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THE STANDARD GIBBS ENERGY OF FORMATION OF Fe(II)Fe(III) HYDROXIDE SULFATE GREEN RUST

Department of Basic Sciences and Environment, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C., Denmark

^* E-mail address of corresponding author: kayala{at}life.ku.dk

Mixed Fe^IIFe^III 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 in these systems. Slow acid titration of crystalline green rust sulfate (GR_SO4) with the formation of magnetite was used as a novel method to determine the standard Gibbs energy of formation of GR _SO4,_fG^o(GR _SO4). Aqueous suspensions of GR _SO4, with pH slightly >8, were titrated slowly with 1 M H₂SO₄ until pH = 3 under strict anoxic conditions. Powder X-ray diffraction and Mössbauer 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 Fe²⁺ release to consumption of protons confirmed the stoichiometry of dissolution of GR_SO4 and the formation of magnetite at equilibrium conditions. The estimate of the absolute value of _fG^o(GR_SO4) was –3819.4336.44 kJ mol^–1 + yx[_fG^o(H₂O_(l))], where y is the number of interlayer water molecules per formula unit. The logarithm of the solubility product, log K_sp, was estimated to be –139.234.8 and is invariable with y. Using the new value for _fG^o(GR_SO4), the reduction potentials of several GR_SO4 -Fe oxide couples were evaluated, with the GR_SO4 -magnetite half cell showing the smallest redox potential at pH 7 and free ion activities of 10^–3.

Key Words: Acid Titration • Gibbs Energy of Formation • Green Rust • Layered Double Hydroxides • Magnetite • Mössbauer Spectroscopy • Solubility Product