Zero-valent iron regulates hydrothermal carbonization pathways toward Fe-C interfacial hydrochar formation

Hydrothermal carbonization (HTC) offers a sustainable route for converting wet biomass-derived precursors into functional carbonaceous materials, yet mechanistic control over carbonization pathways, heteroatom incorporation, and metal-carbon interfacial evolution remains challenging. Here, we use glucose and glycine as representative carbohydrate and amino acid precursors, respectively, to elucidate how zero-valent iron (Fe⁰) regulates HTC chemistry under mild hydrothermal conditions. Rather than serving only as a performance-enhancing additive, Fe⁰ acts as a multifunctional regulator that directs hydrothermal transformation, enabling the controlled construction of Fe-C interfacial architectures. At 200 °C, Fe⁰ promotes the transformation of labile hydroxyl groups into more stable oxygen-containing interfacial moieties associated with C-O-C/Fe-O-C environments, enriches pyridinic-N species, and facilitates the formation of Fe₃O₄ nanophases embedded within the carbon matrix. Combined solid- and liquid-phase analyses reveal a Fe-mediated dissolution-recrystallization process, in which dissolved Fe species migrate from the aqueous phase to the growing hydrochar and participate in Fe-C interface construction. Adsorption experiments using cationic and anionic probes further demonstrate that Fe⁰-regulated HTC increases the specific surface area and changes the surface chemistry of the hydrochar, leading to selective affinity toward cationic species. This work provides mechanistic insights into Fe⁰-mediated hydrothermal carbonization and clarifies how iron regulates carbon, nitrogen, and oxygen redistribution during Fe-C interfacial hydrochar formation.