Thermodynamic Equilibria in Aqueous Suspensions of Synthetic and Natural Fe(II)−Fe(III) Green Rusts: Occurrences of the Mineral in Hydromorphic Soils

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Abstract

Synthetic green rusts, GRs, are prepared by oxidation of Fe(OH)2 incorporating Cl-, SO42-, or CO32- ions. Eh−pH diagrams are drawn, and thermodynamic data are derived. A GR incorporating OH- ions, GR1(OH-), is suspected to exist like similar other M(II)−M(III) compounds. GRs form as corrosion products of steels, implying microbially induced corrosion. Mössbauer and Raman spectroscopies allowed the identification of GR in samples extracted from hydromorphic soils scattered over Brittany, France. This mineral has a varying Fe(III)/Fe(II) ratio. At Fougères, it increases with depth till the oc currence of more oxidized ferric oxyhydroxide. In the same sites, soil solutions are collected and prevented from any oxidation and photoreduction. In large ranges of pH, pe, and Fe(II) concentration variations, soil solutions are in equilibrium with a Fe(II)−Fe(III) compound, a GR1 mineral with pyroaurite-like structure incorporating OH- ions and having the formula [FeII(1-x)FeIIIx(OH)2]+x·[xOH]-x ≡ Fe(OH)(2+x). Computation of ionic activity products (IAP) of the equilibria between minerals and solutions leads to molar ratio x from 1/3 to 2/3, in agreement with the Fe(III)/Fe(II) ratios obtained from Mössbauer spectroscopy. The GR mineral plays a key role for controlling iron in soil solutions, and equilibria between soil and suspension constrain the Fe(III)/Fe(II) ratios of the iron(II)−iron(III) hydroxide.

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