Schwertmannite

This compound can be synthesized in a simple inorganic manner from a sulfate- containing Fe solution (see below). If an acid FeS04 solution is used, oxidation can only be carried out in a controlled bioreactor using the bacterium Thiobacillus ferooxidans (Plate VIII Bigham et al., 1990). 

Preheat 2 L of distilled water to 60 °C in an oven, quickly add 10.8 g Feds 6 H2O and 3 g of Na2S04 (1000 mg SO4/L) and heat for a further 12 min at 60 °C. After cooling at room temperature, dialyse the suspension for a period of 30 days and finally freeze-dry the solid. 

The method produces ca. 2 g of a light ochreous powder which has an XRD pattern with ca. 8 broadened lines (Fig. 14-1) and a surface area of ca. 250 m /g. The SEM photo shows hedge-hog type crystals (Fig. 14-1) and TEM indicates that the spicules are only a few nm across (Fig. 14-2). The IR-spectrum shows intense bands of 804 . The Mossbauer spectra taken at various temperatures (Fig. 14-3) show the gradual development of a sextet, i, e. magnetic ordering at between 60 and 4.2 K. 

Schwertmannite is stable with respect to ferrihydrite in acid, sulfate-rich waters but metastable with respect to goethite. In water, schwertmannite transforms to goethite by a hydrolytic dissolution reaction  

As seen from this equation the pH drops and counteracts the transformation. If the pH is raised towards neutral, as in neutralization ponds, ferrihydrite will form. In an air-dry state schwertmannite can be stored unchanged for years. 

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Carlson, L., Bigham, J.M., Schwertmann, U., Kyek, A., Wagner, F. 2002. Scavenging of As from acid mine drainage by schwertmannite and ferrihydrite a... 

KEYWORDS Acid mine drainage (AMD),chemical instability, ochre-precipitates, schwertmannite, goethite. 

AMD are often very heterogeneous and reactive systems. Therefore, to describe their properties is often an issue of analytical concern as it is difficult to obtain representative samples of the affected watercourses. The instability of AMD is related to the formation and transformations of ochre-products, such as jarosite, schwertmannite and goethite (Bigham et al. 1996). Mineralogical transformations involving these ochre-precipitates have been the subject of intensive research (Kawano Tomita 2001 Kim et al. 2002 Knorr Blodau... 

Kawano Tomita 2001) Reactor C -using the synthetic solution, but inoculated with two small fragments of natural schwertmannite (0.05 g). 

After 45 days of experiment, the precipitate showed an XRD pattern typical of a poorly ordered mineral, such as schwertmannite. Identification of this AMD mineral was confirmed by its distinctive spike morphology, observed by SEM-SE (Fig. 3). After 240 days, XRD detected the resolved reflection at d spacing of 4.18 A, which is characteristic of goethite. Therefore, the maintenance of the precipitate in contact with the solution enabled its evolution to a more crystalline state. The trend of decrease in pH that is observed by the end of the experiment reflects the production of acidity, associated with the transformation of schwertmannite into goethite as it was observed by Bigham etal. (1996) (eq. 1). 

It is expected that schwertmannite would be stable at this low temperature (Kumpulainen et al. 2008). Furthermore, Jonsson et al. (2005) referred to periods of time of > 514 or even several years to promote transformation at pH 3.0. This experiment confirms that mineralogical evolution may be highly variable in AMD systems and still requires further research. 

After 45 days, XRD indicated the formation of schwertmannite in reactors A and C. There were no significant chemical changes in the reactor B. 

These experiments suggest that the addition of schwertmannite to the... 

With time, schwertmannite progressed to a more crystalline mineral (low crystalline goethite), even under unfavourable kinetic conditions (T<4°C) ... 

Addition of natural schwertmannite to a synthetic AMD solution induced similar changes to that observed in the natural... 

