Selective transformation of oxidation-derived dissolved organic matter during biofiltration with implications for DBP precursor control.

Oxidation-biofiltration is widely applied to improve dissolved organic matter (DOM) removal in drinking water treatment, yet the molecular determinants governing the fate of oxidation-derived products remain unclear. This laboratory-scale study investigated the selective fate of oxidation-derived DOM molecules during subsequent biofiltration and identified the molecular characteristics associated with their removal or persistence. Three oxidation processes (O3, O3/H2O2, and O3/UV) were applied to generate compositionally distinct oxidation-derived DOM pools, followed by granular activated carbon and anthracite biofiltration. Oxidation shifted DOM toward more oxygenated and less aromatic compositions, with O3/UV showing stronger depletion of CHO-dominated fractions and less net accumulation of oxidation-derived CHON formulas than O3 and O3/H2O2. During biofiltration, commonly removable formulas were more oxygenated and dominated by CHO than persistent formulas that were enriched in nitrogen and sulfur and had lower O/C ratios. The treatment relevance of these molecular fate patterns was further reflected in disinfection byproduct (DBP) precursor responses: oxidation generally reduced carbonaceous DBP formation potential (C-DBP FP) but increased halonitromethane formation potential (HNM-FP) by 1.7-6.1-fold, whereas subsequent biofiltration reduced both DBP FP and the estimated DBP-associated cytotoxicity index. The pronounced decrease in HNM-FP was consistent with the preferential transformation of more oxygenated N-containing formula-level pools. These findings deepen the molecular-level understanding of oxidation-derived DOM transformation and its selective removal during biofiltration, providing a molecular basis for optimizing DBP precursor control in oxidation-biofiltration processes.