The Gibbs free energies and enthalpies of formation of U (super 6+) phases; an empirical method of prediction

Chen, Fanrong; Ewing, Rodney C.; Clark, Sue B.

doi:10.2138/am-1999-0418

Cited by 110 publications

(94 citation statements)

References 59 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The thermodynamic geochemical code MINTEQA2 was used to evaluate the aqueous speciation of uranium in solution over the pH range being investigated using updated thermodynamic databases from various literature sources (Sergeyeva et al 1972;Langmuir 1978;Alwan and Williams 1980;O'Hare et al 1976;O'Hare et al 1988;Vochten 1990;Nguyen et al 1992;Grenthe et al 1992;Finch 1997;Chen et al 1999;Kalmykov and Choppin 2000). It is important to note that because of the complex chemistry of uranium, there is significant debate within the literature regarding the stoichiometry and the thermodynamic values assigned to aqueous uranium species and secondary mineral phases.…”

Section: 211mentioning

confidence: 99%

“…The thermodynamic geochemical code Geochemist's Workbench was used to evaluate the stability of meta-autunite over the pH range being investigated using updated thermodynamic databases from various literature sources (Bernhard et al 2001;Brooks et al 2003;Sergeyeva et al 1972;Langmuir 1978;Alwan and Williams 1980;O'Hare et al 1976;O'Hare et al 1988;Vochten 1990;Nguyen et al 1992;Grenthe et al 1992;Finch 1997;Chen et al 1999;Kalmykov and Choppin 2000;Delaney and Lundeen 1990). Figure 4.13 presents the stability of Ca-meta-autunite as calculated in Hanford groundwater as a function of pH and the log of the ratio of phosphate to bicarbonate.…”

Section: Effect Of Aqueous Phosphate On Uranium-rich Calcitementioning

confidence: 99%

See 1 more Smart Citation

DNA Profile Analysis and Interpretation

2014

DNA Analysis for Missing Person Identification in Mass Fatalities

View full text Add to dashboard Cite

SummaryThe Hanford Site is a former nuclear defense production facility. A groundwater plume containing uranium, originating from a combination of purposeful discharges of wastewater to cribs, trenches, and ponds, along with some accidental leaks and spills related to nuclear fuel fabrication activities, has persisted beneath the Hanford Site 300 Area for many years. Despite the cessation of uranium releases and the removal of shallow vadose zone source materials, the remedial action objective to lower the concentration of groundwater uranium to the U. S. Environmental Protection Agency maximum contaminant level concentration of 30 µg/L has not been achieved within the anticipated 10-year time period. Some unknown amount of contamination remains in the vadose zone beneath the lower extent of the excavation activities. Additional contamination also may remain beneath buildings and facilities in the southern portion of the 300 Area, which has not been decontaminated and decommissioned. The use of polyphosphate technology for source treatment in the vadose zone and capillary fringe is expected to accelerate the natural attenuation of uranium to thermodynamically stable uranium-phosphate minerals. This effort will complement the current 300 Area treatability test being conducted within the saturated zone (e.g., 300 Area aquifer) for in situ treatment of uranium-contaminated groundwater.Polyphosphate technology has been demonstrated for in situ precipitation of phosphate phases to control the long-term fate of uranium. A critical component of the development and testing is detailed evaluation to determine if polyphosphate technology could be modified for infiltration from ground surface or some depth of excavation to stabilize source uranium phases. This report presents results from bench-scale treatability studies conducted under site-specific conditions to optimize the polyphosphate amendment for implementation of a field-scale technology demonstration to stabilize uranium within the 300 Area vadose zone and capillary fringe of the Hanford Site. Documented in this report are data related to 1) the retardation of polyphosphate as a function of water content and pore water velocity, 2) the reaction between uranium-bearing solid phases and aqueous polyphosphate remediation technology as a function of polyphosphate composition and concentration, 3) the mechanism of autunite formation via the reaction of solid-phase calcite-bound uranium and aqueous polyphosphate remediation technology, 4) the transformation mechanism and reaction kinetics between uranyl-carbonate and -silicate minerals with the polyphosphate remedy under advective conditions, and 5) the extent and rate of uranium released and immobilized as a function of polyphosphate composition and the infiltration rate of the polyphosphate remedy. Kinetic rate law parameters were determined from single-pass flow-through experiments. Pressurized unsaturated flow tests were used to determine the effect of polyphosphate composition, concentration, and infiltration rate...

