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.