Static Batch Experiments of Uranyl Peroxide Nanospheres

2. Environmental Importance of Uranium These experiments are very environmentally important, because they deal with the disposal of nuclear waste. Waste from the creation of nuclear energy comes from a variety of sources. One is the mining and milling of uranium, which creates a large amount of waste for the production of nuclear fuel or weapons. Milling � When uranium is mined from the ground, the daughter products left over and are now much more concentrated at the surface. They are also now exposed to an oxidizing environment. Daughter products left over at the surface can be up to 10,000 times more radioactive than before mining and milling. Currently, 10-15 million tons of mill tailings are produced each year. Enrichment � Nuclear waste is produced both from the enrichment of uranium ore, and from the use of the enriched fuel pellets for nuclear energy. Natural uranium is .7% U235, the fissionable atom, which must be enriched to 4% for nuclear fuel, and 20-90% for weapons. This process leaves vast amounts of depleted uranium to be disposed of. Then the enriched uranium is used in a nuclear reactor for about 3-4 years. There are currently 49,000 metric tons of SNF in the US. Remediation - These storage tanks at the Hanford Site in Washington were constructed to store liquid, high-level waste. After construction was completed, the earth was replaced to bury the tanks underground. The Hanford Site in Washingon was a plutonium plant for nuclear weapons from 1943 to 1987. Since 1987, Hanford�s only focus has been on remediation of the waste produced. Geologically, Hanford is characterized by a layer of sediment anywhere from 20-200 m thick. The vadose zone ranges from over 100 m in the center of the site to only 3 m near the Columbia river. An unconfined aquifer exists within the near-surface sediments, with groundwater flowing generally towards the east and discharging into the river. At Hanford 56 million gallons of highly radioactive liquid and solid waste are stored in 177 underground tanks. 68 of those tanks have leaked millions of gallons of radioactive waste into the subsurface. The plume of actinides is moving much faster than expected. Storage - Yucca Mountain, Nevada - Proposed geologic repository Currently in the United States there are 49,000 tons of SNF in the US. Yucca Mountain could possibly hold 70,000 metric tons of SNF and high-level waste. There is already enough waste present to fill a geologic repository at YM. Yucca Mountain is located 90 miles from private land. The climate is arid and the repository is well above the water table. Geologically, Yucca Mountain is composed of welded tuff. These experiments are very environmentally important, because they deal with the disposal of nuclear waste. Waste from the creation of nuclear energy comes from a variety of sources. One is the mining and milling of uranium, which creates a large amount of waste for the production of nuclear fuel or weapons. Milling � When uranium is mined from the ground, the daughter products left over and are now much more concentrated at the surface. They are also now exposed to an oxidizing environment. Daughter products left over at the surface can be up to 10,000 times more radioactive than before mining and milling. Currently, 10-15 million tons of mill tailings are produced each year. Enrichment � Nuclear waste is produced both from the enrichment of uranium ore, and from the use of the enriched fuel pellets for nuclear energy. Natural uranium is .7% U235, the fissionable atom, which must be enriched to 4% for nuclear fuel, and 20-90% for weapons. This process leaves vast amounts of depleted uranium to be disposed of. Then the enriched uranium is used in a nuclear reactor for about 3-4 years. There are currently 49,000 metric tons of SNF in the US. Remediation - These storage tanks at the Hanford Site in Washington were constructed to store liquid, high-level waste. After construction was completed, the earth was replaced to bury the tanks underground. The Hanford Site in Washingon was a plutonium plant for nuclear weapons from 1943 to 1987. Since 1987, Hanford�s only focus has been on remediation of the waste produced. Geologically, Hanford is characterized by a layer of sediment anywhere from 20-200 m thick. The vadose zone ranges from over 100 m in the center of the site to only 3 m near the Columbia river. An unconfined aquifer exists within the near-surface sediments, with groundwater flowing generally towards the east and discharging into the river. At Hanford 56 million gallons of highly radioactive liquid and solid waste are stored in 177 underground tanks. 68 of those tanks have leaked millions of gallons of radioactive waste into the subsurface. The plume of actinides is moving much faster than expected. Storage - Yucca Mountain, Nevada - Proposed geologic repository Currently in the United States there are 49,000 tons of SNF in the US. Yucca Mountain could possibly hold 70,000 metric tons of SNF and high-level waste. There is already enough waste present to fill a geologic repository at YM. Yucca Mountain is located 90 miles from private land. The climate is arid and the repository is well above the water table. Geologically, Yucca Mountain is composed of welded tuff.

