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Dawn M. Wellman - One of the best experts on this subject based on the ideXlab platform.

  • The effect of bicarbonate on the microbial dissolution of Autunite mineral in the presence of gram-positive bacteria.
    Journal of environmental radioactivity, 2015
    Co-Authors: Paola M. Sepulveda-medina, Yelena Katsenovich, Dawn M. Wellman, Leonel Lagos
    Abstract:

    Bacteria are key players in the processes that govern fate and transport of contaminants. The uranium release from Na and Ca-Autunite by Arthrobacter oxydans strain G968 was evaluated in the presence of bicarbonate ions. This bacterium was previously isolated from Hanford Site soil and in earlier prescreening tests demonstrated low tolerance to U(VI) toxicity compared to other A. oxydans isolates. Experiments were conducted using glass serum bottles as mixed bioreactors and sterile 6-well cell culture plates with inserts separating bacteria cells from mineral solids. Reactors containing phosphorus-limiting media were amended with bicarbonate ranging between 0 and 10 mM and meta-Autunite solids to provide a U(VI) concentration of 4.4 mmol/L. Results showed that in the presence of bicarbonate, A. oxydans G968 was able to enhance the release of U(VI) from Na and Ca Autunite at the same capacity as other A. oxydans isolates with relatively high tolerance to U(VI). The effect of bacterial strains on Autunite dissolution decreases as the concentration of bicarbonate increases. The results illustrate that direct interaction between the bacteria and the mineral is not necessary to result in U(VI) biorelease from Autunite. The formation of secondary calcium-phosphate mineral phases on the surface of the mineral during the dissolution can ultimately reduce the natural Autunite mineral contact area, which bacterial cells can access. This thereby reduces the concentration of uranium released into the solution. This study provides a better understanding of the interactions between meta-Autunite and microbes in conditions mimicking arid and semiarid subsurface environments of western U.S.

  • Comparison of the Kinetic Rate Law Parameters for the Dissolution of Natural and Synthetic Autunite in the Presence of Aqueous Bicarbonate Ions
    Chemical Geology, 2013
    Co-Authors: R. Gudavalli, Yelena Katsenovich, Dawn M. Wellman, Melina Idarraga, Leonel Lagos, Berrin Tansel
    Abstract:

    Abstract This research evaluated the effect of aqueous hydrogen carbonate solutions on the uranium rate of release from natural Ca-Autunite and quantified the process kinetic rate law for a better prediction of the stability of Autunite-group minerals. Testing was accomplished via a single-pass flow-through (SPFT) apparatus using buffered aqueous bicarbonate solutions (0.0005 to 0.003 M) at temperatures of 23–90 °C and pH values of 7–11. The release rate of uranium from Ca-Autunite was directly correlated to increasing concentrations of hydrogen carbonate solutions and showed strong pH dependency. Ca-Autunite kinetic rate law parameters were compared to the values obtained for synthetic Na-Autunite. The power law coefficient and intrinsic rate constant were higher at pH 9–11 for Ca-Autunite than for Na-Autunite. The lower stability of Ca-Autunite was attributed to the high Ca-Autunite surface cracking, fractures and basal plane cleavages as compared to Na-Autunite and the combined effect of the formation of aqueous uranyl–carbonate and calcium uranyl carbonate species as a driving force for uranium(VI) detachment and the formation of secondary Ca–P hydroxyapatite and uranyl phosphate mineral phases as a driving force for phosphate and calcium detachment controlling the net release of elements.

  • Quantification of kinetic rate law parameters of uranium release from sodium Autunite as a function of aqueous bicarbonate concentrations
    Environmental Chemistry, 2013
    Co-Authors: R. Gudavalli, Yelena Katsenovich, Dawn M. Wellman, Leonel Lagos, Berrin Tansel
    Abstract:

    Environmental context Uranium is a key contaminant of concern because of its high persistence in the environment and toxicity to organisms. The bicarbonate ion is an important complexing agent for uranyl ions and one of the main variables affecting its dissolution. Results from this investigation provide rate law parameters for the dissolution kinetics of synthetic sodium Autunite that can influence uranium mobility in the subsurface. Abstract Hydrogen carbonate (also known as bicarbonate) is one of the most significant components within the uranium geochemical cycle. In aqueous solutions, bicarbonate forms strong complexes with uranium. As such, aqueous bicarbonate may significantly increase the rate of uranium release from uranium minerals. Quantifying the relationship of aqueous bicarbonate solutions to the rate of uranium release during dissolution is critical to understanding the long-term fate of uranium within the environment. Single-pass flow-through experiments were conducted to estimate the rate of uranium release from Na meta-Autunite as a function of bicarbonate solutions (0.0005–0.003M) over the pH range of 6–11 and temperatures of 5–60°C. Consistent with the results of previous investigations, the rate of uranium release from sodium Autunite exhibited minimal dependency on temperature, but was strongly dependent on pH and increasing concentrations of bicarbonate solutions. Most notably at pH 7, the rate of uranium release exhibited a 370-fold increase relative to the rate of uranium release in the absence of bicarbonate. However, the effect of increasing concentrations of bicarbonate solutions on the release of uranium was significantly less under higher pH conditions. It is postulated that at high pH values, surface sites are saturated with carbonate, thus the addition of more bicarbonate would have less effect on uranium release. Results indicate that the activation energies were unaffected by temperature and bicarbonate concentration variations, but were strongly dependent on pH conditions. As the pH increased from 6 to 11, the activation energy values were observed to decrease from 29.94 to 13.07kJmol–1. The calculated activation energies suggest a surface controlled dissolution mechanism.

