Combining Fission-Track Radiography and Scanning Electron Microscopy to Elucidate Uranium Mobility Controls

doi:10.21203/rs.3.rs-2338954/v1

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Combining Fission-Track Radiography and Scanning Electron Microscopy to Elucidate Uranium Mobility Controls

https://doi.org/10.21203/rs.3.rs-2338954/v1

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published 04 Jan, 2024

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Residual solid-phase uranium from former mill tailings leachate can contribute to persistent concentrations of uranium in groundwater that exceed regulatory levels. Microscale characterization of uranium-contaminated sediment samples is lacking due to the challenges of detecting uranium at the parts-per-million level and identifying its associations with co-occurring elements. An emerging methodology, fission-track radiography, was applied to detect low-level solid-phase uranium. Scanning electron microscopy and energy dispersive x-ray spectroscopy were used to elucidate uranium associations with co-occurring aluminum, iron, and phosphorous. Uranium-contaminated sediments were collected from the upgradient source zone and downgradient plume zone aquifer sediments at Riverton, Wyoming, USA. The combined microscopic analyses showed that the uranium primarily co-occurred with amorphous aluminum hydroxide and ferric hydroxide coatings in the source zone as opposed to proximal crystalline Fe-rich grains. In the plume zone, uranium primarily co-occurred with apatite as opposed to proximal iron sulfides. The unique geochemical associations of solid-phase uranium with co-occurring aluminum hydroxide, ferric hydroxide, and apatite, as opposed to other proximal minerals, suggested that a select suite of equilibrium and kinetic reactions controls its persistence in groundwater. The combined methodology applied in this study pinpointed the potential suite of uranium reactions that can be used to inform geochemical models for further mechanistic insight and forward simulations of the fate and transport of uranium at contaminated sites.

Uranium mill tailings

Uranium mobility

Fission-track radiography

Aluminum hydroxide coating

Ferric hydroxide coating

Uranium (U) is a long-term environmental concern at sites impacted by the legacy of uranium mining and radioactive waste disposal. Groundwater contamination of uranium by former mine and mill tailings continues to pose a challenge to site restoration efforts even after the primary sources of contamination have been removed due to the persistence of residual uranium associated with aquifer materials (Campbell et al. 2012). Site assessments often rely on site conceptual models to inform clean-up approaches of the secondary sources. Uncertainty regarding the behavior or location of additional uranium sources can cause significant risks to stakeholders. Consequently, accurate characterization of secondary sources and their potential for uranium mobilization is needed to develop effective remediation strategies and time frames.

It is long-established that the dominant factor for the retention of uranium in the subsurface under oxic conditions is sorption to mineral surfaces, amorphous phases, and organic matter (Fenton and Waite 1996; Jové Colón et al. 2006; Stubbs et al. 2009). It is challenging to apply sorption models to mixtures of minerals found in natural sediments because of the insufficient knowledge of the chemical composition of surface regions and their interaction with uranium. Micron-thick coatings of secondary mineral phases and organic matter can form on mineral surfaces and serve as efficient sorption sites with a higher adsorption capacity than the substrate (Jové Colón et al. 2006). Thus, the presence of coating can potentially affect uranium sorption and transport in porous media, and this knowledge gap can add substantial uncertainty to radionuclide desorption predictions from sediments (Davis et al. 2004). In addition, uranium can coprecipitate with iron oxides (Duff et al. 2002), and aluminum hydroxides (Luo et al. 2009) which can produce elevated uranium concentrations if geochemical conditions become favorable towards mineral dissolution (Noubactep et al. 2006). Accurately predicting uranium fate and transport requires conceptual models consistent with molecular to macro-scale experimental evidence (Arai et al. 2007). Identifying the solid-phase sinks for uranium is crucial for conceptual modeling and successful remediation approaches (Jové Colón et al. 2006; Stubbs et al. 2009).

