Lepidocrocite formation

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],... 

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). 

From the X-ray diffraction, XRD pattern in Fig. 19.2, the pre-rasted sample was found to consist of mainly lepidocrocite and magnetite and traces of geothite. The XRD pattern indicated the reduction of several lepidocrocite peaks in favom of ferric-tannate formation after the addition of mangrove tannins. 

Only three values of log Kso are available for lepidocrocite and these are not in very good agreement. Hashimoto and Misawa (1973) measured the solubility of lepidocrocite produced by anodic deposition from Fe" solution on a platinum electrode and obtained a value for log Kso of 2.50. This agrees with the value calculated from the free energy of formation (Blesa et al., 1994). Mohr et al. (1972) quote a value of 2.72 and Lindsay (1979) gives 1.59. 

Dos Santos Alfonso and Stumm (1992) suggested that the rate of reductive dissolution by H2S of the common oxides is a function of the formation rate of the two surface complexes =FeS and =FeSH. The rate (10 mol m min ) followed the order lepidocrocite (20) > magnetite (14) > goethite (5.2) > hematite (1.1), and except for magnetite, it was linearly related to free energy, AG, of the reduction reactions of these oxides (see eq. 9.24). A factor of 75 was found for the reductive dissolution by H2S and Fe sulphide formation between ferrihydrite and goethite which could only be explained to a small extent by the difference in specific surface area (Pyzik Sommer, 1981). 

Under otherwise similar conditions, low oxidation rates appear to promote magnetite and goethite, whereas high rates favor lepidocrocite. Magnetite formation probably requires slow oxidation because complete dehydroxylation of the precursor (green rust) prior to complete oxidation is only possible if sufficient time is available if, on the other hand, complete oxidation is fast and precedes dehydroxylation, lepidocrocite forms in preference to magnetite (Schwertmann Taylor, 1977). Dehydroxylation and oxidation appear to be competing reaction steps. 

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. 

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... 

Fe(III)- and Fe(II)-derived oxyhydroxides. Fn-viron. Sci. Technol. 17 709-713 Cumplido, J. Barron,V. Torrent, J. (2000) Ff-fect of phosphate on the formation of nanophase lepidocrocite from Fe(II) sulfate. Clays Clay Min. 48 503-510... 

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