  • Enhanced U(VI) release from Autunite mineral by aerobic Arthrobacter sp. in the presence of aqueous bicarbonate
    Chemical Geology, 2012
    Co-Authors: Yelena Katsenovich, Dawn M. Wellman, Denny A. Carvajal, Leonel Lagos
    Abstract:

    article The bacterial effect on U(VI) release from the Autunite mineral (Ca((UO2)(PO4))2·3H2O) was investigated to provide a more comprehensive understanding of the important microbiological processes affecting Autunite stability within subsurface bicarbonate-bearing environments. Experiments were performed in a culture of the Arthrobacter oxydans G975 strain, herein referred to as G975, a soil bacterium previously isolated from Hanford Site soil. 91 mg of Autunite powder and 50 mL of phosphorous-limiting sterile media were amended with bicarbonate (ranging between 1 and 10 mM) in glass reactor bottles and inoculated with the G975 strain after the dissolution of Autunite was at steady state. SEM observations indicated that G975 formed a biofilm on the Autunite surface and penetrated the mineral cleavages. The mineral surface colonization by bacteria tended to increase concomitantly with bicarbonate concentrations. Additionally, a sterile culture- ware with inserts was used in non-contact dissolution experiments where Autunite and bacteria cells were kept separately. The data suggest that G975 bacteria is able to enhance the release of U(VI) from Autunite without direct contact with the mineral. In the presence of bicarbonate, the damage to bacterial cells caused by U(VI) toxicity was reduced, yielding similar values for total organic carbon (TOC) degradation and cell density compared to U(VI)-free controls. The presence of active bacterial cells greatly enhanced the release of U(VI) from Autunite in bicarbonate-amended media. Published by Elsevier B.V.

  • 300 Area Uranium Stabilization Through Polyphosphate Injection: Final Report
    2009
    Co-Authors: Vincent R. Vermeul, Dawn M. Wellman, Bruce N. Bjornstad, Brad G. Fritz, Jonathan S. Fruchter, Rob D. Mackley, Darrell R. Newcomer, Donaldo P. Mendoza, Mark L. Rockhold, Mark D. Williams
    Abstract:

    The objective of the treatability test was to evaluate the efficacy of using polyphosphate injections to treat uranium-contaminated groundwater in situ. A test site consisting of an injection well and 15 monitoring wells was installed in the 300 Area near the process trenches that had previously received uranium-bearing effluents. This report summarizes the work on the polyphosphate injection project, including bench-scale laboratory studies, a field injection test, and the subsequent analysis and interpretation of the results. Previous laboratory tests have demonstrated that when a soluble form of polyphosphate is injected into uranium-bearing saturated porous media, immobilization of uranium occurs due to formation of an insoluble uranyl phosphate, Autunite [Ca(UO2)2(PO4)2•nH2O]. These tests were conducted at conditions expected for the aquifer and used Hanford soils and groundwater containing very low concentrations of uranium (10-6 M). Because Autunite sequesters uranium in the oxidized form U(VI) rather than forcing reduction to U(IV), the possibility of re-oxidation and subsequent re-mobilization is negated. Extensive testing demonstrated the very low solubility and slow dissolution kinetics of Autunite. In addition to Autunite, excess phosphorous may result in apatite mineral formation, which provides a long-term source of treatment capacity. Phosphate arrival response data indicate that, under site conditions, the polyphosphate amendment could be effectively distributed over a relatively large lateral extent, with wells located at a radial distance of 23 m (75 ft) reaching from between 40% and 60% of the injection concentration. Given these phosphate transport characteristics, direct treatment of uranium through the formation of uranyl-phosphate mineral phases (i.e., Autunite) could likely be effectively implemented at full field scale. However, formation of calcium-phosphate mineral phases using the selected three-phase approach was problematic. Although amendment arrival response data indicate some degree of overlap between the reactive species and thus potential for the formation of calcium-phosphate mineral phases (i.e., apatite formation), the efficiency of this treatment approach was relatively poor. In general, uranium performance monitoring results support the hypothesis that limited long-term treatment capacity (i.e., apatite formation) was established during the injection test. Two separate overarching issues affect the efficacy of apatite remediation for uranium sequestration within the 300 Area: 1) the efficacy of apatite for sequestering uranium under the present geochemical and hydrodynamic conditions, and 2) the formation and emplacement of apatite via polyphosphate technology. In addition, the long-term stability of uranium sequestered via apatite is dependent on the chemical speciation of uranium, surface speciation of apatite, and the mechanism of retention, which is highly susceptible to dynamic geochemical conditions. It was expected that uranium sequestration in the presence of hydroxyapatite would occur by sorption and/or surface complexation until all surface sites have been depleted, but that the high carbonate concentrations in the 300 Area would act to inhibit the transformation of sorbed uranium to chernikovite and/or Autunite. Adsorption of uranium by apatite was never considered a viable approach for in situ uranium sequestration in and of itself, because by definition, this is a reversible reaction. The efficacy of uranium sequestration by apatite assumes that the adsorbed uranium would subsequently convert to Autunite, or other stable uranium phases. Because this appears to not be the case in the 300 Area aquifer, even in locations near the river, apatite may have limited efficacy for the retention and long-term immobilization of uranium at the 300 Area site..

