Toward More Sustainable Magnetic Nanoparticle Synthesis\nBased on Microwave-Assisted Continuous-Flow Processes

This work examines\nthe translation of a well-known production method\nfor uniform single core magnetite nanoparticles based on the oxidative\nprecipitation of FeSO<sub>4</sub> in aqueous media into a continuous-flow\nprocess, which implements complete control of operating parameters\nand products’ quality, managing to minimize the duration of\nthe rate-controlling thermal aging step. Particularly, the transformation\nof the green rust intermediate form into Fe<sub>3</sub>O<sub>4</sub> nanoparticles was carried out by heating in a microwave cavity adapted\nto a continuous-flow reactor. The role of reaction parameters was\nfirst investigated in batch experiments, which indicated a particle\nsize variation in the range of 20–35 nm following the use of\nan iron precursor concentration from 0.02 to 0.2 M and the adjustment\nof reaction time from hours to minutes at a reaction temperature of\n90 °C. The continuous-flow approach initially involved a vessel\nreactor placed into the microwave oven, continuously fed with the\ngreen rust precursor. Such an arrangement was sufficient to produce\nFe<sub>3</sub>O<sub>4</sub> nanoparticles with dimensions up to around\n50 nm in less than 20 min of heating and significantly reduced the\npercentage of ethanol as solvent. Surprisingly, by using a plug-flow\nreactor (PFR), an extremely high heating rate was succeeded and well-defined\nmagnetic nanoparticles with size around 30 nm were produced with a\nheating period of less than 30 s. Optimization of the continuous-flow\nprocedures was assisted by the computational fluid dynamics study\nof the heating reactor’s thermal profile. The techno-economic\nand life cycle evaluation of the developed procedure was performed,\nindicating a total production cost of 2 € per g of dried nanoparticles\nand a high sustainability degree compared to other reported methods.