bk-2017-1263.ch009

Chapter 9

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Recent Advancements in the Radiochemistry of Elements Pertaining to Select Nuclear Materials and Wastes Eric S. Eitrheim, Andrew W. Knight, Michael K. Schultz, Tori Z. Forbes,* and Andrew W. Nelson Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States *E-mail: [email protected]. Phone: 319-384-1320.

A 2012 National Academy of Sciences report indicated insufficient numbers of post-graduate level radiochemists and nuclear scientists to fulfill the current and future national needs in this sector. This report prompted the creation of a radiochemistry graduate program at the University of Iowa with research efforts in energy, environmental, and medical applications. These research projects, outlined here, have included a focus on understudied elements whose chemistry is still poorly understood. The first study targets methodology to separate gallium (Ga) and plutonium (Pu) isotopes for nuclear fuel applications. Second, we explore the presence of naturally occurring radioactive material (uranium (U), thorium (Th), radium (Ra), lead (Pb), bismuth (Bi), and polonium (Po)) in drill cuttings associated with unconventional drilling operations. Last, the fundamental chemistry of protactinium (Pa) relating to geochronometry and the nuclear fuel cycle is discussed, specifically focusing on separation and dating techniques. The University of Iowa radiochemistry program has aided in this exploration with anticipation that additional research will continue to increase our understanding of these elements in the future.

© 2017 American Chemical Society Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Introduction In 2012, the National Academy of Sciences released a report indicating that there were dwindling numbers of radiochemists and nuclear scientists graduating from U.S. graduate programs (1). The numbers of radiochemists and nuclear scientists graduating from Ph.D. programs is particularly low and expected to be well below the demand by employers in the coming decade. In response to the low rate of Ph.D.-trained radiochemists, the University of Iowa launched an interdisciplinary radiochemistry program that included research and training in medical, environmental, and energy applications of radiochemistry (2). The first cohort of three analytical radiochemistry trainees began in 2012, with research focusing on understudied radionuclides that were ripe for new discoveries. In particular, the research focused on gallium (Ga) and plutonium (Pu) with applications in mixed-oxide (MOX) fuels, naturally occurring uranium-series radionuclides in the environment, and fundamental protactinium (Pa) chemistry. The following chapter will highlight what is known, what the University of Iowa cohort of three discovered, and what still remains to be explored for select radioactive elements, including: plutonium (Pu), protactinium (Pa), uranium (U), thorium (Th), radium (Ra), lead (Pb), bismuth (Bi), and polonium (Po).

Plutonium Nuclear Materials and Applications in Nuclear Forensics Plutonium (Pu) nuclear materials originated shortly after discovery of the element by Glenn T. Seaborg and colleagues in December of 1940 (3). This discovery was not initially announced due to the understanding that this new element had application in nuclear weapons technologies (3). Plutonium was initially produced at the Hanford Site in Washington State using a nuclear reactor; most notably reactor B (4). While production and isolation of gram quantities of Pu began almost immediately, additional materials engineering and processing was necessary to stabilize Pu for use in nuclear weapons. One of the most important advancements in this area was the discovery of plutonium-gallium alloys during the Manhattan Project, which effectively stabilized the material for use in nuclear weapons (5). Pure Pu has six allotropes and the thermodynamically stable phase at standard temperatures and pressures is the high density (19.86 g/cm3) α-form. The lowest density phase is δ-Pu, which is approximately 25% less dense (16.0 g/cm3) than the α-phase and can only be stabilized at temperatures between 310 and 425 °C. Super criticality is only reached through a compression based δ-α transformation, thus maintaining the δ-Pu material under standard conditions is crucial for the design of a functioning nuclear weapon. Creating a Pu alloy stabilizes the δ-phase at room temperature and this is typically achieved by adding ~0.8-1 mol% Ga to the Pu metal. Alloying Ga with Pu also improves the mechanical properties by decreasing corrosion rates, improving the ability to cast, and creating low thermal expansion properties leading to better material processing for nuclear material production. 174

Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Thousands of nuclear weapons have been created using Ga-Pu alloys, and consequently these materials pose a nonproliferation risk (6). According to the International Atomic Energy Agency (IAEA), eight incidences of theft or loss of plutonium-based nuclear materials have been reported globally, excluding Pu-based smoke detectors, from 1993 through 2015 (7). When the authorities confiscate these now-illicit Pu nuclear materials, identifying isotopic and elemental "signatures" linking them to their last legal owner, production location, facility, or method of production is considered crucial for accountability and legal prosecution (8). As Pu used in weapons production has been alloyed with Ga, the elemental signature of interest is the Ga/Pu ratio because it is unique to each specific production facility and can be used to pinpoint the initial material processing facility. This method’s usefulness goes beyond nuclear forensics applications and can also be valuable for nuclear energy-producing industries. Decommissioning of the nuclear arsenal has prompted the U.S. to start dismantling numerous nuclear weapons and using the Pu in MOX (mixed-oxide) nuclear reactors (9). MOX fuel is the mixture of plutonium and uranium oxides and can be used in place of the traditional isotopically-enriched uranium dioxide fuels. However, the initial alloy has significant amounts of Ga, which is incompatible with practical MOX fuel applications even in small quantities, because it can migrate from the fuel into the zirconium (Zr) cladding, making that cladding more brittle and subject to corrosion (10–13). In order to down-blend these weapon pits for MOX applications, removal of Ga to approximately a 105 decontamination factor is needed to avoid these complications, requiring a final Ga concentration of 0.1 ppm within the solid oxide material (14). To use MOX fuel from down-blended Pu-bearing nuclear materials in nuclear reactors, there is a need for advancements in separations technologies. Several methods have been shown to effectively separate Ga and Pu in nuclear weapon pits for application as MOX fuel. For example, a separation of Pu and Ga by ion-exchange chromatography was proposed by DeMuth at Los Alamos National Laboratory that uses aqueous-based ion-exchange chromatography (15, 16). This method focused on large scale separations instead of isotopic analysis, making it less applicable for nuclear forensics applications (15, 16). A second method utilized high-temperature thermal removal of gallium for industrial MOX fuel production and again focused on large quantities necessary in the nuclear energy industry (17). These methods were not designed for, nor are they suitable for, the analytical investigation of gallium, plutonium, or other candidate actinides of interest for nuclear forensics of MOX fuel analysis. In order to address this critical need, methodology was developed at the University of Iowa that provided analysis of various actinide elements and gallium within plutonium nuclear materials, such as nuclear weapon pits and MOX nuclear fuels. Developing a dependable, fast, and accurate analysis method for stable gallium in Pu nuclear materials could be a powerful tool for both nuclear forensics applications and nuclear energy-producing industries. Combining the analysis of Ga with the determination of isotopes of the actinides Pu, U, Th, and Am would increase the efficiency of nuclear forensics investigations. 175



Using extraction chromatography paired with isotope dilution techniques, a complete radiochemical separation of Ga, Pu, Am, U, and Th was developed for nuclear forensics applications of Pu-bearing nuclear materials (18). Extraction chromatographic resins (TEVA and TRU Resins from Eichrom Technologies, LLC) were used to prepare elementally pure fractions for isotope analysis (Figure 1) (18). These chromatographic resins have organic extractants adhered to a solid substrate to allow for a chemical separation. Preparing and separating samples for analysis requires an accurate and precise method that determines the yield of Ga, even though the complex matrix of Pu-bearing nuclear materials is a hindrance to the analysis of gallium concentrations. A notable contribution to developing these methods was the use of a nuclear medical radioisotope, 68Ga, as an improved alternative to trace stable Ga yields in these separations (18). Innovative medical radioisotopes, such as 68Ga, are becoming available and affordable due to recent advancements in their production and isolation (19). Combining ideas from various areas of radiochemistry, such as nuclear medicine and nuclear forensics, can have substantial impacts on methodology development. Utilizing a medical radioisotope to provide stable gallium signatures in nuclear-forensics applications allows quick and accurate determination of Ga concentrations. The 68Ga nuclear medical radiotracer allows precise, stable gallium determination without interfering with the separation and quantification of other isotopes of radioactive actinides.

