One-step chemical-free fabrication of magnetic rice-husk-derived carbon composites from industrial iron wastes for solar photocatalysis

The sustainable conversion of iron-rich industrial wastes into functional carbon-supported materials remains challenging because conventional preparation routes commonly require soluble metal precursors, reducing agents, or other chemical additives. Herein, a simple chemical-free subcritical-water strategy is introduced for the simultaneous valorization of rice-husk-derived carbon (RH) and two industrial iron wastes, sintering waste (SW) and rolling mill scale waste (RMSW). Treatment in water at 180 °C resulted in substantial redistribution of waste-derived Fe-containing species over the carbonaceous matrix. SEM/EDX and elemental mapping confirmed the presence and distribution of Fe-containing species within the resulting composites, while VSM measurements demonstrated their magnetic response. The composites also exhibited broad visible-light absorption with optical band gaps of approximately 2.2 eV. Under natural solar irradiation, RH-SW achieved approximately 96.4% methylene blue removal within 20 min at pH 7, whereas RH-RMSW reached approximately 94.2% at pH 12 after 40 min. Additional experiments using Congo red, a chemically different anionic azo dye, further demonstrated the functional applicability of the composites. Overall, this work introduces a straightforward chemical-free route for converting biomass-derived carbon and iron-rich industrial wastes into magnetically responsive, visible-light-active functional materials, providing a sustainable waste-to-resource strategy with potential application in wastewater treatment.