Species and formation characteristics of halogenated DBPs in chloramination of tannic acid after biodegradation

Tannic acid is widely found in source water and wastewater, and it is also a typical degradation precursor of natural organic matter. In this study, focused on chloramination, the formation characteristics of halogenated DBPs from tannic acid biodegradation products were examined. Fifty-nine polar emerging DBPs (including four nitrogenous DBPs) were detected and forty of them were identified for the first time; meanwhile, their formation pathways were tentatively proposed. In general, much more polar emerging DBPs were formed at the early biodegradation stage than those at the later stage, while commonly observed aliphatic DBPs presented an exactly inverse trend, because initially-formed emerging DBPs can be transformed to those aliphatic DBPs by residual chloramine. Interestingly, while the relative formation level of brominated species in overall halogenated polar emerging DBPs maintained at high level at the later biodegradation stage during chlorination, it decreased significantly later during chloramination. The discrepancy may be due to that hydrolysis effects became dominant at this period in chloramination, whereas DBP formation from the reactions between slow reactive sites and hypohalous acids prevailed in chlorination. In addition, the calculated toxicity drivers among the 21 aliphatic DBPs were found to be haloacetonitriles, although they contribute mildly to the total concentration.