Green rusts

Iron mixed-valence materials such as green rust and fougerite are sensitive to air exposure and soil pollution by nitrates. A MIMOS instrument autonomously lowered or raised within a plexiglas tube, which was put down a bore hole, allowed... 

Only one naturally relevant abiotic Se(VI) reduction process has been documented to date. Se(Vl) can be reduced to Se(TV) and ultimately to Se(0) by green rust , an Fe(II)- and Fe(lll)-bearing phase with sulfate occupying interlayer spaces (Myneni et al. 1997). Johnson and Bullen (2003) obtained an ese(vi)-se(iv) value of 7.4%o ( 0.2) for the Se(VI) reduction reaction. The result was not sensitive to changes in pH or solution composition within the ranges over which green rust is stable. 

Because this reaction must involve two steps, diffusion of selenate into the interlayer spaces of the green rust followed by electron transfer from Fe(ll) green rust, Johnson and Bullen (2003) interpreted this result using a two-step model similar to that discussed above. The diffusion step presumably has very little isotopic fractionation associated with it. Step 2 might be expected to involve a kinetic isotope effect similar to that observed in the HCl reduction experiments. As is discussed above, if the diffusion step is partially rate-limiting, the isotopic fractionation for the overall process should be less than the kinetic isotope effect occurring at the reduction step. This appears to be the case, as the ese(vi)-se(iv) value of 7.4%o is somewhat smaller than that observed for reduction by strong HCl (12%o). 

Johnson TM, Bullen TD (2003) Selenium isotope fractionation during reduction by Fe(ll)-Fe(lll) hydroxide-sulfate (green rust). Geochim Cosmochim Acta 67 413-419 Johnson TM, Bullen TD, Zawislanski PT (2000) Selenium stable isotope ratios as indicators of sources and cycling of selenium Results from the northern reach of San Francisco Bay. Environ Sci Tech 34 ... 

Wiesli RA, Beard BL, Johnson CM (2003b) Experimental determination of Ee isotope fractionation between aq. Ee(ll), green rust , and siderite. Geochim Cosmochim Acta 67 A533... 

Fig. 14 View of the ordered interlayer structure of the green rust with stoichiometry [Fe 4Fe 2(OH)i2]S04- ca. 8H2O. Reprinted with permission from [157]. Copyright Elsevier SAS...

The structure of a green rust LDH-type material with the formula [Fe 4Fe 2(OH)i2]S04-ca. 8H2O has been determined by Rietveld analysis [157]. The material exists as a one-layer polytype with the interlayers containing two planes of sulfate and water molecules giving a basal spacing of 1.1011 nm. The sulfate ions are oriented with their C3 axes perpendicular to the layers and alternate anions point up and down (as shown earlier in Fig. 14) and form a superlattice with parameter a = UoV5 = 0.5524 nm (Fig. 39). 

There is evidence that mixed Fe(II)-Fe(III) hydroxides are formed. These can be produced easily in vitro by partial oxidation of pure Fe(II) hydroxy salts and they have some of the observed properties of the solid phase Fe(II) found in reduced soils, including the grayish-green colours characteristic of reducing conditions in soils. This material is green rust and has the general formula Fe(II)6Fe(III)2(OH)i8 with Al + partly substituted for Fe + and Cl, S04 and C03 substituted for OH . 

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...

Fe(OH)2 does not exist as a mineral. The Fe is divalent and the structure which is based on a hep anion array, is similar to that of brucite. Pure Fe(OH)2 is white. It is, however, readily oxidized, upon which it develops into greenish-blue, so-called, green rust or, on further oxidation, into black magnetite. 

Green Rusts are a group of Fe oxides consisting of layers of Fe OH octahedra in which some Fe is replaced by Fe " and, to maintain neutrality, anions, especially Cr and SO4 are bound between the layers. Green rusts are formed as corrosion products (hence the name). 

Fig. 2.1 Relationship between the inter-layer spacing and the intra-layer spacing (0-0 distance) in the hexagonal close packed and cubic close packed iron oxide structures (Mackay, 1961, with permission). CR I, CRN Green rust I and II...

Tab. 2.5 X-ray data for Green Rusts containing different interlayer anions (data from Lewis, 1997 with permission)...

A small amount of Cr could be incorporated in wiistite at 1350 °C (Bogdandy Engell, 1971) and MgO and MnO were completely miscible with FeO the mixed phases are important in the reduction of iron ores. Wiistite can be doped with small amounts of Mn, Mg, Ca and <10 g kg Si or Al to promote reduction (Moukassi et al., 1984). In green rust Fe has been replaced by Ni" (Refait Genin, 1997) and by Mg (Refait et al. 2001). 

Fe(OH)2 exists as hexagonal plates as do the green rusts (Feitknecht Keller, 1950 Bernal et ak, 1959). The basic morphology of wiistite is cubic, but this compound is frequently obtained as very irregular particles. It is formed as irregular rounded crystals 20-100 (xm across by reduction of hematite with H2/H2O at 800 °C (Moukassi et al., 1984). 

Hematite 34-664 Magnetite 19-629 Maghemite 25-1402 Wustite 6-615 fi-FejOj 33-646 51 Bernalite Green rust 1 Green rust 11 Fe(OH) ... 

Chromate adsorbed from solution by green rust was reduced to Cr with the green rust being simultaneously converted to ferrihydrite or possibly a Cr substituted ferrihydrite (Loyaux-Launiezak et al., 2000). 

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