Goethite Structure Research Articles

Thermal and Mineral Properties of Al-, Cr-, Mn-, Ni- and Ti-Substituted Goethite

AbstractMineralogical and thermal characteristics of synthetic Al-, Cr-, Mn-, Ni- and Ti-bearing goethites, synthesized via alkaline hydrolysis of metal-ferrihydrite gels, were investigated by powder X-ray diffraction and differential thermal analysis. Shifts in unit-cell dimensions were consistent with size of substituent metal ions and confirmed the incorporation of Al3+, Cr3+, Mn3+, Ni2+ and Ti4+ in the goethite structure. A weight loss of 6.2 wt.% for goethite containing 12.2 mol.% Ti, being significantly less than for stoichiometric goethite, is consistent with the replacement of Fe by Ti in the goethite structure coupled with the substitution of O2− ions for OH− (i.e. proton loss). These data provide the first confirmation of the direct replacement of Fe by Ti within goethite. Formation of multiple dehydroxylation endotherms for goethite containing 4.5 mol.% Al, 15.3 mol.% Mn and 12.2 mol.% Ti was not attributed to the decomposition of surface OH groups or related simply to the crystallinity of precursor goethite (‘high-a’ vs. ‘low-a’) as defined by the magnitude of a. Instead, endotherm doublet formation was associated with weight loss due to the dehydroxylation of goethite remaining after initial phase transformation to protohematite and to the evolution of OH− associated with the rapid increase in crystallite size of protohematite directed primarily along the a direction. Development of the first endotherm is due to initial dehydroxylation and transformation to protohematite. With continued heating of well ordered goethite or goethite containing moderate to high levels of substituent cations, domain growth along the a direction is delayed or inhibited to a critical point that provides enough thermal energy to enable goethite transformation to proceed to completion and for proto-hematite domain growth to occur. This results in the formation of a second endotherm. For less well ordered goethite and/or goethite containing only low levels of foreign metal cations, protohematite domain growth is not inhibited and proceeds continuously with heating to give only a single endotherm.

Structural characterization and chemical reactivity of synthetic Mn-goethites and hematites

Several goethites were obtained through the hydrolysis at 60 °C of Fe(III) solutions containing variable amounts of Mn(II) ions. The obtained samples were thermally treated at temperatures ranging from 180 to 310 °C until the complete phase transformation to hematite was achieved. The effect of Mn in the dehydroxylation process was investigated using X-ray diffraction (XRD) and the Rietveld refinement of XRD data together with scanning electron microscopy (SEM), differential thermogravimetric analysis (DTA) and Fourier transform infrared spectroscopy (FTIR). In all cases, the formed hematites retained the acicular shape of the precursor goethite. The dehydroxylation temperature increased with the increase of the Mn content in the parent goethite. The cell parameters of both phases decreased with the thermal treatment, however the decrease in the goethite b-parameter was more pronounced. This fact could be attributed to the distortion in the goethite structure by the presence of manganese. The band shifts in the FT-IR spectra of the goethites with different Mn substitution were analysed. The intensities of the hydroxyl vibrations were indicative of the degree of dehydroxylation. The chemical reactivity of all the samples, before and after the thermal treatment, was also studied. The kinetic experiments were carried out at 40 °C in 4 mol dm − 3 HCl. The acid dissolution of all Mn-goethites showed a congruent behavior indicative of a homogeneous distribution of Mn in the goethite crystals, this trend was not observed in the formed hematites presenting a high Mn content. The dissolution rate in goethites increased with the increase of Mn content, the opposite effect was observed in the corresponding hematites. The activation energy in both phases was also obtained and indicated that the Mn substitution produces an opposite effect on goethite- and hematite-phases. Different kinetic laws were applied in order to explain the dissolution behavior, but the modified first-order Kabai equation described the dissolution data best.

