Comparison of vacuum ultraviolet-based oxidation as ultrafiltration pretreatment in real water: Performance, risk assessment, and mechanistic insights.

Natural organic matter (NOM), a ubiquitous foulant in aquatic environments, is commonly regarded as a predominant driver of ultrafiltration (UF) membrane fouling. Here, a comparative study was innovatively conducted on vacuum ultraviolet (VUV), VUV/hydrogen peroxide (H2O2), VUV/persulfate (PS), and VUV/chlorine as UF membrane pretreatment technologies using Jialing River water (JRW) as the real water matrix, evaluating their performance, mechanisms, and risks. The results demonstrated that all four pretreatments exhibited high efficacy in mitigating membrane fouling, degrading organic matter, and improving UF effluent water quality, with the VUV/PS process achieving the best performance. During chlorination of UF effluent, unlike VUV, VUV/H2O2, and VUV/chlorine, the VUV/PS process reduced disinfection byproducts (DBPs) formation and acute toxicity. Although residual oxidants and anions were present after oxidation, their low concentrations posed a negligible risk of significant secondary pollution. Competitive kinetic tests revealed that VUV/H2O2 achieved the highest HO• yield, while VUV/PS produced minimal HO• yet substantial sulfate radical (SO4•-) due to high PS utilization efficiency. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that reductive substances in JRW were preferentially oxidized. VUV/PS achieved the removal of a wider range of NOM molecules than VUV, VUV/H2O2, and VUV/chlorine processes. During oxidation, reactive oxygen species formation led to an increase in oxygen-containing functional groups and saturated compounds. Interpretable machine learning elucidated the correlation between NOM reactivity and molecular features. The mass difference analysis further identified dealkylation, decarboxylation, deamination, and oxygen addition as the dominant reaction pathways. Overall evaluation indicated that VUV/PS emerged as the preferred option among the four oxidation pretreatments, offering the best combination of water treatment efficiency and risk management.