A variety of sophisticated spectroscopic and diffraction techniques have been used to analyze uranium associations with complex mineral assemblages in sediments from some of the U.S. Department of Energy’s legacy sites, e.g., the Hanford site (e.g., Arai et al. 2007; Catalano et al. 2004; Singer et al. 2009; Stubbs et al. 2009), Naturita site (e.g., Jové Colón et al. 2006), and Grand Junction site (e.g., Johnson et al. 2021). Specifically, X-ray absorption fine structure spectroscopy (XAFS) and X-ray absorption near edge structure spectroscopy (XANES) have been commonly used to determine uranium speciation onto solid phases. However, these methods only provide bulk measurements of uranium speciation in absorption patterns that require the deconvolution of complex spectra. Consequently, XAFS and XANES often cannot detect uranium associations at low concentrations or with poorly or non-crystalline solids such as amorphous surface coatings or organic matter when performed on natural sediments (Catalano et al. 2004; Singer et al. 2009). Thus, these techniques rarely give direct information relevant to the trace concentration conditions typical of uranium mill tailings repositories. Secondary-ion mass spectrometry (SIMS) has a sensitivity of a detection limit down to ppb level, but it requires prior knowledge of the significant uranium sinks (Jové Colón et al. 2006). Fission-track analysis is a methodology that can identify areas with trace-level uranium concentrations within a thin section (Wollenberg 1971; Zielinski and Budahn 1998) and allows us to determine the significant sinks for uranium. This method has been applied to various geological problems (Galagher et al. 1998), e.g., Quaternary geochronology (Dumitru and Stockli 1998), thermochronology (Wolf et al. 1997), and provenance studies (Hurford and Carter 1991). However, despite its potential, it has not been extensively used to elucidate uranium associations in natural sediments to inform conceptual site models. Johnson et al. (2021) applied fission-track technology on trace uranium characterization at Grand Junction, Colorado site; however, much remains unknown in direct observation of uranium hosts at a larger site scale.

In this study, fission-track radiography was combined with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to pinpoint major sinks for trace uranium in natural sediments spatially. The samples analyzed were from a transect of cores in a uranium-contaminated field site in Riverton, Wyoming, including the source zone in the upgradient and plume zone in the downgradient away from the source zone, allowing us to observe uranium sorption sites along the migration pathway. The upgradient and downgradient aquifer sediments were analyzed for uranium co-associations in this study. Uranium below the detection limit of SEM and EDS (0.1% mass concentration) was detected by fission-track radiography. The characterization of the area with elevated uranium, identified by fission-track radiography, was done using SEM and EDS. With the combination of these methods, primary uranium hosts in natural sediments were identified, and probable uranium transport mechanisms along the plume were outlined. The results of the study will aid in modifying conceptual models, and surface adsorption approach in reactive transport models.

Site Description

Sediments for this study were collected from a former uranium mill tailings site located 3.9 kilometers southwest of Riverton, Wyoming, United States of America. The mill processed 816,470 tons of uranium ore from 1958 to 1963 (Merritt 1971). The tailings slurry from the uranium milling process was the primary source that contaminated the shallow groundwater beneath and downgradient of the site (Narasimhan et al. 1986; White et al. 1984). The uranium in the plume in the surficial aquifer is mill-related, as confirmed by uranium activity ratios (²³⁴U/²³⁸U) (U.S. Department of Energy 2016). The plume persists even after rehabilitation of the tailing’s repository was completed in 1989. Annual monitoring and a flood in 2010 indicated that the uranium concentrations were not declining as predicted (Dam et al. 2015) in the site conceptual model formulated in the 1990s. Subsequent work (Dam et al. 2015; U.S. Department of Energy 2016) focused on the secondary source characterizations by lab-scale analyses and new conceptual model construction for residual uranium. The Riverton site has been influenced by periodic flooding, as it is on an alluvial terrace between the Wind River in the north and the Little Wind River in the East (Fig. 1). Most of the contamination in downgradient is in the shallow alluvial aquifer composed of river-transported sands and gravels.

For this study, the site is divided into two zones based on the distribution of residual uranium in a surficial aquifer (Fig. 1):

(1) Source zone is beneath the former tailings area where the tailings pile from the uranium mill was located (Dam et al. 2015). It defines the most upgradient area with remaining solid-phase uranium contamination (Fig. 1 and Fig. 2). Borings 859 and 860 (the blue rectangle in Fig. 1) have been analyzed from this location.

(2) The plume zone is in the downgradient area within the plume where white evaporitic mineral deposits are visually evident along the bank of the Little Wind River (U.S. Department of Energy, 2014). Boring 858 (the orange rectangle in Fig. 1) represents this location in this study.

The plume edge zone is outside the uranium contaminant plume, and the uranium concentration is approximately 0.044 mg/L, which is considered background. Boring 852 represents this location (the green rectangle in Fig. 1). The background boring has not been analyzed in this study.

