Ralph M. Bolanz

Structural Incorporation of As5+ into Hematite

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.

The effect of antimonate, arsenate, and phosphate on the transformation of ferrihydrite to goethite, hematite, feroxyhyte, and tripuhyite

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.

Chromate adsorption on selected soil minerals: Surface complexation modeling coupled with spectroscopic investigation

This study investigates the mechanisms of Cr(VI) adsorption on natural clay (illite and kaolinite) and synthetic (birnessite and ferrihydrite) minerals, including its speciation changes, and combining quantitative thermodynamically based mechanistic surface complexation models (SCMs) with spectroscopic measurements. Series of adsorption experiments have been performed at different pH values (3-10), ionic strengths (0.001-0.1M KNO3), sorbate concentrations (10(-4), 10(-5), and 10(-6)M Cr(VI)), and sorbate/sorbent ratios (50-500). Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy were used to determine the surface complexes, including surface reactions. Adsorption of Cr(VI) is strongly ionic strength dependent. For ferrihydrite at pH <7, a simple diffuse-layer model provides a reasonable prediction of adsorption. For birnessite, bidentate inner-sphere complexes of chromate and dichromate resulted in a better diffuse-layer model fit. For kaolinite, outer-sphere complexation prevails mainly at lower Cr(VI) loadings. Dissolution of solid phases needs to be considered for better SCMs fits. The coupled SCM and spectroscopic approach is thus useful for investigating individual minerals responsible for Cr(VI) retention in soils, and improving the handling and remediation processes.

Hematite (α-Fe2O3)–A potential Ce4+ carrier in red mud

Cerium is the most abundant rare earth element (REE) within the waste product of alumina production (red mud), but its speciation in this complex material is still barely understood. Previous studies showed evidence for a correlation between Ce and the main constituent of red mud, iron oxides, which led us to investigate the most abundant iron oxide in red mud, hematite, as possible carrier phase for Ce. Synthetic hematite can incorporate up to 1.70±0.01wt% Ce, which leads to a systematical increase of all unit cell parameters. Investigations by extended X-ray absorption fine structure spectroscopy suggest an incorporation of Ce4+O6 into the hematite structure by a novel atomic arrangement, fundamentally different from the close-range order around Fe3+ in hematite. Samples of red mud were taken in Lauta (Saxony), Germany and analyzed by powder X-ray diffraction, inductively coupled plasma mass and optical emission spectrometry, electron microprobe analysis and X-ray absorption near-edge structure spectroscopy. Red mud samples consist of hematite (Fe2O3) (34-58wt%), sodalite (Na8Al6Si6O24Cl2) (4-30wt%), gibbsite (Al(OH)3) (0-25wt%), goethite (FeOOH) (10-23wt%), böhmite (AlOOH) (0-11wt%), rutile (TiO2) (4-8wt%), cancrinite (Na6Ca2Al6Si6O24(CO3)2) (0-5wt%), nordstrandite (Al(OH)3) (0-5wt%) and quartz (SiO2) (0-4wt%). While the main elemental composition is Fe>Al>Na>Ti>Ca (Si not included), the average concentration of REE is 1109±6mg/kg with an average Ce concentration of 464±3mg/kg. The main carrier of Ce was located in the Fe-rich fine-grained fraction of red mud (0.10wt% Ce2O3), while other potential Ce carriers like monazite, lead oxides, secondary Ce-minerals and particles of potentially anthropogenic origin are of subordinated relevance. Cerium in red mud occurs predominantly as Ce4+, which further excludes Ce3+ minerals as relevant sources.

Structural incorporation of As5+ into rhomboclase ((H5O2)Fe3+(SO4)2·2H2O) and (H3O)Fe(SO4)2

Iron sulfates represent an essential sink for the toxic element arsenic in arid and semi-arid mining areas with high evaporation rates. Information about the structural incorporation of As(5+) in iron sulfates, however, remains scarce. Here we present evidence for the heterogeneous substitution of S(6+) by As(5+) in the crystal structure of rhomboclase ((H5O2)Fe(3+)(SO4)2 · 2H2O) and its dehydration product (H3O)Fe(SO4)2. Rhomboclase (Rhc) was synthesized in the presence of As(5+) with molar As/Fe ratios of 0, 0.25, 0.5, 0.75 and 1.0, resulting in As loads of 0.0, 0.93, 1.44, 1.69 and 1.87 wt.%, respectively. The unit cell parameters of Rhc increase from 9.729(6), 18.303(2), and 5.432(1) Å for a, b, and c, to 9.745(9), 18.332(5), and 5.436(8) Å when Rhc is crystallized at a molar As/Fe ratio of 1. Simultaneously, the crystallite size decreased from 304 to 176 nm. In situ dehydration of Rhc to (H3O)Fe(SO4)2, investigated by powder X-ray diffraction, shows that Rhc starts to dehydrate at 76 °C, which is completed at 86 °C. The presence of As(5+) does not impact the start or end temperatures of Rhc dehydration but does accelerate the dehydration. X-ray absorption fine structure spectroscopy (EXAFS) reveals that S(6+), in the Rhc and (H3O)Fe(SO4)2 structure, is replaced by As(5+), while the polymerization of AsO4-tetrahedra and FeO6-octahedra during the formation of (H3O)Fe(SO4)2 results in a strong distortion of the AsO4-tetrahedron.

Incorporation of molybdenum(VI) in akaganéite (β-FeOOH) and the microbial reduction of Mo–akaganéite by Shewanella loihica PV-4

Among all highly-crystalline iron oxides present in the environment, akaganeite (β-FeO(OH, Cl)) possesses one of the most unconventional structural setups and is a known scavenger for large quantitates of molybdenum (Mo6+). The factors controlling the exact mechanism for Mo6+ incorporation into the akaganeite crystal structure are poorly understood and the ability of dissimilatory Fe(III)-reducing microorganisms to reduce pure akaganeite or Mo-carrying iron oxides is not well characterized. In the current study, we investigated the short-range order around Mo6+ in akaganeite and the fate of Mo6+ under microbially-mediated Fe(III)-reducing conditions. We found that akaganeite can incorporate up to 14.11 ± 0.22 wt% Mo, while the Fe content decreases from 59.70 ± 0.31 to 40.40 ± 0.24 wt%, which indicates a loss of 2–3 Fe atoms for each Mo incorporated. Simultaneously, the crystal structure unit cell parameters a, b and c decrease, while β increases with increasing Mo content. Surprisingly, dissolution of akaganeite by Shewanella loihica PV-4 showed higher dissolution rates of Mo-bearing akaganeite compared to Mo-free akaganeite. Moreover, these results suggest the reduction of Mo6+ is most likely microbially-induced (Fe3+ → Fe2+, Mo6+ + 2Fe2+ → Mo4+ + 2Fe3+). Furthermore, X-ray absorption spectra collected at the Mo L3-edge show a peak-splitting of the white line with a splitting gap of 2.7 eV and an increased amplitude for the first peak. This observation indicates Mo6+ is octahedrally coordinated by oxygen, assuming a strongly distorted MoO6-octhaedron. Fitting of the short-range order around Mo6+ in akaganeite supports the presence of a strongly distorted MoO6-octahedron in a coordination environment similar to the Fe position in akaganeite and the formation of Fe-vacancies close to the newly incorporated Mo6+.