Free energy of formation for green rust sodium sulphate (NaFeII6FeIII3(OH)18(SO4)2(s))

In a recent study, sulphate-bearing green rust (GRSO4)(GRSO4) was shown to incorporate Na+ in its structure (NaFeII6FeIII3(OH)18(SO4)2(s); GRNa,SO4GRNa,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 (ΔGf0).Freshly synthesised GRNa,SO4GRNa,SO4 was titrated with 0.5 M H2SO4 in an inert atmosphere at 25 °C, producing dissolved Fe2+ and magnetite or goethite. Solution concentrations, PHREEQC and the MINTEQ database were used to calculate reaction constants for the reactions: 2NaFeII6FeIII3(OH)18(SO4)2(s)+12H+(aq)⇆9Fe2+(aq)+2Na+(aq)+4SO42-(aq)+3FeIIFeIII2O4(s)+24H2O(l),K=1054.5±3.0 and NaFeII6FeIII3(OH)18(SO4)2(s)+9H(aq)+⇆6Fe2+(aq)+Na+(aq)+2SO42-(aq)+3α-FeOOH(s)+12H2O(l),K=1042.5±3.7.From the determined equilibrium constants and published ΔGf0 values for the other compounds, we derived ΔGf0 = −6366 ± 18 kJ/mol for anhydrous GRNa,SO4GRNa,SO4. The solubility product at 25 °C and atmospheric pressure is K = 10−210.5±3.2. It is not yet known if the extent of Na+ incorporation in GRSO4GRSO4 depends on formation pathway; it cannot be excluded that both Na-free GRSO4GRSO4 and GRNa,SO4GRNa,SO4 exist. If so, uncertainty in ΔGf0 determined from acid titration is such that the EH–pH stability fields of the two phases are statistically indistinguishable for Na+ concentrations as low as ∼30 μM (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 GRNa,SO4GRNa,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.