Sample Collection and Uranium Extraction

Sonic drilling was used to collect 6-inch diameter continuous core samples from borings 852, 859, 860, and 858, starting from the ground surface to the top of the first confining zone, which is the top of the bedrock (U.S. Department of Energy 2016). The core from the drilling was vibrated out of the drill stem, placed in a plastic core sleeve, and then cut open to attain lithologic information. To obtain geochemical parameters, each core was first subdivided at approximately 1 ft intervals. Then to collect a sufficient sample volume for laboratory analyses, approximately 3 inches of the core was selected and thoroughly mixed in a pan to homogenize the sample. The sample volume necessary for lab analyses was placed in a plastic bottle, and the rest was preserved in a plastic bag. Soil samples were analyzed by a Department of Energy Consolidated Audit Program (DOECAP)-approved laboratory for U.S. Nuclear Regulatory Commission (NRC)-approved Contaminants of Potential Concern (COPC) (manganese [Mn], molybdenum [Mb], sulfate [SO42-], and uranium [U]), major cations (calcium [Ca], magnesium [Mg], potassium [K], and sodium [Na]), and additional major anions (chloride [Cl]). Iron (Fe), total organic carbon, and total inorganic carbon were included as additional analytes. Total acid digestion using EPA method SW-846 3050B was used for uranium extraction. EPA methods SW-846 6010B and SW-846 6020 were used for analyses of metals and major cations (U.S. Environmental Protection Agency 2014). All the well logs, lithologic descriptions, and final data are available from the U.S. Department of Energy (2016).

Thin Section Preparation

Based on solid-phase uranium concentrations, relatively high uranium concentration intervals were chosen to prepare the thin sections by polishing a thin section of an epoxy-impregnated sample of interest. Standard-sized thin section slides (27 × 46 millimeters [mm]) were modified by grinding the edges slightly (25 × 46 mm) to fit in plastic containers for irradiation. Polished thin sections are preferred because they permit the most intimate planar contact between the thin section surface and the detector.

Fission-Track Radiography

Fission-track radiography is a method that determines abundance and location of uranium as discussed in (Wollenberg 1971; Zielinski and Budahn 1998). In brief, this method exposes a thin section to radioactivity, and the resulting radioactive decay of uranium creates “fission tracks” in an overlying mica sheet. The prepared thin section slides were placed in polyethylene tubes and irradiated in a research reactor at the U.S. Geological Survey in Denver, Colorado after a mica was placed over the thin sections following the methods of Johnson et al. (2021). Samples were irradiated at a neutron flux of 2 × 10¹² neutrons per square centimeter per second for 8 hours for a total maximum neutron dose of ~ 11.6 × 10¹⁶ neutrons (less time or a lower neutron flux [or both] is required for higher concentrations of uranium). The samples are then allowed to undergo radioactive decay in storage for about 6 weeks or more to allow for the decay of some shorter-lived neutron activation products. Images of the thin section and associated mica plate were completed using a high-resolution scanner. Paired areas of fission tracks on mica and thin sections were photographed for documentation using cameras mounted to microscopes. Standard petrographic techniques were used to examine the fission tracks and thin sections with a microscope under plain polarized light (PPL). This technique allows for the microscale identification of uranium from the fission tracks and a comparison to the overall grain mineralogy.

SEM-EDS Analysis

The irradiated thin sections were first carbon coated using an Edwards 306A vacuum coater fitted with carbon fiber (Catalog #91202, Ted Pella Inc.) A Hitachi S-4800 Ultra High-Resolution Cold Cathode Field Emission Scanning Electron Microscope (FE-SEM) equipped with a photodiode backscattered electron (PD-BSE) detector and a Bruker Quantax Energy Dispersive X-Ray Spectroscopy (EDS) system was used to examine the thin sections after coating. The FE-SEM parameters during the analysis varied depending on the samples. In general, a 15 kilovolt (kV) accelerating voltage, a 15 mm WD (the analytical working distance), and a 15-micro ampere (µA) emission current were used to analyze the slides. After a region of interest (ROI) was found within the individual thin sections, guided by fission-track radiography images, a BSE image of the location was captured. The BSE image was used as a guide to analyze bright spots, which indicated the presence of high-Z elements. The same ROI was analyzed with EDS to determine the elemental composition of the sample. After a spectrum was obtained, dot mapping was performed to provide an overall idea of the types and distribution of the elements in the ROI; afterward, a point analysis was performed to identify mineral composition based on the spectrum generated. The EDS spectra from point map analysis show intensity (counts per second per electron volt [cps/eV]) and energy (kiloelectron volts [keV]) for respective elemental occurrences, which are used to determine the mineralogical composition of areas with high fission-track intensity.

