Abstraet~Storage of ferrihydrite in aqueous suspensions at 24~ and pHs between 2.5 and 12 for as long as three years resulted in the formation of goethite and hematite. The proportions and crystallinity of these products varied widely with the pH. Maximum hematite was formed between pH 7 and 8, and maximum goethite at pH 4 and at pH 12. The crystallinity of both products, as indicated by X-ray powder diffraction line broadening and magnetic hyperfine field values and distribution widths, was poorer, the lower the proportion of the corresponding product in the mixture. The existence of two competitive formation processes is suggested: goethite is formed via solution, preferably from monovalent Fe(III) ions [Fe(OH)z § and Fe(OH)4-], and hematite by internal rearrangement and dehydration within the ferrihydrite aggregates. This concept relates the proportions of goethite and hematite to the activity of the Fe(III) ion species in solution, and implies that conditions favorable for the formation of goethite are unfavorable for that of hematite and vice versa.

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Iron is the fourth most common element by mass in the Earth's crust and forms compounds in several oxidation states. Iron (hydr)oxides, some of which form inherently and exclusively in the nanometre-size range, are ubiquitous in nature and readily synthesized. These facts add up to render many Fe (hydr)oxides suitable as catalysts, and it is hardly surprising that numerous studies on the applications of Fe (hydr)oxides in catalysis have been published. Moreover, the abundant availability of a natural Fe source from rocks and soils at minimal cost makes the potential use of these as heterogeneous catalyst attractive.Besides those Fe (hydr)oxides that are inherently nanocrystalline (ferrihydrite, Fe5HO8.4H2O, and feroxyhyte, δ’-FeOOH), magnetite (Fe3O4) is often used as a catalyst because it has a permanent magnetization and contains Fe in both the divalent and trivalent states. Hematite, goethite and lepidocrocite have also been used as catalysts in their pure forms, doped with other cations, and as composites with carbon, alumina and zeolites among others.In this review we report on the use of synthetic and natural Fe (hydr)oxides as catalysts in environmental remediation procedures using an advanced oxidation process, more specifically the Fenton-like system, which is highly efficient in generating reactive species such as hydroxyl radicals, even at room temperature and under atmospheric pressure. The catalytic efficiency of Fe (hydr)oxides is strongly affected by factors such as the Fe oxidation state, surface area, isomorphic substitution of Fe by other cations, pH and temperature.

Schwertmannlte is a new oxyhydroxysulphate of iron from the Pyh~isalmi sulphide mine, Province of Oulu, Finland. It occurs there, and elsewhere, as an ochreous precipitate from acid, sulphate-rich waters. Associated minerals at other localities may include jarosite, natrojarosite, goethite and ferrihydrite. Schwertmannite is a poorly crystalline, yellowish brown mineral with a fibrous morphology under the electron microscope. A high specific surface area in the range of 100 to 200 m2/g, rapid dissolution in cold, 5 M HCI or in ammonium oxalate at pH 3, and pronounced X-ray diffraction line broadening are consistent with its poorly crystalline character.Colour parameters for the type specimen as related to CIE illuminant C are L ~ = 53.85, a* = + 15.93, and b ~ = +47.96. Chemical analysis gives FezO3, 62.6; SO3, 12.7; CO2, 1.5; HzO-, 10.2; H20 +, 12.9; total 99.9 wt.%. These data yield an empirical unit cell formula of FelsOIs(OH)9.6(SO4)3.2.10H20 after exclusion of CO2 and HzO-. The most general simplified formula is Fe16O16(OH)y(SO4)z.nH20, where 16 -y = 2z and 2.0 ~< z ~< 3.5. Schwertmannite has a structure akin to that of akaganrite (nominally 13-FeOOH) with. a doubled c dimension. Its X-ray powder diffraction pattern consists of eight broad peaks [dobs in A(Iobs) (hk/)] 4.86(37)(200,111); 3.39(46)(310); 2.55 (100)(212); 2.28(23)(302); 1.95(12)(412); 1.66(21)(522); 1.51(24)(004) M6ssbauer data show the Fe in schwertmannite to be exclusively trivalent and in octahedral coordination; it has a Nrel temperature of 75 __+ 5 K and a saturation magnetic hyperfine field of about 45.6 T. Pronounced asymmetry of the M6ssbauer spectra indicates different locations for Fe atoms relative to SO4 groups in the structure. The name is for Udo Schwertmann, professor of soil science at the Technical University of Munich.

Ferrihydrite is a poorly crystalline, natural Fe3+ oxide which occurs in ochreous spring precipitates and hydromorphic soils of humid temperate climates. The identification of ferrihydrite in soils is complicated by its association with goethite, quartz, and layer silicates.The following criteria were used to identify ferrihydrite in Fe‐oxide accumulations from soils: high solubility in acid oxalate, five to six broad x‐ray diffraction lines, and the existence of a typical magnetic hyperfine field distribution at 4K in Mössbauer spectra rather than a discrete field value. Identification of low concentrations of ferrihydrite (≲ 20% oxalate‐soluble Fe) by x‐ray diffraction was made possible by subtracting diffraction data obtained after oxalate treatment from data obtained before such a treatment (differential XRD). Oxalate treatment preferentially dissolves ferrihydrite over goethite. This led to an increase in the quadrupole splitting observed in Mössbauer spectra from ∼0.10 to ∼0.23 mm s−1, resulted in a significantly narrower field distribution, and intensified the goethite DTA peak.

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Both aluminium substitution and poor crystallinity reduce the magnetic hyperfine field of goethite. M6ssbauer spectra taken at 4.2 K show that the effect of poor crystallinity is similar to that of AI substitution, i.e. it reduces the saturation hyperfine field. A multiple correlation was found to exist between the magnetic hyperfine field at 4.2 K as a dependent variable vs AI substitution and crystallinity as independent variables. If a hyperfine field is to be interpreted with respect to either A1 substitution or crystallinity, it is therefore necessary to have knowledge of the other variable.