Synthetic Akaganeite Research Articles

Reaction of Akaganeite with Mars-Relevant Anions

Akaganeite is an Fe(III) (hydr)oxide with a tunnel structure typically occupied by chloride. The mineral can undergo anion-exchange reactions in aqueous solution, resulting in incorporation of other anions together with Cl– into the tunnels. Identification of anions present in akaganeite tunnels may permit characterization of solution compositions in which akaganeite precipitated and/or existed. Akaganeite has been reported in several locations on Mars, including Yellowknife Bay in Gale crater. However, the nature of the tunnel anions has not been investigated. In order to constrain the nature of the tunnel anions in martian akaganeite, synthetic akaganeite (72 mg/g total Cl– content) was reacted with Mars-relevant anions (F–, OH–, and SO42–). Release of Cl– into solution was monitored with ion chromatography. Anion-reacted akaganeite was characterized with instruments analogous to instruments onboard robotic space crafts including X-ray diffraction (XRD), Mossbauer spectroscopy, thermal and evolved gas a...

Precipitation and Transformation of Secondary Fe Oxyhydroxides in a Histosol Impacted by Runoff from a Lead Smelter

AbstractSecondary Fe(III) oxyhydroxides play a key role in controlling the mobility and bioavailability of trace metals in acidic, sulfate-rich soils, such as mining and smelter sites. Schwertmannite, jarosite, goethite, and ferrihydrite are the most common mineral phases identified in such soils. A good understanding of the precipitation and transformation of these minerals in soils is very important for predicting the mobility and long-term stability of trace metals associated with them. In the present study, bulk powder X-ray diffraction (XRD), scanning electron microscopy (SEM), synchrotron based micro- X-ray diffraction (µ-XRD), and micro X-ray fluorescence (µ-SXRF) spectroscopy were used to investigate precipitates fromthe surface horizon of an organic soil (Histosol) at a site that once contained a lead smelter. Soil samples were collected from 0 to ≈10 cm depth during both wet and dry seasons. Goethite and akaganeite were identified as the major mineral phases in this soil. Schwertmannite and jarosite were also occasionally identified, particularly in the soil samples from dry periods. The peaks in the akaganeite XRD pattern were significantly broadened and the relative intensities of some major peaks were distinctly different compared with the diffraction pattern of synthetic akaganeite, possibly due to the effects of pH and the incorporation of sulfate. The SEM and µ-XRD data support the hypothesis that the goethite in the precipitates is not the product of direct precipitation froms olution but the transformation of previously precipitated schwertmannite or akaganeite.

New analysis of the Mössbauer spectra of akaganeite

The Mössbauer spectra of akaganeite have always been interpreted considering both thetetragonal structure and the chlorine content. However, very recently it has been suggestedthat the crystallographic structure is not tetragonal but monoclinic, thus anotherinterpretation for the Mössbauer spectra is required. For this purpose, we have prepared andcharacterized by several techniques synthetic akaganeite. Our results suggest that the twocrystallographic sites required by the monoclinic symmetry are not distinguishable in theparamagnetic state as previously assumed, but they are only discernible in the lowtemperature magnetic region. At room temperature the spectrum is fitted with twodoublets whose origin is related to the chlorine content, i.e. one Fe site assigned toFe3+ ions located close to chloride ions and the other Fe site to those located close to chloridevacancy sites. The low temperature spectra can be adequately fitted with four sextets,whose hyperfine parameters must be subjected to some constraints. The origin of thesecomponents is related to the two different crystallographic sites and to the chlorine content.In-field Mössbauer spectrometry at low temperature suggests that the magneticstructure behaves as a system which consists of two asperimagnetic-like structuresantiferromagnetically coupled, and not as a collinear antiferromagnet as usually assumed.

Adsorption of Cations on Hydrous Oxides of Iron

The interfacial behavior of two hydrous oxides of iron, namely an amorphous variety (amor. FeOOH) and a synthetic akaganeite (β-FeOOH), in different electrolyte media has been studied by potentiometric titration (both rapid and batch procedures) and particle electrophoresis methods. The background electrolyte medium was varied from simple KNO3 and NaCl solutions to a complex electrolyte solution resembling seawater. In KNO3 solution the pHpzc and the pHiep are almost the same for both samples. In NaCl solution, the pHiep shifts by about a pH unit toward the acid side for amor. FeOOH due to specific sorption of anions; for β-FeOOH the pHiep shifts slightly toward the alkaline side due to the small specific sorption of cations. Consequently, the pHpzc of amor. FeOOH in 0.5 M NaCl, although it shifts slightly to the acid side in the presence of MgCl2, has no corresponding shift to the alkaline side observable in the presence of Na2SO4 solution. Calculations for surface species in different electrolyte media suggest that while surface ionization is stronger than complexation for amor. FeOOH, surface complexation is the stronger phenomenon contributing to the total surface charge on β-FeOOH. All these observations are in conformity with the ΔpKinta value of amor. FeOOH being smaller than that of β-FeOOH.

