This paper reports methods for obtaining time-dependent reduced isotropic Raman spectra of aqueous species in quartz capillary high-pressure optical cells under hydrothermal conditions, as a means of determining quantitative speciation in hydrothermal fluids. The methods have been used to determine relative Raman scattering coefficients and to examine the thermal decomposition kinetics of the non-complexing anions bisulfate (HSO4(-)), perchlorate (CIO4(-)), perrhenate (ReO4(-)), and trifluoromethanesulfonate, or "triflate" (CF3SO3(-)) in acidic and neutral solutions at temperatures up to 400°C and 30 MPa. Arrhenius expressions for calculating the thermal decomposition rate constants are also reported. Thermal stabilities in the acidic solutions followed the order HSO4(-) (stable) > ReO4(-) > CIO4(-) > CF3SO3(-), with half-lives (t1/2) > 7 h at 300°C. In neutral solutions, the order was HSO4(-) (stable) > CF3SO3(-) > ReO4(-) > CIO4(-), with t1/2 > 8 h at 350°C. CF3SO3(-) was extremely stable in neutral solutions, with t1/2 > 11 h at 400°C.
Rare earth elements (REEs), the 15
naturally occurring lanthanides
plus yttrium and scandium, are ubiquitously used in modern life as
they are critical components of many advanced devices and technologies.
However, the demand for REEs is not equal, with the heavy rare earth
elements (HREEs) having a higher demand. Xenotime (HREEPO4) is an important HREE ore mineral and globally is an economical
source of HREE. Most of the crystallographic and thermodynamic properties
of xenotime endmembers have been elucidated by calorimetric, solubility,
and high-pressure studies. Yet, in natural systems, endmembers are
rarely encountered, and instead, REE solid solutions are more commonly
observed. In this work, we characterize the crystal chemistry, thermodynamics
of HREE mixing, and high-temperature material behaviors and thermochemistry
of a synthetic erbium (Er)–ytterbium (Yb) binary xenotime solid
solution (Er(x)Yb(1–x)PO4) using a suite of experimental techniques,
including X-ray fluorescence spectroscopy, synchrotron X-ray powder
diffraction implemented with Rietveld analysis, Fourier transform
infrared spectroscopy coupled with attenuated total reflectance, Raman
spectroscopy, thermogravimetric analysis coupled with differential
scanning calorimetry, and high-temperature oxide melt drop solution
calorimetry. Our results shed light on the formation of natural xenotimes
and lay the foundation for their industrial applications as thermal
coating materials.
Quantitative understanding of uranium transport by high temperature fluids is crucial for confident assessment of its migration in a number of natural and artificially induced contexts, such as hydrothermal uranium ore deposits and nuclear waste stored in geological repositories. An additional recent and atypical context would be the seawater inundated fuel of the Fukushima Daiichi Nuclear Power Plant. Given its wide applicability, understanding uranium transport will be useful regardless of whether nuclear power finds increased or decreased adoption in the future. The amount of uranium that can be carried by geofluids is enhanced by the formation of complexes with inorganic ligands. Carbonate has long been touted as a critical transporting ligand for uranium in both ore deposit and waste repository contexts. However, this paradigm has only been supported by experiments conducted at ambient conditions. We have experimentally evaluated the ability of carbonate-bearing fluids to dissolve (and therefore transport) uranium at high temperature, and discovered that in fact, at temperatures above 100 °C, carbonate becomes almost completely irrelevant as a transporting ligand. This demands a re-evaluation of a number of hydrothermal uranium transport models, as carbonate can no longer be considered key to the formation of uranium ore deposits or as an enabler of uranium transport from nuclear waste repositories at elevated temperatures.
Quantitative
first and second formation constants of aqueous uranyl
sulfate complexes were obtained from Raman spectra of solutions in
fused silica capillary cells at 25 MPa, at temperatures ranging from
25 to 375 °C. Temperature-dependent values of the symmetric O–U–O
vibrational frequencies of UO2
2+(aq), UO2SO4
0(aq), and UO2(SO4)2
2–(aq) were determined from
the high-temperature spectra. Temperature-independent Raman scattering
coefficients of UO2
2+(aq) were calculated directly
from uranyl triflate spectra from 25 to 300 °C, while those of
UO2SO4
0(aq) and UO2(SO4)2
2–(aq) were derived from spectroscopic
data at 25 °C and concentrations calculated using the formation
constants of Tian and Rao (J. Chem. Thermodyn.200941569574), together
with the Specific Ion Interaction Theory (SIT) activity coefficient
model. Chemical structures and vibrational frequencies predicted from
Density Functional Theory (Gaussian 09) were employed to interpret
the Raman spectra. Values of the cumulative formation constants ranged
from log β1 = 3.23 ± 0.08 and log β2 = 4.22 ± 0.15 at 25 °C, to log β1 = 12.35 ± 0.22 and log β2 =
14.97 ± 0.02 at 350 °C. This is the first reported use of
high-pressure fused silica capillary cells to determine formation
constants of metal ligand complexes from their reduced isotropic Raman
spectra under hydrothermal conditions.
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