Interpretation of the formation of unstable halogen-containing disinfection by-products based on the differential absorbance spectroscopy approach

Concentrations of several toxic disinfection by-products (DBP), notably haloacetonitriles (e.g., trichloroacetonitrile, TCAN) and haloketones (e.g., di- and trichloropropanone, DCPN and TCPN, respectively) are affected by chlorination conditions and the inherent instability of these DBPs. In this study, effects of temperature, chlorine dose and reaction time on the formation of TCAN, DCPN and TCPN were interpreted using the approach of differential absorbance spectroscopy. Experimental data obtained for a wide range of water quality conditions demonstrate that in some cases the concentrations of some of the unstable DBPs increased rather than decreased at low temperatures and realistically long contact times. Despite the presence of pronounced changes of the kinetics of generation and degradation of these DBPs at varying temperatures and chlorine doses, their concentrations were strongly correlated with the concurrent changes of spectroscopic properties of DOM quantified via differential absorbance measurements at 272 nm (ΔA272). The maximum values of TCAN, DCPN and TCPN concentrations observed for the chlorination of eight different surface waters occur at the relative decreases of absorbance at 272 nm (defined as RΔA272) values of ca. 0.32 (±0.03), 0.24 (±0.05), and 0.42 (±0.03), respectively. The activation energies of degradation reactions of unstable DBPs were examined and the results indicate that TCAN and TCPN are caused by their hydrolysis with OH− while the degradation of DCPN is mainly caused by halogenation reaction with HOCl. These results in this study may be important for controlling the formation of unstable DBPs and further optimization of drinking water treatment.