Selective adsorption of compositionally distinct microplastic-derived dissolved organic matter governs MnO2 aggregation.

Microplastic-derived dissolved organic matter (MPs-DOM) has emerged as an important component of aquatic dissolved organic matter, yet its influence on the aggregation of environmentally relevant mineral colloids such as manganese dioxide (MnO2) remains poorly understood. Here, the effects of polystyrene-derived DOM (PS-DOM) and tire wear particle-derived DOM (TWP-DOM) on MnO2 aggregation were investigated. PS-DOM promoted MnO2 aggregation, whereas TWP-DOM inhibited aggregation and enhanced colloidal stability. Integrated molecular compositional and spectroscopic analyses revealed that these contrasting behaviors were governed by the selective adsorption of compositionally distinct DOM fractions. PS-DOM, enriched in aromatic and high-molecular-weight constituents, progressively evolved from hydrophobic adsorption to surface complexation and Ca2+-mediated interparticle bridging, thereby promoting aggregate growth. In contrast, TWP-DOM, enriched in oxygen-containing functionalities, exhibited a distinct interfacial interaction pathway that weakened direct DOM-MnO2 coordination and suppressed aggregation. Notably, MnO2 aggregation was governed not by the overall oxygen content of DOM but by the interfacial roles of oxygen-containing functionalities during adsorption and structural evolution. These composition-dependent mechanisms persisted in both artificial and natural freshwaters, providing molecular-level insights into DOM-regulated mineral aggregation and improving our understanding of the environmental behavior of MnO2.