The incorporation of transition metals into hematite may limit the aqueous concentration and bioavailabity of several important nutrients and toxic heavy metals. Before predicting how hematite controls metal-cation solubility, we must understand the mechanisms by which metal cations are incorporated into hematite. Thus, we have studied the mechanism for Ni2+ and Mn3+ uptake into hematite using extended X-ray absorption fine structures (EXAFS) spectroscopy. EXAFS measurements show that the coordination environment of Ni2+ in hematite corresponds to that resulting from Ni2+ replacing Fe3+. No evidence for NiO or Ni(OH)2 was found. The infrared spectrum of Ni-substituted hematite shows an OH-stretch band at 3168 cm−1 and Fe-OH bending modes at 892 and 796 cm−1. These vibrational bands are similar to those found in goethite. The results suggest that the substitution of Ni2+ for Fe3+ is coupled with the protonation of one of the hematite oxygen atoms to maintain charge balance.

The solubility of Mn3+ in hematite is much less extensive than that of Ni2+ because of the strong Jahn-Teller distortion of Mn3+ in six-fold coordination. Structural evidence of Mn3+ substituting for Fe3+ in hematite was found for a composition of 3.3 mole % Mn2O3. However a sample with nominally 6.6 mole % Mn2O3 was found to consist of two phases: hematite and ramsdellite (MnO2). The results indicate that for cations, such as Mn3+ showing a strong Jahn-Teller effect, there is limited substitution in hematite.

The 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.

Deep sediments from the Red Sea have been studied extensively and provide a rich resource for understanding mineral transformations under hydrothermal conditions. Interrelationships among various sampling sites, however, are still rather incomplete. The purpose of the present study was to increase understanding of these systems by characterizing and comparing the Fe-Mn oxyhydroxides from the southern Atlantis II, Chain A, Chain B, and Discovery Deeps, using high-resolution transmission electron microscopy. Some of the hydrothermal sediments of Chain A are dominated by Si-associated Fe oxides (ferrihydrite, goethite, lepidocrocite, and short-range ordered, rounded particles) resembling the hydrothermal sediments of the SW basin in the Atlantis II Deep, indicating sub-bottom connections between the Deeps. Although some of the sediments of the Discovery Deep show a similar trend; short-range ordered, rounded particles were not detected in these sediments, implying that crystallization of this short-range ordered phase is sensitive to the Si/Fe ratio in the brine and only at elevated ratios does it crystallize out of the brine. Silicon-associated and Fe-enriched Mn oxyhydroxides such as groutite, manganite, todorokite, and Mn-dominated lathlike layers occasionally contain Ca and Mg impurities. Manganese substitutes for Fe and vice versa, leading to a solid-solution series between goethite and groutite and Mn-enriched ferrihydrite. Hematite is the only Fe oxide in the hydrothermal sediments that is found to be lacking in impurities, which is probably due to its formation by recrystallization from other Fe oxides.

Much attention has been paid to the adsorption of Fe(II) onto mineral surfaces as it is a crucial step in enhancing the reductive activity of Fe(II) species. The present study elucidates the role of Fe(II) adsorbed on Fe (oxyhydr)oxides (γ-FeOOH, α-FeOOH, and α-Fe2O3) for the reductive transformation of 2-nitrophenol (2-NP), using cyclic voltammetry (CV). Studies of Fe(II) adsorption and 2-NP reduction kinetics showed that an increase in pH gave rise to an elevated density of adsorbed Fe(II) on mineral surfaces, which further resulted in an enhanced reaction rate of 2-NP reduction. In addition, CV tests showed that the enhanced activity of Fe(II) species is attributed to the negative shift of peak oxidation potential (EP) of the Fe(III)/Fe(II) couple. The dependence of adsorbed Fe(II) reactivity on pH values was proven by the three linear correlations obtained (ln k vs. pH, EPvs. pH, and ln k vs. EP). The present study demonstrated that the reductive activity of adsorbed Fe(II) species can be indicated by the EP value of active Fe(II) species. Moreover, the electrochemical approach can be used as an effective tool to study the reductive activity of adsorbed Fe(II) species in subsurface environments.

