Solubility Model for Ferrous Iron Hydroxide, Hibbingite, Siderite, and Chukanovite in High Saline Solutions of Sodium Chloride, Sodium Sulfate, and Sodium Carbonate

A solubility model is presented for ferrous iron hydroxide (Fe­(OH)₂(s)), hibbingite (Fe₂Cl­(OH)₃(s)), siderite (FeCO₃(s)), and chukanovite (Fe₂CO₃(OH)₂(s)). The Pitzer activity coefficient equation was utilized in developing the model to account for the excess free energies of aqueous species in the background solutions of high ionic strength. Solubility limiting minerals were analyzed before and after experiments using X-ray diffraction. Formation of Fe­(OH)₂(s) was observed in the experiments that were initiated with Fe₂Cl­(OH)₃(s) in Na₂SO₄ solution. Coexistence of siderite and chukanovite was observed in the experiments in Na₂CO₃ + NaCl solutions. Two equilibrium constants that had been reported by us for the dissolution of Fe­(OH)₂(s) and Fe₂Cl­(OH)₃(s) (Nemer et al.) were rederived in this paper, using newer thermodynamic data selected from the literature to maintain internal consistency of the series of our data analyses in preparation, including this paper. Three additional equilibrium constants for the following reactions were determined in this paper: dissolution of siderite and chukanovite and dissociation of the aqueous species Fe­(CO₃)₂^–2. Five Pitzer interaction parameters were derived in this paper: β⁽⁰⁾, β⁽¹⁾, and <i>C</i>^φ parameters for the species pair Fe⁺²/SO₄^–2; β⁽⁰⁾ and β⁽¹⁾ parameters for the species pair Na⁺/Fe­(CO₃)₂^–2. Our model predicts that, among the four inorganic ferrous iron minerals, siderite is the stable mineral in two WIPP-related brines (WIPP: Waste Isolation Pilot Plant), i.e., GWB and ERDA6 (Brush and Domski), and the electrochemical equilibrium between elemental iron and siderite provides a low oxygen fugacity (10^–91.2 atm) that can keep the actinides at their lowest oxidation states. (Nemer et al., Brush and Domski; references numbered 1 and 2 in the main text)