Desorption of cadmium from goethite: Effects of pH, temperature and aging

Cadmium is perhaps environmentally the most significant heavy metal in soils. Bioavailability, remobilization and fate of Cd entering in soils are usually controlled by adsorption–desorption reactions on Fe oxides. Adsorption of Cd on soil colloids including Fe oxides has been extensively studied but Cd desorption from such soil minerals has received relatively little attention. Some factors that affect Cd adsorption on goethite include pH, temperature, aging, type of index cations, Cd concentrations, solution ionic strength and presence of organic and inorganic ions. This research was conducted to study the influence of pH, temperature and aging on Cd desorption from goethite. Batch experiments were conducted to evaluate Cd desorption from goethite with 0.01 M Ca(NO 3) 2. In these experiments Cd desorption was observed at 20, 40 and 70 °C in combination with aging for 16 h, 30, 90 and 180 d from goethite that adsorbed Cd from solutions containing initial Cd concentrations of 20, 80 and 180 μM. Following the adsorption step Cd desorption was measured by 15 successive desorptions after aging at various temperatures. At the lowest amount of initially adsorbed Cd and equilibrium pH 5.5, cumulative Cd desorption decreased from 71% to 17% with aging from 16 h to 180 d and the corresponding decrease at equilibrium pH 6.0 was from 32% to 3%. There was a substantial decrease in Cd desorption with increasing equilibration temperature. For example, in goethite with the lowest amount of initial adsorption at equilibrium pH 5.5, cumulative Cd desorption decreased from 71% to 31% with increase in temperature from 20 to 70 °C, even after 16 h. Dissolution of Cd adsorbed goethite in 1 M HCl, after 15 successive desorptions with 0.01 M Ca(NO 3) 2, indicated that approximately 60% of the Cd was surface adsorbed. Overall, dissolution kinetics data revealed that 23% to 88% Cd could not be desorbed, which could possibly be diffused into the cracks and got entrapped in goethite crystals. At elevated temperature increased equilibrium solution pH favoured the formation of CaCO 3 and CdCO 3 which reasonably decreased Cd desorption. Cadmium speciation showed the formation of calcite and otavite minerals at 40 and 70 °C due to increase in pH (>9.5) during aging. X-ray diffraction analysis (XRD) of these samples also revealed the formation of CaCO 3 at elevated temperatures with aging. While mechanisms such as Cd diffusion and/or entrapment into fissures and cracks in goethite structure with increase in temperature and aging are possible.

Investigating Sorption on Iron−Oxyhydroxide Soil Minerals by Solid-State NMR Spectroscopy: A 6Li MAS NMR Study of Adsorption and Absorption on Goethite

High-resolution 2H MAS NMR spectra can be obtained for nanocrystalline particles of goethite (alpha-FeOOH, particle size approximately 4-10 nm) at room temperature, facilitating NMR studies of sorption under environmentally relevant conditions. Li sorption was investigated as a function of pH, the system representing an ideal model system for NMR studies. 6Li resonances with large hyperfine shifts (approximately 145 ppm) were observed above the goethite point of zero charge, providing clear evidence for the presence of Li-O-Fe connectivities, and thus the formation of an inner sphere Li+ complex on the goethite surface. Even larger Li hyperfine shifts (289 ppm) were observed for Li+-exchanged goethite, which contains lithium ions in the tunnels of the goethite structure, confirming the Li assignment of the 145 ppm Li resonance to the surface sites.

Chemical and physical properties of iron hydroxide precipitates associated with passively treated coal mine drainage in the Bituminous Region of Pennsylvania and Maryland

Changes in precipitate mineralogy, morphology, and major and trace element concentrations and associations throughout 5 coal mine drainage (CMD) remediation systems treating discharges of varying chemistries were investigated in order to determine the factors that influence the characteristics of precipitates formed in passive systems. The 5 passive treatment systems sampled in this study are located in the bituminous coal fields of western Pennsylvania and northern Maryland, and treat discharges from Pennsylvanian age coals. The precipitates are dominantly (>70%) goethite. Crystallinity varies throughout an individual system, and lower crystallinity is associated with enhanced sorption of trace metals. Degree of crystallinity (and subsequently morphology and trace metal associations) is a function of the treatment system and how rapidly Fe(II) is oxidized, forms precipitates, aggregates and settles. Precipitates formed earlier in the passive treatment systems tend to have the highest crystallinity and the lowest concentrations of trace metal cations. High surface area and cation vacancies within the goethite structure enable sorption and incorporation of metals from coal mine drainage-polluted waters. Sorption affinities follow the order of Zn > Co ≈ Ni > Mn. Cobalt and Ni are preferentially sorbed to Mn oxide phases when these phases are present. As pH increases in the individual CMD treatment systems toward the pH pzc of goethite, As sorption decreases and transition metal (Co, Mn, Ni and Zn) sorption increases. Sulfate, Na and Fe(II) concentrations may all influence the sorption of trace metals to the Fe hydroxide surface. Results of this study have implications not only for solids disposal and resource recovery but also for the optimization of passive CMD treatment systems.