The As sorption capacity of a natural Mn and Fe mineral-containing sample from the Iron Quadrangle province, Brazil, was investigated. A detailed mineralogical identification was obtained by combining X-ray diffraction analyses (with Rietveld refinement), X-ray fluorescence spectroscopy, optical microscopy, and scanning electron microscopy coupled with X-ray energy dispersive spectrometry-EDS. The oxidation state of the adsorbed As species was determined by X-ray absorption near edge structure spectroscopy. The results demonstrate that the presence of naturally occurring Mn oxides promotes the effective oxidation of As (III) to As(V). Also, the Mn minerals show a significant uptake of both the trivalent and pentavalent As species. This study demonstrates that the combined influences of As(III) depletion by oxidation and adsorption on a natural oxide sample consisting of Mn minerals and Fe oxides may effectively contribute to the reduction the As concentration in waters.

Maghemite (γ-Fe2O3) is a ferrimagnetic Fe oxide commonly found in tropical and subtropical soils, especially in the topsoil where it is usually a product of burning. Isomorphic substitution (IS) of the Fe in maghemite by different metals (mainly Al3+) can modify its mineralogical and chemical attributes, and these modifications may be important to understanding the formation and properties of this mineral in soils and sediments. The objective of this work was to evaluate the crystallochemical alterations of synthetic, Al-substituted maghemites prepared by the precipitation of magnetites from alkaline aqueous media containing FeSO4·7H2O with increasing amounts of Al2(SO4)3·7H2O to obtain hypothetical Al3+ for Fe3+ substitutions ranging from 0.0 to 40.0 mol %. The Al-substituted magnetites were washed and dried, and then heated to 250ºC for 4 h to form yellowish red maghemites that were characterized by total chemical analysis, X-ray diffraction, specific surface area (SSA), mass-specific magnetic susceptibility, infrared spectroscopy, transmission electronic microscopy, and color. Increasing Al3+ substitution to an experimental maximum of 15.9 mol % decreased both the a0 dimension of the cubic unit cell (a0 = 0.8339 − 396.157 × 10−16 Al, r2 = 0.99) and the mean crystallite dimension (MCD = 76.4–3.15Al, r2 = 0.79) of the maghemites. With the decrease in MCD came a more yellowish color, an increase in SSA, and a decrease in crystallinity as measured through extraction of the samples with acid ammonium oxalate. The mass-specific magnetic susceptibility of the maghemites increased with Al3+ substitution up to 5.3 mol % and then decreased with further replacement of Fe by Al. Solid-phase aluminum in excess of 16 mol % substitution appeared to occur as a separate, poorly crystalline phase that was X-ray amorphous.

The formation of hard hematite in steam generators with relatively high levels (5–10 µg/L) of dissolved oxygen at temperatures around 280–290°C and pressures around 6–8 MPa can serve as an analog for the formation of hard hematite in sedimentary processes. Furthermore, in steam generators, as well as in nature, hematite is an effective cementing agent, capable of incorporating as much as twice its own weight of other solids to form a hard composite material. Laboratory simulations showed ferrihydrite to be the likely starting material for the formation of hard, dense hematite at temperatures much lower than those required for sintering of anhydrous hematite. These laboratory simulations, performed at temperatures around 260°C and pressures of ∼500 MPa, resulted in the formation of hard hematite or hematite-based composite solids over periods of 3–5 h, compared with several months in steam generators and many years in nature. The amount of water present during the synthesis (10–15% of the weight of dry ferrihydrite) and the gradual removal of water proved to be key parameters in the formation of hard, dense hematite. The mechanism, studied by means of X-ray diffractometry, Mössbauer spectroscopy and infrared spectroscopy, appeared to involve build-up, then gradual condensation of OH bridges, leading to the conversion of ferrihydrite to hydrohematite with approximately 4–5% of residual water. The presence of other solids, such as copper and its oxides, alumina and silica, in large quantities, resulted in smaller grain size of the hydrohematite product but did not affect its mechanical properties. On the other hand, the use of hydrazine to provide a reducing environment produced goethite during the precursor synthesis stage and soft magnetite during the pressing stage. However, whenever hematite was produced, it could not be subsequently reduced to magnetite by hydrazine under the reaction conditions specified above. The mechanical properties as well as the spectroscopic characteristics of the product of pressing agreed with observations on sedimentary hematite-cemented rocks.