Peter C. Burns - One of the best experts on this subject based on the ideXlab platform.

  • thermochemical studies of x npo2 po4 h2o 3 x k rb neptunium analogs of the Autunite meta Autunite group
    Journal of Solid State Chemistry, 2020
    Co-Authors: Lei Zhang, Samuel N. Perry, Jennifer E. S. Szymanowski, Ginger E. Sigmon, Peter C. Burns
    Abstract:

    Abstract Actinide phosphate materials with Autunite or meta-Autunite type sheets consisting of vertex-sharing actinyl square bipyramids and phosphate tetrahedra have low aqueous solubility and potential applications for actinide immobilization in the environment. Neptunyl phosphates, X(NpO2)(PO4)(H2O)3, X ​= ​K+ and Rb+, have been synthesized by hydrothermal reactions and characterized for their structures and purities. Direct thermochemical studies were performed using high temperature oxide melt drop solution calorimetry. After correcting for the presence of residual fluorapatite from the synthesis, the formation enthalpies of K(NpO2)(PO4)(H2O)3 and Rb(NpO2)(PO4)(H2O)3 are determined to be −3235 ​± ​21 ​kJ/mol and −3270 ​± ​11 ​kJ/mol, respectively. The thermodynamic data presented here may be important for understanding neptunium mobility after disposal of nuclear waste in a geological repository.

  • Thermochemical studies of X(NpO2)(PO4)(H2O)3 (X = K+, Rb+), neptunium analogs of the Autunite/meta-Autunite group
    Journal of Solid State Chemistry, 2020
    Co-Authors: Lei Zhang, Samuel N. Perry, Jennifer E. S. Szymanowski, Ginger E. Sigmon, Peter C. Burns
    Abstract:

    Abstract Actinide phosphate materials with Autunite or meta-Autunite type sheets consisting of vertex-sharing actinyl square bipyramids and phosphate tetrahedra have low aqueous solubility and potential applications for actinide immobilization in the environment. Neptunyl phosphates, X(NpO2)(PO4)(H2O)3, X ​= ​K+ and Rb+, have been synthesized by hydrothermal reactions and characterized for their structures and purities. Direct thermochemical studies were performed using high temperature oxide melt drop solution calorimetry. After correcting for the presence of residual fluorapatite from the synthesis, the formation enthalpies of K(NpO2)(PO4)(H2O)3 and Rb(NpO2)(PO4)(H2O)3 are determined to be −3235 ​± ​21 ​kJ/mol and −3270 ​± ​11 ​kJ/mol, respectively. The thermodynamic data presented here may be important for understanding neptunium mobility after disposal of nuclear waste in a geological repository.

  • High-temperature calorimetric measurements of thermodynamic properties of uranyl arsenates of the meta-Autunite group
    Chemical Geology, 2018
    Co-Authors: Ewa A. Dzik, Haylie L. Lobeck, Lei Zhang, Peter C. Burns
    Abstract:

    Abstract Five uranyl arsenates of the meta-Autunite group with mono (Li, K, Cs) and divalent (Sr and Cu) cations were synthesized at room temperature and characterized. Their thermodynamic properties were measured using high-temperature oxide melt calorimetry in a molten 3Na2O·4MoO3 solvent at 976 K. Measured enthalpies of drop solution of each compound were used to calculate their respective enthalpies of formation from oxides and elements. Calculated standard state enthalpies of formation from the elements are −3301 ± 14 kJ/mol for LiUAs, −3089 ± 14 kJ/mol for KUAs, −5607 ± 15 kJ/mol for CsUAs, −6638 ± 21 kJ/mol for SrUAs, and −6161 ± 24 kJ/mol for CuUAs. These values are evaluated relative to acid-base interactions using oxide acidity expressed on Smith's scale, as well as the normalized charge deficiency per anion (NCDA), which probes relationships between structural units and interstitial complexes. Linear relationships exist between the heats of formation, acidity of binary oxides, and bond requirements within these meta-Autunite type structures. The new thermodynamic data combined with published results yield insights into the thermodynamic behavior of uranyl arsenates. Thermodynamic properties presented here are important for understanding the genesis of uranium deposits and uranium mobility in the subsurface.

