The Experts below are selected from a list of 249 Experts worldwide ranked by ideXlab platform
Arthur Lieu - One of the best experts on this subject based on the ideXlab platform.
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uptake and speciation of uranium in Synthetic Gypsum caso4 2h2o applications to radioactive mine tailings
Journal of Environmental Radioactivity, 2018Co-Authors: Jacques K Desmarais, Ning Chen, Renfei Feng, Patrick Zhang, Dien Li, Arthur LieuAbstract:Abstract PhosphoGypsum formed from the production of phosphoric acid represents by far the biggest accumulation of Gypsum-rich wastes in the world and commonly contains elevated radionuclides, including uranium, as well as other heavy metals and metalloids. Therefore, billions-of-tons of phosphoGypsum stockpiled worldwide not only possess serious environmental problems but also represent a potential uranium resource. Gypsum is also a major solid constituent in many other types of radioactive mine tailings, which stems from the common usage of sulfuric acid in extraction processes. Therefore, management and remediation of radioactive mine tailings as well as future beneficiation of uranium from phosphogysum all require detailed knowledge about the nature and behavior of uranium in Gypsum. However, little is known about the uptake mechanism or speciation of uranium in Gypsum. In this study, synthesis experiments suggest an apparent pH control on the uptake of uranium in Gypsum at ambient conditions: increase in U from 16 μg/g at pH = 6.5 to 339 μg/g at pH = 9.5. Uranium L 3 -edge synchrotron X-ray absorption spectroscopic analyses of Synthetic Gypsum show that uranyl (UO 2 ) 2+ at the Ca site is the dominant species. The EXAFS fitting results also indicate that uranyl in Synthetic Gypsum occurs most likely as carbonate complexes and yields an average U-O distance ∼0.25 A shorter than the average Ca-O distance, signifying a marked local structural distortion. Applications to phosphoGypsum from the New Wales phosphoric acid plant (Florida, USA) and uranium mine tailings from the Key Lake mill (Saskatchewan, Canada) show that Gypsum is an important carrier of uranium over a wide range of pH and controls the fate of this radionuclide in mine tailings. Also, development of new technologies for recovering U from phosphoGypsum in the future must consider lattice-bound uranyl in Gypsum.
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Uptake and speciation of uranium in Synthetic Gypsum (CaSO4•2H2O): Applications to radioactive mine tailings.
Journal of Environmental Radioactivity, 2017Co-Authors: Jacques K Desmarais, Ning Chen, Renfei Feng, Patrick Zhang, Dien Li, Arthur LieuAbstract:Abstract PhosphoGypsum formed from the production of phosphoric acid represents by far the biggest accumulation of Gypsum-rich wastes in the world and commonly contains elevated radionuclides, including uranium, as well as other heavy metals and metalloids. Therefore, billions-of-tons of phosphoGypsum stockpiled worldwide not only possess serious environmental problems but also represent a potential uranium resource. Gypsum is also a major solid constituent in many other types of radioactive mine tailings, which stems from the common usage of sulfuric acid in extraction processes. Therefore, management and remediation of radioactive mine tailings as well as future beneficiation of uranium from phosphogysum all require detailed knowledge about the nature and behavior of uranium in Gypsum. However, little is known about the uptake mechanism or speciation of uranium in Gypsum. In this study, synthesis experiments suggest an apparent pH control on the uptake of uranium in Gypsum at ambient conditions: increase in U from 16 μg/g at pH = 6.5 to 339 μg/g at pH = 9.5. Uranium L 3 -edge synchrotron X-ray absorption spectroscopic analyses of Synthetic Gypsum show that uranyl (UO 2 ) 2+ at the Ca site is the dominant species. The EXAFS fitting results also indicate that uranyl in Synthetic Gypsum occurs most likely as carbonate complexes and yields an average U-O distance ∼0.25 A shorter than the average Ca-O distance, signifying a marked local structural distortion. Applications to phosphoGypsum from the New Wales phosphoric acid plant (Florida, USA) and uranium mine tailings from the Key Lake mill (Saskatchewan, Canada) show that Gypsum is an important carrier of uranium over a wide range of pH and controls the fate of this radionuclide in mine tailings. Also, development of new technologies for recovering U from phosphoGypsum in the future must consider lattice-bound uranyl in Gypsum.
