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

Mixed (FeFeIII)-Fe-II 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),Delta(f)G(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 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 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 Delta(f)G(o)(GR(SO4)) was -3819.43 +/- 6.44 kJ mol(-1) + y x [Delta(f)G(o)(H2O(1))], where y is the number of interlayer water molecules per formula unit. The logarithm of the solubility product, log K-sr, was estimated to be -139.2 +/- 4.8 and is invariable with y. Using the new value for Delta(f)G(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 it pH 7 and free ion activities of 10 (3).