Rare earth elements (REE) are critical elements found in monazite, xenotime, and hydrated REE phosphates which typically form in hydrothermal mineral deposits. Accurate predictions of the solubility of these REE phosphates and the speciation of REE in aqueous fluids are both key to understanding the controls on the transport, fractionation, and deposition of REE in natural systems. Previous monazite and xenotime solubility experiments indicate the presence of large discrepancies between experimentally derived solubility constants versus calculated solubilities by combining different data sources for the thermodynamic properties of minerals and aqueous species at hydrothermal conditions. In this study, these discrepancies were resolved by using the program GEMSFITS to optimize the standard partial molal Gibbs energy of formation (ΔfG°298) of REE aqueous species (REE3+ and REE hydroxyl complexes) at 298.15 K and 1 bar while keeping the thermodynamic properties fixed for the REE phosphates. A comprehensive experimental database was compiled using solubility data available between 25 and 300 °C. The latter permits conducting thermodynamic parameter optimization of ΔfG°298 for REE aqueous species. Optimal matching of the rhabdophane solubility data between 25 and 100 °C requires modifying the ΔfG°298 values of REE3+ by 1–6 kJ/mol, whereas matching of the monazite solubility data between 100 and 300 °C requires modifying the ΔfG°298 values of both REE3+ and REEOH2+ by ∼ 2–10 kJ/mol and ∼ 15–31 kJ/mol, respectively. For xenotime, adjustments of ΔfG°298 values by 1–26 kJ/mol are only necessary for the REE3+ species. The optimizations indicate that the solubility of monazite in acidic solutions is controlled by the light (L)REE3+ species at <150 °C and the LREEOH2+ species at >150 °C, whereas the solubility of xenotime is controlled by the heavy (H)REE3+ species between 25 and 300 °C. Based on the optimization results, we conclude that the revised Helgeson-Kirkham-Flowers equation of state does not reliably predict the thermodynamic properties of REE3+, REEOH2+, and likely other REE hydroxyl species at hydrothermal conditions. We therefore provide an experimental database (ThermoExp_REE) as a basic framework for future updates, extensions with other ligands, and optimizations as new experimental REE data become available. The optimized thermodynamic properties of aqueous species and minerals are available open access to accurately predict the solubility of REE phosphates in fluid-rock systems.
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pH swing cycle for CO 2 capture electrochemically driven through proton-coupled electron transfer
We perform a thermodynamic analysis of the energetic cost of CO 2 separation from flue gas (0.1 bar CO 2 (g)) and air (400 ppm CO 2 ) using a pH swing created by electrochemical redox reactions involving proton-coupled electron transfer from molecular species in aqueous electrolyte. In this scheme, electrochemical reduction of these molecules results in the formation of alkaline solution, into which CO 2 is absorbed; subsequent electrochemical oxidation of the reduced molecules results in the acidification of the solution, triggering the release of pure CO 2 gas. We examined the effect of buffering from the CO 2 –carbonate system on the solution pH during the cycle, and thereby on the open-circuit potential of an electrochemical cell in an idealized four-process CO 2 capture-release cycle. The minimum work input varies from 16 to 75 kJ mol CO2 −1 as throughput increases, for both flue gas and direct air capture, with the potential to go substantially lower if CO 2 capture or release is performed simultaneously with electrochemical reduction or oxidation. We discuss the properties required of molecules that would be suitable for such a cycle. We also demonstrate multiple experimental cycles of an electrochemical CO 2 capture and release system using 0.078 M sodium 3,3′-(phenazine-2,3-diylbis(oxy))bis(propane-1-sulfonate) as the proton carrier in an aqueous flow cell. CO 2 capture and release are both performed at 0.465 bar at a variety of current densities. When extrapolated to infinitesimal current density we obtain an experimental cycle work of 47.0 kJ mol CO2 −1 . This result suggests that, in the presence of a 0.465 bar/1.0 bar inlet/outlet pressure ratio, a 1.9 kJ mol CO2 −1 thermodynamic penalty should add to the measured value, yielding an energy cost of 48.9 kJ mol CO2 −1 in the low-current-density limit. This result is within a factor of two of the ideal cycle work of 34 kJ mol CO2 −1 for capturing at 0.465 bar and releasing at 1.0 bar. The ideal cycle work and experimental cycle work values are compared with those for other electrochemical and thermal CO 2 separation methods.
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- Award ID(s):
- 1914543
- NSF-PAR ID:
- 10312649
- Date Published:
- Journal Name:
- Energy & Environmental Science
- Volume:
- 13
- Issue:
- 10
- ISSN:
- 1754-5692
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Experimental determination of quartz solubility in H2O-CaCl2 solutions at 600–900 °C and 0.6–1.4 GPamore » « less
Abstract Fluid-mediated calcium metasomatism is often associated with strong silica mobility and the presence of chlorides in solution. To help quantify mass transfer at lower crustal and upper mantle conditions, we measured quartz solubility in H2O-CaCl2 solutions at 0.6–1.4 GPa, 600–900 °C, and salt concentrations to 50 mol%. Solubility was determined by weight loss of single-crystals using hydrothermal piston-cylinder methods. All experiments were conducted at salinity lower than salt saturation. Quartz solubility declines exponentially with added CaCl2 at all conditions investigated, with no evidence for complexing between silica and Ca. The decline in solubility is similar to that in H2O-CO2 but substantially greater than that in H2O-NaCl at the same pressure and temperature. At each temperature, quartz solubility at low salinity (XCaCl2 < 0.1) depends strongly on pressure, whereas at higher XCaCl2 it is nearly pressure independent. This behavior is consistent with a transition from an aqueous solvent to a molten salt near XCaCl2 ~0.1. The solubility data were used to develop a thermodynamic model of H2O-CaCl2 fluids. Assuming ideal molten-salt behavior and utilizing previous models for polymerization of hydrous silica, we derived values for the activity of H2O (aH2O), and for the CaCl2 dissociation factor (α), which may vary from 0 (fully associated) to 2 (fully dissociated). The model accurately reproduces our data along with those of previous work and implies that, at conditions of this study, CaCl2 is largely associated (<0.2) at H2O density <0.85 g/cm3. Dissociation rises isothermally with increasing density, reaching ~1.4 at 600 °C, 1.4 GPa. The variation in silica molality with aH2O in H2O-CaCl2 is nearly identical to that in H2O-CO2 solutions at 800 °C and 1.0 GPa, consistent with the absence of Ca-silicate complexing. The results suggest that the ionization state of the salt solution is an important determinant of aH2O, and that H2O-CaCl2 fluids exhibit nearly ideal molecular mixing over a wider range of conditions than implied by previous modeling. The new data help interpret natural examples of large-scale Ca-metasomatism in a wide range of lower crustal and upper mantle settings.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0EE01834A