3. Uranium Oxidation States Uranium occurs primarily in the 4 and 6+ oxidation states. It also occurs less commonly in the 5+ state.Uranium with a 4+ charge occurs under reducing conditions and primarily produces uraninite, which is the most common uranium-bearing mineral. Uraninite is the natural analog of commercial spent nuclear fuel.Uranium with a charge of 6+ occurs in oxidizing conditions. It allows bonding to two oxygen molecules, creating the uranyl ion. There are over 200 uranyl minerals known.Understanding the oxidation states of uranium is important for understanding the evolution of spent nuclear fuel at Yucca Mountain. Waste stored will initially have a 4+ valence state, but will be stable with a 6+ charge, therefore causing much more varied and soluble forms of uranium. Ultimately, the alteration phases that form at the 6+ oxidation state will determine the effect of nuclear waste on the environment. peroxides, oxyhydrates, carbonates, phosphates, silicates, sulfates, vanadates Uranium occurs primarily in the 4 and 6+ oxidation states. It also occurs less commonly in the 5+ state.Uranium with a 4+ charge occurs under reducing conditions and primarily produces uraninite, which is the most common uranium-bearing mineral. Uraninite is the natural analog of commercial spent nuclear fuel.Uranium with a charge of 6+ occurs in oxidizing conditions. It allows bonding to two oxygen molecules, creating the uranyl ion. There are over 200 uranyl minerals known.Understanding the oxidation states of uranium is important for understanding the evolution of spent nuclear fuel at Yucca Mountain. Waste stored will initially have a 4+ valence state, but will be stable with a 6+ charge, therefore causing much more varied and soluble forms of uranium. Ultimately, the alteration phases that form at the 6+ oxidation state will determine the effect of nuclear waste on the environment. peroxides, oxyhydrates, carbonates, phosphates, silicates, sulfates, vanadates

4. Comparison of Uranium Alteration Phase Sequences These illustrations show the comparative reaction sequences for uranium alteration phases. Most research has shown the paragenetic sequence of uranium alteration products go from from uraninite, SNF and UO2 to uranyl oxide hyrdates, then to uranyl alkaline silica hydrates. The first illustration shows the sequence of phases for ten-year drip tests of water on UO2 pellets, and the second is the sequence observed at he natural Nopal I deposit in Mexico. Recent research, however has suggested a new direction for the alteration of spent nuclear fuel. Research has shown the formation of uranyl peroxide minerals as the sole alteration phase, in the absence of all other phases.These illustrations show the comparative reaction sequences for uranium alteration phases. Most research has shown the paragenetic sequence of uranium alteration products go from from uraninite, SNF and UO2 to uranyl oxide hyrdates, then to uranyl alkaline silica hydrates. The first illustration shows the sequence of phases for ten-year drip tests of water on UO2 pellets, and the second is the sequence observed at he natural Nopal I deposit in Mexico. Recent research, however has suggested a new direction for the alteration of spent nuclear fuel. Research has shown the formation of uranyl peroxide minerals as the sole alteration phase, in the absence of all other phases.

5. Uranyl Peroxide Minerals Because of the possibility of uranyl peroxides forming as alteration on spent nuclear fuel, there has recently there has much research done on these minerals. There are only two known peroxide minerals, both of which are uranium-bearing - studtite and metastudtite. A study done by McNamara, et al., found that a SNF pellet that was in contact with water for 1.5 years resulted in a studtite-covered surface. Sattonay, et al., bombarded UO2 pellets with an aplpha beam, and after 6 hours the UO2 was corroded and metastudtite was observed on the surface (peroxide provided by alpha-radiolysis of water). Amme also did an experiment involving UO2 pellets in contact with different peroxide concentrations. Within 48 hours, they found that alteration in the form of studtite had appeared on the surface.Because of the possibility of uranyl peroxides forming as alteration on spent nuclear fuel, there has recently there has much research done on these minerals. There are only two known peroxide minerals, both of which are uranium-bearing - studtite and metastudtite. A study done by McNamara, et al., found that a SNF pellet that was in contact with water for 1.5 years resulted in a studtite-covered surface. Sattonay, et al., bombarded UO2 pellets with an aplpha beam, and after 6 hours the UO2 was corroded and metastudtite was observed on the surface (peroxide provided by alpha-radiolysis of water). Amme also did an experiment involving UO2 pellets in contact with different peroxide concentrations. Within 48 hours, they found that alteration in the form of studtite had appeared on the surface.

