Electron donating capacity reduction of dissolved organic matter by solar irradiation reduces the cytotoxicity formation potential during wastewater chlorination.

After treated wastewater is discharged into surface water for unplanned indirect potable reuse, solar irradiation transforms the dissolved organic matter (DOM), which would alter the formation of disinfection byproducts (DBPs) and change the cytotoxicity formation potential (CtFP) during post–chlorination in drinking water treatment plants. This study investigated the effects of solar irradiation on the CtFP and total organic halogen formation potential (TOXFP) of wastewater during post–chlorination. Exposure to natural sunlight decreased the formation potential of cytotoxicity to Chinese Hamster Ovary cells. Under 24 h simulated solar irradiation, CtFP and TOXFP decreased by more than 40%. X–ray photoelectron spectra and Fourier transformation infrared spectra suggested solar irradiation destroyed the key DBP precursors containing phenolic hydroxyl moieties (Ph–OH). The destruction of Ph–OH under solar irradiation was reflected by a decrease in the electron donating capacity (EDC) of DOM and the post–chlorination decreased the EDC further. Increasing the irradiation–consumed EDC abated the chlorine–consumed EDC, while the chlorine–consumed EDC was positively correlated to the CtFP and TOXFP by means of the electrophilic substitution–aromatic ring cleavage. Solar irradiation thus reduced the CtFP and TOXFP in wastewater during post–chlorination. This study revealed that solar irradiation decreased the risks of treated wastewater for unplanned indirect potable reuse and provided a strategy of controlling CtFP and TOXFP via reducing EDC of DOM in pretreatments.