Low-temperature solvent-free synthesis of polycrystalline hematite nanoparticles via mechanochemical activation and their adsorption properties for Congo red

A novel low-temperature solvent-free process for synthesizing polycrystalline hematite (α-Fe2O3) nanoparticles via mechanochemical activation was developed. A powder mixture of iron nitrate nonahydrate (Fe(NO3)3·9H2O) and urea ((NH2)2CO) was high-energy ball-milled and then heated at different temperatures under static conditions. The X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM) analyses of the products confirmed that the amount of urea and the heating temperature adjusted the crystallite size and particle size. The experimental results demonstrated that both the milling treatment and the solvent-free condition are required to promote the dehydration of goethite (α-FeOOH). The thermogravimetry/differential thermal analysis (TG/DTA) study for the milled and unmilled precursors showed that activation of the precursor could inhibit the formation of the iron–urea complex, resulting in a rapid progress of the hematite formation reactions. As a practical application, the hematite nanoparticles were applied to a batch adsorption of Congo red. The adsorption isotherm and kinetic data were well fitted with the Langmuir isotherm model and pseudo-second-order model equations, respectively. Polycrystalline hematite nanoparticles can be used as an effective adsorbent for the adsorptive removal of Congo red. The present study may provide a new method for the environmentally friendly synthesis of hematite as well as other metal oxide nanoparticles.