Elevated risk of haloacetonitrile formation during post-chlorination when applying sulfite/UV advanced reduction technology to eliminate bromate

The formation of haloacetonitriles (HANs) during chlorination after sulfite/ultraviolet (UV) treatment of bromate (BrO 3 − ) in the presence of amino acids (AAs) was investigated. During sulfite/UV treatment, the primary species hydrated electrons (e aq − ) and hydrogen atom radicals (H ) dominated the reduction of BrO 3 − to bromide (Br − ), whereas the sulfite anion radicals (SO 3 − ) and H degraded AAs to produce the intermediates HN=C(CH 3 )–COOH, CH 3 –CH=NH, and CH 3 –C≡N via α‑hydrogen abstraction and NH 2 –hydrogen abstraction mechanisms. During post-chlorination, Br − was converted to HBrO/BrO-, and the HN=C(CH 3 )–COOH, CH 3 –CH=NH, and CH 3 –C≡N groups featured higher bromine utilization factor (BUF) and chlorine utilization factor (CUF) values than AAs, enhancing the formation of dibromoacetonitrile (DBAN) and dichloroacetonitrile (DCAN). The energetic feasibility of the transformation pathway, that is, HN=C(CH 3 )–COOH, CH 3 –CH=NH, and CH 3 –C ≡ N formation via hydrogen abstraction by SO 3 − and H and their further conversion to HANs, was proved by density functional theory calculations, which showed stepwise negative Gibbs free energy changes (ΔG < 0). The effects of pH and water matrices (e.g., HCO 3 − , Cl − , Fe 3+ , and natural organic matter) were comprehensively evaluated. Although 72% of BrO 3 − was removed by sulfite/UV treatment in the presence of AAs, the cytotoxicity index (CTI) and genotoxicity index (GTI) during post-chlorination increased by 213% and 125%, respectively, due to the formation of 24 CX 3 R-type disinfection by-products (DBPs), especially brominated DBPs. Accordingly, more attention should be given to the formation of brominated DBPs during post-chlorination when using sulfite/UV processes to remove BrO 3 − in the presence of AAs. As a solution, using monochloramine instead of chlorine as a disinfectant after the sulfite/UV process could significantly lower the CTI and GTI values by alleviating the formation of brominated DBPs.