Novel Production of Magnetite Particles via Thermochemical Processing of Digestate From Manure and Food Waste

Sustainable management of food waste has become a global priority because of the significant environmental impacts associated with conventional disposal methods, including landfilling. Thermochemical processing is a food-waste-to-energy conversion technology in which food waste materials are converted to biofuel in a reduced O
₂
environment at elevated temperatures. Another conversion technology is anaerobic digestion, in which microorganisms digest biodegradable material, producing biofuel and solid byproducts “digestate.” We measured the physical properties of “biochar” produced by combining these approaches: digestate was used as feedstock for a commercial-scale thermochemical processing system. Magnetite (Fe
₃
O
₄
) particles were produced during the food-waste-to-energy conversion process. This was particularly unexpected because none of the input materials were ferromagnetic, and no specific Fe precursors were introduced in the process. The Fe
₃
O
₄
was identified through a combination of X-ray fluorescence and dc magnetometry. Zero-field cooled magnetization-temperature curves reveal a Verwey transition at ~125 K across samples prepared under various conditions. Room temperature magnetization-field loops show a Langevin-like curve, technical saturation, and coercive fields of H
_C
= 98-130 Oe across various samples. Clear Verwey transition, room temperature hysteresis, and an irreversibility temperature well above room temperature indicate that particles are multidomain. We attribute the presence of Fe
₃
O
₄
to the relatively high concentration of Fe naturally present in the solid digestate and the operating parameters of the thermochemical conversion process. High surface area magnetic biochar has a variety of potential applications, including the adsorption of heavy metals, wastewater treatment, supercapacitors, and conductive polymer composites.