Oxidation lepidocrocite

Synthesis from Fe systems involves oxidative hydrolysis of Fe" solutions. The initial precipitate may be a so-called green rust (Bernal et al., 1959 Taylor, 1980). As various Fe oxides (lepidocrocite, goethite, fer-oxyhyte and magnetite) may be produced by this method, careful control of factors such as the rate of oxidation, pH and the nature of the anion present is necessary to ensure formation of pure goethite. 

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. 

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%). 

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. 

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. 

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... 

The orange coloured lepidocrocite, y-FeOOH, is named after its platy crystal shape (lepidos scale) and its orange colour (krokus = saffron). It occurs in rocks, soils, biota and rust and is often an oxidation product of Fe ". It has the boehmite (y-AlOOH) structure which is based on cubic close packing (ccp) of anions. 

Oxidation of FeCl2 solutions at pH 7-8 in the presence of soluble Al led to Al substituted lepidocrocite with an Al/(Fe-i-Al) of up to 0.14 the a, h and c unit cell edge lengths fell linearly as the Al content increased (Schwertmann Wolska, 1990). The Bhf at 4.2K also decreased linearly over the same range of Al substitution, i. e. 

Synthetic crystals of lepidocrocite are platy or lath-like, elongated in the a-direction and terminate in 101 faces. The predominant face is 010 and crystals often lie on this face. Lepidocrocite is commonly formed by oxidation of Fe systems. The crystal... 

Fig. 4.14 Synthetic lepidocrocite produced by oxidation of a FeCl2 solution, a) Monodomainic, lath-shaped crystals, produced by oxidation with 100 ml air min " at 50°C and pH 7.5 shadowed with 5 nm chromium at 45° (Courtesy R.Ciova-noli). b) Multidomainic crystals obtained at pH 7-7.5 and room temperature (see Schwertmann Taylor, 1972a). c) Crystal aggregates produced in the presence of urotropin (courtesy R. Ciova-noli). d) Very small crystals showing (010) lattice fringes of 1 nm (Schwertmann. Taylor, 1979, with permission), e) Cubic crystals formed after ageing multidomainic crystals shown in (b) in M KOH containing 3.32 10 M Si at 80°C for 1749 h (Schwertmann Taylor, 1972, with per-...

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