Examination of disinfection by-product (DBP) formation in source waters: A study using log-transformed differential spectra

Abstract

Formation of disinfection by-products (DBPs) in ten drinking source waters located in the United States was examined in this study. DBP generation was interpreted in the context of halogenation-induced changes of log-transformed absorbance spectra of dissolved organic matter (DOM) present in the waters. This approach allows probing the behavior of relatively minor structures that can be highly sensitive towards any process of interest, notably DOM halogenation. This concept was applied to examine effects of chlorination time on the kinetics of chlorine consumption and release of several DBP groups such as total trihalomethanes (THM4, including CHCl3, CHCl2Br, CHClBr2 and CHBr3), haloacetic acids (HAA9, including MCAA, MBAA, DCAA, TCAA, BCAA, DBAA, BDCAA, DBCAA and TBAA), haloacetonitriles (THAN4, including TCAN, DCAN, BCAN and DBAN), haloketones (HK2, including DCP and TCP), chloral hydrate (CH) and chloropicrin (CPN). Two alternative parameters, namely the differential logarithm of DOM absorbance at 350 nm (DLnA350) and change of the spectral slope in the range of wavelengths 325–375 nm (DSlope325-375) were introduced to quantify individual DBP species formed and Cl2 consumption. DLnA350 and DSlope325-375, especially DLnA350 were determined to be more reliable than differential absorbance at 272 nm that was utilized in prior applications of differential spectroscopy to characterize DBP formation. Strong linear relationships between DLnA350 values and concentrations of major groups of and individual DBP species (e.g. THM4, HAA9, HAN4 and CPN were found to exist (mostly, R2 > 0.95) and the intercept of these correlations with the y-axis was near zero for the examined water sources. Correlations between DLnA350 values and concentrations of CH and HK2 were also strong but they were nonlinear. The slope of the correlations between the concentrations of major groups of DBP species vs -DLnA350 were also well correlated with SUVA254 and LnA350 for all the examined source waters. It indicates that log-transformations of the absorbance spectra of surface water and parameters based on such transformations (e.g., DLnA350 and DSlope325-375) have a potential to provide an alternative reliable approach to monitor the halogenation of DOM and attendant formation of individual DBP species.