Ning Chen - One of the best experts on this subject based on the ideXlab platform.
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uptake and speciation of uranium in Synthetic Gypsum caso4 2h2o applications to radioactive mine tailings
Journal of Environmental Radioactivity, 2018Co-Authors: Jacques K Desmarais, Ning Chen, Renfei Feng, Patrick Zhang, Dien Li, Arthur LieuAbstract:Abstract PhosphoGypsum formed from the production of phosphoric acid represents by far the biggest accumulation of Gypsum-rich wastes in the world and commonly contains elevated radionuclides, including uranium, as well as other heavy metals and metalloids. Therefore, billions-of-tons of phosphoGypsum stockpiled worldwide not only possess serious environmental problems but also represent a potential uranium resource. Gypsum is also a major solid constituent in many other types of radioactive mine tailings, which stems from the common usage of sulfuric acid in extraction processes. Therefore, management and remediation of radioactive mine tailings as well as future beneficiation of uranium from phosphogysum all require detailed knowledge about the nature and behavior of uranium in Gypsum. However, little is known about the uptake mechanism or speciation of uranium in Gypsum. In this study, synthesis experiments suggest an apparent pH control on the uptake of uranium in Gypsum at ambient conditions: increase in U from 16 μg/g at pH = 6.5 to 339 μg/g at pH = 9.5. Uranium L 3 -edge synchrotron X-ray absorption spectroscopic analyses of Synthetic Gypsum show that uranyl (UO 2 ) 2+ at the Ca site is the dominant species. The EXAFS fitting results also indicate that uranyl in Synthetic Gypsum occurs most likely as carbonate complexes and yields an average U-O distance ∼0.25 A shorter than the average Ca-O distance, signifying a marked local structural distortion. Applications to phosphoGypsum from the New Wales phosphoric acid plant (Florida, USA) and uranium mine tailings from the Key Lake mill (Saskatchewan, Canada) show that Gypsum is an important carrier of uranium over a wide range of pH and controls the fate of this radionuclide in mine tailings. Also, development of new technologies for recovering U from phosphoGypsum in the future must consider lattice-bound uranyl in Gypsum.
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Uptake and speciation of uranium in Synthetic Gypsum (CaSO4•2H2O): Applications to radioactive mine tailings.
Journal of Environmental Radioactivity, 2017Co-Authors: Jacques K Desmarais, Ning Chen, Renfei Feng, Patrick Zhang, Dien Li, Arthur LieuAbstract:Abstract PhosphoGypsum formed from the production of phosphoric acid represents by far the biggest accumulation of Gypsum-rich wastes in the world and commonly contains elevated radionuclides, including uranium, as well as other heavy metals and metalloids. Therefore, billions-of-tons of phosphoGypsum stockpiled worldwide not only possess serious environmental problems but also represent a potential uranium resource. Gypsum is also a major solid constituent in many other types of radioactive mine tailings, which stems from the common usage of sulfuric acid in extraction processes. Therefore, management and remediation of radioactive mine tailings as well as future beneficiation of uranium from phosphogysum all require detailed knowledge about the nature and behavior of uranium in Gypsum. However, little is known about the uptake mechanism or speciation of uranium in Gypsum. In this study, synthesis experiments suggest an apparent pH control on the uptake of uranium in Gypsum at ambient conditions: increase in U from 16 μg/g at pH = 6.5 to 339 μg/g at pH = 9.5. Uranium L 3 -edge synchrotron X-ray absorption spectroscopic analyses of Synthetic Gypsum show that uranyl (UO 2 ) 2+ at the Ca site is the dominant species. The EXAFS fitting results also indicate that uranyl in Synthetic Gypsum occurs most likely as carbonate complexes and yields an average U-O distance ∼0.25 A shorter than the average Ca-O distance, signifying a marked local structural distortion. Applications to phosphoGypsum from the New Wales phosphoric acid plant (Florida, USA) and uranium mine tailings from the Key Lake mill (Saskatchewan, Canada) show that Gypsum is an important carrier of uranium over a wide range of pH and controls the fate of this radionuclide in mine tailings. Also, development of new technologies for recovering U from phosphoGypsum in the future must consider lattice-bound uranyl in Gypsum.
