Secondary 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.

The transformation of schwertmannite to goethite was studied by ageing one synthetic and five natural schwertmannites in water at room temperature. Additionally, one synthetic and two natural schwertmannites were kept at variable pH (2, 4, 6 and 8). After one year, only the synthetic sample and one natural schwertmannite had transformed to goethite. However, the oxalate solubility of Fe and trace elements in all the samples decreased, whereas the total Fe/S ratios and specific surface areas of all samples increased. Arsenic and organic matter appeared to suppress the schwertmannite-to-goethite phase transformation. At pH 2, synthetic schwertmannite fully-transformed to goethite, but at pH 4–6 only minor transformation occurred. Depending on pH, many trace elements were released into solution during ageing of the natural schwertmannites. In general, Co, Mn, Zn and Si were released to solution, whereas As was enriched in the remaining iron oxide fraction. Al was dissolved at pH <4.

Mine-drainage waters and associated precipitates from several active and abandoned lignite mines and mine dumps in the Sokolov Basin, northwestern Czech Republic, were sampled and analysed. The data showed considerable variations of effluent composition and pH that can generally be related to differences in the local microenvironments. Temporal changes such as seasonal fluctuations of precipitation, leading to variations of water infiltration through and runoff over mines and mine dumps, were also observed to noticeably affect effluent composition. These variations have led to a wide range of precipitates, the principal constituents of which are generally one or several of the ferric minerals jarosite, schwertmannite, goethite, ferrihydrite and lepidocrocite.

The present paper consists of two parts: a short review of the genesis and properties of the named minerals, and a study of mine-drainage precipitates formed under different local conditions in various lignite mines and mine dumps of the Sokolov mining district. We show that variations in mine-drainage precipitate mineralogy, such as the presence or absence of specific ‘key’ minerals, can serve as indicators for factors such as the pH and sulphate concentration, and thus — with certain limitations — for the genetic environment during precipitate formation. Such variations in composition are reflected, among other properties, in the precipitate colour, which can therefore be used for a rapid identification and classification — both in the field and by remote sensing — of regions that are potentially prone to acid mine drainage.

At 25°C, synthetic schwertmannite, a common iron compound in acid mine drainage, completely converted in water to a sulphate-containing goethite within ~100 days. The rate of transformation doubled as the pH increased from 4.0 to 7.2. In deviation from its normal acicular habit, the goethite appeared as spherical aggregates.

Schwertmannite is a new oxyhydroxysulphate of iron from the Pyhäisalmi 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 HCl 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 Fe2O3, 62.6; SO3, 12.7; CO2, 1.5; H2O−, 10.2; H2O+, 12.9; total 99.9 wt.%. These data yield an empirical unit cell formula of Fe16O16(OH)9.6(SO4)3.2·10H2O after exclusion of CO2 and H2O−. The most general simplified formula is Fe16O16(OH)y(SO4)z·nH2O, where 16 − y = 2z and 2.0 ⩽ z ⩽ 3.5. Schwertmannite has a structure akin to that of akaganéite (nominally β-FeOOH) with a doubled c dimension. Its X-ray powder diffraction pattern consists of eight broad peaks [dobs in (Iobs) (hkl)] 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); and 1.46(18)(204,542), giving a = 10.66(4), c = 6.04(1) Å, and V = 686(6) Å3 for a primitive, tetragonal unit cell. The probable space group is P4/m. Upon heating, schwertmannite transforms to hematite with Fe2(SO4)3 occurring as an intermediate phase. Bidentate bridging complexes between Fe and SO4 are apparent in infrared spectra. Mössbauer data show the Fe in schwertmannite to be exclusively trivalent and in octahedral coordination; it has a Néel temperature of 75 ± 5 K and a saturation magnetic hyperfine field of about 45.6 T. Pronounced asymmetry of the Mössbauer 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.