A long-standing unresolved puzzle related to the Danish temperate humid climate is the presence of extended areas with large Fe contents, where goethite and ferrihydrite are present in the topsoil along with hematite and maghemite. Hematite and, particularly, maghemite would normally be interpreted as the result of high temperature as found after forest fires. However, a body of evidence argues against these sites having been exposed to fire. In an attempt to get closer to an explanation of this Fe mineralogy, an experimental forest fire was produced. The results showed a clear mineralogical zonation down to 10 cm depth. This was not observed at the natural sites, which contained a mixture of goethite/ferrihydrite, hematite and maghemite down to 20 cm depth. The experimental forest fire left charcoal and ashes at the topsoil, produced high pH and decreased organic matter content, all of which is in contrast to the natural sites. The conclusion from this work is that the mineralogy of these sites is not consistent with exposure to forest fire, but may instead result from long-term transformation in a reducing environment, possibly involving microbiology.

Mössbauer spectroscopy is the technique of recoil-free resonant emission and absorption of gamma rays. It has the advantage of being oblivious to all elements except the one under survey. In the case of materials formed on the earth’s surface, such as soils and clays, the only propitious element is Fe. Iron is the fourth most abundant element in the Earth’s crust, it is essential for life, and almost all environmental materials contain at least some Fe. It is also fortuitous that 57Fe Mössbauer spectroscopy is among the most straightforward to operate. 57Fe Mössbauer spectroscopy thus allows the characterization of iron speciation, and thereby of environmental conditions, over a wide range of concentrations, making it an extremely effective environmental probe.

Straightforward as it may seem, Mössbauer spectroscopy nevertheless has many pitfalls. Besides problems arising from the basic physics, complications can arise among other causes from imperfect crystallinity (small particle size), non-stoichiometry, interparticle effects and isomorphous substitutions.

In this paper a succinct review of the basic principles of Mössbauer spectroscopy is presented, followed by examples of Mössbauer spectra of minerals that are common constituents of clays and soils, and by more complex cases of soils, clays and fired clays.

Soil solution chemistry, especially pH and the presence of multivalent ions, affects the surface charge (SC) of Fe oxides and accordingly colloidal stability and sorption properties. The SC of synthetic goethite and hematite was quantified in the presence of different electrolytes (NaCl, CaCl2, Na2SO4 and CaSO4) by combining the streaming potential with polyelectrolyte titration. The point of zero charge (PZC) for goethite was observed at pH 8.2 and the stability field around the PZC, where colloids are flocculated, is more extended (±1 pH unit) than that of hematite with a PZC at pH 7.1 (±0.5 pH unit). The SC decreases with increasing SO4 concentration, indicating adsorption of SO4 on the oxide, whereas the presence of Ca increases the SC. At pH 4, the addition of 0.1 mmol l–1 Na2SO4 induced a decrease in SC from 1.5 to 0.380 μmolc m–2 for goethite and from 0.85 to 0.42 μmolc m–2 for hematite. In a suspension with 0.1 mmol l–1 Na2SO4, the number of colloids is already reduced, and both oxides flocculate rapidly and completely at >0.5 mmol l–1 Na2SO4. While the addition of SO4 did not affect charge titrations with the cationic polyelectrolyte, the anionic polyelectrolyte formed complexes with Ca, resulting in an overestimation of positive SC. The electrolyte CaSO4 is most efficient at keeping goethite and hematite in the pH range 4–10 in the flocculated state. Besides pH, the presence of multivalent ions should also be considered when predicting colloid mediated transport and adsorption properties of anionic substances by Fe oxides in soil systems.

