Co-precipitation synthesis of hematite (α-Fe2O3) and hausmannite Mn3O4 nanoparticles for superior removal of Congo red and methylene blue

This study reports a simple co-precipitation synthesis of hematite (α-Fe2O3) and hausmannite (Mn3O4) nanoparticles. Systematic optimization of synthesis parameters—yielding an OH⁻/Fe2+ molar ratio of 8:1, a Fe2+/Fe3+ ratio of 1:2, a temperature of 70°C and a 30-min reaction time for hematite, alongside an Mn2+:OH⁻ ratio of 2:1 at 25°C for 1 h for Mn3O4—produced highly crystalline, spherical nanoparticles, as confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Furthermore, this work provides the first demonstration that a facile, low-energy co-precipitation route can produce Mn3O4 nanoparticles capable of removing approximately 95% of 50 mg l−1 Congo red (CR) within 5 min at neutral pH. This uptake rate and efficiency considerably exceed those of previously reported Mn3O4 adsorbents under similar conditions. In parallel, hematite nanoparticles synthesized under the same optimized conditions achieve approximately 80% removal of 50 mg l−1 CR at pH 4, a level of performance in acidic media not previously reported in comparable studies. Investigation using methylene blue (MB, 1 ppm) suggested complex removal mechanisms: the binary oxide facilitated near-complete removal at pH 4, whereas hematite exhibited behaviour indicative of concurrent adsorption and degradation pathways. Taken together, these findings highlight that precise tuning of synthesis parameters directly dictates phase, crystallinity, particle size and morphology, yielding low-cost oxide adsorbents with high kinetic selectivity for targeted dye remediation.