A Synergistic Molecular Mechanism Defines Pyrogenic Dissolved Organic Matter (pyDOM) Coagulation

Climate change is exacerbating wildfires, resulting in an increased influx of pyrogenic dissolved organic matter (pyDOM) into aquatic systems. The distinctive molecular signature of pyDOM, which is markedly different from that of conventional humic substances, is hypothesized to have an impact on its environmental fate, especially during coagulation that is a crucial process for carbon sedimentation and water purification. Nevertheless, the specific removal mechanisms and the resulting floc architectures remain unclear. Through a comparative investigation of pyDOM from subtropical burnt soils and standard humic acid (HA) during aluminum sulfate coagulation, we have discovered a selective coagulation mechanism governed by pyDOM's molecular properties. pyDOM with condensed aromatics and oxygenated aliphatics is removed via synergistic π-π stacking and Al coordination. This stands in stark contrast to the charge neutralization dominated pathway observed for HA. The pyDOM-specific pathway gives rise to larger, denser flocs with a wider size distribution. At the molecular level, aggregation initiates with the condensation of aromatic cores, and oxygen and heteroatom functional groups form stable Al-O-C complexes, acting as chemical cross-links that strengthen floc integrity. This research identifies a previously neglected coagulation paradigm unique to pyDOM, which differs fundamentally from the traditional HA-centric framework, thus enhancing our mechanistic understanding of pyDOM-driven particle assembly and carbon export. These findings are essential for precisely predicting carbon stability in fire-affected watersheds and for devising effective water treatment strategies in wildfire-prone regions.