Figure 1. Tandem column arrangement of the TEVA/TRU developed for the separation and purification of stable Ga and radioactive isotopes of elements Th, Pu, Am, and U. The load and rinse solutions remove common ions and matrix interferences. The columns are then disassembled. Steps 4-6 (TEVA) may be run concurrently with steps 7-9 (TRU). Reproduced with permission from ref. (18). Copyright 2015 Springer. 176 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Advancements in Separations and Nuclear Forensics Applications Methods developed at the University of Iowa provide the separation and analysis of Ga, Pu, U, Th, and Am for the purpose of nuclear forensics and nuclear fuel analysis. Further advancements are needed, however, that include certification of the method for use on various nuclear materials. This method’s applications could potentially be extended into many types of nuclear materials (MOX fuels, Pu-Ga alloys, nuclear wastes), but verification is a crucial step in the process. Accessing nuclear materials is not trivial due to nonproliferation security controls; accessing nuclear materials for peaceful research and development is similarly challenging. Performing this separation on digested nuclear fuels (both pre and post burn-up) as well as Pu-nuclear weapon pits would allow for a more complete understanding of this method’s strengths and limitations. Additional quantification of Ga by ICP-MS would also be a useful improvement as these instruments become more widely used for analysis of trace-level metals. The use of 68Ga as a radiotracer could be performed before the analysis of gallium by ICP-MS, and would likewise provide a precise quantification of the gallium present in a nuclear material. Certifying this method for ICP-MS analysis would widen the scope of sample analysis to include compounds like post-irradiation nuclear materials, which would have a dramatically more complex matrix, including fission products(e.g. 137Cs, 90Sr, 131I, 85Kr, 99Tc, 151Sm, 93Zr, etc.) that complicate analysis of the sample by radiotracers (20). Additional modifications to the method may be needed with the addition of fission products. For example, additional method verification would be required for various isobaric interferences, which could accompany fission products arising from post-burnup MOX fuels. Overall, the use of extraction chromatography with the addition of a medical radioisotope tracer helped to advance our methods regarding Pu-bearing nuclear materials. This work highlights the importance of radiochemistry training programs within the university setting, where advancements in one field can be applied to other fields and students involved in interdisciplinary programs can take advantage of a growing number of interesting isotopes used in a wide range of applications.

Concerns about Radioactivity in Liquid Waste from Hydraulic Fracturing When the University of Iowa program launched in 2012, concerns about naturally occurring radium (Ra) isotopes in Marcellus Shale produced fluids had just begun to surface within the scientific community (21–23). A group at the United States Geologic Survey (USGS) published a detailed study that illustrated the potential for 226Ra and 228Ra to concentrate in produced fluids and brines generated by hydraulic fracturing for natural gas in the Marcellus Shale formation (21). This report raised numerous questions about radioactivity in hydraulic fracturing flowback water and produced fluids, such as the presence of 177 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


naturally occurring radionuclides and environmental impacts of waste disposal. The USGS report was followed by a key study by Warner et al. (2013) that indicate the presence of high levels of Ra isotopes downstream of wastewater treatment plants handling Marcellus Shale produced fluids, suggesting that certain treatment plants were undertreating the radioactivity associated with these fluids (23). After the Warner et al. (2013) and USGS studies, it was clear that there were elevated levels of radioactivity in these fluids and that there was the potential for certain isotopes (226Ra and 228Ra) to contaminate the environment. Despite these two observations, there were no validated methods to detect and characterize radionuclides in hydraulic fracturing-produced fluids and brines. The problem presented by this lack of methodology was twofold: (1) without comprehensive methodology for alpha, beta, and gamma emitters, it was unclear whether other radioactive elements were present in the fluids, and (2) the radioactivity concentrations of reported Ra isotopes remained questionable.

