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Trade-offs in microplastic-adsorbed iopamidol degradation by UV-AOPs: Molecular-level insights into deiodination pathways versus iodinated disinfection by-products formation
Summary
This study examined how a common medical contrast agent called iopamidol behaves when it sticks to microplastics during UV water treatment. The researchers found that different UV treatment methods create a trade-off: one approach breaks down the chemical more effectively but produces toxic byproducts, while another retains harmful iodine compounds. The findings matter because they show that microplastics in water treatment systems can complicate the removal of pharmaceutical pollutants.
The divergent transformation pathways of iopamidol (IPM) adsorbed on polyethersulfone microplastics (PES-MPs) during UV/chlorine (UV/Cl) and UV/peracetic acid (UV/PAA) treatments were elucidated in this study. Molecular-level trade-offs between degradation efficiency and disinfection byproducts (DBPs) toxicity were unraveled through fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis. Radical-mediated cleavage of aromatic C-I bonds was predominantly observed in UV/Cl process, yielding low-molecular weight chlorinated aliphatics (300-500 Da). While iodine retention and polyiodination were promoted through carbon-centered radicals-mediated decarboxylation in UV/PAA process. IPM degradation involved the deiodination, amide hydrolysis, and amino oxidation, yielding intermediates and DBPs precursors (such as TP-274.5, TP-91.5). The formation of chlorinated-DBPs (Cl-DBPs) was predominantly driven by electrophilic substitution mechanisms involving MPs-DOM under UV/Cl treatment. In contrast, the iodinated-DBPs (I-DBPs) exhibited significant accumulation (601.06 μg/L) during UV/PAA treatment, particularly polyiodinated aromatic compounds such as CHNOI, attributed to sequential deiodination and reiodination pathways. Excitation-emission matrix-parallel factor analysis (EEM-PARAFAC) and FT-ICR-MS analyses revealed the enhanced aromatic oxygenation (O/C ratio 0.3-0.5) in UV/Cl process, while stable iodinated intermediates accumulated during UV/PAA treatment. Mass difference analysis identified 33 reaction types, with hydroxylation (+1O) prevailing in UV/Cl system and polyiodination (+2I-2H) dominating in UV/PAA system. Toxicity assessments predicted two- to three-fold higher chronic risks associated with mixed Cl-/I-DBPs compared to the parent compound IPM, underscoring the necessity to balance degradation efficacy with DBPs control in systems contaminated with MPs. This study provides mechanistic insights for optimizing advanced oxidation processes in complex water matrices.
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