Observations of axisymmetric tracer particle orientation during flow through a dilute fixed bed of fibers

Direct microstructural evidence for net tracer particle orientation induced solely by hydrodynamic interactions in a dilute, disordered, fibrous media is reported. A dilute fixed bed of randomly placed fibers was constructed and glycerol/water suspensions of either synthetic akaganeite (βFeOOH, average aspect ratio 6.3) or hematite (αFe2O3, average aspect ratio 1.6) tracer particles were made to flow axially through the bed at prescribed flow rates. Conservative linear dichroism, a noninvasive light scattering technique, was employed to provide a direct measure of the orientational order parameter for the tracer particle population at the end of the bed. The effect of Brownian motion on the hydrodynamically induced order in the suspensions was studied over three orders of magnitude in scaled rotary Peclet number, 5<P<2000. The steady degree of particle alignment exhibited by each suspension as it exited the fiber bed was found to be in good quantitative agreement with theory [Phys. Fluids 31, 728 (1988); ibid. 31, 2769 (1988)]. Time scales characterizing the development of net order induced in the tracer populations were investigated and compared qualitatively to theoretically derived bounds.

Thermal behaviour in air of iron oxyhydroxides obtained from the method of homogeneous precipitation: Part II. Akaganeite sample

Differential scanning calorimetry (DSC), thermogravimetry (TG), infrared spectroscopy (IR) together with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) have been employed to follow the structural and morphological changes happening in the thermal decomposition, in air, of a synthetic akaganeite sample (β-FeOOH). It has been stated that both the water molecules and chloride ions adsorbed within the structural tunnels have an important role in the stability of structure of akaganeite. At 300 °C, after the dehydroxylation, two broad reflections can be detected only in the X-ray diffraction pattern, which shows traces of both akaganeite and haematite. The sharp exothermic peak appearing in the DSC curve at 360 °C is related to both the loss of chloride ions as HC1 and the formation of haematite. At this point there is a change in crystallite shape, and the porosity almost disappears indicating that a sintering process is occurring. From 430 to 700 °C the haematite crystallinity increases. At 700 °C two kinds of crystals, rhombic and hexagonal, can be observed.

Effect of grinding in synthetic akaganeite

The textural and structural changes induced by prolonged mechanical treatment in synthetic akaganeite are studied. Dry grinding in a porcelain ball mill originated the conversion of the oxyhydroxide into hematite. Prior to this transformation, the adsorption data reveal the occurrence of hysteresis phenomena due to the fragmentation and compression of the akaganeite particles. Further grinding causes a progressive loss of porosity. The X-ray coherently diffracting domains suffer small variations in their size throughout the process indicating their complete conversion into hematite in successive fractions of the particles.

Thermal behaviour of synthetic akaganeite under different experimental conditions

The products of the thermal decomposition of β-FeOOH in air and vacuum are compared. While hematite is the only product in air, N2 and O2, the akaganeite sample leads to a different product in vacuum. This product is characterized by X-ray diffraction and chemical analysis, being identified as poorly crystallized maghemite with traces of magnetite. The DSC curve of this product shows a multiple exothermal effect that causes its conversion to hematite.

The βFeOOH to αFe2O3 phase transformation: Structural and magnetic phenomena

It is shown that under ambient atmospheric conditions heating causes the crystal structure of βFeOOH (synthetic akaganeite) to degenerate gradually into a quasi amorphous intermediate state, before the final phase transformation to αFe2O3 (hematite) takes place. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Mossbauer Spectroscopy, this amorphization process is monitored and the structural, morphological and magnetic features of the intermediate phase as a function of the isochronal heating temperature are discussed: the crystallites develop macropores on their surface, the adsorption capacity raises up to 10 percent of the initial mass, a third type of Fe3+ coordination, having an extremely large quadrupole splitting, is created and the Neel temperature, after an initial decrease, exhibits a sharp increase at higher heating temperatures. The magnetic behaviour of this intermediate phase at low temperatures and in high external fields suggests this antiferromagnet undergoes magnetic phase transitions (metamagnetism and spin flop) at unusually low critical fields.