6. Uranyl Peroxide Structures Because of their of their formation on the surface of spent nuclear fuel and UO2, uranyl peroxides are very important. A lot of research has focused on seeing what other uranyl peroxides exist. Most known structures are sheet structures, and isolated and chain structures, such as studtite, have also been discovered. Also, recently, uranyl peroxide nanoclusters have been discovered. These are the largest known clusters in actinides and polyhedron structures of 20, 24, 28 and 32 have been found. Before the uranium nanoclusters were discovered, the largest isolated cluster of organic uranium involved only 4 polyhedra.Because of their of their formation on the surface of spent nuclear fuel and UO2, uranyl peroxides are very important. A lot of research has focused on seeing what other uranyl peroxides exist. Most known structures are sheet structures, and isolated and chain structures, such as studtite, have also been discovered. Also, recently, uranyl peroxide nanoclusters have been discovered. These are the largest known clusters in actinides and polyhedron structures of 20, 24, 28 and 32 have been found. Before the uranium nanoclusters were discovered, the largest isolated cluster of organic uranium involved only 4 polyhedra.

7. Small-angle X-ray scattering X-ray scattering was done on parent solutions of the nanoclusters. The APS at ANL is a national synchrotron-radiation light source research facility that uses high-energy x-rays to look at uranium aggregates in solution.X-ray scattering data was collected on nanocluster solutions that had aged for 2, 28, and 180 days. As you can see in figure one, Figure 1: Evolving aggregation of the uranyl species � y-axis is a function of the intensity of uranium in solution Within two days of synthesis, no nanoclusters were found in solution. At 28 days, there were mixed species and at 6 months, the only species detected was the nanocluster. The 180 day data consistent with a spherical-shell model (red line � model, green line � data) Monodisperse cluster in solution Figure 2: log-log plot Guinier plot of data for 180 day solution. Linear over a wide Q range, is representative fit. The slope of the line is a function of the radius of the cluster, which is 16.2 Angstroms. SO, complete after 180 days, also stable in contact w/ geologic media (quartz, plagioclase, kaolinite, kyanite, biotite, and tuff)X-ray scattering was done on parent solutions of the nanoclusters. The APS at ANL is a national synchrotron-radiation light source research facility that uses high-energy x-rays to look at uranium aggregates in solution.X-ray scattering data was collected on nanocluster solutions that had aged for 2, 28, and 180 days. As you can see in figure one, Figure 1: Evolving aggregation of the uranyl species � y-axis is a function of the intensity of uranium in solution Within two days of synthesis, no nanoclusters were found in solution. At 28 days, there were mixed species and at 6 months, the only species detected was the nanocluster. The 180 day data consistent with a spherical-shell model (red line � model, green line � data) Monodisperse cluster in solution Figure 2: log-log plot Guinier plot of data for 180 day solution. Linear over a wide Q range, is representative fit. The slope of the line is a function of the radius of the cluster, which is 16.2 Angstroms. SO, complete after 180 days, also stable in contact w/ geologic media (quartz, plagioclase, kaolinite, kyanite, biotite, and tuff)

