Iron control by equilibria between hydroxy-Green Rusts and solutions in hydromorphic soils

In order to verify Fe control by solution - mineral equilibria, soil soluti ons were sampled in hydromorphic: soils on granites and shales, where the o ccurrence of Green Rusts had been demonstrated by Mossbauer and Raman spect roscopies. Eh and pH were measured in situ, and Fe(II) analyzed by colorime try. Ionic Activity Products were computed from aqueous Fe(II) rather than total Fe in an attempt to avoid overestimation by including colloidal parti cles. Solid phases considered are Fe(II) and Fe(III) hydroxides and oxides, and the Green Rusts whose general formula is [(Fe1-xFexIII)-Fe-II(OH)(2)]( +x).[x/z A(-z)](-x), where compensating interlayer anions, A(-), can be Cl- , SO42-, CO32- or OH-, and where x ranges a priori from 0 to 1. In large ra nges of variation of pH, pe and Fe(LI) concentration, soil solutions are (i ) oversaturated with respect to Fe(III) oxides; (ii) undersaturated with re spect to Fe(LI) oxides, chloride-, sulphate- and carbonate-Green Rusts; (ii i) in equilibrium with hydroxy-Green Rusts, i.e., Fe(II)-Fe(III) mixed hydr oxides. The ratios, x = Fe(III)/Fe-t, derived from the best fits for equili brium between minerals and soil solutions are 1/3, 1/2 and 2/3, depending o n the sampling site, and are in every case identical to the same ratios dir ectly measured by Mossbauer spectroscopy. This implies reversible equilibri um between Green Rust and solution. Solubility products are proposed for th e various hydroxy-Green Rusts as follows: log K-sp = 28.2 +/- 0.8 for the r eaction Fe-3(OH)(7) + e(-) + 7 H+ = 3 Fe2+ + 7 H2O; log K-sp = 25.4 +/- 0.7 for the reaction Fe-2(OH)(5) + e(-) + 5 H+ = 2 Fe2+ + 5 H2O; log K-sp = 45 .8 +/- 0.9 for the reaction Fe-3(OH)(8) + 2e(-) + 8 H+ = 3 Fe2+ + 8 H2O at an average temperature of 9 +/- 1 degrees C, and 1 atm. pressure. Tentative values for the Gibbs free energies of formation of hydroxy-Green Rusts obt ained are: Delta(f)G degrees (Fe-3(OH)(7), cr, 282.15 K) = -1799.7 +/- 6 kJ mol(-1), Delta(f)G degrees (Fe-2(OH)(5), cr, 282.15 K) = -1244.1 +/- 6 kJ mol(-1) and Delta(f)G degrees (Fe-3(OH)(8), cr, 282.15 K) = -1944.3 +/- 6 k J mol(-1). (C) 1999 Elsevier Science Ltd.