Macroscale Solid-Phase Uranium Associations

A total acid digestion extraction method was used to measure total sediment-associated uranium. The digestion results of the solids indicate that the uranium concentrations range from 0.02 to 22 mg/kg in sediment. A summary of uranium concentrations (mg/kg) with depths (ft) for borings 852, 859, 860, and 858 is provided in Fig. 2. Boring 852 is the background with a uranium concentration of 2 mg/kg above the water table and 1 mg/kg below the water table.

Borings 859 and 860 are in the source zone with above background uranium concentrations of 5.5 mg/kg at 8.5 ft depth in fine sand and 2.7 mg/kg in coarse sand at 15.5 ft depth, respectively, in the saturated zone. The tailings (in addition to 3.9 ft of sub-pile material) from the unsaturated zone of the former tailings area were removed as a part of the DOE reclamation program (U.S. Department of Energy 2016); hence, the unsaturated zones of these two borings are not included in this paper.

Boring 858 is in the plume zone with a uranium concentration of 0.72 mg/kg (below background) in a coarse sand layer at 13 ft depth below the water table. Although the concentration is below the background, it has been selected for this study to understand the uranium transport mechanism between the upgradient source zone to the downgradient plume zone in the saturated layers. This study focused only on the saturated layers. Hence, the unsaturated layer of boring 858 was not studied despite uranium being above the background concentration (7.8 mg/kg). The high uranium concentration (5 mg/kg) at the bottom of boring 858 is due to the higher clay content in that portion of the bedrock.

Thus, the macroscale uranium concentration data from total digestions indicate significant variation in the solid-phase uranium concentration between the former tailings area-source zone and plume zone (as shown with depths, stratigraphy, and water table location in Fig. 2). Based on the total uranium digestions; it is the finer sediments in the saturated zone that contain more uranium than the coarser fractions, similar to other sediments exposed to uranium mill tailings (Davis et al. 2004). The sediments to be analyzed (red rectangle in Fig. 2) in the microscopic methods were selected from the depths based on the solid-phase uranium concentrations from the total acid digestion method.

Microscale Uranium Associations: Uranium Hosts

The microscale analysis of uranium in this section includes findings from fission-track radiography, SEM and EDS. The combined microscopic analyses show solid-phase uranium associations from the upgradient source zone to the downgradient plume zone. Three types of uranium hosts in the contaminated sediments are identified in this study. In the source zone, two distinctly different hosts for uranium are identified in aquifer sediments: Al hydroxide and ferric hydroxide coatings. In the plume zone, uranium is hosted by apatite. The amorphous coatings and apatite minerals do not contain sufficient uranium to be detected using EDS in the electron microprobe. However, fission-track analysis collected in the same thin sections exhibited precise spatial distribution of uranium that worked as a guide in SEM/EDS analysis. It is to be noted that the backscattered electron (BSE) image, fission-track radiography image, transmitted light image, and elemental maps are generated from the same ROI.

Al hydroxide coating

Al-rich coating: The aquifer sediments from boring 859 (Fig. 1 and Fig. 2) located in the source zone contain Al-rich precipitate (Fig. 3 and Fig. 4), as described below. Figures 3 and 4 depict two different sample points from the same thin section (Online Resource Fig. S1) prepared from boring 859. The Al-rich precipitates are found to be a few microns in width and up to a few tens of microns long (Fig. 3). SEM and EDS allowed us to show that these occur both as individual precipitates (Fig. 3) and at the edges of coatings of feldspar (Fig. 3) and quartz (Online Resource Fig. S4). The dried gel appearance with a distinct shrinkage crack (Fig. 3 and Fig. 4) and a consistent color (Online Resource Fig. S3) indicate that the precipitate is Al-rich rather than clay that has a molted appearance (Johnson et al. 2021). The stronger signal of the Al elemental map than the Si map (Fig. 3 and Fig. 4), and the higher Al peak than Si, with trace amounts of Ca, S, and P in the EDS results (Fig. 3, Fig. 4, and Online Resource Fig. S11) at the region of the precipitate support that Al is the primary element in the precipitate and not a component of clay.

