Enabling robust aerobic sludge granulation and self-recovery via electron transfer at Fe₃O₄@SiO₂ interface

Slow granulation and instability remain major bottlenecks hindering the widespread application of aerobic granular sludge (AGS) technology. Here we report a novel approach for enhancing granulation and stability of AGS systems by Fe₃O₄@SiO₂ mediated electron transfer process. Heterostructured Fe₃O₄@SiO₂ acts as a dynamic nucleation site, which reduced granulation time by 20 days and enabled fragmented-sludge recovery within just 8 days. The engineered Fe₃O₄@SiO₂ heterostructures exhibit a distinctive nanoflower-like morphology, providing significantly larger surface area and intrinsic magnetism. These properties create spatially distributed anchorage sites that effectively facilitate microbial colonization and the formation of conductive biofilms. Electrochemical analyses revealed a 5-fold reduction in charge transfer resistance and enhanced redox currents, attributed to upregulated cytochrome C (increased by 127%) and adenosine triphosphate (ATP) synthesis (increased by 151%). This optimized electron flux stems from altered cofactor states (NAD⁺/NADH >1 vs. 0.89 control) and elevated FAD levels, stimulating extracellular polymeric substance (EPS) secretion (increased by 128%) with enriched humic acid-like components. Based on integrated microbiome analysis, which revealed that redirected carbon flux occurs through TCA cycle upregulation and oxidative phosphorylation activation, a syntrophic consortium was consolidated. This enabled our products to achieve extraordinary resilience in pollutant removal, with COD at 92.37%, TP at 92.07%, and NH₄⁺-N above 80%. Such resilience emerged via reprogrammed electron transfer, enhanced microbiome functionality, and rapid system stabilization. This synergy, via optimized microbial aggregation and regulated EPS secretion, conquers AGS's granulation/stability bottlenecks, enabling scalable municipal/industrial deployment to advance global sustainable wastewater treatment.