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arsenic speciation in Synthetic Gypsum caso4 2h2o a synchrotron xas single crystal epr and pulsed endor study
Geochimica et Cosmochimica Acta, 2013Co-Authors: Ning Chen, Mark J NilgesAbstract:Abstract Gypsum (CaSO 4 ·2H 2 O) is a major by-product of mining and milling processes of borate, phosphate and uranium deposits worldwide and, therefore, potentially plays an important role in the stability and bioavailability of heavy metalloids, including As, in tailings and surrounding areas. Gypsum containing 1900 and 185 ppm As, synthesized with Na 2 HAsO 4 ·7H 2 O and NaAsO 2 in the starting materials, respectively, have been investigated by synchrotron X-ray absorption spectroscopy (XAS), single-crystal electron paramagnetic resonance spectroscopy (EPR), and pulsed electron nuclear double resonance spectroscopy (ENDOR). Quantitative analyses of As K edge XANES and EXAFS spectra show that arsenic occurs in both +3 and +5 oxidation states and the As 3+ /As 5+ value varies from 0.35 to 0.79. Single-crystal EPR spectra of gamma-ray-irradiated Gypsum reveal two types of arsenic-associated oxyradicals: [AsO 3 ] 2− and an [AsO 2 ] 2− . The [AsO 3 ] 2− center is characterized by principal 75 As hyperfine coupling constants of A 1 = 1952.0(2) MHz, A 2 = 1492.6(2) MHz and A 3 = 1488.7(2) MHz, with the unique A axis along the S–O1 bond direction, and contains complex 1 H superhyperfine structures that have been determined by pulsed ENDOR. These results suggest that the [AsO 3 ] 2− center formed from electron trapping on the central As 5+ ion of a substitutional (AsO 4 ) 3− group after removal of an O1 atom. The [AsO 2 ] 2− center is characterized by its unique A ( 75 As) axis approximately perpendicular to the O1–S–O2 plane and the A 2 axis along the S–O2 bond direction, consistent with electron trapping on the central As 3+ ion of a substitutional (AsO 3 ) 3− group after removal of an O2 atom. These results confirm lattice-bound As 5+ and As 3+ in Gypsum and point to potential application of this mineral for immobilization and removal of arsenic pollution.
