Free energy of formation for green rust sodium sulphate (NaFe II6Fe III3(OH) 18(SO 4) 2(s))

Abstract

In a recent study, sulphate-bearing green rust ( GR SO 4 ) was shown to incorporate Na + in its structure (NaFe II 6Fe III 3(OH) 18(SO 4) 2(s); GR Na, SO 4 ). 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 ( Δ G f 0 ) . Freshly synthesised GR Na, SO 4 was titrated with 0.5 M H 2SO 4 in an inert atmosphere at 25 °C, producing dissolved Fe 2+ and magnetite or goethite. Solution concentrations, PHREEQC and the MINTEQ database were used to calculate reaction constants for the reactions: 2 NaFe II 6 Fe III 3 ( OH ) 18 ( SO 4 ) 2 ( s ) + 12 H + ( aq ) ⇆ 9 Fe 2 + ( aq ) + 2 Na + ( aq ) + 4 SO 4 2 - ( aq ) + 3 Fe II Fe III 2 O 4 ( s ) + 24 H 2 O ( l ) , K = 10 54.5 ± 3.0 and NaFe II 6 Fe III 3 ( OH ) 18 ( SO 4 ) 2 ( s ) + 9 H ( aq ) + ⇆ 6 Fe 2 + ( aq ) + Na + ( aq ) + 2 SO 4 2 - ( aq ) + 3 α - FeOOH ( s ) + 12 H 2 O ( l ) , K = 10 42.5 ± 3.7 . From the determined equilibrium constants and published Δ G f 0 values for the other compounds, we derived Δ G f 0 = −6366 ± 18 kJ/mol for anhydrous GR Na, SO 4 . 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 GR SO 4 depends on formation pathway; it cannot be excluded that both Na-free GR SO 4 and GR Na, SO 4 exist. If so, uncertainty in Δ 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 ∼30 μM (2 SD level; 0.036 M SO 4 2− concentration). In sea water, where Na + and SO 4 2− concentrations are high, but soluble Fe 2+ 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, SO 4 -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.