Formation of iron sulfide phases on hydrochar through one-pot hydrothermal synthesis: Effects of biomass type and sulfur-to-iron ratio.

Iron sulfide-supported hydrochar (FeS-HTC) was synthesized via a one-pot hydrothermal process by simultaneously reacting microalgae (AG) and sewage sludge digestate (SD) biomass with iron and sulfur precursors. The effects of biomass type, iron and sulfur concentrations, and sulfur-to-iron (S/Fe) molar ratio on the iron sulfide phase formation and oxidation performance of the resulting materials were systematically investigated. X-ray diffraction (XRD) revealed that the S/Fe ratio strongly influenced iron sulfide phase formation: AG-derived FeS-HTC exhibited a phase transformation sequence from pyrrhotite (Fe1-xS) to greigite (Fe3S4) to pyrite (FeS2) as sulfur content increased, while SD-derived samples consistently formed pyrite across all conditions. Surface analysis via XPS and FT-IR confirmed the incorporation of mixed-valence and oxidized iron species (e.g., phosphoferrite and greigite) in AG samples and predominantly reduced species (e.g., pyrite) in SD samples. SEM study showed biomass-dependent morphologies, with AG producing microspheres and SD forming nanoscale particle aggregates. The FeS-HTCs exhibited strong catalytic activity in Fenton-like oxidation of methylene blue under acidic conditions. Enhanced degradation was observed at S/Fe = 4, especially in pyrite-rich samples, due to efficient Fe2+/Fe3+ cycling and hydroxyl radical generation. Notably, SD-derived FeS-HTC demonstrated faster initial reaction rates, while AG-derived samples allowed for selective synthesis of greigite at S/Fe = 3. These findings highlight the importance of biomass composition and sulfur dosage in tuning the structure and reactivity of FeS-HTCs. The materials developed in this study show strong potential for use in advanced oxidation processes for wastewater treatment and other environmental remediation applications.