Structure of titanium-doped goethite rust

To investigate the influence of titanium addition on the formation and structure of goethite (α-FeOOH) rust which is one of main corrosion products of weathering steel, the artificially synthesized α-FeOOH rusts were prepared by hydrolysis of aqueous solutions of Fe(III) containing Ti(IV) at different atomic ratios (Ti/Fe) in the range 0–0.1. The obtained rusts particles were observed by TEM. Characterization by XRD, N 2 absorption, Mössbauer spectroscopy was also done. TEM observation revealed that the α-FeOOH rust particle size increased with the increase of Ti/Fe, and that Ti-enriched poorly crystalline particles were formed around the rust particles. XRD confirmed that the crystallite size increased with the increase of Ti/Fe, while the XRD peaks decreased in intensity. Specific surface area obtained by N 2 absorption increased with the increase of Ti/Fe. It is deduced from the obtained results that the addition of Ti(IV) increases the crystallite size of α-FeOOH, and produces double domain particles consisting of the particle core and a porous poorly crystalline shell. It is thought that such unique rust structure produced by titanium addition contributes to the protective properties of rust layer of the weathering steel.

Effects of Crystal Modification on the Binding of Trace Metals and Arsenate by Goethite (7 pp)

Dedicated to Prof. Dr. Ulrich Forstner on his 65th birthday Goethite (α-FeOOH) as the most frequently occurring iron oxide in the environment plays a significant role in the binding of inorganic pollutants. Accordingly, synthetic goethite is used for the purification of contaminated water. Goethite crystals can be prepared in different shape as porous and non-porous forms. The mineral can also be modified by partial substitution of structural Fe+3 for different foreign elements. The biggest possible substituent known so far is Cd+2 which causes a strong expansion of the unit-cell parameters. An incorporation of Pb+4 generates a permanent positive charge. Goethites with these morphological and structural modifications were selected for sorption experiments with Co2+, Ni2+, Zn2+, Cd2+, Pb2+, and arsenate. It was intended to demonstrate the potential of mineral modification for improving sorption properties. Batch sorption studies were carried out combining each mineral with a single element at different pH and reaction times. Cations were investigated at a single initial concentration only while arsenate was tested over a range of concentrations in order to establish sorption isotherms. The sorption step was followed by an extraction step to characterize time dependent immobilization reactions. A time dependent increase of trace metal and arsenate sorption is attributed to a migration of ions into pores of star-shaped goethite and to a binding by specific sorption sites at the surface. The migration into pores is related to the size of adsorbing cations. The almost identical sorption behaviour of Ni2+ and Co2+ on pure goethite is contrasted by a strong preference of Co over Ni on Cd-goethite. Expansion of the unit-cell dimensions in Cd+2 substituted goethite generates highly specific binding sites at the surface. These are accessible to Co and Zn only. A permanent positive charge in Pb+4 substituted goethite reduces the binding of cations and doubles the sorption capacity for arsenate. Pb-goethite also contains a limited portion of highly specific sites which can only be accessed by Zn2+. Immobilization takes place even after a short contact time of 16 hours. This process results in a growing fraction of non-extractable metals and arsenate with reaction time. Pores and foreign elements in the goethite structure greatly affect the reactivity of the mineral and the ability to immobilize inorganic pollutants. A possible mechanisms for the preferred sorption of Co and Zn by Cd-goethite is seen in the ability of these metal ions to adopt a smaller size: Co by oxidation of Co2+ to Co3+ and Zn by tetrahedral coordination of Zn2+. This kind of binding can be viewed as structural incorporation. The binding properties of modified goethites can well be characterized by sorption tests including an array of elements with different ionic size and charge. There is considerable potential for designing goethite modifications with improved surface reactivity for specific purposes such as water purification and possibly catalysis of reactions.

