Process-Specific Molecular Transformation and Toxicity Evolution of Dissolved Organic Matter in Algae-Laden Source Water Under Coagulation, Ozonation, and Adsorption

Dissolved organic matter (DOM) in algae-laden micro-polluted source water is highly complex, posing major challenges to drinking water treatment and risk control. However, the molecular fate of DOM and its associated toxicity consequences under different treatment processes remains insufficiently understood. In this study, a multi-scale characterization approach combined with toxicity prediction was used to systematically compare the effects of coagulation, ozonation, and adsorption on the molecular transformation and toxicity evolution of DOM. FT-ICR MS analysis assigned 1092 DOM molecular formulae in the raw water, while 741 and 800 assigned formulae remained after coagulation and adsorption, respectively. Both processes showed distinct molecular selectivity: saturated molecules were preferentially removed by both treatments, whereas coagulation showed a stronger preference for oxidized molecules. By comparison, ozonation achieved limited CODMn and DOC reduction but markedly reduced UV254 and increased the number of assigned molecular formulae to 1500. The ozonated effluent was characterized by diverse transformation products, especially oxidized saturated small molecules, accompanied by enhanced bio-origin fluorescence signals and more prominent low-molecular-weight neutral and biopolymer fractions. In addition, ozonation increased the numbers of highly acute and highly chronic toxic molecules by 53.60% and 42.25%, respectively, whereas coagulation and adsorption reduced these high-risk molecules. These findings reveal the process-specific molecular transformation and toxicity evolution of DOM under three classical water treatment processes, providing a theoretical basis for process optimization and ecological risk control.