Understanding the influence of pre-ozonation on the formation of disinfection byproducts and cytotoxicity during post-chlorination of natural organic matter: UV absorbance and electron-donating-moiety of molecular weight fractions.

Pre-ozonation can reduce the formation of disinfection byproducts (DBPs) and related adverse effects during subsequent chlorination, but the change of each molecular weight (MW) fraction during each step of combined pre-ozonation and post-chlorination has not been well illustrated. In this study, it was investigated in terms of electron-donating-moieties (EDMs) and UVA for a representative natural organic matter from Suwanee river (SRNOM). Pre-ozonation suppressed the post-chlorination of SRNOM through oxidation of almost all EDMs (>85%) and UVA (>90%) in high MW fractions (HMW, >3.2 kDa) and moderate EDMs (43%) and UVA (72%) in medium MW fractions (MMW, 1.0-3.2 kDa). Furthermore, pre-ozonation led to comparable abatements of EDMs and UVA for HMW fractions, but lower abatement of EDMs than UVA for MMW fractions. However, when t-BuOH was used as an OH-quencher, pre-ozonation led to a few instances in which there were higher abatements of EDMs than UVA for the MMW fraction. These findings suggested that the HMW fraction dominantly underwent ring-cleavage of phenols via O- or OH-oxidation. Differently, the MMW fraction underwent ring-cleavage of phenols and quinones-formation via O-oxidation, but occasionally underwent hydroxylation and hydro-phenol formation via OH-oxidation. Because of forehand elimination of reactive moieties (e.g. EDMs), pre-ozonation obviously inhibited the formation of representative DBPs (67%-84% inhibition), total organic chloride (51% inhibition) and cytotoxicity (31% inhibition), but may have promoted the formation of carbonyl-DBPs (trichloroacetone and chloral hydrate). When compared with UVA, EDMs would better for surrogate of DBPs formation. EDM abatement surrogated the formation of total organic chlorine (TOCl) and cytotoxicity following a two-stage phase, possibly because the speciation of DBPs and transformation products varied with the abatement of EDMs.