Mechanistic insights into sulfidated nanoscale zero-valent iron enhanced methanogenesis: Electron redistribution and direct interspecies electron transfer-driven metabolic reconfiguration

Sulfidated nanoscale zero-valent iron (S-nZVI) enhances methanogenesis, yet the underlying mechanisms linking its interfacial structure to microbial metabolic responses remain unclear. This study elucidated S-nZVI's role via electron redistribution, microbial syntrophy enhancement, and metabolic pathway reconfiguration. Density functional theory revealed that sulfur-induced Fe-3d and S-3p orbital coupling, bandgap opening, and valence band shift collectively improved interfacial conductivity. At 5 g·L⁻¹, S-nZVI increased methane yield by 15 % and 68 % over nZVI and control, respectively. It also shortened lag phase, promoted extracellular polymeric substances secretion, and shifted electron transfer from cytochrome-based to abiotic pathways. Metagenomics confirmed enrichment of direct interspecies electron transfer (DIET)-associated genera and acetoclastic methanogenesis genes. Furthermore, the in-situ formation of conductive Fe₃O₄ and enhanced microbe colonization collectively reinforced DIET and methanogenesis. Overall, S-nZVI facilitated electron redistribution and drove the reconfiguration of syntrophic metabolism toward more efficient methanogenesis, offering mechanistic insights into material-microbe synergy for enhanced bioenergy recovery.