Effect of far UVC pretreated dissolved organic matter on volatile disinfection byproducts abatement in real surface water disinfection process: Implications from non-targeted high-resolution mass spectrometry and interpretable machine learning.

Ultraviolet (UV) pretreatment can alter the molecular components of dissolved organic matter (DOM) in raw water, thereby influencing the formation potential of disinfection byproducts (DBPs) during free chlorine disinfection of drinking water treatment. This study systematically evaluated the effect of four representative wavelengths of UV irradiation (including far UVC) on DOM components transformation and DBPs formation in real surface water. By combing Fourier transform-ion cyclotron resonance mass spectroscopy (FT-ICR MS) with interpretable machine learning, the key molecular features and phototransformation pathways responsible for DBP precursor behavior under UV irradiation were effectively identified. The results demonstrated that all the four types of UV irradiation promoted DOM transforming from high molecular weight (MW) and aromatic components to low MW and saturated one, which easily reacted with bromide ions and free chlorine to generate bromine-chlorine mixed DBPs. Notably, UV and UV tended to decompose nitrogen- and sulfur-containing components of DOM and reduce their reaction pathways, resulting in a lower yield of nitrogenous DBPs (N-DBPs). By contrast, UV and UV preferred to degrade DOM components with high H/C, resulting in a high dose of carbon-containing DBPs (C-DBPs). The inhibitory effect of UV pretreatment on the formation of trichloronitromethane was lower than that of UV, which could attribute to its lower capability on reducing number of chlorination reaction pathway of aromatic compounds in DOM. Nonetheless, the calculated toxicity of volatile DBPs was lower in chlorinated UV-pretreated waters than in chlorinated raw water, indicating that UV-pretreatment had positive effect on DBPs abatement. These findings provide practical guidance for selecting specific wavelength UV pretreatment strategies to minimize DBPs formation. This work also expands current knowledge by demonstrating the mechanistic differences in DOM phototransformation and DBPs generation across multiple UV wavelengths, advancing the field toward more targeted and efficient disinfection control strategies.