show abstract

Section: 211mentioning

confidence: 99%

Section: Effect Of Aqueous Phosphate On Uranium-rich Calcitementioning

confidence: 99%

DNA Profile Analysis and Interpretation

2014

DNA Analysis for Missing Person Identification in Mass Fatalities

View full text Add to dashboard Cite

show abstract

“…Table 2 lists U(VI) species that can form under these Eh and pH conditions in the aerobic YM repository with calcium-silica saturation. For these major U(VI) species that have been observed, Table 2 also shows their Gibbs energies of formation, G º f,298 (Freeze, Cherry 1979;Bruton, Shaw 1988;Finch 1997;Chen et al 1999). Figure 10 shows that under aerobic condition in the YM repository U(VI) species are expected to form.…”

Section: Silicates Suppress the Formation Of Calcium Hydroxidementioning

confidence: 99%

“…10 represent quartz (lower) and amorphous silica (upper) equilibria. The mineral stability fields defined by the light dashed lines are based on the Gibbs energies for the minerals given by Finch (1997 2 ) vs. log[H4SiO4] diagram for the SiO2-CaO-UO3-H2O system, the predicted stability fields for these phases replace most of the field for uranophane (Chen et al 1999).…”

Section: Silicates Suppress the Formation Of Calcium Hydroxidementioning

confidence: 99%

Cost-Effective Cementitious Material Compatible with Yucca Mountain Repository Geochemistry

Dole¹

2004

View full text Add to dashboard Cite

“…However, through the use of semi-empirical methods for estimating free energies and enthalpies of formation, 15,16 acceptable data for stability of certain uranyl silicate solid phases has been predicted. In this model uranyl hydrolysis products such as schoepite are expected to predominate when dissolved silicate is low.…”

Section: H 2 O)mentioning

confidence: 99%

Conditions Conducive to Forming Crystalline Uranyl Silicates in SRS Evaporators

Oji¹

2004

View full text Add to dashboard Cite

SUMMARYThe laboratory conditions (temperature range of 80-150º C, hydrothermal and oxidizing environment, soluble uranium and silica, atmospheric and pH conditions) under which synthetic uranyl silicates are made are almost identical to SRS evaporator conditions, with the exception of differences in sodium ion (Na + ) concentration. Synthetic uranyl silicates have been made only under low Na + concentration (<0.02 M), while attempts to synthesize uranyl silicates in this study in the presence of high Na + concentration (5.6 M), which are typical of SRS evaporators have proved unfruitful.In the presence of soluble silica and uranyl ion, uranyl silicates (sodium weeksite, sodium boltwoodite and uranophane) have been synthesized at moderately low to high pH in temperature ranges of 80-150 ºC and at less than 0.02 M Na + ion concentration in the reaction mixtures. However, in the presence of high Na + concentration the main product distribution for the same soluble silica-uranium reaction mixture shifts towards the formation of clarkeite (Na[(UO 2 )O(OH)](H 2 O) 0-1 ), a hydrated sodium uranate and not towards the formation of uranyl silicates.There is a threshold Na + concentration requirement above which uranyl silicates are not formed under laboratory conditions that are quite similar to SRS evaporator conditions. This threshold Na + concentration, which is yet to be determined, may be influenced by the sodium-to-uranium ratio in a reaction mixture.Therefore, based on the synthesis information presented in this study the following conclusions are made:• Crystalline uranyl silicates are readily formed in the presence of dissolved uranium and silica.• The presence of high Na + concentration in a reaction mixture of dissolved uranium and silica inhibits the formation of crystalline uranyl silicates.• The reaction path way favors the formation of hydrated sodium uranates over the uranyl silicates in a reaction mixture with high Na + concentration.• Crystalline uranyl silicate minerals are not easily produced under SRS evaporator conditions because of high Na + concentration in the evaporators and in the evaporator feed tanks.• The principal uranium minerals which could be produced under evaporator conditions are sodium uranium minerals or clarkeite, which could also exist in the dehydrated form called sodium uranate.

show abstract

The Gibbs free energies and enthalpies of formation of U (super 6+) phases; an empirical method of prediction

Cited by 110 publications

References 59 publications

DNA Profile Analysis and Interpretation

DNA Profile Analysis and Interpretation

Cost-Effective Cementitious Material Compatible with Yucca Mountain Repository Geochemistry

Conditions Conducive to Forming Crystalline Uranyl Silicates in SRS Evaporators

Contact Info

Product

Resources

About