Mechanistic insight into Fe(III)-DOM complexes modulated UV/chlorine for promoting oxidative capacity and reducing...

The presence of Fe(III) in aquatic environments tends to form Fe(III)-dissolved organic matter (DOM) complexes, but their role in enhancement of decontamination and curtailment of disinfection by-products (DBPs) formation remains insufficiently understood. Herein, we systematically elucidated the pivotal role and underlying mechanism of structure-dependent complexation between Fe(III) and DOM in promoting carbamazepine (CBZ) degradation and curtailing DBPs formation in UV/chlorine treatment. The results indicated that Fe(III) readily formed stable complexes with hydroxyl and carboxyl functional groups in DOM and showed higher conditional stability constant (logK_ML) with fulvic acid (FA) than humic acid (HA). Although the introduction of DOM significantly suppressed the oxidation efficiency of UV/chlorine system, it had a negligible impact on the UV/chlorine/Fe(III) system, demonstrating that Fe(III)-DOM complexation effectively alleviated the DOM-induced quenching of reactive species and resulted in a more than 4-fold higher rate constant for CBZ oxidation. The oxidative efficiency of UV/chlorine/Fe(III)-DOM system was governed by DOM structure, where the abundance and arrangement of carboxyl and phenolic hydroxyl groups dictated Fe(III) complexation capacity, subsequently determining Fe(III) reducibility and reactive species formation. The complexes formed between Fe(III) and HA suppressed triplet-state DOM (³DOM*) formation while substantially promoting the generation of ∙OH, Cl∙ and other reactive species (including O₂·^-,¹O₂ and Fe(IV)), among which ∙OH and Cl∙ were identified as the dominant reactive species responsible for CBZ degradation. The variations in steady-state concentrations and contributions of radicals were clarified under various conditions. Compared to UV/chlorine/HA, UV/chlorine/Fe(III)-HA pretreatment significantly suppressed the formation potential and cytotoxicity of total DBPs under varying pretreatment time, pH levels and Br^- concentrations, with cytotoxicity reduced by 10.7-45.0%, 35.6-48.6% and 56.8-74.4%, respectively, demonstrating that Fe(III)-HA complexation effectively mitigated toxicity risks by altering HA transformation pathways and reactive species generation. Molecular-level analysis revealed that UV/chlorine/Fe(III)-HA pretreatment steered the transformation of HA towards highly oxidized, less reactive molecular compositions, thereby reducing DBPs formation potential.