Schwertmannite and the chemical modeling of iron in acid sulfate waters. Geochimica et Cosmochimica Acta, 60, 2111-2121. Jonsson, J. Persson, P., Sjoberg, S., Lovgren, L. 2005. Schwertmannite precipitated from acid mine drainage phase transformation, sulphate release and surface properties. Applied Geochemistry, 20, 179-191. 

Kawano, M. Tomota, K. 2001. Geochemical modeling of bacterially induced mineralization of schwertmannite and jarosite in sulfuric acid spring water. American Mineralogist, 86, 1156-1165 Kim, J. Kim, S. Tazaki, K. 2002. 

Mineralogical characterization of microbial ferrihydrite and schwertmannite and non-biogenic Al-sulfate precipitates from acid mine drainage in the Donghae mine area, Korea. Environmental Geology, 42, 19-31. Knorr, K. Blodau, C. 2007. Controls on schwertmannite transformation rates and products. Applied Geochemistry, 22, 2006-2015. 

Jrs jarosite, Kank4cankite, Lpc=lepidocrocite, Fhar iharmacosiderite, Rgr=realgar, Schw=schwertmannite, TooeMooeleite, Yuk=yukonite... 

L. Carlson, J.M. Bigham, U. Schwertmann, A. Kyek, F. Wagner (2002) Scavenging of As from Acid mine Drainage by Schwertmannite and Ferrihydrite A comparison with synthetic analogues.- Environm. Sci. Technol., preprint... 

Mineral name Goethite Lepidocrocite Akaganeite Schwertmannite Feroxyhyte... 

Schwertmannite, Fei60is(0H)y(S04)z nHyO, has the same basic structure as akaganeite, but contains sulphate instead of chloride ions. This recently recognized mineral frequently occurs in nature as an oxidation product of pyrite and can be... 

Almost all the iron oxides, hydroxides and oxide hydroxides are crystalline. The degree of structural order and the crystal size are, however, variable and depend on the conditions under which the crystals were formed. All Fe oxides display a range of crystallinities except for ferrihydrite and schwertmannite which are poorly crystalline. 

In addition to sulphate, selenate (J. M. Bigham, unpubl.) and chromate (S. Regen-spurg unpubl.) can also be incorporated in the tunnels of synthetic schwertmannite. Whether or not two different Se-O distances (based on EXAFS) attributable to surface and tunnel selenate, respectively, exist in the Se-form is still under discussion (Waychunas et al., 1995, 1995 a). The Cr form has the bulk composition Fei6Oi6(OH)i0.23(CrO4)2.gg. In fact, synthetic schwertmannite formed in the sul-phate/arsenate system tolerates arsenate only up to a As/(As-rS) mole ratio of ca. 0.5, and it is likely that most of this arsenate is surface-bound. Above this ratio, a new, very poorly ordered Fe-hydroxy arsenate with two broad XRD peaks at ca. 0.31 and 0.16 nm and BhfS at 4.2K and ca. 1.5 K of 41.6 and 47.3T, respectively, forms (Carlson et al. 2002). From this one may conclude that, whereas the tetrahedral oxyanions with hexavalent central cations (S Se Cr) can be accomodated in the tunnel positions, the pentavalent cations can not, or not as easily. Schwertmannite from acid mine water contained between 6 and 70 g kg As (Carlson et al. 2002). 

Fig. 4.16 Upper SEM micrograph of crystal aggregates of schwertmannite from a mine drainage wetland (Cagliano et al. 2003, with permission) Lower TEM micrograph of schwertmannite (Bigham et al., 1990 with permission). 

Data concerning the surface area of schwertmannite is limited. Due to the poor crystallinity, the areas are very high and range from 240-320 m g (EGME) for synthetic samples and from 125-225 m g for natural ones (Bigham et al., 1990). 

Goethite 29-713 Lepidocrocite 44-1415 Akaganeite 13-157 Schwertmannite 6-FeOOH > Feroxyhyte Ferrihydrite 29-712 HP FeOOH > ... 

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