. NHCl photolysis generated an aminyl radical (NH) and a chlorine radical (Cl) that further transformed to a chlorine dimer (Cl) and a hydroxyl radical (HO). The second-order rate constants for Cl and Cl reacting with NH were estimated as 2.59 × 10 Ms and 3.45 × 10 Ms at pH 3.9, respectively. Cl, Cl, and HO contributed 95.2%, 3.5%, and 1.3% to NH removal, respectively, at the condition of 3 mM NHCl and pH 7.5. Major products included nitrite and nitrate, possibly accompanied by nitrogen-containing gases. This investigation provides insight into the photochemistry of NH degradation in the UV/NHCl process and offers an alternative method for drinking water production.