Ferric precipitates in the water column at the San Telmo acidic mine pit lake in the Iberian Pyrite Belt, southwest Spain, have been studied by scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction, X-ray fluorescence, inductively coupled plasma massspectrometry and other complementary techniques. These Fe(III) precipitates were recovered from sediment traps which were left at different depths (25, 35, 40 and 100 m) in the lake for several months. Seasonal variations in the water chemistry were recorded to link the mineralogical findingsto vertical and temporal changes in aqueous composition. The results indicate that schwertmannite is the first Fe(III) mineral to crystallize after the oxidation of Fe(II), in agreement with previous studies. Schwertmannite is kinetically favoured in comparison to other Fe(III) minerals, andit buffers the pH at 2.6–3.0. It is metastable, and alters to a (H3O+)- and (K+)-bearing jarosite (containing 58 mol.% H3O+ and 42 mol.% K+ on average) at lower pH (e.g. at pH 2.2–2.5 in the summer season), eitherin the water column (during settling) and/or in the benthic sediments, in a time period of weeks to months. The extent of hydronium substitution at the alkali site in the jarosite reflects the higher activity of free aqueous protons in solution (10–2.2 to 10–3.0)in comparison to the activities of K+ (10–4.5) and Na+ (10–3.2). Microscopic examination of mixed schwertmannite–jarosite precipitates found in the water column suggest that some textural and compositional features of metastableschwertmannite (e.g. the internal 'pincushion' arrangement and incorporation of trace amounts of Mg, Al, As and Pb) are conserved in the jarosite during the early stages of the mineralogical transformation, but many of these relics are lost in the later stages of crystal growth. Despite thehydronium-rich nature of the jarosite solid solution, this material is also an important sink for K+, which decreases in concentration with decreasing pH unlike most of the other major cations in the water column (notably Na+, Mg2+, Ca2+, Al3+,Fe3+, Cu2+, Zn2+). In addition to the release of Fe3+ to the aqueous solution, the conversion of schwertmannite to (H3O+, K+)-bearing jarosite consumes protons and thus may represent an additional pH control at SanTelmo and other acidic mine pit lakes of the area.

Approaches currently used for remediating acid mine drainage (chiefly active chemical treatment and passive bioremediation systems) have a number of major detractions, including their failure to recover potentially valuable metals from these waters. Bioremediation strategies that utilizereactor-housed microorganisms can circumvent this problem, but have tended not to be widely used due to their relatively high costs. We have devised innovative approaches for remediating mine waters that use acidophilic bacteria to remove metals either as oxidized or reduced phases, usingmodular bioreactors that are designed to operate at minimal cost and to generate products that have commercial value. A composite system is described that combines microbial oxidation of ferrous iron with abiotic precipitation of ferric iron as schwertmannite, a mineral that has commercialvalue as an absorbent of arsenate and other environmental pollutants, and as a pigment. Sulfidogenic bioreactors maintained at acidic pH values are used to selectively precipitate metal sulfides, such as CuS. Tests with synthetic mine drainage containing mixtures of soluble metals confirmedthat these systems can generate relatively pure mineral deposits from complex acid waters. The units are designed to be configured differently, according to the nature of the mine water requiring treatment.

Selective precipitation of schwertmannite was identified in the water column of a chemically stratified acidic lake (pH 2.0 2.5). A set of analytical techniques was used to identify accurately the mineral phase (X-ray diffraction, X-ray fluorescence spectrometry, scanning electron microscopy coupled with energy dispersive spectroscopy). The lake has two clearly differentiated layers: a thin superficial layer that extends to ∼2 m in depth with less dissolved solid content and, therefore, lower density, which contrasts with a thicker lower layer that extends to 34 m in depth. The upper layer is strongly influenced by climatic factors, triggering dilution (rainfall), concentration (dry season) and intense seasonal thermal fluctuations, while the lower layer is more compositionally and thermally stable.

Schwertmannite precipitate was initially observed only in the upper layer, adhered to a plastic buoy and the anchor line. The latter was confirmed through a test involving a precipitation device that consisted of maintaining a series of plastic plates at different depths over a period of several months. Only the plates located in the upper layer became covered with precipitate, whereas the plates that were submerged in the lower layer remained clean. These observations clearly differed from the saturation indices of schwertmannite (SIsch) calculated using PHREEQC and using Bigham et al. (1996) or Kawano and Tomita (2001) solubility products. Schwertmannite would not precipitate in the lake with the former but would precipitate in both layers with the latter. It is the constant provided by Yu et al. (1999) that makes SIsch in both layers approach the observed behaviour: SIsch>0 displays oversaturation in the upper layer while SIsch<0 displays undersaturation in the lower layer. The value of the product of solubility that better adjusts to this situation is log Ksch = 9.5 10. Using the method by Yu et al. (1999) to establish the apparent solubility, a range of log Ksch values was defined between 10.5 and 11.5 for the system studied.