Solubility Products of Chloride and Sulfate Green Rust During Transformation to Magnetite

• Author / Creator

  MacPherson, Neil

• Green rust (GR) is a group of mixed FeII and FeIII hydroxide minerals and is an important phase in both modern and ancient geochemical systems. For this reason, establishing the thermodynamic properties of GR is vital to understanding and modelling its role in natural systems. In this study, batch solubility experiments were conducted on fully characterized GR(Cl) and GR(SO4) to determine their solubility products (Ksp). While the solubility products calculated here are not true solubility products because GR is a metastable phase that transforms into magnetite over the course of days, the kinetics of this transformation is slow enough to make the Ksp values useful in predicting aqueous iron concentrations. Triplicate solubility experiments of GR(Cl) and GR(SO4) were performed in pure water and simulated seawater (0.56 M ionic strength) at three intervals of GR aging: 0, 2, and 7 days. X-ray diffraction (XRD) and Mössbauer spectroscopy analyses evidenced a progressive transformation of GR into magnetite with time. The solubility experiments yielded an average Ksp for the three GR age intervals. Variation between the average Ksp as a function of GR age were statistically insignificant, thus the resulting average Ksp for each GR age were averaged to produce a grand mean log Ksp for GR(Cl) and GR(SO4) in pure water and simulated seawater. The grand mean log Ksp (Cl) in pure water and 0.56 M IS water is calculated to be -75.07  1.39 and -76.22  0.42, respectively. The grand mean log Ksp (〖SO〗4) in pure water and 0.56 M IS water are -119.72  1.03 and -126.60  1.07, respectively. Chemical speciation modelling of the concomitant dissolution of GR and magnetite showed that solution Fe concentrations were almost entirely controlled by GR dissolution. This is a key finding in predicting the role of GR in geochemical systems, as many existing models assume that magnetite controls dissolved iron concentrations rather than GR.

• Subjects / Keywords

  • iron
  • modelling
  • magnetite
  • transformation
  • sulfate green rust
  • solubility
  • chloride green rust

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-fa27-ps93

• License

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