Iron control by equilibria between hydroxy-Green Rusts and solutions in hydromorphic soils |
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| Institution: | 1. INRA - U.R. Sol et Agronomie, 65 rue de Saint Brieuc, F 35042 Rennes Cedex, France;2. Université de Rennes 1, France;3. Laboratoire de Chimie Physique pour l’Environnement, UMR 7564 CNRS-Univ. H. Poincaré, Equipe sur la Réactivité des Espèces du Fer and Department of Materials Science, ESSTIN, Université H. Poincaré Nancy 1, 405, rue de Vandoeuvre, F 54600 Villers-lès-Nancy, France;4. Ecole Nationale Supérieure Agronomique de Rennes, France;1. Department of Physical Sciences, Grand Rapids Community College, Grand Rapids, MI, USA;2. Department of Physics, Oklahoma State University, Stillwater, OK, USA;3. School of Meteorology, University of Oklahoma, Norman, OK, USA;1. School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, Shaanxi, China;2. Collaborative Innovation Center of Information Sensing and Understanding at Xidian University, China;3. School of Electronics and Information Engineering, Hunan University of Science and Engineering, Yongzhou, 425199 Hunan, China;1. Department of Physics, MIT Campus, Anna University, Chennai 600044, India;2. Department of Physics, Pondicherry Engineering College, Puducherry 605014, India;1. Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional Unidad Saltillo, Av. Industria Metalúrgica 1062, Parque Industrial Ramos Arizpe, Ramos Arizpe, Coahuila C.P.25000, México;2. Red de Estudios Moleculares Avanzados, Instituto de Ecología A.C., Carretera Antigua a Coatepec 351, El Haya, 91070 Xalapa, Veracruz, Mexico;3. Centro Brasileiro de Pesquisas Físicas, Río de Janeiro 22290-180, Brazil;4. Mailstop 70A1150, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA |
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| Abstract: | In order to verify Fe control by solution - mineral equilibria, soil solutions were sampled in hydromorphic soils on granites and shales, where the occurrence of Green Rusts had been demonstrated by Mössbauer and Raman spectroscopies. Eh and pH were measured in situ, and Fe(II) analyzed by colorimetry. Ionic Activity Products were computed from aqueous Fe(II) rather than total Fe in an attempt to avoid overestimation by including colloidal particles. Solid phases considered are Fe(II) and Fe(III) hydroxides and oxides, and the Green Rusts whose general formula is FeII1?xFeIIIx(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 ranges of variation of pH, pe and Fe(II) concentration, soil solutions are (i) oversaturated with respect to Fe(III) oxides; (ii) undersaturated with respect to Fe(II) oxides, chloride-, sulphate- and carbonate-Green Rusts; (iii) in equilibrium with hydroxy-Green Rusts, i.e., Fe(II)-Fe(III) mixed hydroxides. The ratios, x = Fe(III)/Fet, derived from the best fits for equilibrium between minerals and soil solutions are 1/3, 1/2 and 2/3, depending on the sampling site, and are in every case identical to the same ratios directly measured by Mössbauer spectroscopy. This implies reversible equilibrium between Green Rust and solution. Solubility products are proposed for the various hydroxy-Green Rusts as follows: log Ksp = 28.2 ± 0.8 for the reaction Fe3(OH)7 + e? + 7 H+ = 3 Fe2+ + 7 H2O; log Ksp = 25.4 ± 0.7 for the reaction Fe2(OH)5 + e? + 5 H+ = 2 Fe2+ + 5 H2O; log Ksp = 45.8 ± 0.9 for the reaction Fe3(OH)8 + 2e? + 8 H+ = 3 Fe2+ + 8 H2O at an average temperature of 9 ± 1°C, and 1 atm. pressure. Tentative values for the Gibbs free energies of formation of hydroxy-Green Rusts obtained are: ΔfG° (Fe3(OH)7, cr, 282.15 K) = ?1799.7 ± 6 kJ mol?1, ΔfG° (Fe2(OH)5, cr, 282.15 K) = ?1244.1 ± 6 kJ mol?1 and ΔfG° (Fe3(OH)8, cr, 282.15 K) = ?1944.3 ± 6 kJ mol?1. |
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