Regression model and pathways of halonitromethane formation from amino acids in the presence of bromide ion under UV/monochloramine disinfection

Abstract

Halonitromethanes (HNMs) including chlorinated and brominated HNMs, a category of nitrogenous disinfection by-products with high toxicity, were formed from amino acids (AAs) in the presence of bromide ion (Br-) under UV/monochloramine (UV/NH2Cl) disinfection. This study developed a regression model using water characteristics (pH, HCO3– and Br-) and disinfection conditions (disinfection time and NH2Cl doses) to predict HNM formation from AAs (tyrosine (Tyr), tryptophan (Trp) and aspartic acid (Asp)) in the presence of Br- under UV/NH2Cl disinfection. Main contributors to the abatement of Tyr and Trp were UV (6.1%-18.7%), HO (18.5%-62.3%), Cl2− (0.3%-17.9%), Br (2.4%-41.3%, kBr•,AA=∼109-1010 M−1·s−1), Br2− (1.3%-11.0%, kBr2•-,AA=∼108-109 M−1·s−1), NO (2.6%-43.0%) and CO3− (0%-43.2%) based on the first-principle kinetic model of UV/NH2Cl/Br- process, while the removal ratio of Asp was low. It was responsible for higher HNM concentrations from Tyr and Trp than those from Asp. Due to the decline in HNM concentrations with prolongation of reaction time, HNM concentrations were considered as the quadratic function of disinfection time with coefficients related to water characteristics and NH2Cl doses. The regression model with backward stepwise regression described HNM formation well. After introducing specific UV absorbance at 254 nm (SUVA254) and interaction terms associated with SUVA254, HNM formation from three AAs was successfully predicted by one regression equation (R2 = 0.91, mean absolute error = 0.46, mean relative error = 37%, root-mean-square error = 0.81). Using the optimal regression model, increasing NH2Cl doses was found to advance the peak time for HNM formation. Besides, obtained heatmaps of HNM formation revealed that a high NH2Cl dose with a high UV dose or low NH2Cl dose helped to limit HNM formation. Lastly, the possible formation pathways of HNMs were presented.