Direct Evidence from Multiscale Measurements Explains the Nonmonotonic Deposition of Microplastics in the Presence of Dissolved Organic Matter: Overlooked Contribution of Molecular Self-Assembly

Microplastics (MPs) pollution constitutes a critically escalating global environmental challenge. Understanding dissolved organic matter (DOM)-MPs interactions and their transport behavior in soil and aquatic systems is of fundamental significance. However, contradictory observations regarding the impact of DOM on the fate of MPs exist in the literature, hindering our understanding of their risk and ecological impacts. To date, quantitative evidence explaining how DOM self-assembly influences the transport kinetics of MPs or other emerging particulate contaminants has not been explored. Here, we employed excitation-emission matrix spectroscopy, colloidal atomic force microscopy, and microfluidic systems to systematically elucidate DOM-mediated mechanisms controlling the fate of pristine and aged MPs from molecular, nano, and interfacial scales in a coherent manner. Direct evidence demonstrated that interactions among DOM components mediated the process of self-assembly on MPs and hematite surface, resulting in nonmonotonic deposition behavior in response to an increase in DOM concentration. The pristine MPs prefer to adsorb hydrophobic DOM fractions via nonspecific interactions, whereas aged MPs undergo a triphasic DOM self-assembly process due to the variation in DOM concentration and composition through specific interactions. Consequently, their deposition fluxes on hematite vary accordingly: pristine MPs flux decreased to (49.8 ± 2.9) × 10⁻⁴ μm/min then increased to (64.0 ± 4.5) × 10⁻⁴ μm/min, whereas aged MP flux decreased to (12.4 ± 1.8) × 10⁻⁴ μm/min, subsequently increased to (22.1 ± 0.8) × 10⁻⁴ μm/min, and finally declined to (4.0 ± 2.0) × 10⁻⁴ μm/min. Critical DOM concentrations serving as the "inflection point" of deposition fluxes for both pristine MPs and aged MPs were identified. Overall, this study establishes a novel framework for investigating the deposition mechanisms of MPs and other particulate pollutants, highlighting the critical role of DOM self-assembly in regulating the fate of contaminants in DOM-rich environments. The observed molecular fractionation of DOM, dictated by both concentration and composition, determines its interactions with minerals, thereby regulating the subsequent stabilization and biogeochemical cycling of carbon within the earth's critical zone.