8. Batch Experiments So, we�ve seen these nanoclusters in solution. But, in order to understand their environmental importance, we decided to do a series of batch experiments to better understand the transport of uranium in porous media. These experiments were static and allowed us to compare the sorption of just uranium to a parent solution of U-24 nanoclusters in contact with geologic media consisting of minerals and tuff. Through small-angle x-ray scattering, proved that a solution 180 days old only contains uranium in the form of nanoclusters. So, for experiments, used a solution 1 day old in which uranium had not evolved into nanoclusters, and a solution 180 days old that was just nanoclusters, then compared data. Through x-ray scattering data, we also know that the nanoclusters are stable at a pH = 11.4, that at which the experiments were conducted. To begin the experiments, we took a 10-fold dilute (H2O, H2O2, LiOH, and [UO2(NO3)2(H2O)6]) and put 1 mL in contact with each of 12 different minerals and 5 grain sized of tuff. We prepared 4 batches of experiments for each of the two phases of uranium, at 12, 24, 48, 72 hours. The experiments were then placed on a shaker table for the allotted number of hours. To control the experiments, we also maintained a pH range and used the same uranium and lithium concentrations.So, we�ve seen these nanoclusters in solution. But, in order to understand their environmental importance, we decided to do a series of batch experiments to better understand the transport of uranium in porous media. These experiments were static and allowed us to compare the sorption of just uranium to a parent solution of U-24 nanoclusters in contact with geologic media consisting of minerals and tuff. Through small-angle x-ray scattering, proved that a solution 180 days old only contains uranium in the form of nanoclusters. So, for experiments, used a solution 1 day old in which uranium had not evolved into nanoclusters, and a solution 180 days old that was just nanoclusters, then compared data. Through x-ray scattering data, we also know that the nanoclusters are stable at a pH = 11.4, that at which the experiments were conducted. To begin the experiments, we took a 10-fold dilute (H2O, H2O2, LiOH, and [UO2(NO3)2(H2O)6]) and put 1 mL in contact with each of 12 different minerals and 5 grain sized of tuff. We prepared 4 batches of experiments for each of the two phases of uranium, at 12, 24, 48, 72 hours. The experiments were then placed on a shaker table for the allotted number of hours. To control the experiments, we also maintained a pH range and used the same uranium and lithium concentrations.

9. Minerals Used in Batch Experiments Minerals � ground into powder common minerals found at Hanford � quartz, plagioclase and biotite, (proven that nanoclusters are stable when in contact with several different minerals.) Using these minerals gives us ability to see not just cation interaction in a uranium structure, but also use common geologic media found at nuclear waste disposal sites. They have a wide structural and chemical diversity. We are using silicates, carbonates, sulfates, feldspars, amphiboles, clay minerals. Minerals � ground into powder common minerals found at Hanford � quartz, plagioclase and biotite, (proven that nanoclusters are stable when in contact with several different minerals.) Using these minerals gives us ability to see not just cation interaction in a uranium structure, but also use common geologic media found at nuclear waste disposal sites. They have a wide structural and chemical diversity. We are using silicates, carbonates, sulfates, feldspars, amphiboles, clay minerals.

10. Tuff Used in Batch Experiments Used different grain sizes of tuff like that at YM to understand sorption as a function of grain size.Used different grain sizes of tuff like that at YM to understand sorption as a function of grain size.

11. Experimental Method XRD 1 � verify the mineral and its structure SEM/EDS � verify the exact chemical composition of the minerals (not accounting for trace elements) Experiments: took .1 g. (gravimetrically) aliquots of each of the solutions from the batch experiment preparation, and diluted them 100-fold to 10 g. of total solution with ultrapure water. ICP � to analyze the samples of uranium solution XRD 2 � verify that the final composition of the minerals was the same as the beginning. XRD 1 � verify the mineral and its structure SEM/EDS � verify the exact chemical composition of the minerals (not accounting for trace elements) Experiments: took .1 g. (gravimetrically) aliquots of each of the solutions from the batch experiment preparation, and diluted them 100-fold to 10 g. of total solution with ultrapure water. ICP � to analyze the samples of uranium solution XRD 2 � verify that the final composition of the minerals was the same as the beginning.

12. ICP Analysis PICS � 1) Calibration curve analyte 385.958: a blank, 5 standards and U24 original concentrationICP runs by: ICP OES is a technique that measures the light emitted from excited atoms or ions. An argon plasma torch vaporizes a small amount of sample that is run to the nebulizer at a constant rate. Atoms and ions are excited, and as electrons return to the ground state, light is emitted. Before the analysis of samples is begun, the ICP must be calibrated with standards of known concentration and checked repeatedly throughout the analysis with control samples. We first created a standard curves using standards ranging from 0-10 ppm. We then ran ICP/OES at 5 different wavelenths to obtain Kd values.PICS � 1) Calibration curve analyte 385.958: a blank, 5 standards and U24 original concentrationICP runs by: ICP OES is a technique that measures the light emitted from excited atoms or ions. An argon plasma torch vaporizes a small amount of sample that is run to the nebulizer at a constant rate. Atoms and ions are excited, and as electrons return to the ground state, light is emitted. Before the analysis of samples is begun, the ICP must be calibrated with standards of known concentration and checked repeatedly throughout the analysis with control samples. We first created a standard curves using standards ranging from 0-10 ppm. We then ran ICP/OES at 5 different wavelenths to obtain Kd values.

