Kinetic Synthesis of Nanomaghemite Mediated by Arsenate

The formation, transformation, and behavior of elements are complex for Fe (oxyhydr)oxides. In this study, we developed a micro-titration method to control the growth kinetics and transformation of Fe (oxyhydr)oxides. Under all conditions, ferrihydrite was the main phase and the minor phases were controlled by the synthesis rate and arsenate addition. In experiments without As, nanomaghemite was formed at high titration rates, while goethite and hematite were formed at low rates. In experiments at As/Fe = 0.1, nanomaghemite was formed under all conditions and the size of the nanomaghemite particles increased as the titration rate decreased. The results indicate that nanomaghemite is a common phase in the precipitation of ferrihydrite, although it is difficult to detect because of its high instability. Anions that are compatible with nanomaghemite, such as phosphate and arsenate, can increase its stability. Our study has important implications for soil magnetism and industrial material design.