Synergistic removal of ammonium by monochloramine photolysis

The presence of ammonium (NH₄⁺) in drinking water treatment results in inhibition of disinfection efficiency and formation of nitrogenous disinfection by-products. Our previous study found monochloramine (NH₂Cl) photolysis under 254 nm UV irradiation can be effective for removal of NH₄⁺; however, the mechanisms of NH₄⁺ degradation in this process were unknown. The kinetics and fundamental radical chemistry responsible for NH₄⁺ removal in the UV/NH₂Cl process were investigated in this study. The results showed that the pseudo first-order rate constant for NH₄⁺ degradation in the UV/NH₂Cl process ranged between 3.6 × 10^-4 to 1.8 × 10^-3 s^-1. Solution pH affected radical conversion and a higher NH₄⁺ 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 NH₄⁺ removal. NH₂Cl photolysis generated an aminyl radical (NH₂^•) and a chlorine radical (Cl^•) that further transformed to a chlorine dimer (Cl₂^•-) and a hydroxyl radical (HO^•). The second-order rate constants for Cl^• and Cl₂^•- reacting with NH₄⁺ were estimated as 2.59 × 10⁸ M^-1s^-1 and 3.45 × 10⁵ M^-1s^-1 at pH 3.9, respectively. Cl^•, Cl₂^•-, and HO^• contributed 95.2%, 3.5%, and 1.3% to NH₄⁺ removal, respectively, at the condition of 3 mM NH₂Cl and pH 7.5. Major products included nitrite and nitrate, possibly accompanied by nitrogen-containing gases. This investigation provides insight into the photochemistry of NH₄⁺ degradation in the UV/NH₂Cl process and offers an alternative method for drinking water production.