Effect of Mn(II) incorporation on the transformation of ferrihydrite to goethite

A series of Mn-substituted goethites were obtained by the addition of Mn(II) to ferrihydrite in alkaline media, at different times. The total aging period was 24 h. Chemical analysis indicated that the Mn mol fraction ( χ Mn) remained practically constant (ca. 8 mol%) in the oxalate–ammonium non-extracted samples. In the extracted samples, the χ Mn values increased with the earlier addition of the Mn(II) solution to the iron–oxyhydroxide suspension. XRD patterns of the obtained solids showed that the unique phase present was goethite. Although, in most of the experiments, the Mn(II) was added when the goethite phase was already formed, variations in the unit cell parameters were observed. The changes in the cell parameters followed the trend reported for coprecipitated samples. An enlargement of the acicular crystals is in line with the higher Mn incorporation. Lattice parameters and cell volume for the extracted samples were obtained by the Rietveld simulation of XRD data. Kinetics measurements indicate that the initial dissolution rate increases with the Mn content in the goethite structure, except in the samples where Mn was added later. Dissolution–time curves show a better fit with the two-dimensional contracting geometry law than with the cubic root law, this fact is attributed to the presence of two more exposed faces, both of different reactivity, in the acicular crystals of the Mn-substituted goethite. The dissolution has also well been described by the Kabai equation. Deviation from congruence indicates an inhomogeneous distribution of Mn into the goethite crystals due to the more belated addition of Mn to the solid phase.

The Nature of Co in Synthetic Co-substituted Goethites

AbstractThe crystallochemical features of Co in Co-substituted goethite solid-solutions prepared by two different procedures have been studied using infrared, X-ray photoelectron and electron energy loss spectroscopies. It was found that the path followed for the synthesis of Co-substituted goethite determines the oxidation state of Co in the goethite structure. Thus, in the solid-solution prepared by precipitation with Na2CO3 of an Fe(II) aqueous solution containing Co(II) cations, followed by the aerial oxidation of the precipitate, the Co cations were found to be divalent, whereas trivalent Co was incorporated into the goethite obtained by ageing a solution containing Fe(III) and Co(II) cations precipitated by the addition of KOH. This different behavior is explained by the higher pH of goethite formation in the latter case, which favors the oxidation of the Co(II) cations.

Cadmium adsorption and desorption behaviour on goethite at low equilibrium concentrations: effects of pH and index cations

The transport and bioavailability of cadmium is governed mainly by its adsorption–desorption reactions with minerals such as goethite—a common iron oxide mineral in variable charged and highly weathered tropical soils. Soil factors such as pH, temperature, solution Cd concentration, ionic strength and ageing affect Cd adsorption on goethite. The desorption behaviour of Cd from goethite at low concentrations is not fully understood. This study investigates the adsorption–desorption of Cd at low Cd concentrations (Cd adsorbed on goethite from 20 to 300 μM Cd solutions) in Na and Ca nitrate solutions of 0.03 M nominal ionic strengths. Synthetic goethite prepared by ageing a ferric hydroxide gel at high pH and room temperature was used for Cd adsorption and desorption studies. For desorption experiment 10 successive desorptions were made for the whole range of initial Cd concentrations (20–300 μM) in the presence of 0.01 M Ca(NO 3) 2 or 0.03 M NaNO 3 solutions. Cadmium adsorption was found to be higher in Na + than Ca 2+ probably due to the competition of Ca 2+ ions with Cd 2+ ions for adsorption sites on the surfaces of goethite. The effect of index cation on Cd adsorption diminished with increase in pH from 5.0 to 6.0. Cadmium desorption decreased with increase in pH from 5.0 to 6.0 in both Na and Ca systems. After 10 successive desorptions with 0.03 M NaNO 3 at the lowest initially adsorbed Cd approximately 45%, 20% and 7% of the adsorbed Cd was desorbed at pH 5.0, 5.5 and 6.0, respectively. The corresponding desorptions in the presence of 0.01 M Ca(NO 3) 2 were 49%, 22% and 8%, respectively. The Freundlich parameter, k, based on each progressive step of desorption at different adsorbed concentration increased with increasing desorption step, which may indicates that a fraction of Cd was resistant to desorption. Low Cd desorbability from goethite may be due to its specific adsorption and/or possibly as a result of Cd entrapment in the cracks or defects in goethite structure.

