Impact of ClO pre-oxidation on the formation of CXR-type DBPs from tyrosine-based amino acid precursors during chlorination and chloramination.

ClO is frequently used as a pre-oxidant in water treatment plants. However, the effects of ClO pre-oxidation on disinfection by-product (DBP) formation, especially the highly toxic nitrogenous DBPs, during subsequent chlor (am)ination have not been studied thoroughly. There is also limited information about DBP formation from combined amino acids (AAs), which are more abundant than free AAs in source waters. Many typical DBPs (including representative N-DBPs) have a similar structure of "CXR" (X = H, Cl, Br or I). In the study, tyrosine and forms representing its reactivity in combined AAs (tyrosine tert-butyl ester and Boc-tyrosine) were selected as model precursors. The formation of various regulated and unregulated CXR-type DBPs from ClO pre-oxidation and subsequent chlor (am)ination were studied at a wide-range of ClO and chlor (am)ine doses (ClO/precursors and chlor (am)ine/precursors are at the range of 0-2.5 and 1-20 [Mol/Mol], respectively). Chloroform and chloral hydrate (CH) yields increased with chlorine dose, while haloacetonitrile and haloacetamide maximized at median chlorine dose (Cl/Precursors = 10). All DBP yields increased with chloramine dose. ClO pre-oxidation increased chloroform, haloacetonitrile, trichloronitromethane and CH yields during chlorination, but ClO increased chloroform, CH, trichloroacetamide while decreased dichloroacetonitrile and trichloronitromethane yields during chloramination. The overall toxicity of the formed DBPs was evaluated by cytotoxicity index (CTI). ClO pre-oxidation increased CTI from all precursors during post-chlorination while reduced it during post-chloramination. Results imply that ClO is probably more suitable for use in combination with chloramination disinfection, rather than chlorination, in the integrated control of CXR-type DBPs from source waters abundant in AAs.