Low-level hydrophilic carbon dots disintegrate the intracellular-extracellular polymeric structure of sludge-EPS conjugates during thermal hydrolysis: solid-aqueous interfacial insights and active water-soluble molecular evolution

Polymeric structure and component complexity of sludge-EPS conjugates represent a formidable challenge for the regeneration of water-soluble resources. The goal of this work is to understand how model compound-derived carbon dots (i.e., melanoidins) modulate the microstructure disintegration and component evolution of sludge-EPS conjugates during thermal hydrolysis. Low-dose (5–20 mg/kg total solids (TS)) melanoidins altered the non-covalent forces between extracellular components, leading to the transfer of total interface energies toward higher repulsion and lower attraction, thereby imposing the disintegration of extracellular structures and high-Mw components. Crucially, the alteration of non-covalent forces was relevant to the formation of active protein-like species (pyrrolic-N, sulfurized N_1–5 proteins, and sulfurous amino acids), which serve as the key redox mechanism triggers for intracellular-extracellular ROS generation, cell inactivation, and electron transfer activity. Furthermore, we demonstrated that low-dose melanoidins efficiently attacked the protein structural barrier and confined protein-humic acid binding, thus accelerating the hydrolysis of labile protein-N and stable quaternary-N. This led to the damage of structural morphology and detachment of hydrophilic-hydrophobic groups in sludge-EPS conjugates, consequently facilitating the release of water-soluble molecules. By tracking the metabolic stream of water-soluble molecules, we found that melanoidins mediated the reductive pathway from saturated molecules to active unsaturated aromatic molecules. However, the positive regulation of high-dose melanoidins (40–80 mg/kg TS) was impeded, attributed to the non-covalent binding of excessive MLD with released water-soluble components, leading to solid-phase adsorption and therefore restraining intracellular component release. We extended the traditional single-limited cell lysis perspective to the solid-aqueous interface and intracellular-extracellular domain, this work establishes a comprehensive framework for understanding the spatial structure cleavage and water-soluble molecule recovery of sludge-EPS conjugates.