Iron oxide, precipitation lepidocrocite

Goethite is synthesised by a wide variety of methods that are described in Cornell and Schwertmarm (1996). The main pigment variety is Mars yellow, which was produced from oxidation of iron(II) sulfate ( green vitriol or copperas, q.v.) mixed with alum and precipitated by means of an alkah. Other methods include the oxidation of iron chloride, and the conversion of the iron oxide hydroxides lepidocrocite and ferrihydrite (qq.v.) in alkaline solutions. 

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%). 

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],... 

Figure 9. Isotherm fits of the surface precipitation model for the sorption of ferrous iron on iron oxides ( ) magnetite, ( ) goethite, ( ) lepidocrocite, (A) hematite (for parameter values see Table I, Fe(II)soi = concentration of dissolved ferrous iron, rpe= concentration of surface-bound ferrous iron per total concentration of iron oxide, pH 7.2,1=20 mM, T=25 C, 25 m L l, teq=15 min) adapted from (7).

Of particular interest was the way in which detailed information could be derived from voltammetric studies of clay minerals treated aerobically with iron (11) solutions. This caused the precipitation of a thin, active layer of iron (HI) oxi(hydroxides) which could later be used for the sorption of arsenate(V) ions [69]. By employing the voltammetry of immobilized microparticles, it was possible to distinguish different iron species, namely (i) ion-exchangeable, labile, or sorbed iron (HI) ions (ii) ferrihydrite or lepidocrocite and (iii) crystalline hematite or goethite. Cepria et al. subsequently employed the voltammetry of immobilized microparticles in the phase analysis of iron (III) oxides and oxi(hydroxides) in binary mixtures, as well as in cosmetic formulations [70]. 

Lepidocrocite has most often been precipitated in the laboratory at pH s between 4 and 7 by the oxidation of aqueous or solid ferrous iron species. The mineral is usually accompanied by goethite. Akaganeite is the rarest of the oxyhydroxides under natural conditions. Perhaps the only published descriptions of natural occurrences are by Van Tassel (5) and Chandy (6). The mineral has been precipitated in the laboratory at room temperatures by the slow hydrolysis of 0.02-0.08M FeCl3 solutions (7). 

Despite these limitations, the technique has been applied in the past to characterize the forms of iron occurring in different plants [62-65]. More recently, a detailed study of iron uptake and translocation in rice [66] grown in anaerobic FeCb-enriched nutrient solutions showed primarily the presence of Fe(lll) hydrous oxide components precipitated on the root cell wall (ferrihydrite and/or lepidocrocite). No evidence of Fe(ll) was found in the leaf tissue, the spectra were characteristic of Fe(lll) present in ferritin and in other complexed forms, not further identified by the authors. Iron biomineralization was also observed In a perennial grass grown in extreme acidic environment with a high content of metals [67,68]. In this case, the Mossbauer spectral analysis indicated that iron accumulated in this plant mainly as jarosite and ferrihydrite (ferritin). ]arosite accumulated In roots and rhizomes, while ferritin was detected in all the structures. 

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