Maximizing Reducing Potential of Fe3O4 Nanoparticles for Efficient Removal of Cr(VI) in Drinking Water

The dimensions and the reduction capacity of Fe3O4 nanoparticles are considered to be the key parameters in achieving the successful, efficient removal of hexavalent chromium, aiming for drinking water purification. This research study focuses on the optimization of reaction parameters during the oxidative precipitation of FeSO4 carried out in a microwave-heated plug-flow reactor, to realize the preparation of Fe3O4 nanoparticles with an increased reduction potential as reflected in the Fe2+/Fe3+ ratio by approximating the ideal value of 0.5. In particular, the coupling of synthesis with features that allow for control of the oxidation extent, and include the addition of a reducing agent, an increase in ageing temperature, and inhibition of aggregation, were tested as potential approaches to tune the reducing potential and overcome reported Cr(VI) capture efficiencies provided by Fe3O4 nanoparticles. The evaluation results showed that adding a reductant after nanoparticle formation inhibits spontaneoussurface oxidation, bringing an improvement in the Cr(VI) uptake capacity for a residual concentration equal to the new EU regulation limit, by around 40%, reaching a value of 2.15 mg/g. However, working at an ageing temperature of around 100 °C resulted in an even better performance with an uptake increase of 120% and a capacity value of 3.45 mg/g. Finally, adding nanoparticles in the form of a dispersion instead of a dried powder provides an extra 10% improvement as a consequence of limited aggregation.