Iron oxides lepidocrocite

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

Infrared spectroscopy showed that SO2 adsorbed on goethite to form sulphito (OSO2) species and on lepidocrocite to form sulphate and sulphite complexes (Ka-neko and Matsumoto, 1989). XPS measurement suggested that H2S adsorbed on iron oxide to give both Sg and SH3 as surface species, whereas only Sg formed if the surface had been pre-exposed to water vapour (Prasad et ah, 1993). 

Leland and Bard (1987) found that the different iron oxides induced photooxidation of oxalate and sulphite at rates that varied by up to two orders of magnitude. For oxalate, the rate was greater for maghemite than for hematite, but this order was reversed for sulphite. Lepidocrocite (layer structure) induced faster oxidation of both compounds that did the other polymorphs of FeOOH (tunnel structures) the authors considered that the rate differences were probably associated with structural differences between the adsorbents. 

The physical and chemical parameters which influence iron oxide formation vary with time and space, e. g. through changing water/air content. Microenvironments exist in pores of different sizes and with different degrees of filling. For example, hematite was identified in coatings at the (dry) surface of a basalt boulder, whereas goethite occurred in a nearby (moister) crack (Bender-Koch et al., 1995 a). In another case, goethite was the dominant oxide next to the root surface, whereas lepidocrocite predominated a few mm away from it (Schwertmann Fitzpatrick, 1977). Often, however, the exact conditions under which Fe oxides form are difficult to determine. 

Processings of iron oxides at room temperature. II. Mechanochemical reaction effects on the structure and surface of pure, synthetic lepidocrocite. Mat. Res. Bull. 17 1017-1023... 

Murad, E. Schwertmann, U. (1984) The influence of crystallinity on the Mdssbauer spectrum of lepidocrocite. Min. Mag. 48 507-511 Murad, E. Schwertmann, U. (1986) Influence of Al substitution and crystal size on the room-temperature Mdssbauer spectrum of hematite. Clays Clay Min. 34 1-6 Murad, E. Schwertmann, U. (1988) Iron oxide mineralogy of some deep-sea ferromanganese crusts. Am. Min. 73 1395-1400 Murad, E. Schwertmann, U. (1993) Temporal stability of a fine-grained magnetite. Clays Clay Min. 41 111-113... 

Schwertmann, U. Wolska, E. (1990) The influence of aluminum on iron oxides. XV. Al-for-Fe substitution in synthetic lepidocrocite. Clays Clay Min. 38 209-212... 

Orange iron oxide with the lepidocrocite structure (y-FeOOH) is obtained if dilute solutions of the iron(II) salt are precipitated with sodium hydroxide solution or other alkalis until almost neutral. The suspension is then heated for a short period, rapidly cooled, and oxidized [3.22], [3.23],... 

Iron frequently has been postulated to be an important electron acceptor for oxidation of sulfide (58, 84,119, 142, 152). Experimental and theoretical studies have demonstrated that Fe(III) will oxidize pyrite (153-157). Reductive dissolution of iron oxides by sulfide also is well documented. Progressive depletion of iron oxides often is coincident with increases in iron sulfides in marine sediments (94, 158, 159). Low concentrations of sulfide even in zones of rapid sulfide formation were attributed to reactions with iron oxides (94). Pyzik and Sommer (160) and Rickard (161) studied the kinetics of goethite reduction by sulfide thiosulfate and elemental S were the oxidized S species identified. Recent investigations of reductive dissolution of hematite and lepidocrocite found polysulfides, thiosulfate, sulfite, and sulfate as end products (162, 163). 

Recently we presented (23) the results of an experimental study on the kinetics and mechanisms of the reaction of lepidocrocite (y-FeOOH) with H2S. With respect to the interaction between iron and sulfur, lepidocrocite merits special attention. It forms by reoxidation of ferrous iron under cir-cumneutral pH conditions (24), and it can therefore be classified as a reactive iron oxide (19). The concept of reactive iron was established by Canfield (19), who differentiated between a residual iron fraction and a reactive iron fraction (operationally defined as soluble in ammonium oxalate). The reactive iron fraction is rapidly reduced by sulfide or by microorganisms. 

In this study we performed initial rate experiments, reacting H2S with lepidocrocite (23). The consumption of H2S was measured continuously by using a pH2S electrode cell (25). To avoid interferences of pH buffer solutions with the iron oxide surface, the pH was stabilized by using a pH-stat that added appropriate amounts of HC1 to the solution. The added volume, which was also continuously monitored, provided information about the amount of protons consumed during the reaction. Dissolved iron was measured only in some runs. 

In addition to a better understanding of the reaction of sulfide with ferric oxides and its role in pyrite formation, a more exact definition of the term reactive iron is critical. Does reactive iron mean a different iron oxide fraction for bacterial dissolution (e.g., weathering products such as goethite or hematite) than for reaction with sulfide (e.g., reoxidized lepidocrocite) In other words, is there a predigestion of ferric oxides by bacteria that allows a subsequent rapid interaction of sulfide with ferric oxides ... 

Iron oxides Geothite, hematite, magnetite, lepidocrocite... 

In water logged soils radial oxygen loss from the root raises the redox potential in the rhizosphere as a consequence of which iron oxide plaques are seen to develop on root surfaces. Bacha and Hossner (1977) removed the coatings on rice roots growing under anaerobic conditions. The coatings were composed primarily of the iron oxide mineral lepidocrocite (y-FeOOH) as the only crystalline component. St-Cyr and Crowder (1990) studied the iron oxide plaque on roots of Phragmites and detected both Fe and Mn. The Fe Mn ratio of the plaque resembled the ratio of Fe Mn in substrate carbonates. The plaque material also contained Cu. 

Adsorption of Cyclohexane. The adsorption of cyclohexane was determined on synthetic lepidocrocite and its decomposition products prepared by heating it in vacuo for varying intervals of time at 190°, 300°, 400°, and 500°, and also on synthetic goethite and its decomposition products obtained in a similar manner by heating at 150°, 180°, 250°, 300°, and 500°. The adsorption was found to be physical in nature, and the isotherms are Type II of the Brunauer classification (3) in all cases except on iron oxide prepared from goethite at 300° and 500° here the isotherms are Type III. This finding is in contrast to the Type IV isotherms common to ferric oxide gels the difference may be due to the crystalline nature of the parent material. 

Typical results are shown in Figures 1 and 2. Thus Figure 1 represents the isotherms of cyclohexane on the two parent materials lepidocrocite and goethite at 35°, while Figure 2 represents the isotherms at the same temperature on iron oxide prepared from lepidocrocite by heating in vacuo at 300° for 0.5, 1, 3, 8, and 18 hours. The results agree with the equation of Brunauer, Emmett, and Teller (4) ... 

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