Thermochemical regeneration of iron-based materials using syngas derived from carbon dioxide gasification of digested sludge biochar.

Conventional regeneration of iron-based materials heavily relies on fossil-derived reducing gases, necessitating sustainable thermochemical alternatives. This study investigates a CO2-assisted thermochemical regeneration strategy that integrates CO2 gasification with iron redox cycling, simultaneously enabling sludge valorization. Commercial activated carbon and digested sludge biochar were evaluated under CO2/steam atmospheres to characterize syngas generation as a function of the steam-to-CO2 ratio and the gas flow rate. The generated CO-rich syngas was utilized for the thermochemical reduction of oxidized iron species (Fe2O3 and oxidized zero-valent iron, ZVI), followed by functionality assessment via H2 production performance. Both carbon sources drove a stepwise reduction pathway (Fe2O3 → Fe3O4 → FeO → Fe), and biochar-derived syngas promoted more rapid progression toward metallic Fe under the specific conditions of the in-situ XRD experiment. Extensive Fe2O3 reduction toward metallic Fe was achieved under specific conditions, producing submicron metallic Fe particles that exhibited higher cumulative H2 production than did virgin ZVI (<150 μm). Pre-oxidized ZVI samples were also functionally restored; both the FeCO3 formed via reactions of ZVI with H2O and CO2 and that generated in the presence of sludge residues exhibited viable H2 production capacities after regeneration. These results indicate that effective utilization of CO2-derived syngas for iron-material regeneration provides a potential pathway for sludge-derived carbon valorization and integrated resource recovery.