Effect of Mo and V on the Hydrothermal Crystallization of Hematite from Ferrihydrite: An in Situ Energy Dispersive X-ray Diffraction and X-ray Absorption Spectroscopy Study

Abstract

In this study we used in situ synchrotron-based energy dispersive X-ray diffraction (EDXRD) to follow the transformation of ferrihydrite to hematite at pH ∼ 8 and ionic strength between 0.1 and 0.7. In addition, the effects of coprecipitated molybdenum (Mo) and vanadium (V) on the transformation were evaluated both through EDXRD and X-ray absorption spectroscopy (XAS). The transformation end product in all experiments was hematite with small amounts of goethite as an intermediate phase. XAS results revealed that Mo and V were initially adsorbed and coprecipitated onto/with ferrihydrite as molybdate and vanadate ions, respectively. After the ferrihydrite transformed to hematite these metals were sequestered into the hematite structure. The kinetic results showed that the presence of Mo and V in the ferrihydrite structure had little to no effect on the kinetics of the ferrihydrite transformation. The transformation however occurred ∼30% faster at higher ionic strength.