Ferrihydrite association with goethite

The rapid oxidation of Fe " close to the surface and in the presence of a fair supply of organic matter and dissolved Si, conditions which hinder crystallization, leads to ferrihydrite instead of goethite. The ferrihydrite is, however, often associated with goethite and it is still unknown whether the two minerals have formed simultaneously or in sequence. Simultaneous formation seems more likely for two reasons in the first place, low-temperature hydrolysis of Fe " or oxidation of Fe ", both, led to mixtures of the two oxides in different proportions if the rate of hydrolysis/oxidation was varied (Schwertmann et al. 1999 Schwertmann Cornell, 2000). Secondly, the transformation of ferrihydrite, especially in the presence of Si, appears to be extremely sluggish. 

Hm hematite, Gt goethite, Ch chamosite, Sd siderite, Im ilmenite, Fh ferrihydrite in which volcanism is associated with clastic sedimentation... 

The laboratory derived model of hematite formation in soils via ferrihydrite has received general acceptance. So far, it is the only way to produce hematite at ambient temperatures and in the pH range of soils. Support from soil analysis, however, is meagre. Hematite is usually associated with other Fe oxides, mainly with goethite but not with ferrihydrite. There seems to be only one report of a ferrihydrite-hema-tite association (based on XRD and Mossbauer spectra) viz. in several andisols formed from basalt in the warm and moist climate of Hawaii (Parfitt et al., 1988). In this case, in addition to the low age of the soils, high release of Si may retard the transformation of ferrihydrite to hematite, whereas normally, the rate of transformation of ferrihydrite seems to be higher than that of ferrihydrite formation, so that this mineral does not persist. 

Carlson, L. Schwertmann, U. (1981) Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochim. Cosmochim. Acta 45 421-429 Carlson, L. Schwertmann, U. (1987) Iron and manganese oxides in Finnish ground water treatment plants. Wat. Res. 21 165-170 Carlson, L. Schwertmann, U. (1990) The effect of CO2 and oxidation rate on the formation of goethite versus lepidocrocite from an Fe(II) system at pH 6 and 7. Clay Min. 25 65-71... 

The ratio of fuivic to humic acid varies between soils and between horizons of the same soil. Humic fractions are involved with solubilization of the sesquioxides (especially gibbsite, AI(OH)3 goethite, FeOOH haematite, Fe Oj and ferrihydrite, Fe Oj.nH O). It is therefore desirable to determine the Al and Fe associated with these fractions. The scheme of separations is shown in Fig. 5.1. 

Purple hematite. Hematite crystals several j,m in size have a purple color. Such crystals can be produced by holding ferrihydrite under very strongly alkaline condition (5 M NaOH) at 70 °C for 8 days. The crystals are idiomorpliic. Some goethite is usually associated with the hematite. 

In another report. Hansel et al. (2002) studied Fe plaque associated with the rhizosphere of Typha latifolia and Phalaris arundinacea from a mine waste-contaminated site that was high in As. For both plants, the Fe and As concentrations associated with roots were approximately 10-fold concentrated on roots relative to their abundances in the bulk soil. XRF revealed that Fe plaque consisted primarily of ferrihydrite but also had appreciable levels of goethite and siderite T. latifolia also had a significant amount of lepidocrocite. Their analysis suggested that As was sequestered fairly homogeneously within the Fe plaque. A XANES analysis indicated that approximately 80% of the As was As(V) and 20% As(III). Blute et al. (2004) reported similar ratios for As(V) and As(III)... 

Ferrihydrite is a disordered high surface area, iron hydroxide formed by ferrolysis at ambient temperatures. With time ferrihydrite becomes unstable and converts to goethite (Waychunas et al. 1993). Arsenical ferrihydrite are produced by the neutralization of effluents which contain As together with ferric iron. Low solubilities are associated with products with an Fe As ratio of >3 1. Lower Fe As ratios tend to produce a highly soluble product (Krause Ettel 1985, 1989 Harris Krause 1993 MIRO 1994). Adsorption of oxyanions in mildly acidic conditions is an effective method of attenuating some elements such as arsenic (Bowell 1994). However any environmental changes will lead to liberation of the contained species. 

Arsenic and iron concentrations in the sediment porewaters were found to be closely correlated. Although these concentrations are considerably elevated at depth in the sediment column—reaching 17 pM (1.3 mg/L) for arsenic and 1.6 mM (90 mg/L) for iron—only a small fraction of the iron and arsenic deposited to the sediments needs to be remobilized to support these concentrations. XAS analysis of the sediments indicated that arsenic in the solid phase is reduced from As(V) to As(III) above the depth at which arsenic is released into the porewater. Iron in solid phase remains as Fe(III). XAS analysis showed no evidence of conversion to magnetite (though conversion of ferrihydrite to goethite could not be excluded). Sequential extractions indicated that most of the arsenic can be released from sediment by treatment with magnesium chloride or phosphate solutions this treatment does not release iron, behavior that is consistent with sorption as a mode of association for the majority of the arsenic with the sediment. The remainder of the arsenic is released, along with almost all the iron, by treatment with hydrochloric acid.15... 

As shown in Figure 9, Fe and Pb are closely associated in a rind on the root surface which consists of 63% ferrihydrite, 32% goethite, and 5% siderite, as determined by later XAFS analysis. More detailed XAFS analysis of these samples carried out at SSRL showed that Pb is complexed with organic functional groups, most likely those of bacterial biofilms. Arsenic, which also appears to be associated in part with iron, is present as a combination of two sorbed As species ( 82% As(V) and 18% As(III)). Mn and Zn occur as isolated nodules of mixed-metal carbonate (rhodochrosite/hydrozincite) on the root surface. 

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