Effect of cationic substituents on particle morphology of goethite and the magnetic properties of maghemite derived from substituted goethite

Preparation of nanoparticles with the desired shape and size by the wet-chemical precipitation process is a challenging task. Thus, the effects of different substitutional impurities such as Al, Cr, Co or Ni on the particle morphology and phase stability of goethite have been investigated. Goethite is prepared by air oxidation of $Fe(OH)_2 · xH_2O$ gel under near neutral conditions. Below certain concentration levels of dopants $(Al^{3+} \leq 10; Cr^{3+} \leq 5; Co^{2+} \leq 10$ and $Ni^{2+} \leq 5 at.%)$ the samples remained monophasic as revealed by XRD, TEM, and IR studies. Above these levels, the substituents produce traces of secondary phases such as lepidocrocite, spinel ferrite and $M^n(OH)_n$. The goethite structure is stable in spite of the iso- or aliovalent substitutions. The individual additives have divergent influence on the particle morphology; $Al^{3+}$ and $Cr^{3+}$ decreases the particle size to <50 nm and aspect ratio (AR) <2. Co-substitution produces slender particles with AR as high as 25. Whereas, $Ni^{2+}$ does not have any influence on the particle morphology. The attributable factors in morphology control are the increased nucleation rate, restricted growth along needle axes, and the strain induced in the goethite lattice as a result of difference in ionic radii. Maghemite, $\gamma -Fe_2O_3– \delta$, particles are obtained from goethite wherein the topotactic conversion renders the retention of the particle morphology of the precursor. Maghemite with substituted impurities showed substantial differences in magnetic properties. Saturation magnetization $(\sigma s)$ and coercivity $(H_ c)$ go down to very low values due to relaxation of spins on the surface atoms as revealed by Mossbauer spectroscopy. Decrease in coercivity is by way of the presence of diamagnetic ion $(Al^{3+})$. Whereas, Co-substituted maghemite has enhanced $H_ c$ as a result of high magnetocrystalline anisotropy accompanied by the shape anisotropy.

Paleo-gossans within the lateritic iron crust: example of the nickeliferous prospect of Bonga, Burkina Faso

Bonga is a new example of nickel concentration developed by lateritic weathering of Paleoproterozoic ultramafic rocks. It is comparable with the residual nickel deposits of Côte d’Ivoire. However, within the lateritic iron crust at Bonga, massive to rubbly ferruginous nodules occur, most of which are highly nickeliferous (>7000 ppm). The lateritic iron crust itself however does not contain more than 300 to 400 ppm Ni. Their macroscopic facies (nodules), mineralogical and chemical compositions suggest that they could represent paleo-gossans. In the nodules, nickel is mainly associated with the poorly crystallized forms (plasma or amorphous phase) in goethite, maghemite and hematite. In the well-crystallized forms, nickel is mainly associated with maghemite and goethite, slightly more with the ferri-chromite, but not with hematite. With separation of goethite from the other lateritic phases (controlling by X-ray diffraction), and in regard to microprobe goethite analysis, a Ni-bearing pre-edge X-ray absorption spectrum of goethite sample shows an octahedral coordination for Ni, and the Möfissbauer experiment gives evidences of Ni incorporation in the structure of goethite.

Microwave heating behaviour of nickeliferous limonitic laterite ores

The nickeliferous laterite ores, in which the nickel occurs in oxide form, represent a significant potential resource of metallic nickel. However, in comparison to the nickel-containing sulfide ores, the extraction costs are relatively high and thus it will be necessary to develop new processing techniques, which are both technically and economically viable. In the present research, the potential application of microwaves for the heating of a nickeliferous limonitic laterite ore ((Fe,Ni)O(OH) · nH2O) was investigated. Firstly, since the nickeliferous limonitic laterite ore contains considerable moisture, both free and combined, then thermogravimetric analysis (TGA) was performed in order to characterize the changes, which result from the dehydration processes. Derivative thermogravimetric analysis (DTGA) curves were calculated from the TGA data. Secondly, the real (ε′) and imaginary (ε″) permittivities of the ore were measured at frequencies of 912 and 2460 MHz at temperatures up to about 1000 °C using the cavity perturbation technique and these results were related to the DTGA curves. Also, the loss tangent (tanδ=ε″/ε′) was calculated from the permittivity data. Finally, the microwave heating behaviour of the nickeliferous limonitic laterite ore was determined at 2460 MHz. The results show that the both the real (ε′) and imaginary permittivities (ε″) and the loss tangent (tanδ) increase with temperature and change as both the free and the combined moisture are removed. The permittivities (ε′ and ε″) increased with increasing slope of the TGA curve and vice versa during the goethite to hematite dehydroxylation reaction, where there was a maximum in the permittivities (ε′ and ε″). It is proposed that these changes, which occur during the dehydroxylation reaction, are a result of the liberation of hydroxyl units from the goethite structure. Furthermore, some hydroxyl units were retained in the hematite structure (i.e. hydrohematite (α-Fe2−x/3O3−xOHx)) even after the goethite to hematite transition and thus the permittivities of the dehydroxylated ore were higher than that of normal hematite. With regards, to their microwave heating behaviour, it is shown that despite the relatively low permittivities (ε′ and ε″) of these materials at low temperatures, they can be heated using microwaves. The microwave heating process can be improved by conventional preheating of the sample or by the addition of charcoal as a susceptor or by selecting a crucible material, which acts as a susceptor. There is a rapid increase in the permittivities over the temperature range of about 600–800 °C and this combined with the low thermal conductivities of these oxides, can result in rapid internal heating of the sample and thermal runaway.

