Iron oxide, precipitation goethite

Examples of both homogeneous and heterogeneous nucleation exist in the iron oxide system. Goethite precipitates directly from soluble ferric species in solution. Its formation can, however, be assisted by addition of seed crystals of goethite to the system. As the interplanar spacings in the... 

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).

Iron Oxide Reds. From a chemical point of view, red iron oxides are based on the stmcture of hematite, a-Fe202, and can be prepared in various shades, from orange through pure red to violet. Different shades are controlled primarily by the oxide s particle si2e, shape, and surface properties. Production. Four methods are commercially used in the preparation of iron oxide reds two-stage calcination of FeS047H2 O precipitation from an aqueous solution thermal dehydration of yellow goethite, a-FeO(OH) and oxidation of synthetic black oxide, Fe O. ... 

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%). 

Fig. 16.12 Pseudo-first-order rate constants, for the transformation of CFBr - 6.5pM) in suspensions of goethite (25Fe(II)j j of ImM at 25 °C ionic strength = 20mM) as a function of solution pH ( ). The contact time of Fe(II) with iron oxide before addition of CFBr3 was >24 hours. Also shown are k values for the control experiments in the absence of goethite ( ). The precipitate formed in the pH 8.9 control (arrow) was identified as a form of green rust. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalogenated methanes by surface-bound Fe(ll) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741. Copyright 2002 American Chemical Society...

From these data it follows that when iron is precipitated in acid and neutral environments the first products should be X-ray-amorphous highly dispersed iron hydroxides, which in the course of time acquire the crystal structure of goethite or hematite. The mechanism of this process depends on kinetic factors (rate of oxidation of Fe " ), form of migration of the iron (ionic or colloidal), and acidity of the parent solution. In neutral environments ferrihydrite possibly is formed as an intermediate metastable phase, especially if the iron migrates in colloidal form or in the form of the Fe ion. The products of diagenesis of such a sediment may be both goethite (in the case of low Eh values typical of the Precambrian iron-ore process) and dispersed hematite (in the case of deposition of the oxide facies of BIF). 

At pH values much above 3 the iron(III) precipitates as the common iron(III) oxide, goethite (FeOOH) ... 

---