Identification of the phase of Al-rich coating: The exact phases of the Al-rich precipitate could not be identified using SEM-EDS alone. However, based on a study by White et al. (1984) in Riverton, Wyoming, and EDS data with high Al peaks than that of Si (Fig. 3 and Fig. 4), it can be postulated that the Al-rich precipitate is Al hydroxide. According to White et al. (1984), the low-pH process waters caused the dissolution of calcite and the production of CO₂ during contact with the underlying calcite-rich aquifer. White et al. (1984) hypothesized that the buffering of the mill tailing effluents in the groundwater caused significant Al and Fe hydroxide precipitation, indicated by the dramatic decrease of dissolved Al and Fe concentrations at near-neutral pH conditions.

Al hydroxide as a uranium host: Fission-track technology pinpointed trace uranium associations with corresponding hosts in the thin sections by overlapping the fission-track maps with the BSE images and EDS elemental maps. The fission tracks of uranium occur along the Al hydroxide precipitate that revealed the co-association of uranium with Al hydroxide precipitates and coatings around feldspar and quartz (Fig. 3 and Fig. 4). Fe-rich grains (Fig. 3), and iron oxide (Fig. 4) are also observed in proximity to Al hydroxide precipitate. Corresponding image (b) and Fe elemental map in Fig. 3 and Fig. 4 show no uranium fission tracks with the Fe-rich grains. Ferric hydroxide phases are known to have a high affinity towards uranium and are assumed to be one of the primary sorbing phases of uranium (Duff and Amrhein 1996; Walter et al. 2003; Davis et al. 2004). This suggests that Al hydroxide coating is a preferential reservoir for trace-concentration uranium.

Mechanism with Al hydroxide coating: Uranium is known to coprecipitate with a variety of hydroxides and carbonates, including aluminum hydroxides (Luo et al. 2009). Boring 859 contains a low carbonate concentration (alkalinity total as CaCO3 is 314 mg/L) and a higher pH (>5) that are favorable to form uranium coprecipitate with Al hydroxide (Luo et al. 2009). According to Luo et al. (2020), this precipitate is stable against dissolution. Moreover, the fact that uranium is associated with Al hydroxide precipitates, but not Fe-rich phases indicates that the residual uranium in boring 859 sediments is also unlikely to release by desorption from Fe oxides under ambient conditions. However, the long-term stability of coprecipitated cases depends on weathering of soil minerals (Noubactep et al. 2004). Thus, the coprecipitated uranium might release into groundwater if the Al hydroxide-coated feldspar or quartz weathers. Nevertheless, the significantly higher sedimentary uranium concentration (5.5 mg/kg) (Fig. 2) than the dissolved uranium concentration (~0.09 mg/L) (U.S. Department of Energy 2016) indicates a relatively slower uranium release.

Ferric hydroxide coating

Fe-rich coating: The aquifer sediments from boring 860 (Fig. 1 and Fig. 2), which is also located in the source zone, contain Fe-rich coating (Fig. 5 and Fig. 6) in contrast to boring 859 which contain Al-rich coating. Figures 5 and 6 depict two different sample points (Online Resource Fig. S14) from the same thin section prepared from boring 860. The Fe-rich coatings are a few microns thick (Fig. 4 and Fig. 5) and appear around the edges of feldspar (Fig. 5 and 6). At least two morphologies of the Fe-rich coating are observed in the two sample points- purely amorphous (Fig. 5) and particulates (Fig. 6). A stronger Fe signal and weaker Al and Si signals observed in the coatings (Fig. 5 and Fig. 6, Online Resource Fig. S18, and S19) depict the coating as Fe-rich. However, the morphology of feldspar (Fig. 5 and Fig. 6) indicates that it is chemically weathered around the edges. A significant amount of Al and Si with high Fe peaks (Fig. 5 and Online Resource Fig. S21) on the coating around weathered feldspar indicates the possibility of Fe-rich coating being intermixed with weathered kaolinite from feldspar.

Identification of phases of the Fe-rich coating: Goldberg. (1989) showed that Fe-oxides carry a sufficient positive charge at low pH and can precipitate on clay surfaces. These coatings are stable at high pHs and form oxide phases separate from clays. Charlet et al. (2002) found that the surface coatings on clay particles frequently encountered in soil particles are very likely a product of heterogeneous oxidative precipitation of iron by a variety of natural electron acceptors. Moreover, the ferric hydroxide precipitates are prominent under former tailings areas (White et al. 1984). Although the exact phases of the Fe-rich coating could not be identified with the methods used in this study, however, based on the above discussion, it can be postulated that the Fe-rich coating with clay are ferric hydroxides.