  • thermodynamic properties of phosphate members of the meta Autunite group a high temperature calorimetric study
    The Journal of Chemical Thermodynamics, 2017
    Co-Authors: Ewa A. Dzik, Haylie L. Lobeck, Lei Zhang, Peter C. Burns
    Abstract:

    Abstract Samples of synthetic analogs of uranyl phosphate minerals have been prepared at room temperature by slow mixing of reactants by a diffusion method. Reaction products were analyzed using powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), inductively coupled plasma optical emission spectrophotometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS). Calorimetric measurements have been performed in a Calvet-type twin calorimeter using sodium molybdate (3Na2O-4MoO3) solvent at 976 K as a flux. The enthalpy of formation from the binary oxides, ΔHf-ox, at 298 K was calculated for each compound from the respective drop solution enthalpies, ΔHds. Calculated standard enthalpies of formation from the elements, ΔH0f, at 298 K are −3425 ± 9 kJ/mol for meta-ankoleite (KUP), −6233 ± 17 kJ/mol for meta-Autunite (CaUP), −6921 ± 13 kJ/mol for meta-torbernite (CuUP), −7254 ± 17 kJ/mol for meta-saleeite (MgUP), −3264 ± 12 kJ/mol for Rb-meta-Autunite (RbUP), −3580 ± 7 kJ/mol for meta-natro-Autunite (NaUP), −3692 ± 11 kJ/mol for Li-meta-Autunite (LiUP), −6402 ± 5 kJ/mol for meta-uranocircite (BaUP), −3277 ± 6 kJ/mol for Cs-meta-Autunite (CsUP), and −7109 ± 19 kJ/mol for Co-meta-Autunite (CoUP). The results exhibit trends of the thermodynamic stability of these compounds. The normalized charge deficiency per anion (NCDA) approach relates the thermodynamic stability of these compounds to their crystal structures. The thermodynamic stability of uranyl phosphate minerals is important for understanding their formation in Nature, as well as their fate in a geological repository for nuclear waste, and their existence in the subsurface of anthropogenically contaminated environments.

  • Thermodynamic characterization of synthetic Autunite
    American Mineralogist, 2017
    Co-Authors: Ewa A. Dzik, Haylie L. Lobeck, Lei Zhang, Peter C. Burns
    Abstract:

    Autunite, Ca[(UO 2 )(PO 4 )] 2 (H 2 O) 11 , is a common uranyl mineral found in oxidized portions of uranium deposits, as well as subsurface environments contaminated by uranium. Enthalpies of formation of Autunite were obtained via high-temperature oxide melt calorimetry using a 3Na 2 O·4MoO 3 solvent at 976 K. The synthetic analog of Autunite was prepared using slow mixing by diffusion into an aqueous barrier solution at room temperature. Prior to calorimetric measurements, the material was characterized using powder X-ray diffraction (PXRD), inductively coupled plasma optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), and Raman spectroscopy, to ensure purity. The calculated enthalpy of formation from binary oxides of Autunite is −579.92 ± 21.68 kJ/mol; the enthalpy of formation from the elements is −8311.32 ± 21.79 kJ/mol. The measured drop solution enthalpy allowed calculation of the enthalpy of the reaction of dehydration of Autunite to meta-Autunite. The results demonstrate that Autunite is a metastable phase and explain the observed rapid dehydration to meta-Autunite, a lower hydrate, as well as the common occurrence of the latter mineral in nature.

Steven W. Forrester - One of the best experts on this subject based on the ideXlab platform.

  • Dissolution kinetics of synthetic and natural meta‐Autunite minerals, X3−n(n)+ [(UO2)(PO4)]2 · xH2O, under acidic conditions
    Geochemistry Geophysics Geosystems, 2007
    Co-Authors: Dawn M. Wellman, Jonathan P. Icenhower, Katie M. Gunderson, Steven W. Forrester
    Abstract:

    [1] Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO22+) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exerts a strong influence on uranium transport. Autunite group minerals, X3 − n(n)+ [(UO2)(PO4)]2 · xH2O, have been identified as the long-term uranium-controlling phases in many systems of geochemical interest. Anthropogenic operations related to uranium mining operations have created acidic environments exposing uranyl phosphate minerals to low-pH groundwaters. Investigations regarding the dissolution behavior of Autunite group minerals under acidic conditions have not been reported; consequently, knowledge of the longevity of uranium-controlling solids is incomplete. The purpose of this investigation was threefold: (1) to quantify the dissolution kinetics of natural calcium meta-Autunite, Ca[(UO2)2(PO4)2] · 3H2O, and synthetic sodium meta-Autunite, Na2[(UO2)2(PO4)2] · 3H2O, under acidic conditions; (2) to measure the effect of temperature and pH on meta-Autunite mineral dissolution; and (3) to investigate the formation of secondary uranyl phosphate phases as long-term controls on uranium migration. Single-pass flow-through (SPFT) dissolution tests were conducted over the pH range of 2 to 5 and from 5° to 70°C. Results presented here illustrate meta-Autunite dissolution kinetics are strongly dependent on pH but are relatively insensitive to temperature variations. In addition, the formation of secondary uranyl-phosphate phases such as uranyl phosphate, (UO2)3(PO4)2 · x H2O, may serve as a secondary phase limiting the migration of uranium in the environment.

