THERMODYNAMIC EQUILIBRIA IN AQUEOUS SUSPENSIONS OF SYNTHETIC AND NATURAL FE(II)-FE(III) GREEN RUSTS - OCCURRENCES OF THE MINERAL IN HYDROMORPHIC SOILS

Synthetic green rusts, GRs, are prepared by oxidation of Fe(OH)(2) inc orporating Cl-, SO42-, or CO32- ions. E-h-pH diagrams are drawn, and t hermodynamic data are derived. A GR incorporating OH- ions, GR1(OH-), is suspected to exist like similar other M(II)-M(III) compounds. GRs f orm as corrosion products of steels, implying microbially induced corr osion. Mossbauer and Raman spectroscopies allowed the identification o f GR in samples extracted from hydromorphic soils scattered over Britt any, France. This mineral has a varying Fe(III)/Fe(II) ratio. At Fouge res, it increases with depth till the occurrence of more oxidized ferr ic oxyhydroxide. In the same sites, sail solutions are collected and p revented from any oxidation and photoreduction. In large ranges of pH, pe, and Fe(II) concentration variations, soil solutions are in equili brium with a Fe(II)-Fe(III) compound, a GR1 mineral with pyroaurite-li ke structure incorporating OH- ions and having the formula [Fe-(1-x(II ))Fe-III(OH)(2)](+x.)[xOH](-x) = Fe(OH)((2+x)). Computation of ionic a ctivity products (IAP) of the equilibria between minerals and solution s leads to molar ratio x from 1/3 to 2/3, in agreement with the Fe(III )/Fe(II) ratios obtained from Mossbauer spectroscopy. The GR mineral p lays a key role for controlling iron in soil solutions, and equilibria between soil and suspension constrain the Fe(III)/Fe(II) ratios of th e iron(II)-iron(III) hydroxide.