The clay mineralogy and chemical composition of the white veins, red matrix and both Fe- and Mn-bearing nodules occurring in a laterite profile in Hubei, south China were investigated using X-ray diffraction, scanning electron microscopy equipped with an energy-dispersive spectrometer, and high-resolution transmission electron microscopy. The results show that the mineral components of the red matrix are mainly quartz, kaolinite, halloysite, goethite and minor illite, whereas the white net-like veins contain mostly quartz, kaolinite, halloysite, and illite. In the net-like horizon, the chemical index of alteration (CIA, the ratio of Al2O3/(Al2O3+CaO+K2O+Na2O)) and the TiO2/Al2O3 ratio are 89.8% and 0.021 for the white vein and 90.7% and 0.025 for the red matrix, respectively. Both white-vein and red-matrix components have similar TiO2/Al2O3 ratios, and are similar to the ratio 0.027 of the unaltered bedrock. The similarity in TiO2/Al2O3 values indicates that all three portions of the laterite soil share the same origin. Also, although the white-vein and red-matrix components differ in Fe2O3 abundance, the similar CIA values do imply similar degrees of alteration. The Fe-bearing and Mn-bearing nodules were produced by the local accumulation of Fe2O3 and MnO, respectively. Halloysite in the weathering profile occurs in two different morphologies, tubular and platy crystals. Tubular halloysite occurs both in the red matrix and the Fe-bearing nodule whereas platy halloysite occurs only in the white vein and Mn-bearing nodule assemblages. Crystallization of small tubular halloysite from Si and Al concretions in the red matrix is observed, indicating that the morphology of these crystals in the weathering environment is mainly controlled by Fe3+ cations, whereas platy halloysite may be derived from the hydration of kaolinite.

Vivianite [Fe3(PO4)2·8H2O] is easily oxidized in the presence of air. In this work, we studied the oxidation and incongruent dissolution of vivianite in a calcareous medium containing an anion-exchange resin (AER) that acted as a sink for phosphate. Freshly prepared vivianite suspensions with calcite sand and an AER membrane were oxidized and stirred by bubbling air or CO2-free O2. Experiments were finished when oxidation rate and P removal by the AER became slow, which was at 53 days (air system) or 28 days (CO2free O2 system). At these times, the respective values of the Fe(II)/total Fe ratio were 0.29 and 0.12, and the respective values of the atomic P/Fe ratio were 0.15 and 0.11. The final product of the oxidation was poorly crystalline lepidocrocite in the form of thin (1 ­ 5 nm), irregular lamellae that were soluble in acid oxalate. The unit-cell edge lengths of this lepidocrocite were a= 0.3117, b= 1.2572, and c= 0.3870, vs. a= 0.3071, b= 1.2520 and c= 0.3873 nm for the reference lepidocrocite. The lepidocrocite lamellae contained occluded P (P/Fe atomic ratio = 0.03 ­ 0.04). The results of this and a previous study made us hypothesize that this occluded P is structural and occupies tetrahedral sites adjacent to the empty octahedral sites in the sheets extending on ﹛010﹜.

Alternative Fenton and Fenton-like systems based on natural Fe oxides are described. The collected materials were modified through controlled reduction with H2 and were characterized by chemical analysis, X-ray diffraction, saturation magnetization measurements, and Mössbauer spectroscopy at 298 and 110 K. The catalytic activities of these original and modified materials were tested by studying the decomposition of H2O2 and the discolouration of methylene blue. Iron oxides present in the samples were mainly hematite and subordinate goethite which, after controlled reduction, were converted to metallic iron and magnetite. The mixture of Fe0 and magnetite in one of these materials was significantly more efficient at H2O2 decomposition and the discolouration of methylene blue than the original Fe3+ oxides. These results suggest that Fe2+ is essential to produce an active Fenton system.

Air pollution control (APC) facilities at waste incinerator plants produce large quantities of solid residues rich in salts and heavy metals. Heavy metals are readily released to water from the residues and it has, therefore, been found suitable to apply a rapid co-precipitation/adsorption process as a means to immobilize the toxic elements. In the ‘Ferrox process’, this immobilization is based on co-precipitation with an Fe(III) oxide formed by oxidation of Fe(II) by air in an aqueous slurry with the APC residue at alkaline pH. In this work we have undertaken a Mössbauer spectroscopy study of the Fe oxide phase formed by precipitation at room temperature and of the oxides present after heating to 600 and 900°C. The only Fe oxide observed in the Ferrox product at room temperature is a very poorly crystalline ferrihydrite. Analytical transmission electron microscopy showed that the main elements associated with the ferrihydrite are Si and Ca. Following heating to 600°C the oxide is still characterized as an amorphous Fe oxide, and it is probable that Si associated with the ferrihydrite is decisive in preventing crystallization. After the 900°C treatment a transformation into defect maghemite is observed. Reducing gases produced from carbon in the samples probably induces this transformation. It eases, thus, the reduction of Fe(III) and the consequent formation of magnetite that eventually oxidizes to maghemite during cooling in air.