Lepidocrocite

Hydroxide Fe(OH)3 (Fe + plus OH ) has definite existence and there are many ill-deiined hydrates used as pigments. FeOOH has two forms goethite and lepidocrocite. Colloidal Fe(OH)3 is easily obtained as a deep red sol. Many Fe(III) hydroxy complexes are known. Fe(OH)2 may be formed from Fe and OH" in the absence of O2 but it is very readily oxidized. 

This is oxidised to ferric hydroxide Fe(OH)3, which is a simple form of rust. The final product is the familiar reddish brown rust Fe203 -H20, of which there are a number of varieties, the most common being the a form (goethite) and the 7 form (lepidocrocite). In situations where the supply of oxygen is restricted, Fe3 04 (magnetite) or 7 FejOj may be formed. 

Figure 6.4 (a) Large-area STM image (620A x 620 A, + 0.48V, 1.4 nA) of a singledomain lepidocrocite nanosheet on (1 x 2)-Pt(l 1 0). The central brighter area is separated from the lower terrace by a substrate mono-atomic step. Inset (14 x 4) LEED pattern. 

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%). 

First attempts to incorporate pre-formed magnetite colloids within alginate/silica nanocomposites via a spray-drying process have been described, but formation of lepidocrocite y-FeOOH and fayalite Fe2Si04 was observed, attributed to Fe2+ release during the aerosol thermal treatment [53],... 

Farquhar ML, Chamock JM, Livens FR, Vaughun DJ (2002) Mechanisms of arsenic uptake from aqueous solution by interaction with goethite, lepidocrocite, mackinawite and pyrite as X-ray absorption spectroscopy study. Environ Sci Tecnol 36 1757-1762... 

Sung, W. and J. J. Morgan, 1981, Oxidative removal of Mn(II) from solution catalysed by the y-FeOOH (lepidocrocite) surface. Geochimica et Cosmochimica Acta 45, 2377-2383. 

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

Reactivity of Fe(III)(hydr)oxide as measured by the reductive dissolution with ascorbate. "Fe(OH)3" is prepared from Fe(II) (10 4 M) and HCO3 (3 10 4 M) by oxygenation (po2 = 0.2 atm) in presence of a buffer imidazd pH = 6.7 (Fig. a) and in presence of TRIS and imidazol pH = 7.7 (Fig. b). After the formation of Fe(III)(hydr)oxide the solution is deaerated by N2, and ascorbate (4.8 10 2 M) is added. The reactivity of "Fe(OH)3 differs markedly depending on its preparation. In presence of imidazole (Fig. a) the hydrous oxide has properties similar to lepidocrocite (i.e., upon filtration of the suspension the solid phase is identified as lepidocrocite). In presence of TRIS, outer-sphere surface complexes with the native mononuclear Fe(OH)3 are probably formed which retard the polymerization to polynuclear "Fe(OH)3" (von Gunten and Schneider, 1991). 

Enhanced Reactivity of Fe(III) formed at Surfaces. Another way to keep the Fe(III) hydroxide formed from oxygenation of Fe(II) from extensive polymerization, is to oxidize (02) the adsorbed Fe(II). Apparently the Fe(III) formed on the surface (or part of it), plausibly because of a different coordinative arrangement of the adsorbed ions, does not readily polymerize fully to a "cross-linked" three-dimensional structure and is thus more reactive than freshly formed lepidocrocite. 

Similar photo-induced reductive dissolution to that reported for lepidocrocite in the presence of citric acid has been observed for hematite (a-Fe203) in the presence of S(IV) oxyanions (42) (see Figure 3). As shown in the conceptual model of Faust and Hoffmann (42) in Figure 4, two major pathways may lead to the production of Fe(II)ag i) surface redox reactions, both photochemical and thermal (dark), involving Fe(III)-S(IV) surface complexes (reactions 3 and 4 in Figure 4), and ii) aqueous phase photochemical and thermal redox reactions (reactions 11 and 12 in Figure 4). However, the rate of hematite dissolution (reaction 5) limits the rate at which Fe(II)aq may be produced by aqueous phase pathways (reactions 11 and 12) by limiting the availability of Fe(III)aq for such reactions. The rate of total aqueous iron production (d[Fe(aq)]T/dt = d [Fe(III)aq] +... 

Oxide composition and lattice structure influences the coordin-ative environment of surface sites, and should have an impact on rates of ligand substitution. Hematite (Fe203), goethite (a-FeOOH), and lepidocrocite (y-FeOOH), for example, are all Fe(III) oxide/ hydroxides, but may exhibit different rates of surface chemical... 

Mn(II) Oxidation in the Presence of Lepidocrocite The Influence of Other Ions... 

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

This paper discusses the oxidation of Mn(II) in the presence of lepidocrocite, y-FeOOH. This solid was chosen because earlier work (18, 26) had shown that it significantly enhanced the rate of Mn(II) oxidation. The influence of Ca2+, Mg2+, Cl", SO,2-, phosphate, silicate, salicylate, and phthalate on the kinetics of this reaction is also considered. These ions are either important constituents in natural waters or simple models for naturally occurring organics. To try to identify the factors that influence the rate of Mn(II) oxidation in natural waters the surface equilibrium and kinetic models developed using the laboratory results have been used to predict the... 

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