Method Development By the summer of 2013, the U.S. Environmental Protection Agency (USEPA) had funded a project at the University of Iowa to develop and validate a method to detect gross alpha and beta particles in these fluids (24). Development of the gross alpha and beta method required a technique that could isolate key radionuclides, including U, Th, Pa, Ra, Pb, Bi, and Po isotopes. Numerous methods to isolate these radionuclides in drinking water exist (e.g. EPA 900.0 (25) & ASTM method #D7283) (25, 26); however, these methods are intended for waters with low concentrations of dissolved solids. Water samples with higher levels of dissolved solids are generally unsuitable for a “gross alpha and beta” method due to 1) the need to remove interfering stable elements that would absorb or attenuate the ionizing radiation, and 2) the challenges of separating the numerous radionuclides from stable elements. The sample of Marcellus Shale produced fluids provided to the University of Iowa had exceptionally high concentrations of dissolved solids (~278,000 mg/L) (27). Furthermore, the fluids contained high levels of barium (Ba) salts (>9,000 mg/kg) (28). The problem with Ba for radiochemical separations is that Ba and Ra have very similar chemistry and often co-elute or co-precipitate during chemical separations (28–31). The high levels of dissolved solids (in particular the high concentrations of Ba) suggested that a low-cost, rapid, practical separation of Ra from the Marcellus Shale fluids would not be possible. Given that Ra had been previously established as a key radionuclide in Marcellus Shale produced fluids and flowback (21, 32), this suggested that a single gross alpha/beta method was impractical. Thus, we had to split the sample up to measure key alpha emitters (238U, 234U, 232Th, 230Th, 228Th, 210Po) by alpha spectrometry and key beta emitters by gamma spectrometry (228Ac, 212Bi, 212Pb, 214Pb, 214Bi). Note that most key beta emitters of concern in this case also emitted gamma rays (ex: 210Pb; 46 keV) (33), or had short-lived daughters that could be measured by gamma spectrometry, which would allow for modeling their activity (ex: 226Ra released 214Bi and 214Pb). 178



Radiochemical Disequilibrium During the development of the isotope-specific method, we discovered several interesting radiochemical parameters. First, we observed that U levels in the produced fluids were exceptionally low, suggesting that U was insoluble. This is to be expected given that the Marcellus Shale is a reducing zone, meaning that U would be found in the +4 valence state and immobile (34). The second discovery was that 234U radioactivity concentrations exceeded 238U radioactivity concentrations. This result was surprising at first, given that a system as old as the Marcellus Shale is expected to be in secular equilibrium (meaning 238U and 234U should have the exact same radioactivity concentration; for further reading on radioactive equilibrium we recommend the following references) (35–37). The discrepancy in radioactivity levels is believed to be attributed to a phenomenon known as alpha recoil enrichment, which results in higher-than-expected concentrations of decay products (38, 39). The third discovery was that Ra activities, as measured by chemical separations, were greatly underestimated. We attributed this discovery to Ba interference during chemical separations, which is further discussed in the following references (27, 29). Lastly, and perhaps of greatest interest to us, was the apparent absence of 226Ra decay products from the fluids (40). Given the solubility of Pb in these fluids (27), we expected 210Pb to be detectable; however, models suggested that 210Pb was absent at the time of extraction and thereby insoluble in the fluids. More studies are needed to fully understand the chemical reasoning for the 210Pb insolubility in these fluids, but we currently hypothesize that this is caused by the partitioning of 222Rn gas (41). The absence of 210Po was expected due to the lack of its parent 210Pb.