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Arsenic speciation in Synthetic Gypsum (CaSO4·2H2O): A synchrotron XAS, single-crystal EPR, and pulsed ENDOR study
Geochimica et Cosmochimica Acta, 2013Co-Authors: Ning Chen, Mark J NilgesAbstract:Abstract Gypsum (CaSO 4 ·2H 2 O) is a major by-product of mining and milling processes of borate, phosphate and uranium deposits worldwide and, therefore, potentially plays an important role in the stability and bioavailability of heavy metalloids, including As, in tailings and surrounding areas. Gypsum containing 1900 and 185 ppm As, synthesized with Na 2 HAsO 4 ·7H 2 O and NaAsO 2 in the starting materials, respectively, have been investigated by synchrotron X-ray absorption spectroscopy (XAS), single-crystal electron paramagnetic resonance spectroscopy (EPR), and pulsed electron nuclear double resonance spectroscopy (ENDOR). Quantitative analyses of As K edge XANES and EXAFS spectra show that arsenic occurs in both +3 and +5 oxidation states and the As 3+ /As 5+ value varies from 0.35 to 0.79. Single-crystal EPR spectra of gamma-ray-irradiated Gypsum reveal two types of arsenic-associated oxyradicals: [AsO 3 ] 2− and an [AsO 2 ] 2− . The [AsO 3 ] 2− center is characterized by principal 75 As hyperfine coupling constants of A 1 = 1952.0(2) MHz, A 2 = 1492.6(2) MHz and A 3 = 1488.7(2) MHz, with the unique A axis along the S–O1 bond direction, and contains complex 1 H superhyperfine structures that have been determined by pulsed ENDOR. These results suggest that the [AsO 3 ] 2− center formed from electron trapping on the central As 5+ ion of a substitutional (AsO 4 ) 3− group after removal of an O1 atom. The [AsO 2 ] 2− center is characterized by its unique A ( 75 As) axis approximately perpendicular to the O1–S–O2 plane and the A 2 axis along the S–O2 bond direction, consistent with electron trapping on the central As 3+ ion of a substitutional (AsO 3 ) 3− group after removal of an O2 atom. These results confirm lattice-bound As 5+ and As 3+ in Gypsum and point to potential application of this mineral for immobilization and removal of arsenic pollution.
Mark J Nilges - One of the best experts on this subject based on the ideXlab platform.
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arsenic speciation in Synthetic Gypsum caso4 2h2o a synchrotron xas single crystal epr and pulsed endor study
Geochimica et Cosmochimica Acta, 2013Co-Authors: Ning Chen, Mark J NilgesAbstract:Abstract Gypsum (CaSO 4 ·2H 2 O) is a major by-product of mining and milling processes of borate, phosphate and uranium deposits worldwide and, therefore, potentially plays an important role in the stability and bioavailability of heavy metalloids, including As, in tailings and surrounding areas. Gypsum containing 1900 and 185 ppm As, synthesized with Na 2 HAsO 4 ·7H 2 O and NaAsO 2 in the starting materials, respectively, have been investigated by synchrotron X-ray absorption spectroscopy (XAS), single-crystal electron paramagnetic resonance spectroscopy (EPR), and pulsed electron nuclear double resonance spectroscopy (ENDOR). Quantitative analyses of As K edge XANES and EXAFS spectra show that arsenic occurs in both +3 and +5 oxidation states and the As 3+ /As 5+ value varies from 0.35 to 0.79. Single-crystal EPR spectra of gamma-ray-irradiated Gypsum reveal two types of arsenic-associated oxyradicals: [AsO 3 ] 2− and an [AsO 2 ] 2− . The [AsO 3 ] 2− center is characterized by principal 75 As hyperfine coupling constants of A 1 = 1952.0(2) MHz, A 2 = 1492.6(2) MHz and A 3 = 1488.7(2) MHz, with the unique A axis along the S–O1 bond direction, and contains complex 1 H superhyperfine structures that have been determined by pulsed ENDOR. These results suggest that the [AsO 3 ] 2− center formed from electron trapping on the central As 5+ ion of a substitutional (AsO 4 ) 3− group after removal of an O1 atom. The [AsO 2 ] 2− center is characterized by its unique A ( 75 As) axis approximately perpendicular to the O1–S–O2 plane and the A 2 axis along the S–O2 bond direction, consistent with electron trapping on the central As 3+ ion of a substitutional (AsO 3 ) 3− group after removal of an O2 atom. These results confirm lattice-bound As 5+ and As 3+ in Gypsum and point to potential application of this mineral for immobilization and removal of arsenic pollution.