  • dissolution kinetics of synthetic and natural meta Autunite minerals x3 n n uo2 po4 2 xh2o under acidic conditions
    Geochemistry Geophysics Geosystems, 2007
    Co-Authors: Dawn M. Wellman, Jonathan P. Icenhower, Katie M. Gunderson, Steven W. Forrester
    Abstract:

    [1] Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO22+) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exerts a strong influence on uranium transport. Autunite group minerals, X3 − n(n)+ [(UO2)(PO4)]2 · xH2O, have been identified as the long-term uranium-controlling phases in many systems of geochemical interest. Anthropogenic operations related to uranium mining operations have created acidic environments exposing uranyl phosphate minerals to low-pH groundwaters. Investigations regarding the dissolution behavior of Autunite group minerals under acidic conditions have not been reported; consequently, knowledge of the longevity of uranium-controlling solids is incomplete. The purpose of this investigation was threefold: (1) to quantify the dissolution kinetics of natural calcium meta-Autunite, Ca[(UO2)2(PO4)2] · 3H2O, and synthetic sodium meta-Autunite, Na2[(UO2)2(PO4)2] · 3H2O, under acidic conditions; (2) to measure the effect of temperature and pH on meta-Autunite mineral dissolution; and (3) to investigate the formation of secondary uranyl phosphate phases as long-term controls on uranium migration. Single-pass flow-through (SPFT) dissolution tests were conducted over the pH range of 2 to 5 and from 5° to 70°C. Results presented here illustrate meta-Autunite dissolution kinetics are strongly dependent on pH but are relatively insensitive to temperature variations. In addition, the formation of secondary uranyl-phosphate phases such as uranyl phosphate, (UO2)3(PO4)2 · x H2O, may serve as a secondary phase limiting the migration of uranium in the environment.

  • effects of ph temperature and aqueous organic material on the dissolution kinetics of meta Autunite minerals na ca 2 1 uo2 po4 2 3h2o
    American Mineralogist, 2006
    Co-Authors: Dawn M. Wellman, Jonathan P. Icenhower, Amy P. Gamerdinger, Steven W. Forrester
    Abstract:

    Autunite minerals have been frequently identified in contaminated sediments as the long-term controlling phase of uranium. Under these conditions the mobility of uranium in subsurface pore waters is limited by the rate of dissolution of Autunite and meta-Autunite group minerals, [(UO2)(PO4)]2 ? xH2O. Single-pass flow-through (SPFT) tests were conducted to quantify the dissolution kinetics of natural calcium meta-Autunite, Ca[(UO2)2(PO4)2]2 ? 3H2O, and synthetic sodium meta-Autunite, Na2[(UO2)(PO4)]2 ? 3H2O, as a function of pH (7 -10) and temperature (5 ? 70 C) in the presence and absence of aqueous organic material. The data indicate that release of uranium and phosphorus are non-stoichiometric over the range of experimental conditions investigated. In a 0.1 M NH4OH buffer solution, acquisition of valid dissolution rate data was limited by uramphite solubility, NH4[(UO2)(PO4)]2 ? xH2O. Dissolution rates obtained in a 0.01 M TRIS [tris (hydroxymethyl) aminomethane] buffered solution increased by a factor of {approx}100X over the pH interval of 7 to 10 (? = 0.90?0.08), irrespective of temperature. At constant pH the rate data showed a minor increase with temperature. Data from experiments using a more concentrated 0.05 M TRIS buffer exhibited a {approx}35-fold increase in rates compared to those in a 0.01 M TRIS buffermore » at constant temperature and pH. The difference in rate between interlayer cation (Na+ or Ca2+) and uranium release is {approx}10,000 in neutral solutions; however, the difference diminishes to {approx}10 at higher pH values. The combination of structural dissolution and ion exchange explain these trends in interlayer cation behavior. Data presented here illustrate the significance of pH and dissolved organic material on the dissolution of Autunite minerals.« less

Lei Zhang - One of the best experts on this subject based on the ideXlab platform.

  • thermochemical studies of x npo2 po4 h2o 3 x k rb neptunium analogs of the Autunite meta Autunite group
    Journal of Solid State Chemistry, 2020
    Co-Authors: Lei Zhang, Samuel N. Perry, Jennifer E. S. Szymanowski, Ginger E. Sigmon, Peter C. Burns
    Abstract:

    Abstract Actinide phosphate materials with Autunite or meta-Autunite type sheets consisting of vertex-sharing actinyl square bipyramids and phosphate tetrahedra have low aqueous solubility and potential applications for actinide immobilization in the environment. Neptunyl phosphates, X(NpO2)(PO4)(H2O)3, X ​= ​K+ and Rb+, have been synthesized by hydrothermal reactions and characterized for their structures and purities. Direct thermochemical studies were performed using high temperature oxide melt drop solution calorimetry. After correcting for the presence of residual fluorapatite from the synthesis, the formation enthalpies of K(NpO2)(PO4)(H2O)3 and Rb(NpO2)(PO4)(H2O)3 are determined to be −3235 ​± ​21 ​kJ/mol and −3270 ​± ​11 ​kJ/mol, respectively. The thermodynamic data presented here may be important for understanding neptunium mobility after disposal of nuclear waste in a geological repository.

