Abstract
Among all iron oxides, hematite (α-Fe2O3), goethite (α-FeOOH), and ferrihydrite (FeOOH⋅nH2O) 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 akaganéite. X-ray absorption near edge structure (XANES) measurements at the Mo L3-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.
Highlights
Molybdenum (Mo) occurs as a trace element in most soils but it is essential for most known life forms on Earth (Neunhäuser et al 2001; Mendel and Bittner 2006)
Six-line ferrihydrite, and goethite have the ability to control the bioavailability of micro-nutrients such as Mo and contaminants in soils (Cornell and Schwertmann 2003)
Surface adsorption might play a significant role in short-term retention of such substances, previous studies proposed that structural incorporation might likewise be an important process to contain these substances but for prolonged periods
Summary
Molybdenum (Mo) occurs as a trace element in most soils but it is essential for most known life forms on Earth (Neunhäuser et al 2001; Mendel and Bittner 2006). Both in the global biogeochemical oxidation-reduction cycles of carbon, nitrogen, and sulfur (Stiefel 2002; Williams and Frausto da Silva 2002) and in the metabolism of living organisms (Bortels 1930) Mo holds a key position in the active sites of a wide range of metalloenzymes (Norrish 1975; Mendel and Bittner 2006).
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