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Arsenic speciation in Synthetic Gypsum (CaSO4·2H2O): A synchrotron XAS, single-crystal EPR, and pulsed ENDOR study
Geochimica et Cosmochimica Acta, 2013Co-Authors: Ning Chen, Mark J NilgesAbstract:Abstract Gypsum (CaSO 4 ·2H 2 O) is a major by-product of mining and milling processes of borate, phosphate and uranium deposits worldwide and, therefore, potentially plays an important role in the stability and bioavailability of heavy metalloids, including As, in tailings and surrounding areas. Gypsum containing 1900 and 185 ppm As, synthesized with Na 2 HAsO 4 ·7H 2 O and NaAsO 2 in the starting materials, respectively, have been investigated by synchrotron X-ray absorption spectroscopy (XAS), single-crystal electron paramagnetic resonance spectroscopy (EPR), and pulsed electron nuclear double resonance spectroscopy (ENDOR). Quantitative analyses of As K edge XANES and EXAFS spectra show that arsenic occurs in both +3 and +5 oxidation states and the As 3+ /As 5+ value varies from 0.35 to 0.79. Single-crystal EPR spectra of gamma-ray-irradiated Gypsum reveal two types of arsenic-associated oxyradicals: [AsO 3 ] 2− and an [AsO 2 ] 2− . The [AsO 3 ] 2− center is characterized by principal 75 As hyperfine coupling constants of A 1 = 1952.0(2) MHz, A 2 = 1492.6(2) MHz and A 3 = 1488.7(2) MHz, with the unique A axis along the S–O1 bond direction, and contains complex 1 H superhyperfine structures that have been determined by pulsed ENDOR. These results suggest that the [AsO 3 ] 2− center formed from electron trapping on the central As 5+ ion of a substitutional (AsO 4 ) 3− group after removal of an O1 atom. The [AsO 2 ] 2− center is characterized by its unique A ( 75 As) axis approximately perpendicular to the O1–S–O2 plane and the A 2 axis along the S–O2 bond direction, consistent with electron trapping on the central As 3+ ion of a substitutional (AsO 3 ) 3− group after removal of an O2 atom. These results confirm lattice-bound As 5+ and As 3+ in Gypsum and point to potential application of this mineral for immobilization and removal of arsenic pollution.
J.h. Potgieter - One of the best experts on this subject based on the ideXlab platform.
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A comparison of the performance of various Synthetic Gypsums in plant trials during the manufacturing of OPC clinker
Cement and Concrete Research, 2004Co-Authors: J.h. Potgieter, S.s Potgieter, Robert I. MccrindleAbstract:Abstract This paper compares the plant performances of various Synthetic Gypsums used as set regulators in cement. The decision about the suitability of a specific Gypsum was based on measurements and comparisons of the specific areas (Blaine), initial setting time (min), final setting time (h), SO 3 content and compressive strength of the OPC clinker mixed with it. The results from the trials run with Synthetic Gypsum that was wet milled and treated with milk of lime showed no significant difference in compressive strengths at all ages, but a delay of approximately 35% in initial and 50% in final setting times occurred. Gypsum from a Tioxide plant can be successfully used as set retarders for OPC cement if the handling problems can be solved. In this case, no significant difference in either initial or final setting times or the compressive strengths, were observed. The ultrasonically treated phosphoGypsum displayed a large variability of 50% or more in final and initial setting times, as well as compressive strengths at all ages, and is unsuccessful in rendering the Gypsum usable as a set retarder for OPC cement.
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Thermogravimetric analysis of the reaction between carbon and CaSO4·2H2O, Gypsum and phosphoGypsum in an inert atmosphere
Thermochimica Acta, 1999Co-Authors: E. M. Van Der Merwe, Christien A. Strydom, J.h. PotgieterAbstract:Abstract Heating stoichiometric amounts of carbon and pure CaSO 4 , Synthetic Gypsum or phosphoGypsum in a nitrogen atmosphere, results in the formation of CaS between 700°C and 1100°C. Different heating rates were used to investigate the reaction, and the amount of CaS formed depends on the heating rate used. A quantitative XRD method was used to determine the amounts of CaSO 4 , CaS, CaO and C in the samples. More CaS formed with increasing heating rate. Addition of 5% Fe 2 O 3 and 5% ZnO as catalysts lowers the temperature range, as well as the activation energy of the reaction. The relationship between the activation energy values and degree of conversion ( α ) for the reaction between carbon and CaSO 4 indicates that it is a complex reaction, and that simultaneous competitive reactions are taking place.