  • Thermochemical studies of X(NpO2)(PO4)(H2O)3 (X = K+, Rb+), neptunium analogs of the Autunite/meta-Autunite group
    Journal of Solid State Chemistry, 2020
    Co-Authors: Lei Zhang, Samuel N. Perry, Jennifer E. S. Szymanowski, Ginger E. Sigmon, Peter C. Burns
    Abstract:

    Abstract Actinide phosphate materials with Autunite or meta-Autunite type sheets consisting of vertex-sharing actinyl square bipyramids and phosphate tetrahedra have low aqueous solubility and potential applications for actinide immobilization in the environment. Neptunyl phosphates, X(NpO2)(PO4)(H2O)3, X ​= ​K+ and Rb+, have been synthesized by hydrothermal reactions and characterized for their structures and purities. Direct thermochemical studies were performed using high temperature oxide melt drop solution calorimetry. After correcting for the presence of residual fluorapatite from the synthesis, the formation enthalpies of K(NpO2)(PO4)(H2O)3 and Rb(NpO2)(PO4)(H2O)3 are determined to be −3235 ​± ​21 ​kJ/mol and −3270 ​± ​11 ​kJ/mol, respectively. The thermodynamic data presented here may be important for understanding neptunium mobility after disposal of nuclear waste in a geological repository.

  • High-temperature calorimetric measurements of thermodynamic properties of uranyl arsenates of the meta-Autunite group
    Chemical Geology, 2018
    Co-Authors: Ewa A. Dzik, Haylie L. Lobeck, Lei Zhang, Peter C. Burns
    Abstract:

    Abstract Five uranyl arsenates of the meta-Autunite group with mono (Li, K, Cs) and divalent (Sr and Cu) cations were synthesized at room temperature and characterized. Their thermodynamic properties were measured using high-temperature oxide melt calorimetry in a molten 3Na2O·4MoO3 solvent at 976 K. Measured enthalpies of drop solution of each compound were used to calculate their respective enthalpies of formation from oxides and elements. Calculated standard state enthalpies of formation from the elements are −3301 ± 14 kJ/mol for LiUAs, −3089 ± 14 kJ/mol for KUAs, −5607 ± 15 kJ/mol for CsUAs, −6638 ± 21 kJ/mol for SrUAs, and −6161 ± 24 kJ/mol for CuUAs. These values are evaluated relative to acid-base interactions using oxide acidity expressed on Smith's scale, as well as the normalized charge deficiency per anion (NCDA), which probes relationships between structural units and interstitial complexes. Linear relationships exist between the heats of formation, acidity of binary oxides, and bond requirements within these meta-Autunite type structures. The new thermodynamic data combined with published results yield insights into the thermodynamic behavior of uranyl arsenates. Thermodynamic properties presented here are important for understanding the genesis of uranium deposits and uranium mobility in the subsurface.

  • thermodynamic properties of phosphate members of the meta Autunite group a high temperature calorimetric study
    The Journal of Chemical Thermodynamics, 2017
    Co-Authors: Ewa A. Dzik, Haylie L. Lobeck, Lei Zhang, Peter C. Burns
    Abstract:

    Abstract Samples of synthetic analogs of uranyl phosphate minerals have been prepared at room temperature by slow mixing of reactants by a diffusion method. Reaction products were analyzed using powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), inductively coupled plasma optical emission spectrophotometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS). Calorimetric measurements have been performed in a Calvet-type twin calorimeter using sodium molybdate (3Na2O-4MoO3) solvent at 976 K as a flux. The enthalpy of formation from the binary oxides, ΔHf-ox, at 298 K was calculated for each compound from the respective drop solution enthalpies, ΔHds. Calculated standard enthalpies of formation from the elements, ΔH0f, at 298 K are −3425 ± 9 kJ/mol for meta-ankoleite (KUP), −6233 ± 17 kJ/mol for meta-Autunite (CaUP), −6921 ± 13 kJ/mol for meta-torbernite (CuUP), −7254 ± 17 kJ/mol for meta-saleeite (MgUP), −3264 ± 12 kJ/mol for Rb-meta-Autunite (RbUP), −3580 ± 7 kJ/mol for meta-natro-Autunite (NaUP), −3692 ± 11 kJ/mol for Li-meta-Autunite (LiUP), −6402 ± 5 kJ/mol for meta-uranocircite (BaUP), −3277 ± 6 kJ/mol for Cs-meta-Autunite (CsUP), and −7109 ± 19 kJ/mol for Co-meta-Autunite (CoUP). The results exhibit trends of the thermodynamic stability of these compounds. The normalized charge deficiency per anion (NCDA) approach relates the thermodynamic stability of these compounds to their crystal structures. The thermodynamic stability of uranyl phosphate minerals is important for understanding their formation in Nature, as well as their fate in a geological repository for nuclear waste, and their existence in the subsurface of anthropogenically contaminated environments.