Microwave heating behaviour of nickeliferous limonitic laterite ores

The nickeliferous laterite ores, in which the nickel occurs in oxide form, represent a significant potential resource of metallic nickel. However, in comparison to the nickel-containing sulfide ores, the extraction costs are relatively high and thus it will be necessary to develop new processing techniques, which are both technically and economically viable. In the present research, the potential application of microwaves for the heating of a nickeliferous limonitic laterite ore ((Fe,Ni)O(OH) · nH 2O) was investigated. Firstly, since the nickeliferous limonitic laterite ore contains considerable moisture, both free and combined, then thermogravimetric analysis (TGA) was performed in order to characterize the changes, which result from the dehydration processes. Derivative thermogravimetric analysis (DTGA) curves were calculated from the TGA data. Secondly, the real ( ε ′) and imaginary ( ε ″) permittivities of the ore were measured at frequencies of 912 and 2460 MHz at temperatures up to about 1000 °C using the cavity perturbation technique and these results were related to the DTGA curves. Also, the loss tangent (tan δ= ε ″/ ε ′) was calculated from the permittivity data. Finally, the microwave heating behaviour of the nickeliferous limonitic laterite ore was determined at 2460 MHz. The results show that the both the real ( ε ′) and imaginary permittivities ( ε ″) and the loss tangent (tan δ) increase with temperature and change as both the free and the combined moisture are removed. The permittivities ( ε ′ and ε ″) increased with increasing slope of the TGA curve and vice versa during the goethite to hematite dehydroxylation reaction, where there was a maximum in the permittivities ( ε ′ and ε ″). It is proposed that these changes, which occur during the dehydroxylation reaction, are a result of the liberation of hydroxyl units from the goethite structure. Furthermore, some hydroxyl units were retained in the hematite structure (i.e. hydrohematite (α-Fe 2− x/3 O 3− x OH x )) even after the goethite to hematite transition and thus the permittivities of the dehydroxylated ore were higher than that of normal hematite. With regards, to their microwave heating behaviour, it is shown that despite the relatively low permittivities ( ε ′ and ε ″) of these materials at low temperatures, they can be heated using microwaves. The microwave heating process can be improved by conventional preheating of the sample or by the addition of charcoal as a susceptor or by selecting a crucible material, which acts as a susceptor. There is a rapid increase in the permittivities over the temperature range of about 600–800 °C and this combined with the low thermal conductivities of these oxides, can result in rapid internal heating of the sample and thermal runaway.

Acid and thermal treatments of lateritic bauxites

H2SO4 and HCl processes accompanied by thermal treatments at different stages are described for the industrial extraction of alumina from lateritic clay minerals, especially from kaolinite, an important component of lateritic bauxites. The effects of calcination at high temperatures on the pretreatments of the raw lateritic materials, as well as on the thermal transformations of the products (salts) resulting from the acid treatments of lateritic bauxites are analyzed. Beside energetic considerations, separation methods of Al from Fe and from other lateritic-metallic components are emphasized in the extraction and purification processes. The mechanism of a controlled HCl-extracting treatment performed on iron-bearing kaolinite is described, as well as its characterization by IR and DTA ad hoc methods, enabling to distinguish between Al and Fe in the structure of lateritic kaolinite. The location and quantification of Al in the structure of lateritic goethite is also considered.

