Facile fabrication of magnetite (Fe3O4) nanoparticles by hydrothermal carbonization of waste iron supplements†

We report a novel, time-dependent synthesis of magnetic magnetite (Fe₃O₄) and maghemite (Fe₂O₃) nanoparticles (MNPs) embedded in hydrochar via hydrothermal carbonization (HTC) of expired pharmaceutical waste containing iron-based supplements. This unique circular reuse and waste valorization proposition offers pharmaceutical waste as a sustainable iron-rich feedstock for MNP fabrication. The synthesis was conducted at a fixed temperature of 275 °C with residence times of 6 and 12 h, producing maghemite- and magnetite-dominant phases, respectively. Unlike conventional methods that rely on high temperatures, toxic reagents, or complex protocols, our approach offers a low-cost, sustainable route for functional nanomaterial production. The structural, morphological, and surface chemical characteristics of the MNP hydrochar were elucidated using XRD, SEM, and FTIR analyses. To evaluate environmental sustainability, we performed a comparative life cycle assessment (LCA) against co-precipitation and pyrolysis/gasification methods. The HTC route exhibited the lowest environmental impact across multiple TRACI 2.1 impact categories. Finally, the adsorption performance of the fabricated MNP hydrochar was assessed in a model methylene blue (MB) wastewater system, demonstrating >95% removal efficiency and an adsorption capacity of 1.38 mg g⁻¹. These findings present a viable pathway for integrating waste valorization, green nanomaterial synthesis, and sustainable wastewater treatment.