Radium Decay Products As we were investigating the absence of 226Ra decay products in these fluids, Warner et al. (2013) documented high levels of Ra isotopes downstream of a wastewater treatment facility processing Marcellus Shale produced fluids but the study did not address Ra decay products.To further explore the environmental transport and mobility of Ra decay products, particularly 210Po and 210Pb, we embarked on a year-long field study near Mannington, West Virginia (42). Local citizens expressed concerns that the Northern West Virginia Water Treatment Facility was discharging undertreated Marcellus Shale waste into the Hibbs Reservoir. We later discovered through a Freedom of Information Act (FOIA) request that this facility only received coal wastes and that all documented discharges appeared in the range of issued permits (43). Regardless, we saw this as a unique opportunity to investigate the fate and transport of 226Ra decay products in the environment. After three sampling trips, we discovered that 210Po radioactivity concentrations were approximately 2.3 ± 0.4 (n=12) fold greater than the parent 226Ra (42). In one sample, the 210Po radioactivity concentration exceeded that of cleanup goals for surface sediments at USEPA CERCLA sites. We were hesitant to publish this result without an appropriate local control site, but many of the lakes in the surrounding area were impacted by oil and natural gas or coal extraction. Therefore, we sampled a lake at F.W. Kent State Park near 179


Oxford, Iowa that was of similar size, but had no documented oil and natural gas or coal extraction in its watershed to serve as a control site (44). We were shocked to discover that 210Po exceeded 226Ra concentrations by 2.8 ± 0.5 (n=5) at the Iowa site and that the activity concentration was very similar to what we observed at impacted Hibbs Reservoir in West Virginia. The only apparent source of 210Po for F.W. Kent Lake is background 222Rn, suggesting that 210Po accumulates in lakes in areas with high background 222Rn.


Drill Cuttings Although the bulk of discussion on radioactivity related to drilling activities in the Marcellus Shale has focused on liquid waste, we understood that all the radioactivity in liquid waste ultimately came from solids in the subsurface—i.e., the source rock necessarily containing the parents 238U and 232Th (40). As we were finishing our research on liquid wastes, reports were emerging in popular press outlets that radioactivity levels in solid wastes were potentially at elevated and actionable levels. Specifically, several news sources noted that radioactivity alarms at landfills were activated by trucks carrying drill cuttings (solid waste) (45). These radioactivity alarms were characterized by dose rates, but did not define the radiochemical profile of the drill cuttings. In early 2014, we began efforts to obtain drill cuttings so that we could characterize the disequilibrium status of these cuttings, but were not able to obtain materials until a drilling company directly supplied us with them in late 2015. During this time, Pennsylvania and West Virginia released studies that characterized certain aspects of these drill cuttings (gamma spectrometry-based results); however, the cuttings’ radiochemical profiles were not released due to a lack of methodology for alpha spectrometry. Thus we saw a critical need to develop methodologies for examining these materials and to distribute these methods to other researchers through the peerreview process. Due to the chemical characteristics and high organic content of the cuttings, we tailored these methods from techniques previously developed for asphalt. The method development process is described further in Eitrheim et al. (2016) (41). The most interesting aspect of the study is that 226Ra decay products were in disequilibrium with the supporting parent radionuclides. This continual trend of 238U/210Po disequilibrium in environmental radioactivity measurement raises several questions: Where is 210Po going?; How is it getting there?; What are the health implications of 210Po exposure? The following section of this chapter will explore these questions regarding the fate, transport, and health implications of 210Po exposure.

