Unraveling the Evolution of Organohalogens and Their Associated Toxicity Changes in the Chlorination of Preozonated Reclaimed Water Containing Bromide

Ozonation is widely used to remove precursors and control disinfection byproducts (DBPs) in postchlorination. However, ozonation itself forms highly toxic Br-DBPs in the presence of bromide, yet little is known about the evolution of toxicity and Br-DBPs during postchlorination. Ten mg/L O₃ of wastewater containing 2 mg/L bromide increased the cytotoxicity from 3.5 to 6.8 to 5.7–21.4 mg-phenol/L (genotoxicity from 0.25 to 2.1 μg-4-NQO/L). Postchlorination (10 mg of Cl₂/L) further increased the cytotoxicity to 19.8–34.7 mg of pentol/L (genotoxicity to 2.59 μg-4-NQO/L). Such a “two-stage toxicity increase” corresponded well with the formation of individual DBPs and total organic halogens. Fourier transform ion cyclotron resonance mass spectrometry unveiled that preozonation mainly formed highly toxic phenolic and unsaturated aliphatic 1Br-DBPs (200–500 Da). During postchlorination, precursors with higher unsaturation and aromaticity mainly generated DBPs via electrophilic addition, while those with higher saturation underwent electrophilic substitution and formed phenolic 1Br-DBPs and 2Br-DBPs with less Cl-DBPs. Network computational analysis of the precursor-product pairs demonstrated that HOCl might only compete with HOBr to combine the unhalogenated sites in DOM, rather than substitute bromine in Br-DBPs that has been formed during preozonation. Byproducts in the composite active halogen system (HOBr and HOCl) tended to present in the form of more toxic Br-DBPs.