Synergistic removal of ammonium by monochloramine photolysis

摘要

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 x 10(-4) to 1.8 x 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 center dot) and a chlorine radical Cl that further transformed to a chlorine dimer (Cl-2(center dot-)) and a hydroxyl radical (HO center dot). The second-order rate constants for Cl-center dotbold /boldand Cl-2(center dot) reacting with NH4+ were estimated as 2.59 x 10(-8) M(-1)s(-1) and 3.45 x 10(-5) M(-1)s(-1) at pH 3.9, respectively. Cl-center dot, Cl-2(center dot), and HO center dotbold /boldcontributedbold /bold95.2%, 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. (C) 2019 Elseiver Ltd. All rights reserved.