Future Directions of Research: Enhanced Understanding of 210Po in Natural Environments Polonium Radiochemistry and Sources Observations from our unconventional drilling studies suggest that 210Po poses an uncharacterized environmental health concern. Moreover, Po is among 180 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


the least characterized and poorly studied elements in the periodic table, in part due to the inherent challenges studying short-lived radionuclides. There are no stable isotopes of Po; all known isotopes are radioactive and have relatively short half-lives. The longest-lived isotope is the artificially produced 209Po (half-life 109 years) (46) and the longest lived naturally occurring isotope is 210Po (half-life of 138 days) (33). 210Po is the most important isotope of Po with respect to environmental nutrient cycling and public health. As the final radioactive product in the 238U decay series, 210Po is ubiquitous in nature and found along long-lived 238U series radionuclides. For example, 210Po is often observed in geologic and industrial sources of 238U (such as phosphate rocks, marine black shales, and uranium ores), 226Ra (phosphogypsum, hydraulic fracturing wastes, pipe scale), and 210Pb (coal fly ash, tobacco leaves, marine organisms) (34, 40, 47–50). Despite its high concentration in some environmental media (concentration factors greater than 10-fold over parent radionuclides), there are significant challenges with studying 210Po in the environment. The short half-life of 210Po results in very small quantities; masses of 210Po rarely exceed a few femtograms per gram or liter of environmental materials (51). For comparison, the half-life of 238U and 210Po is 4.5 billion years and 138 days, respectively, which means that with the same radioactivity concentration there will be 14 billion more atoms of 238U than 210Po. Laboratory-based studies on 210Po chemistry and biochemistry are thus largely based on trace-level masses due to the radiological hazards and challenges of obtaining gram quantities of 210Po. One gram of 210Po has a specific activity of 166 terabecquerels (TBq) per gram (~4500 Curies per gram), which corresponds to a lethal dose of 210Po of less than 10 micrograms. Furthermore, gram quantities of Po lack environmental relevance, i.e., trace-level experiments are more informative of environmental processes. Such trace levels of Po (10 femtograms or less) dictate that radiochemical methods must be used as masses are below the reasonable detection limits of conventional chemical characterization techniques (35). Polonium Water Chemistry Po has a unique aqueous chemistry that has variable redox chemistry that is not completely understood within natural systems. Its most stable valence state in the environment is tetravalent Po (Po(IV)), which forms strong hydroxide complexes to produce insoluble Po(OH)4. Under oxidizing conditions, Po is very particle-reactive and readily adsorbs to mineral surfaces (51). Reduction to divalent species can occur in natural waters, leading to the formation of more soluble forms and higher mobility. Organisms such as sulfate-reducing bacteria can take advantage of this redox process, as the Po(IV) likely serves as a reducible electron acceptor for either dissimilatory or assimilatory metabolic processes that produce the more mobile Po(II) oxidation state. However, studies have suggested that the mobilization of Po occurs in a narrow range of biogeochemical and geochemical conditions and in more reducing conditions resulting in the formation of insoluble Po(0) forms. The unique chemistry of Po is exploited during radiochemical separations and measurements, as Po is the only known naturally occurring radionuclide that autodeposits onto Ag, Ni, and Cu metals 181


under reducing, acidic conditions. While the basic understanding of Po chemistry has been explored, behavior in more complex environmental systems deserves additional investigations. Polonium Health Concerns Although very small mass quantities of 210Po are present in the environment, presents significant public health hazards. 210Po is known to bioaccumulate in the body and target the soft tissues, where it can deliver significant doses of high-energy alpha particles (52). Both acute exposures (ingestion of few micrograms) and extremely low-level environmental exposures (ingestion of few femtograms per day) are of concern (53, 54). As environmental health practitioners become more aware of the ubiquity and cancer hazards of 210Po, there are increasing concerns over possible routes for exposure (53). Although some of this attention may be attributed to the assassination of Alexander Litvinenko in 2006, acute radiotoxic exposures to 210Po are exceptionally rare (55). On the other hand, environmental and occupational exposures for 210Po are commonplace, with major pathways of exposure including inhalation (for examples, smoking tobacco products, 222Rn contaminated buildings) and ingestion (consumption of aquatic species, ingestion of groundwater) (47, 51, 56). Of all exposure pathways, ingestion of 210Po from contaminated groundwater is the least characterized and represents the most critical need for future research.


