The phase purity of a series of ZnAl4(OH)12SO4•nH2O layered double hydroxides (ZnAl4-LDH) obtained from a reaction of bayerite (Al(OH)3) with an excess of zinc(II) sulfate under hydrothermal conditions was investigated as a function of the reaction temperature, the duration of the hydrothermal treatment, and the zinc(II) concentration. The product quality, i.e., crystalline impurities, Al impurities and bulk Zn:Al ratio, were assessed by powder X-ray diffraction (PXRD), 27 Al MAS NMR, and elemental analysis. Structural characterization of a stoichiometric ZnAl4-LDH (120 °C, 9 days, and 2.8 M Zn(II)) showed a well-defined structure of the metal ion layer as evident by a single, well-defined Zn environment, i.e., no Zn substitution on the Al sites according to Zn K-edge EXAFS and PXRD. Furthermore, nearly all of the twelve different 1 H in the-OH groups and 4 27 Al resonances could be assigned using 1 H, 27 Al NMR correlation experiments recorded with ultra-fast MAS. The interlayer water content is variable based thermogravimetric analysis and changes in the 1 H MAS NMR spectra with temperature. A composition of ZnAl4(OH)12(SO4)⋅2.6H2O was obtained from combination of these techniques and confirmed that ZnAl4-LDH is isostructural to the mineral nickelalumite (NiAl4(OH)12SO4⋅3H2O).

Hematite (α-Fe2O3) is one of the most common iron oxides and a sink for the toxic metalloid arsenic. Arsenic can be immobilized by adsorption to the hematite surface; however, the incorporation of As in hematite was never seriously considered. In our study we present evidence that, besides adsorption, the incorporation of As into the hematite crystals can be of great relevance for As immobilization. With the coupling of nanoresolution techniques and X-ray absorption spectroscopy the presence of As (up to 1.9 wt %) within the hematite crystals could be demonstrated. The incorporated As(5+) displays a short-range order similar to angelellite-like clusters, epitaxially intergrown with hematite. Angelellite (Fe4As2O11), a triclinic iron arsenate with structural relations to hematite, can epitaxially intergrow along the (210) plane with the (0001) plane of hematite. This structural composite of hematite and angelellite-like clusters represents a new immobilization mechanism and potentially long-lasting storage facility for As(5+) by iron oxides.

Iron oxides, typical constituents of many soils, represent a natural immobilization mechanism for toxic elements. Most iron oxides are formed during the transformation of poorly crystalline ferrihydrite to more crystalline iron phases. The present study examined the impact of well known contaminants, such as P(V), As(V), and Sb(V), on the ferrihydrite transformation and investigated the transformation products with a set of bulk and nano-resolution methods. Irrespective of the pH, P(V) and As(V) favor the formation of hematite (α-Fe2O3) over goethite (α-FeOOH) and retard these transformations at high concentrations. Sb(V), on the other hand, favors the formation of goethite, feroxyhyte (d’-FeOOH), and tripuhyite (FeSbO4) depending on pH and Sb(V) concentration. The elemental composition of the transformation products analyzed by inductively coupled plasma optical emission spectroscopy show high loadings of Sb(V) with molar Sb:Fe ratios of 0.12, whereas the molar P:Fe and As:Fe ratios do not exceed 0.03 and 0.06, respectively. The structural similarity of feroxyhyte and hematite was resolved by detailed electron diffraction studies, and feroxyhyte was positively identified in a number of the samples examined. These results indicate that, compared to P(V) and As(V), Sb(V) can be incorporated into the structure of certain iron oxides through Fe(III)-Sb(V) substitution, coupled with other substitutions. However, the outcome of the ferrihydrite transformation (hematite, goethite, feroxyhyte, or tripuhyite) depends on the Sb(V) concentration, pH, and temperature.

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Among all highly-crystalline iron oxides present in the environment, akaganéite (β-FeO(OH, Cl)) possesses one of the most unconventional structural setups and is a known scavenger for large quantitates of molybdenum (Mo6+).

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.