Effects of Formation Conditions on the Physicochemical Properties, Aggregation, and Phase Transformation of Iron Oxide Nanoparticles

In this work, hematite transformation from a precursor 6-line ferrihydrite phase was investigated by systematically altering the forced hydrolysis hematite synthesis. Specifically, we used a combination of in situ and ex situ characterization techniques to examine the effects of varying the Fe-III injection rates and cooling methods on the hematite and 6-line ferrihydrite nanoparticle size, isoelectric point, mineral phase, and aggregation. Finally, As(V) adsorption experiments were performed to determine how the two iron oxide phases existed in the reaction system. Nanoparticle synthesis thermodynamics and kinetics were found to control the extent of distinct 6-line ferrihydrite phases in the iron oxide nanoparticle solutions, as well as the particle size and isoelectric point. Conversion of 6-line ferrihydrite to hematite was greatly influenced by the degree of aggregation (determined by synthesis conditions) during drying. As(V) adsorption experiments revealed that 6-line ferrihydrite and hematite exist as a linear combination of two separate phases. These results provide unique information regarding how in situ iron oxide nanoparticle properties can direct their ex situ behavior.