Ultraviolet irradiation alters carbonyl structures to promote toxic halogenated carbonyl disinfection byproduct formation in drinking water.

Ultraviolet (UV) disinfection is widely used in drinking water treatment, yet its effects on halogenated carbonyl disinfection by-product (DBP) formation during subsequent chlorination remain insufficiently understood. This study integrated pilot-scale field experiments, bench-scale UV fluence experiments, high-resolution mass spectrometry (HRMS), p-toluenesulfonyl hydrazide (TSH) carbonyl derivatization, targeted carbonyl analysis, and model-compound chlorination to link UV-induced dissolved organic matter (DOM) transformation with halogenated aldehyde (HAL) and halogenated ketone (HK) formation. In pilot-scale field tests, UV irradiation at 160 mJ/cm2 increased the formation potentials of dichloroacetaldehyde and trichloroacetaldehyde from 0.81 to 1.67 μg/L and from 0.82 to 0.98 μg/L, respectively, while regulated trihalomethanes and haloacetic acids decreased. The calculated DBP-associated cytotoxicity, estimated from detected DBP concentrations and literature-reported Chinese hamster ovary cell LC50 values, increased by 75.17%. Bench-scale experiments showed class-specific fluence responses: HALs peaked at 160 mJ/cm2, whereas HKs continued to increase under intensified UV exposure, with 1,1,1-trichloropropan-2-one increasing from 1.03 to 15.30 μg/L at 2400 mJ/cm2. FT-ICR MS showed that UV irradiation followed by chlorination increased detectable CHOCl formulas from 85 to 180, while TSH-Orbitrap MS revealed an increase in detectable carbonyl-related formulas from 54 to 151. Targeted and model-compound experiments further identified aromatic aldehyde-like structures as strong HK precursors; benzaldehyde and 2-phenylacetaldehyde primarily formed 1,1,3-trichloropropan-2-one, whereas acetophenone produced chloropropanone. These findings demonstrate that UV irradiation can shift DBP formation from regulated THMs/HAAs toward unregulated halogenated carbonyl DBPs by increasing carbonyl-containing precursor availability in DOM.