On the influence of the dual iron co-ordination on the hyperfine field in βFeOOH

57Fe Mössbauer measurements of βFeOOH (synthetic akaganeite) between 4.2 and 56 K are reported, in order to study the compound's low temperature antiferromagnetic state. Two of the three observed hyperfine fields are identified and characterized in terms of the two different iron co-ordinations, while the third contribution is shown to be a thermally induced effect.

Changes in crystallite size and microstrains of hematite derived from the thermal decomposition of synthetic akaganeite

The thermal decomposition of akaganeite, β-FeOOH, in air is studied. The structural properties of the dehydroxylation product, identified as hematite, are determined at different temperatures. From the results on crystallite size and microstrains, a change of crystallite shape has been shown to occur in the range of temperatures of 300 to 485°C. A marked increase in crystallite size and a diminution in microstrains is observed at 525°C. This change is related to the occurrence of an exothermic peak in the DTA and DSC curves of akaganeite at about 512°C. The peak is thus ascribed to a recrystallization process of hematite. Electron micrographs support the most relevant observations of changes in particle size and shape.

Thermal decomposition of synthetic akaganeite (β-FeOOH)

The thermal decomposition of synthetic akaganeite (β-FeOOH) has been investigated using differential thermal analysis, thermogravimetry, electron microscopy and X-ray diffraction. The results show that β-FeOOH undergoes a dehydroxylation reaction at 250–300°C in common with other polymorphs of FeOOH, but unlike them undergoes a strongly exothermic peak after dehydroxylation. The size and temperature of this peak is strongly dependent on the atmosphere surrounding the sample and appears to be associated with the loss of chloride, an unavoidable impurity in the synthetic material. Morphological and chemical data suggest that a process of chemical transport occurs during thermal decomposition in a nitrogen atmosphere.

The porous structure of synthetic akaganeite

We report in this paper the synthesis and characterization of ß-FeOOH and its decomposition products in vacuum. IR spectra, electron microscopy, electron and X-ray diffraction and adsorption isotherms of N 2 at 77 K demonstrate that ß-FeOOH is stable up to 150°C. Having been treated at this temperature, the material has no molecular water in the hollandite type tunnels existing in its structure. This empty tunnels make anhidrous ß-FeOOH a good adsorbent for small molecules. This structural microporosity is different from that produced when treating ß-iron oxyhidroxide at temperatures over 150°C. In the course of this treatment ß-FeOOH decompose to γ-Fe 2O 3, when a single macropore is present in every crystal. At the final stage of the decomposition α-Fe 2O 3 is formed and the surface area becomes very small.

The nature of iron in Akaganéite (β-FeOOH)

The Mossbauer spectra of a size range of synthetic akaganeite (β-FeOOH) samples containing chloride, and one sample containing fluoride, in the structural tunnels, have been obtained over 4-295 K. At 295 K the spectra consist of overlapping paramagnetic doublets which indicate the presence of at least two sites for iron. Typical parameters for a spectrum of chloride-containing β-FeOOH fitted to two doublets are: (i) δ (relative to iron metal) 0.372 mm s-1, Δ 0.55 mm s-1, Г 0.27mm s-1; (ii) δ 0.375 mm s-1, Δ 0.94mm s-1, Г 0.33 mm s-1, with peak area ratios (i) : (II) of 60 : 40 for lath-shaped particles and 53 : 47 for spindle-shaped particles. At 77 K the spectra consist of overlapping magnetic hyperfine components which indicate the presence of at least three sites for iron, a three-component model giving internal magnetic fields of about 470 kOe, 460 kOe, and 435 kOe. The spectra of fluoride-containing β-FeOOH are very similar to those for chloride-containing β- FeOOH. The results are explained in terms of two types of octahedra within the lattice, FeO3(OH)3 and FeO2(OH)4, the extra proton in the second type balancing the charge on halide ions within the tunnels. Secondary effects may arise from the interaction of the halide ions with adjacent octahedra. The effects of modifying the surfaces of β-FeOOH particles (heating, phosphate sorption) are consistent with modification of the outer surfaces of the particles only, and it is not necessary to invoke the presence of large internal pores, of the order of 30Ǻ by 30Ǻ, as has been suggested by some earlier workers.

Nitrogen adsorption on synthetic akaganeite and its structural implications

AbstractComparison of the adsorptive properties of two samples of akaganéite prepared by different methods shows that a structural scheme based upon solid rods is much more likely than a previously proposed scheme involving tubes. Specific surface areas, qualitative and quantitative assessments of porosity' and electron micrographs strongly support this structure.