The standard gibbs energy of formation of Fe(II)Fe(III) hydroxide sulfate green rust
Ayala-Luis, Karina Barbara (University of Copenhagen. Department of Basic Sciences and Environment)
Koch, Bender Christian (University of Copenhagen. Department of Basic Sciences and Environment)
Hansen, Hans Christian Bruun (University of Copenhagen. Department of Basic Sciences and Environment)

Date:	2008
Abstract:	Mixed FeFe hydroxides, commonly referred to as 'green rusts' (GRs), are important reactive phases in both man-made and natural geochemical systems. Determinations of the standard Gibbs energy of formation of GRs are needed to understand and predict the occurrence and possible reactions of GRs inthese systems. Slow acid titrationof crystalline greenrust sulfate (GR ) with the formation of magnetite was used as a novel method to determine the standard Gibbs energy of formation of GR,ΔG(GR). Aqueous suspensions of GR, with pH slightly >8, were titrated slowly with 1 M HSO until pH = 3 under strict anoxic conditions. Powder X-ray diffraction and Mossbauer analysis revealed that magnetite was the only solid phase formed during the initial part of the titration, where the equilibrium pH was maintained above 7. 0. The ratio of Fe release to consumption of protons confirmed the stoichiometry of dissolutionof GR and the formation of magnetite at equilibrium conditions. The estimate of the absolute value of ΔG(GRSO) was -3819. 43±6. 44 kJ mol +y × [ΔG(HO)],wherey is the number of interlayer water molecules per formula unit. The logarithm of the solubility product, log K, was estimated to be -139. 2±4. 8 and is invariable with y. Using the new value for ΔG(GR), the reduction potentials of several GR -Fe oxide couples were evaluated, with the GR -magnetite half cell showing the smallest redox potential at pH 7 and free ion activities of 10.
Rights:	Tots els drets reservats.
Language:	Anglès
Document:	Article ; recerca ; Versió publicada
Subject:	Acid Titration ; Gibbs Energy of Formation ; Green Rust ; Layered Double Hydroxides ; Magnetite ; Mössbauer Spectroscopy ; Solubility Product
Published in:	Clays and Clay Minerals, Vol. 56, Núm. 6 (2008) , p. 633-644, ISSN 1552-8367

DOI: 10.1346/CCMN.2008.0560604

12 p, 478.2 KB  UAB restricted access

The record appears in these collections:
Articles > Research articles
Articles > Published articles