Variation of hydrogen bonded O…O distances in goethite at high pressure

In-situ synchrotron X-ray powder diffraction experiments on goethite were performed up to 24.5 GPa using a diamond-anvil cell at BL-18C in the Photon Factory, Japan. The compression behavior of goethite is anisotropic: the a axis, which is close to the direction of the hydrogen bond, is almost twice as compressible as the b and c axes. A second-order Birch-Murnaghan fit (K' = 4) to the unitcell volumes and pressures up to 24 GPa gave a bulk modulus K0 = 111(2) GPa. The pressure medium apparently affects the compression behavior of goethite, as a sudden strain broadening occurs above 11 GPa, where solidification of the 4:1 methanol:ethanol pressure medium occurs. The crystal structure of goethite was refined by means of the Rietveld method at several pressures below 10 GPa. The main compression occurs in the “vacant channels” in the crystal structure, where the OH ···O bridges are located. The hydrogen-bonded O···O distance shortens with increasing pressure at the rate of about -0.023 Å/GPa and reaches 2.60(1) Å at 9 GPa.

Cd-Substituted Goethites — A Structural Investigation by Synchrotron X-ray Diffraction

AbstractThe structural and physical effects of partially substituting Cd for Fe in goethite have been investigated. The solubility of Cd2+ in goethite is ∼10 mol.%, i.e. Fe0.905Cd0.095OOH. The structures of the substituted goethites have been refined, using the Rietveld method, from synchrotron X-ray powder diffraction data. There is a progressive increase in the size of the unit-cell parameters and unit-cell volume, upon the incorporation of much larger Cd2+ ion (0.95 Å) compared with Fe3+ (0.645 Å) in the goethite structure, together with a reduction in crystallinity. Transmission electron microscopy measurements confirm the crystallite size decreases as the Cd2+ content increases in goethite structure.

Structural study of a series of synthetic goethites obtained in aqueous solutions containing cadmium(II) ions

The capacity of goethite for CdIIsubstitution has been explored in a series of synthetic samples prepared from FeIIIand CdIInitrate solutions aged 21 days in alkaline media. The total metal content ([Fe]+[Cd]) was 0.071 M in all preparations. The samples have been characterized by chemical and X-ray diffraction analysis; the morphology of the solids is described. The cell parameters for all samples were obtained by the Rietveld fits to the X-ray diffraction data. Refined structures show that for samples prepared at the final molar ratio μCd≤5.50 (expressed as μCd=100×[Cd]/[Cd]+[Fe]), a (Cd, Fe)-goethite is the only crystalline product. In these samples, the unit cell parameters increased as a function of Cd concentration, indicating Cd incorporation in the structural frame. At the preparative ratio, μCd=7.03, the incorporation of Cd in the goethite structure is drastically reduced and a probable Cd-substituted hematite is formed together with the Fe,Cd-goethite.

Incorporation of Cr, Mn and Ni into goethite (α-FeOOH): mechanism from extended X-ray absorption fine structure spectroscopy

AbstractThe crystal-chemical mechanisms by which transition metals are associated with goethite are fundamental to our understanding of the solubility and bioavailability of micronutrients and heavy metals in soils, and in the formation of laterite ore deposits. Transition metals such as Cr, Mn and Ni may sorb onto goethite by forming surface precipitates, surface complexes or by replacing Fe3+ in the goethite structure. In the work reported here, we investigated the local coordination environment of Cr, Mn and Ni in synthetic goethite using EXAFS spectroscopy. We demonstrate the isomorphous substitution for Fe3+ by Cr3+ (up to 8 mol.%), Mn3+ (up to 15 mol.%) and Ni2+ (up to 5 mol.%). We find, however, that the next-nearest-neighbour coordination environment changes with composition. The perturbations are likely to be responsible for limiting the accommodation of Cr3+, Mn3+ or Ni2+ in the FeOOH structure.

Incorporation of Cr, Mn and Ni into goethite (α-FeOOH): mechanism from extended X-ray absorption fine structure spectroscopy

The crystal-chemical mechanisms by which transition metals are associated with goethite are fundamental to our understanding of the solubility and bioavailability of micronutrients and heavy metals in soils, and in the formation of laterite ore deposits. Transition metals such as Cr, Mn and Ni may sorb onto goethite by forming surface precipitates, surface complexes or by replacing Fe3+ in the goethite structure. In the work reported here, we investigated the local coordination environment of Cr, Mn and Ni in synthetic goethite using EXAFS spectroscopy. We demonstrate the isomorphous substitution for Fe3+ by Cr3+ (up to 8 mol.%), Mn3+ (up to 15 mol.%) and Ni2+ (up to 5 mol.%). We find, however, that the next-nearest-neighbour coordination environment changes with composition. The perturbations are likely to be responsible for limiting the accommodation of Cr3+, Mn3+ or Ni2+ in the FeOOH structure.