13. Results If the Kd value is close to zero or zero, all uranium remains in solution. When the Kd values is great, more uranium is leaving solution, either through sorption or precipitation. The nondramatic differences in Kd values of uranium nanoclusters and just uranium will be evaluated after the experiments have been done over a wide pH range. However, for some of the minerals there are striking differences in Kd values. We found that these results are not dependent on time, and that whatever removal is occurring is taking place rapidly.If the Kd value is close to zero or zero, all uranium remains in solution. When the Kd values is great, more uranium is leaving solution, either through sorption or precipitation. The nondramatic differences in Kd values of uranium nanoclusters and just uranium will be evaluated after the experiments have been done over a wide pH range. However, for some of the minerals there are striking differences in Kd values. We found that these results are not dependent on time, and that whatever removal is occurring is taking place rapidly.

14. Discussion First, we will look at those minerals in which we see little or no removal of uranium from solution. The minerals exhibit exactly what we would expect in this situation. We see less sorption of an anion as pH increases, and these experiments were performed at a pH of 11.4. The dominant aqueous species of the uranyl anion, at this pH, is uranyl tricarbonate, which has a �4 charge.First, we will look at those minerals in which we see little or no removal of uranium from solution. The minerals exhibit exactly what we would expect in this situation. We see less sorption of an anion as pH increases, and these experiments were performed at a pH of 11.4. The dominant aqueous species of the uranyl anion, at this pH, is uranyl tricarbonate, which has a �4 charge.

15. Results We will now discuss those minerals which exhibit significant removal of uranium from solution, but little or no removal from a predominantly-nanocluster solution. Those minerals are calcite, augite, hornblende and gypsum.We will now discuss those minerals which exhibit significant removal of uranium from solution, but little or no removal from a predominantly-nanocluster solution. Those minerals are calcite, augite, hornblende and gypsum.

16. Discussion Now, we will look at those minerals in which we see significant removal of uranium from solution, and little or no removal from a dominantly nanocluster solution. Those minerals are calcite, augite, hornblende, and gypsum, all of which are calcium-bearing. We did single-crystal X-ray diffraction of a precipitate that formed in the gypsum batch, and discovered that a calcium uranyl peroxide was forming. We didn�t observe this phase in the other minerals, but it is possible that trace quantities are forming.Now, we will look at those minerals in which we see significant removal of uranium from solution, and little or no removal from a dominantly nanocluster solution. Those minerals are calcite, augite, hornblende, and gypsum, all of which are calcium-bearing. We did single-crystal X-ray diffraction of a precipitate that formed in the gypsum batch, and discovered that a calcium uranyl peroxide was forming. We didn�t observe this phase in the other minerals, but it is possible that trace quantities are forming.

17. Results Lastly, we will examine the minerals that exhibit significant removal of uranium, and of the nanocluster, from solution.Lastly, we will examine the minerals that exhibit significant removal of uranium, and of the nanocluster, from solution.

18. Discussion Biotite and kaolinite, which are both phyllosilicates, exhibit significant removal of both uranium and a predominantly nanocluster-filled solution. In kaolinite, we see almost no difference between the behavior of the two solutions. In biotite, however, there is almost 4 times more uranium removed from the nanocluster than the uranium solution. It�s possible that as the solution ages, there is less peroxide in solution. So, these results could be a function of the peroxide. Or, it�s possible that it has to do with more nanoclusters being present in the U24 solution. The nanoclusters could possibly be breaking down into smaller clusters in solution.Biotite and kaolinite, which are both phyllosilicates, exhibit significant removal of both uranium and a predominantly nanocluster-filled solution. In kaolinite, we see almost no difference between the behavior of the two solutions. In biotite, however, there is almost 4 times more uranium removed from the nanocluster than the uranium solution. It�s possible that as the solution ages, there is less peroxide in solution. So, these results could be a function of the peroxide. Or, it�s possible that it has to do with more nanoclusters being present in the U24 solution. The nanoclusters could possibly be breaking down into smaller clusters in solution.

19. Future Research

20. Acknowledgements