210Po

Polonium in Groundwater: A Critical Need for Research Contamination of groundwater by 210Po can occur through both anthropogenic and natural processes but the extent of the problem is unclear. 210Po may accumulate in groundwater due to activities associated with extractive industries, such as phosphate fertilizer production and unconventional drilling for natural gas (40, 57, 58). 210Po can also accumulate in groundwater due to natural processes as first reported by the United States Geologic Survey (USGS) when they discovered naturally high levels of 210Po in groundwater—the major source of water for local dairy operations in Fallon, NV (40, 57, 58). The extent of natural 210Po contamination of groundwater is currently unknown for multiple reasons. First, 210Po has been historically neglected in groundwater monitoring regimens, as it is not directly regulated by any federal agency (58). Second, most groundwater users are not required to test groundwater for any radionuclides. Lastly, as a pure alpha-emitter, 210Po cannot be measured by routine radioactivity dose assessment techniques (such as via a Geiger counter) (35). Despite these challenges, several agencies including the USGS and others in Minnesota and Iowa have begun to monitor for 210Po in select groundwater sampling sites with preliminary results anticipated very soon (59, 60). Regional scale environmental monitoring can provide important clues to the geochemistry and biogeochemistry of 210Po. Students in University of Iowa Radiochemistry program will take part in future studies on the fate and transport of 210Po and with the help of local, regional, and international collaborations to further our understanding of the chronic health implications of exposure to low doses of this material. 182



Protactinium: The Element of Witchcraft Protactinium (Pa) is a naturally occurring actinide element, yet its chemistry continues to mystify scientists. Of the naturally occurring elements, Pa was one of the last seven to be discovered, largely in part to a general misunderstanding of its chemistry (61, 62). The chemistry of Pa has been described as puzzling (63), peculiar (64), mysterious (65), and even witchcraft (65), and much of these descriptions still apply today (65). In the late 1950s, a researcher in the National Academy of Sciences, Harold Kirby, postulated that in that decade, much of the mystery surrounding the chemistry of Pa “will be eliminated through the development of a convenient radiotracer (65).” Yet our understanding of its chemistry is still shrouded in mystery and ongoing concerted efforts are only beginning to understand the basic chemical and physical properties of Pa. At the University of Iowa, investigations were driven to establish a better understanding of the chemistry of Pa and apply this new knowledge to applications in nuclear forensics, nuclear energy, and environmental radiochemistry.

The Chemistry of Protactinium and Applications The challenge of understanding the chemistry of Pa lies in its unique chemical and nuclear properties. Interestingly, Pa exists in a critical location within the actinide series (An), where an energy crossover occurs between the 6d and 5f orbitals and computational studies have suggested that these states will be nearly degenerate (66). This leads to distinct bonding characteristics that are distinctive to Pa and chemistry that deviates drastically from other early series actinides like Th, U, Np and Pu (64). A recent study investigating the enthalpy of isomerization of AnO2(H2O)+ to AnO(OH)2+ (for An: Pa-Pu) revealed that this process is distinctively low in energy for Pa in contrast to the other An elements where the formation of AnO2(H2O)+ is considerably more energetically favorable (66). Within this context, the research at the University of Iowa aimed to develop a detailed understanding of the trace-level chemical properties of Pa (67). We focused on the unique properties of Pa in trace-level concentrations to improve our understanding of the chemical behavior and bonding characteristics of Pa (67–69). Pa is not widely used as a radiotracer due to its high radioactivity, low abundance, and poorly understood chemistry, so its primary geochemical application is the assessment of 231Pa/235U ratios (Figure 2) (68, 70–73). Small amounts of 235U occur naturally in geologic deposits where it will decay via alpha-particle emission to 231Pa and the isotopic ratio can be used to determine the time that has elapsed since the initial fractionation (70). While there are numerous dating methods available to geologists to date a variety of sample types, the 230Th/238U and 231Pa/235U isotopic ratios are viable options for determining the precise ages of quaternary materials (events occurring up to 250,000 years ago or ~7 half-lives of 231Pa). The 231Pa/235U chronometer is more sensitive with samples of younger ages due to its shorter half-life compared to 230Th and can be used for more precise measurements within this time period. Most notably, 231Pa/235U has been used to date a human skull from Qafzeh, Israel and Neanderthal bones in 183