  • Thermodynamic characterization of synthetic Autunite
    American Mineralogist, 2017
    Co-Authors: Ewa A. Dzik, Haylie L. Lobeck, Lei Zhang, Peter C. Burns
    Abstract:

    Autunite, Ca[(UO 2 )(PO 4 )] 2 (H 2 O) 11 , is a common uranyl mineral found in oxidized portions of uranium deposits, as well as subsurface environments contaminated by uranium. Enthalpies of formation of Autunite were obtained via high-temperature oxide melt calorimetry using a 3Na 2 O·4MoO 3 solvent at 976 K. The synthetic analog of Autunite was prepared using slow mixing by diffusion into an aqueous barrier solution at room temperature. Prior to calorimetric measurements, the material was characterized using powder X-ray diffraction (PXRD), inductively coupled plasma optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), and Raman spectroscopy, to ensure purity. The calculated enthalpy of formation from binary oxides of Autunite is −579.92 ± 21.68 kJ/mol; the enthalpy of formation from the elements is −8311.32 ± 21.79 kJ/mol. The measured drop solution enthalpy allowed calculation of the enthalpy of the reaction of dehydration of Autunite to meta-Autunite. The results demonstrate that Autunite is a metastable phase and explain the observed rapid dehydration to meta-Autunite, a lower hydrate, as well as the common occurrence of the latter mineral in nature.

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  • dissolution kinetics of synthetic and natural meta Autunite minerals x3 n n uo2 po4 2 xh2o under acidic conditions
    Geochemistry Geophysics Geosystems, 2007
    Co-Authors: Dawn M. Wellman, Jonathan P. Icenhower, Katie M. Gunderson, Steven W. Forrester
    Abstract:

    [1] Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO22+) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exerts a strong influence on uranium transport. Autunite group minerals, X3 − n(n)+ [(UO2)(PO4)]2 · xH2O, have been identified as the long-term uranium-controlling phases in many systems of geochemical interest. Anthropogenic operations related to uranium mining operations have created acidic environments exposing uranyl phosphate minerals to low-pH groundwaters. Investigations regarding the dissolution behavior of Autunite group minerals under acidic conditions have not been reported; consequently, knowledge of the longevity of uranium-controlling solids is incomplete. The purpose of this investigation was threefold: (1) to quantify the dissolution kinetics of natural calcium meta-Autunite, Ca[(UO2)2(PO4)2] · 3H2O, and synthetic sodium meta-Autunite, Na2[(UO2)2(PO4)2] · 3H2O, under acidic conditions; (2) to measure the effect of temperature and pH on meta-Autunite mineral dissolution; and (3) to investigate the formation of secondary uranyl phosphate phases as long-term controls on uranium migration. Single-pass flow-through (SPFT) dissolution tests were conducted over the pH range of 2 to 5 and from 5° to 70°C. Results presented here illustrate meta-Autunite dissolution kinetics are strongly dependent on pH but are relatively insensitive to temperature variations. In addition, the formation of secondary uranyl-phosphate phases such as uranyl phosphate, (UO2)3(PO4)2 · x H2O, may serve as a secondary phase limiting the migration of uranium in the environment.

  • Dissolution kinetics of synthetic and natural meta‐Autunite minerals, X3−n(n)+ [(UO2)(PO4)]2 · xH2O, under acidic conditions
    Geochemistry Geophysics Geosystems, 2007
    Co-Authors: Dawn M. Wellman, Jonathan P. Icenhower, Katie M. Gunderson, Steven W. Forrester
    Abstract:

    [1] Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO22+) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exerts a strong influence on uranium transport. Autunite group minerals, X3 − n(n)+ [(UO2)(PO4)]2 · xH2O, have been identified as the long-term uranium-controlling phases in many systems of geochemical interest. Anthropogenic operations related to uranium mining operations have created acidic environments exposing uranyl phosphate minerals to low-pH groundwaters. Investigations regarding the dissolution behavior of Autunite group minerals under acidic conditions have not been reported; consequently, knowledge of the longevity of uranium-controlling solids is incomplete. The purpose of this investigation was threefold: (1) to quantify the dissolution kinetics of natural calcium meta-Autunite, Ca[(UO2)2(PO4)2] · 3H2O, and synthetic sodium meta-Autunite, Na2[(UO2)2(PO4)2] · 3H2O, under acidic conditions; (2) to measure the effect of temperature and pH on meta-Autunite mineral dissolution; and (3) to investigate the formation of secondary uranyl phosphate phases as long-term controls on uranium migration. Single-pass flow-through (SPFT) dissolution tests were conducted over the pH range of 2 to 5 and from 5° to 70°C. Results presented here illustrate meta-Autunite dissolution kinetics are strongly dependent on pH but are relatively insensitive to temperature variations. In addition, the formation of secondary uranyl-phosphate phases such as uranyl phosphate, (UO2)3(PO4)2 · x H2O, may serve as a secondary phase limiting the migration of uranium in the environment.