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thermogravimetric analysis of the reaction between carbon and caso4 2h2o Gypsum and phosphoGypsum in an inert atmosphere
Thermochimica Acta, 1999Co-Authors: E M Van Der Merwe, C A Strydom, J.h. PotgieterAbstract:Abstract Heating stoichiometric amounts of carbon and pure CaSO 4 , Synthetic Gypsum or phosphoGypsum in a nitrogen atmosphere, results in the formation of CaS between 700°C and 1100°C. Different heating rates were used to investigate the reaction, and the amount of CaS formed depends on the heating rate used. A quantitative XRD method was used to determine the amounts of CaSO 4 , CaS, CaO and C in the samples. More CaS formed with increasing heating rate. Addition of 5% Fe 2 O 3 and 5% ZnO as catalysts lowers the temperature range, as well as the activation energy of the reaction. The relationship between the activation energy values and degree of conversion ( α ) for the reaction between carbon and CaSO 4 indicates that it is a complex reaction, and that simultaneous competitive reactions are taking place.
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Thermogravimetric studies of the synthesis of cas from Gypsum, CaSo_4·2H_2O and phosphoGypsum
Journal of thermal analysis, 1997Co-Authors: C A Strydom, E. M. Groenewald, J.h. PotgieterAbstract:Using a heating rate of 2°C min^−1, CaS reacts with oxygen in air from 700°C to form CaSO_4, with a complete conversion at 1100°C. Synthesis of CaS from the reaction between CaSO_4 containing compounds and carbon compounds in air would not be possible, as the carbon reacts from 600°C with oxygen in the air to give CO_2. Heating stoichiometric amounts of carbon and pure CaSO_4, Synthetic Gypsum or phosphoGypsum in a nitrogen atmosphere, results in the formation of CaS from 850°C. Using a heating rate of 10°C min^−1, the formation of CaS is completed at 1080°C. Addition of 5% Fe_2O_3 as a catalyst lowers the starting temperature of the reaction to 750°C. Activation energy values at different fraction reaction values (α) differ between 340 and 400 kJ mol^−1. The relationship between the activation energy values and conversion (α) indicates that the reaction proceeds via multiple steps.
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The thermal dehydration of Synthetic Gypsum
Thermochimica Acta, 1995Co-Authors: Christien A. Strydom, D.l. Hudson-lamb, J.h. Potgieter, E. DaggAbstract:Mass losses vary between 13.7 and 16.5% and heat of dehydration values between 377 and 420 J g−1 for the dehydration of ground and mixed inhomogeneous Gypsum samples. As for the dehydration of CaSO4·2H2O, the dehydration of Synthetic Gypsum proceeds via multi-step reactions. Using a heating rate of 5°C min−1, the very slow dehydration of CaSO4·2H2O and impurities in the Gypsum samples start at temperatures lower than 95°C. The main dehydration of the calcium sulphate dehydrate part of Synthetic Gypsum occurs between 95 and 170°C (heating rate 5°C min−1) and seems to proceed via a process with an activation energy of 392 ± 100 kJ mol−1 for α-values between 0 and 0.1. For α-values between 0.1 and 0.7, the reaction can be described by a first-order process with autocatalysis with an activation energy value of 100.5 ± 1.2 kJ mol−1. The third part of the reaction (0.7 < α < 1), up to temperatures of 180°C, gives an activation energy value of 96 ± 15 kJ mol−1. Even at temperatures above 250°C, some CaSO4·O.15H2O was still observed. The dehydration of calcium hemihydrate seems to proceed via the formation of CaSO4·0.15H2O.
Jacques K Desmarais - One of the best experts on this subject based on the ideXlab platform.
