INCORPORATION OF Mo6+ IN FERRIHYDRITE, GOETHITE, AND HEMATITE

Among all iron oxides, hematite (alpha-Fe2O3), goethite (alpha-FeOOH), and ferrihydrite (FeOOH.nH(2)O) are the most common mineral species. While immobilization of Mo6+ by surface adsorption on ferric oxides has been studied extensively, the mechanisms of incorporation in their structure have been researched little. The objective of this study was to investigate the relation between Mo content and its structural incorporation in hematite, goethite, and six-line ferrihydrite by a combination of X-ray absorption spectroscopy (XAS), powder X-ray diffraction (pXRD), and inductively-coupled plasma optical emission spectrometry (ICP-OES). Synthesized in the presence of Mo, the hematite, goethite, and six-line ferrihydrite phases incorporated up to 8.52, 0.03, and 17.49 wt. % Mo, respectively. For hematite and goethite, pXRD analyses did not indicate the presence of separate Mo phases. Refined unit-cell parameters correlated with increasing Mo concentration in hematite and goethite. The unit-cell parameters indicated an increase in structural disorder within both phases and, therefore, supported the structural incorporation of Mo in hematite and goethite. Analysis of pXRD measurements of Mo-bearing six-line ferrihydrites revealed small amounts of coprecipitated akaganeite. X-ray absorption near edge structure (XANES) measurements at the Mo L-3-edge indicated a strong distortion of the MoO6 octahedra in all three phases. Fitting of extended X-ray absorption fine structure (EXAFS) spectra of the Mo K-edge supported the presence of such distorted octahedra in a coordination environment similar to the Fe position in the investigated specimen. Incorporation of Mo6+ at the Fe3+-position for both hematite and goethite resulted in the formation of one Fe vacancy in close proximity to the newly incorporated Mo6+ and, therefore, charge balance within the hematite and goethite structures.

Automated summary

This study investigated the structural incorporation of molybdenum in hematite, goethite, and six-line ferrihydrite using a combination of X-ray absorption spectroscopy, powder X-ray diffraction, and inductively-coupled plasma optical emission spectrometry. The results showed that molybdenum could be structurally incorporated in these minerals, forming coordination environments similar to iron. X-ray absorption and diffraction analyses revealed differences in the structural incorporation of molybdenum in different iron oxide minerals.