Israel (74), which helped confirm that humans and Neanderthals did not coexist in the Levant during the same period (75). Furthermore, these two isotopic ratios can be used together to determine sedimentation rates or provide extremely precise ages (76–78).

Figure 2. Radioactive ingrowth of 231Pa to secular equilibrium with 235U and to secular equilibrium with 238U. This figure does not take into account the isotopic abundance of the U isotopes.

230Th

The 231Pa/235U isotopic ratio has also proven itself to be valuable for the agedating of U-special nuclear material for nuclear forensics analysis (79, 80). For these samples, the materials are relatively young (4 M HCl), Pa is selectively extracted into the organic phase whereas the other actinides (Th, U, Np, and Am) remain in the aqueous phase. Slope analysis experiments with respect to the extractant alcohol and the ligand (Cland NO3-) showed a 2:1 relationship between the extractant molecules and the central Pa metal. Furthermore, these studies revealed the approximate speciation of Pa with respect to the ligands. In HCl, the extracted species of Pa appears to be a hexachloro complex, PaCl6-, suggesting that the extraction occurs via the formation of HPaCl6 complex acid and stabilized by the H-bonding network of the alcohols of the hydroxyl groups. In HNO3, Pa speciation is less obvious because NO3- can act as a mono- or bidentate ligand, and slope analysis cannot distinguish between the two forms. The analysis suggests the formation of a 2:1 Pa-nitrate species, but the additional coordinating species (hydroxyl, water, oxo) are not known and must be verified with additional spectroscopic or scattering techniques (84).

Figure 4. Separation methods for the separation of Pa from Np for the tracer preparation, and separation of Th, Pa, and U for isotope dilution alpha spectrometry.

Future Work with Pa Pa was discovered over 100 years ago, but our current understanding of Pa chemistry remains at an introductory level. Current research is focusing on the fundamental chemistry of aqueous complexes (85, 86) and rapid methods 187 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

for isotopic analysis (73, 79), but the overall progress is slow. Only a few investigators around the world are actively engaged in Pa research. In a recent perspective, Wilson highlights the importance of investigating Pa as an avenue to understanding the chemical dependencies of 6d and 5f electronic structures, but notes, “future contributions to (Pa) chemistry may come from where Meitner and Hahn first found it, in silico” (64).


Conclusions As discussed above, the forecasted shortage of radiochemists and radiochemistry graduate programs led to the development of curriculum and expertise at University of Iowa. The first cohort of radiochemistry students focused on Ga and Pu with applications in MOX fuels, naturally occurring uranium-series radionuclides in the environment, and fundamental Pa chemistry. Advancing the complete separation of gallium and actinides in nuclear materials for nuclear forensics applications was accomplished yet developments regarding ICP-MS and certification of these methods is still needed. Research in uranium-series radionuclides highlighted the importance of sound methodology and comprehensive analysis of decay products, including 210Po. From our observations of 210Po in aqueous environments, we have begun to probe deeper into the geochemical cycling of 210Po in the Upper Midwest. Our understanding of Pa has also broadened, potentially allowing better applications for isolation, purification, and separation of Pa for isotopic analysis with applications ranging from nuclear forensics to geochronology. Still a better understanding of fundamental Pa chemistry is needed, which may be aided in the future by better access to this element. Overall, the University of Iowa Radiochemistry program has contributed to the understanding of these various elements. There is high hope that radiochemistry advancements will continue to contribute to future discoveries regarding elements and their chemistry.

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