In a recent study, sulphate-bearing green rust (GRSO4) was shown to incorporate Na+ in its structure (NaFeII6FeIII3(OH)18(SO4)2(s); GRNa,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,SO4 was titrated with 0.5M 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±18kJ/mol for anhydrous GRNa,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 GRSO4 depends on formation pathway; it cannot be excluded that both Na-free GRSO4 and GRNa,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 ∼30μM (2 SD level; 0.036M 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,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.
- Programområde 2: Vandressourcer