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

Abstract

Synthetic green rusts, GRs, are prepared by oxidation of Fe(OH)2 incorporating Cl-, SO4 2-, or CO3 2- ions. E h−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 )FeIII x (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.

Size control of magnetite nanoparticles by organic solvent-free chemical coprecipitation at room temperature

“…When the ferrous/ferric ions molar ratio was higher than the stoichiometric ratio (¼0.5), the reaction rate increased and the single Fe 3 O 4 phase formed. The particle size increased with increasing molar ratio because of the effect of SO [14,15] according to the following reactions [16]. …”

A CRITICAL REVIEW ON THE PROCESS OF CONTAMINANT REMOVAL IN FE0–H2O SYSTEMS

“…The earliest detailed surveys involving contaminant removal by Fe 0 materials in laboratory batch systems raised speculation that the decontamination occurs through a reduction reaction at the surface of the metal [16][17][18][19][20]. In the meantime several other reductants (secondary reductants) have been identified in the system Fe 0 -H 2 O, including green rust, structural Fe II and atomic or molecular hydrogen [21][22][23]. Therefore, contaminant reduction can also occur within the matrix of corrosion products.…”

Humusica 2, article 12: Aqueous humipedons – Tidal and subtidal humus systems and forms

“…The development of redoximorphic features is often associated to the presence of plant roots, which strongly contribute to diffuse oxygen and prevent anoxic conditions, and are often embedded in films of iron oxides (Génin et al, 1998;Richardson et al, 2001). Some examples of these features are shown in Figure 6.…”