  • effects of ph temperature and aqueous organic material on the dissolution kinetics of meta Autunite minerals na ca 2 1 uo2 po4 2 3h2o
    American Mineralogist, 2006
    Co-Authors: Dawn M. Wellman, Jonathan P. Icenhower, Amy P. Gamerdinger, Steven W. Forrester
    Abstract:

    Autunite minerals have been frequently identified in contaminated sediments as the long-term controlling phase of uranium. Under these conditions the mobility of uranium in subsurface pore waters is limited by the rate of dissolution of Autunite and meta-Autunite group minerals, [(UO2)(PO4)]2 ? xH2O. Single-pass flow-through (SPFT) tests were conducted to quantify the dissolution kinetics of natural calcium meta-Autunite, Ca[(UO2)2(PO4)2]2 ? 3H2O, and synthetic sodium meta-Autunite, Na2[(UO2)(PO4)]2 ? 3H2O, as a function of pH (7 -10) and temperature (5 ? 70 C) in the presence and absence of aqueous organic material. The data indicate that release of uranium and phosphorus are non-stoichiometric over the range of experimental conditions investigated. In a 0.1 M NH4OH buffer solution, acquisition of valid dissolution rate data was limited by uramphite solubility, NH4[(UO2)(PO4)]2 ? xH2O. Dissolution rates obtained in a 0.01 M TRIS [tris (hydroxymethyl) aminomethane] buffered solution increased by a factor of {approx}100X over the pH interval of 7 to 10 (? = 0.90?0.08), irrespective of temperature. At constant pH the rate data showed a minor increase with temperature. Data from experiments using a more concentrated 0.05 M TRIS buffer exhibited a {approx}35-fold increase in rates compared to those in a 0.01 M TRIS buffermore » at constant temperature and pH. The difference in rate between interlayer cation (Na+ or Ca2+) and uranium release is {approx}10,000 in neutral solutions; however, the difference diminishes to {approx}10 at higher pH values. The combination of structural dissolution and ion exchange explain these trends in interlayer cation behavior. Data presented here illustrate the significance of pH and dissolved organic material on the dissolution of Autunite minerals.« less

  • Polyphosphate Amendments for In-Situ Immobilization of Uranium Plumes
    2005
    Co-Authors: Dawn M. Wellman, Jonathan P. Icenhower, Eric M. Pierce, Sarah D. Burton, Steven R. Baum, Bruce K. Mcnamara, Keith N. Geiszler, Bart C. Butler
    Abstract:

    A multi-faceted approach has been taken to address basic science questions with regards to the efficacy of utilizing phosphate amendments for subsurface immobilization of uranium plumes. Hydraulically saturated and unsaturated column tests demonstrate the ability of polyphosphate compounds to control the precipitation kinetics of insoluble phosphate minerals and optimize conditions for controlled application of phosphate amendments for subsurface remediation. X-Ray micro-focus tomography results illustrate long-term effects of phosphate mineralization on hydraulic conductivity. 31P NMR has been utilized to quantify the effect of sedimentary and aqueous components on the in-situ hydrolysis kinetics of condensed polyphosphates. Single-pass flow-through (SPFT) tests have been conducted to evaluate the longevity and quantify the effects of aqueous organic material on the dissolution kinetics of Autunite minerals, X1-2[(UO2)(PO4)]2?nH2O. Preliminary results indicate: (1) Autunite minerals will precipitate within 1-2 months given a 0.05 M phosphate concentration and 10-6 M aqueous uranium concentration, under hydraulically saturated conditions; (2) polyphosphate chain lengths can be optimized for specific site conditions, given thorough knowledge of the subsurface environment; (3) the release of uranium from Autunite minerals appears to be 6-7 order of magnitude slower than uranium (UO2) minerals formed by iron barrier reduction; and (4) understanding secondary uranyl-phase formation is necessary formore » predicting the long-term fate of uranium in the environment.« less

  • Synthesis and characterization of sodium meta-Autunite, Na[UO2PO4]·3H2O
    Radiochimica Acta, 2005
    Co-Authors: Dawn M. Wellman, Jeffrey G. Catalano, Jonathan P. Icenhower, Amy P. Gamerdinger
    Abstract:

    In-situ precipitation of Autunite minerals has been proposed as a method for remediating groundwaters, deep within the subsurface, faced with uranium contamination. Recently, it has been to utilizes long-chain sodium polyphosphate compounds as a ''time-released'' source of phosphate for precipitation of uranium-phosphate minerals. Elevated sodium concentrations presented by this technique enhance formation of sodium Autunite relative to the more common calcium Autunite mineral phase. The goal of forming Autunite minerals in-situ to remediate uranium contaminated groundwater requires a thorough understanding of the properties of sodium Autunite minerals to evaluate the longevity and efficacy of a uranium-phosphate barrier. Research presented here is part of a larger effort to quantify the solubility and dissolution properties uranium-phosphate minerals, that may form due to remediation efforts, under environmentally relevant conditions. This paper focuses on the development of a direct synthesis route for precipitating sodium Autunite, and a comparative analysis of the structural properties and differences presented in the direct versus previous indirect methods of precipitation using extended X-ray absorption fine structure (EXAFS) spectroscopy, chemical digestion followed by inductively-coupled plasma-optical emission spectroscopy and inductively-coupled plasma-mass spectroscopy for elemental analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and multi point Brunauer-Emmett-Teller (BET) analyses.