Free energy of formation for green rust sodium sulphate (NaFe6IIFe3III(OH)18(SO4)2(s))

In a recent study, sulphate-bearing green rust (GR(SO4)) was shown to incorporate Na+ in its structure ((NaFe6Fe3III)-Fe-II(OH)(18)(SO4)(2(s)); GR(Na,SO4)). The compound was synthesised by aerial oxidation of Fe(OH)(2(S)) in the presence of NaOH. This paper reports on its free energy of formation (Delta G(f)(0)). Freshly synthesised GR(Na,SO4) was titrated with 0.5 M H2SO4 in an inert atmosphere at 25 degrees C, producing dissolved Fe2+ and magnetite or goethite. Solution concentrations, PHREEQC and the MINTEQ database were used to calculate reaction constants for the reactions: 2NaFe(6)(II)Fe(3)(III)(OH)(18)(SO4)(2(s)) + 12H((aq))(+)reversible arrow 9Fe(aq)(2+) + 2Na((aq))(+) + 4SO(4)(2-) + 3Fe(II)Fe(2)(III)O(4(s)) + 24H(2)O((1)), K = 10(54.5 +/- 3.0) and (NaFe6Fe3III)-Fe-II(OH)(18)(SO4)(2(s)) + 9H((aq))(+)reversible arrow 6Fe((aq))(2+) + Na-(aq)(+) + 2SO(4)((aq))(2-) + 3 alpha-FeOOH(s) + 12H(2)O((1)), K = 10(42.5 +/- 3.7) From the determined equilibrium constants and published Delta G(f)(0) values for the other compounds, we derived Delta G(f)(0) = -6366 +/- 18 kJ/mol for anhydrous GR(Na,SO4). The solubility product at 25 degrees C and atmospheric pressure is K = 10(-210.5 +/- 3.2). It is not yet known if the extent of Na+ incorporation in GR(SO4) depends on formation pathway; it cannot be excluded that both Na-free GR(SO4) and GR(Na,SO4) exist. If so, uncertainty in Delta G(f)(0). determined from acid titration is such that the E-H-pH stability fields of the two phases are statistically indistinguishable for Na+ concentrations as low as similar to 30 mu M (2 SD level; 0.036 M SO42- concentration). In sea water, where Na+ and SO42- concentrations are high, but soluble Fe2+ is low, GR is expected to form where local conditions increase concentration gradients, such as for corrosion of metallic iron and steel. Another example of an environment that would provide GR(Na,SO4)-favourable conditions is a degrading concrete and steel radioactive waste storage facility, where groundwater is saline. Green rust is a well-known sink for redox-active trace components, making it a compound that should be considered in risk assessment modelling of groundwater quality. Phase stability is critical in such simulations. (C) 2010 Elsevier Ltd. All rights reserved.