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uptake and speciation of uranium in Synthetic Gypsum caso4 2h2o applications to radioactive mine tailings
Journal of Environmental Radioactivity, 2018Co-Authors: Jacques K Desmarais, Ning Chen, Renfei Feng, Patrick Zhang, Dien Li, Arthur LieuAbstract:Abstract PhosphoGypsum formed from the production of phosphoric acid represents by far the biggest accumulation of Gypsum-rich wastes in the world and commonly contains elevated radionuclides, including uranium, as well as other heavy metals and metalloids. Therefore, billions-of-tons of phosphoGypsum stockpiled worldwide not only possess serious environmental problems but also represent a potential uranium resource. Gypsum is also a major solid constituent in many other types of radioactive mine tailings, which stems from the common usage of sulfuric acid in extraction processes. Therefore, management and remediation of radioactive mine tailings as well as future beneficiation of uranium from phosphogysum all require detailed knowledge about the nature and behavior of uranium in Gypsum. However, little is known about the uptake mechanism or speciation of uranium in Gypsum. In this study, synthesis experiments suggest an apparent pH control on the uptake of uranium in Gypsum at ambient conditions: increase in U from 16 μg/g at pH = 6.5 to 339 μg/g at pH = 9.5. Uranium L 3 -edge synchrotron X-ray absorption spectroscopic analyses of Synthetic Gypsum show that uranyl (UO 2 ) 2+ at the Ca site is the dominant species. The EXAFS fitting results also indicate that uranyl in Synthetic Gypsum occurs most likely as carbonate complexes and yields an average U-O distance ∼0.25 A shorter than the average Ca-O distance, signifying a marked local structural distortion. Applications to phosphoGypsum from the New Wales phosphoric acid plant (Florida, USA) and uranium mine tailings from the Key Lake mill (Saskatchewan, Canada) show that Gypsum is an important carrier of uranium over a wide range of pH and controls the fate of this radionuclide in mine tailings. Also, development of new technologies for recovering U from phosphoGypsum in the future must consider lattice-bound uranyl in Gypsum.
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Uptake and speciation of uranium in Synthetic Gypsum (CaSO4•2H2O): Applications to radioactive mine tailings.
Journal of Environmental Radioactivity, 2017Co-Authors: Jacques K Desmarais, Ning Chen, Renfei Feng, Patrick Zhang, Dien Li, Arthur LieuAbstract:Abstract PhosphoGypsum formed from the production of phosphoric acid represents by far the biggest accumulation of Gypsum-rich wastes in the world and commonly contains elevated radionuclides, including uranium, as well as other heavy metals and metalloids. Therefore, billions-of-tons of phosphoGypsum stockpiled worldwide not only possess serious environmental problems but also represent a potential uranium resource. Gypsum is also a major solid constituent in many other types of radioactive mine tailings, which stems from the common usage of sulfuric acid in extraction processes. Therefore, management and remediation of radioactive mine tailings as well as future beneficiation of uranium from phosphogysum all require detailed knowledge about the nature and behavior of uranium in Gypsum. However, little is known about the uptake mechanism or speciation of uranium in Gypsum. In this study, synthesis experiments suggest an apparent pH control on the uptake of uranium in Gypsum at ambient conditions: increase in U from 16 μg/g at pH = 6.5 to 339 μg/g at pH = 9.5. Uranium L 3 -edge synchrotron X-ray absorption spectroscopic analyses of Synthetic Gypsum show that uranyl (UO 2 ) 2+ at the Ca site is the dominant species. The EXAFS fitting results also indicate that uranyl in Synthetic Gypsum occurs most likely as carbonate complexes and yields an average U-O distance ∼0.25 A shorter than the average Ca-O distance, signifying a marked local structural distortion. Applications to phosphoGypsum from the New Wales phosphoric acid plant (Florida, USA) and uranium mine tailings from the Key Lake mill (Saskatchewan, Canada) show that Gypsum is an important carrier of uranium over a wide range of pH and controls the fate of this radionuclide in mine tailings. Also, development of new technologies for recovering U from phosphoGypsum in the future must consider lattice-bound uranyl in Gypsum.
