Experiments and Thermodynamic Modeling of Chukanovite (Fe 2 (OH) 2 CO 3 ) to High Ionic Strengths
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Experiments and Thermodynamic Modeling of Chukanovite (Fe2(OH)2CO3) to High Ionic Strengths Sungtae Kim1, Justin Dean1, Jandi Knox1, Leslie Kirkes1 and Je-Hun Jang1 1 Sandia National Laboratories, 4100 National Parks Highway, Carlsbad, NM 88220, U.S.A. ABSTRACT While conducting siderite (FeCO3) solubility experiments in NaCl-Na2CO3 brines, evidence for a second phase was detected. Experiments, in which synthesized siderite was reacted with high ionic strength (0.18 – 7.5 m) solutions at room temperature and high pH (>10), were conducted in a glovebox. As the aging time of siderite-bearing experiments increased, the pH of the solution decreased, signaling formation of a hydroxyl-bearing phase. Decreasing pH values are interpreted to indicate that a hydroxyl-bearing phase, such as chukanovite, is the reaction controlling solid in the solid assemblage. Chukanovite was tentatively identified by XRD analysis. We set out, therefore, to determine the thermodynamic stability of chukanovite under the experimental conditions. Aqueous thermodynamic model parameters were determined with experimentally analyzed Fe(II) solubility data, and subsequently yielded a proposed formation free energy of chukanovite (-1149.8 kJ/mol). INTRODUCTION The Waste Isolation Pilot Plant (WIPP) repository uses steel in waste containers and lead in shielded containers. Dissolved Fe(II) and Pb(II) species interact strongly with anions such as sulfide and carbonate, and thereby compete with the transuranic species for organic ligands such as citrate, EDTA and oxalate in brine. Potential radionuclide releases to the environment after decommissioning require determining the solubility of the radionuclides in the brine. The solubility of radionuclides of concern are affected not only by the availability of complexing ligands, but also by the concentrations of Fe(II) in equilibrium with solubility-limiting phases. The solution chemistry for brines relevant to the WIPP repository is determined by using the Pitzer model to evaluate the activity coefficients for the various components of the brine. As part of updating the WIPP thermodynamic database, the Pitzer interaction parameters necessary for modeling are being determined. Accordingly, the thermodynamic interaction between Fe(II) and carbonate (CO32-) were explored in siderite-added WIPP relevant brines at room temperature and high pH (> 10). During these siderite solubility experiments we detected evidence for the presence of a second phase, tentatively identified as chukanovite [Fe2(OH)2CO3], and we began to explore the potential effects this phase might exert on the availability of organic ligands and, ultimately, the mobility of radionuclide elements. EXPERIMENTAL DETAILS
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To mimic an environment consistent with the expected anoxic WIPP conditions, experiment
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