Deriving formation constants for aqueous metal complexes from XANES spectra: Zn2+ and Fe2+ chloride complexes in hypersaline solutions
-
Weihua Liu
,
Barbara Etschmann
Abstract
The development of numerical modeling of reactive transport relies on the availability of thermodynamic properties for the solid, surface, aqueous, and vapor species stable at the conditions of interest. The lack of experimental studies and comprehensive activity-composition models severely limits the predictive capabilities of these models in systems involving highly saline fluids or low-density volatile-rich fluids. X-ray absorption near-edge structure spectroscopy (XANES) is a powerful technique to study the speciation of transition metals in aqueous fluids: it is element specific; sensitive to the oxidation state of the metal, the ligand field and the coordination geometry of the complex; and it is suitable for measuring trace amounts of metals (<1 wt%) in solutions over a wide pressure and temperature range.
Formation constants for the aqueous complexes of transition metals can be determined from a series of XANES spectra obtained on solutions containing a constant amount of the metal of interest and variable concentrations of a ligand. The method relies on a non-linear, least-squares-fitting approach, with full distribution of species calculations based on a complete thermodynamic model for the experimental system under consideration. The technique is particularly suitable for following octahedral to tetrahedral transitions among weak chloride complexes of transition metals. The log K for the reaction Zn2+(octahedral) + 4Cl- = ZnCl42-(tetrahedral) at 25 °C is retrieved to be 0.1(6), within error of the accepted literature value. The same method applied to Fe2+-chloride complexes shows that the log K for the reaction Fe2+(octahedral) + 4Cl- = FeCl42-(tetrahedral) increases from -6.2(6) at 25 °C to -2.9(3) at 150 °C. This study confirms that tetrahedral chloride complexes play an important role in Fe transport in hypersaline brines especially at elevated temperatures, and shows that XANES is well suited to study systems that may be difficult to study with other techniques.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Single-crystal FTIR and X-ray study of vishnevite, ideally [Na6(SO4)][Na2(H2O)2](Si6Al6O24)
- Crystallographic texture and microstructure of terebratulide brachiopod shell calcite: An optimized materials design with hierarchical architecture
- High-temperature phase relations and topological constraints in the quaternary system MgO-Al2O3-SiO2-Cr2O3: An experimental study
- Tilt and buckling modes, and acoustic anisotropy in layers with post-perovskite connectivity
- Optical absorption study of natural garnets of almandine-skiagite composition showing intervalence Fe2+ + Fe3+ → Fe3+ + Fe2+ charge-transfer transition
- Deriving formation constants for aqueous metal complexes from XANES spectra: Zn2+ and Fe2+ chloride complexes in hypersaline solutions
- Birnessite polytype systematics and identiÞ cation by powder X-ray diffraction
- Improved measurement of fission-track annealing in apatite using c-axis projection
- Improved modeling of fission-track annealing in apatite
- Intercalibration of FTIR and SIMS for hydrogen measurements in glasses and nominally anhydrous minerals
- Order and miscibility in the otavite–magnesite solid solution
- An infrared investigation of the otavite-magnesite solid solution
- Partitioning of calcium, magnesium, and transition metals between olivine and melt governed by the structure of the silicate melt at ambient pressure
- The geometric effects of VFe2+ for VMg substitution on the crystal structures of the grandidierite-ominelite series
- The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis
- Gram-Charlier development of the atomic displacement factors into mineral structures: The case of samsonite, Ag4MnSb2S6
- Superstructure of Challis mordenite with doubled monoclinic unit cell
- Biopyribole evolution during tremolite synthesis from dolomite and quartz in CO2-H2O fluid
- The evolution of diamond morphology in the process of dissolution: Experimental data
- The pearceite-polybasite group of minerals: Crystal chemistry and new nomenclature rules
- Determination of layer stacking microstructures and intralayer transition of illite polytypes by high-resolution transmission electron microscopy (HRTEM)
- Structural behavior of Al3+ in peralkaline, metaluminous, and peraluminous silicate melts and glasses at ambient pressure
- The crystal structure of ingersonite, Ca3Mn2+Sb45+O14, and its relationships with pyrochlore
- Chemical composition, statistical analysis of the unit cell, and electrostatic modeling of the structure of Al-saturated chlorite from metamorphosed rocks
- XANES study of the oxidation state of Cr in lower mantle phases: Periclase and magnesium silicate perovskite
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