Molecular interference for surface sites as a hidden driver of coagulation impairment in wastewater-impacted drinking...

The presence of effluent organic matter (EfOM) in drinking water sources, a consequence of unplanned potable reuse, challenges the efficacy of conventional coagulation processes. This study employs a multi-scale analytical approach to reveal that EfOM does not simply increase organic loading but triggers a molecular-level interference with natural organic matter (NOM) for active sites on floc surfaces, manifested as preferential adsorption of EfOM components. Through integrated application of FT-ICR-MS, X-ray photoelectron spectroscopy, floc characterization, and contact angle measurements, we demonstrate that EfOM components, particularly lipid- and protein-like molecules, exhibit preferential adsorption onto reactive surface hydroxyl groups on aluminum flocs, thereby inhibiting the removal of aromatic, high-molecular-weight NOM fractions. This competitive adsorption enriches floc surfaces with hydrophilic C-O and COOH groups, elevating electrostatic repulsion and impairing inter-floc bridging, ultimately yielding smaller, less-settleable flocs. As a result, finished water contains a more diverse organic portfolio. It shows significant enrichment in heteroatomic (N, S, P) species characterized by high unsaturation and a low oxidation state, which are molecular features that have been previously linked in the literature to enhanced disinfection byproduct formation potential. Our study elucidates the mechanistic pathways linking molecular interference to impaired treatment performance, and underscores the need to develop advanced treatment strategies to mitigate downstream water quality risks.