Polyaniline-modified red mud promotes chain elongation during anaerobic fermentation of waste activated sludge through a hydrated electroactive interface

Medium-chain fatty acids (MCFAs) production from waste activated sludge is constrained by poor substrate release and inefficient utilization of short-chain fatty acids (SCFAs) and reducing equivalents. Polyaniline-modified red mud (RM@PANI) was developed as a hydrated electroactive mineral interface to address these limitations. At 2 g/L, RM@PANI increased MCFAs production to 12.4 g COD/L, 2.27 times the control, and raised the caprylate fraction from 12.6 % to 36.7 %. At 1 g/L, it produced 19.5 % more MCFAs than bare RM. RM@PANI mobilized EPS, sustained hydrolysis and acidogenesis, and enriched taxa associated with hydrolysis and chain elongation (CE), particularly the CE genus Sporanaerobacter. PANI coating increased local acetate hydration and raised the simulated diffusion coefficient from 0.003 to 0.368 Ų/ps. Higher atoE abundance suggested greater genetic potential for SCFA transport. Community restructuring, cell clustering, and faster acetate transport were consistent with tighter coupling between acidogenesis and CE. Meanwhile, RM@PANI increased electron exchange capacity, lowered charge-transfer resistance, expanded the intracellular NAD(H) pool, and enhanced metabolic potential across reverse β-oxidation and fatty acid biosynthesis. Electron transfer efficiency increased from 69 % to 76 %, with more electron equivalents recovered in MCFAs. Several flavin-related genes were more abundant in the RM@PANI group, while DFT calculations showed stronger RF-surface charge transfer, supporting flavin-mediated electron transfer as a potential interfacial process. RM@PANI improved MCFAs production by coupling precursor supply, SCFAs transport, microbial organization, and electron utilization at a hydrated electroactive interface, providing a route for converting industrial residues into functional materials for sludge carbon recovery.