Ferric hydroxide as a uranium host: The fission-track analysis (Fig. 5 and Fig. 6) and BSE images (Fig. 5 and 6) show a relatively high density of uranium fission tracks occurring with ferric hydroxide coating around feldspar. Although ferric hydroxide coating is intermixed with kaolinite, uranium is found to have more affinity towards ferric hydroxide coating than kaolinite. It is more evident when the ferric hydroxide coating occurred as particles (Fig. 6). Corresponding images (a), (b), and (c) in Fig. 6 show that uranium fission tracks are more concentrated where ferric hydroxide particles are apparent (Fig. 6). Kaolinite is known to exhibit much greater uranium sorption due to more available aluminol sites (Bachmaf and Merkel 2011). However, the comparative visualization in the density of fission tracks (Fig. 6) shows that the trace uranium has more affinity towards ferric hydroxide coating than clay. The corresponding image (a) and Fe elemental map in Fig. 5 and Fig. 6 show that no uranium fission tracks are associated with Fe when it is incorporated into an aluminosilicate (Fig. 5) or a Fe-rich crystalline grain (Fig. 6). Although, the grains are in proximity to the ferric hydroxide coating. The morphology and uneven distribution of the elements on the grain in Fig. 5 indicate a crack with a weathering surface that might have been exposed when the thin section was cut. The above discussion and the direct comparative visualization of uranium co-occurrence in the admixture of clays (Fig. 5 and Fig. 6) suggest that in a low temperature environment, the ferric hydroxide coating plays a significant role as a uranium reservoir.

Mechanism with ferric hydroxide coating: Deposition of Fe-rich particles on kaolinite (Fig. 5 and Fig. 6) would increase the surface area independently of the underlying material (Arias, 1995). Hence a significant amount of uranium might get sorbed onto ferric hydroxide coating on feldspar. Typically, when uranium is adsorbed onto aged ferric hydroxide coating, the mechanism is ion exchange which is almost entirely reversible (Noubactep et al. 2004). Thus, the remobilization of sorbed uranium into groundwater is possible via concentration-driven desorption. The aqueous uranium concentrations remain ~0.77 mg/L which is significantly above the mean concentration level (MCL) of 0.044 mg/l (U.S. Department of Energy 2016). Meanwhile, the solid-phase uranium is relatively close to the background concentration of 2 mg/kg (Fig. 2) in boring 860, indicating a significant release of uranium from the solid-phase to the aqueous phase. Concentration-driven desorption of uranium from amorphous ferric hydroxide coating is likely responsible for the continuous release of uranium in the 860 boring.

Apatite

The aquifer sediments from boring 858 located in the plume zone (Fig. 1 and Fig. 2) contain a calcium phosphate mineral incorporated into quartz (Fig. 7). The EDS spectrum 1 in fig. 6 with a high Ca and P peak with traces of F, but no U peak indicates that the mineral is apatite but not autunite. The morphology of apatite (Fig. 7) shows that it is present as a crystalline grain at the edges of quartz rather than an amorphous coating. Although the uranium peak is not identified in EDS spectra generated from apatite, a relatively high density of uranium fission tracks is found to be associated with apatite (Fig. 7). Apatite is one of the most promising phases to immobilize uranium for the long term by accommodating a significant amount of U(VI) in the apatite structure (Rakovan et al. 2002, Dangelmayr et al. 2022). Mackinawite (Fig. 7) is also observed in quartz fractures, suggesting an anoxic environment below the water table. However, uranium fission tracks are found only with apatite. Since uranium did not occur with reduced iron minerals and did not precipitate as autunite (Fig. 6), apatite is the main sink for uranium in the saturated layer of the plume zone. Since the sample has a low uranium concentration in sediments (Fig. 2), it indicates the insignificant presence of apatite-like uranium sorbing phases, thus limiting the retardation of uranium in the saturated zone. However, the uranium concentration (~0.643 mg/L) in groundwater is above MCL. Such elevated uranium concentration can be explained by uranium transport from the source zone and plume stability in groundwater.

Effect on the transport behavior of uranium

The groundwater plume at the Riverton, Wyoming processing site persists to this day despite the completion of surface remediation in 1989 with the removal of the tailings. The persistence of the high groundwater uranium concentrations is hypothesized to result from the secondary sources in the saturated zone of the source zone. In this study, unique hosts of solid-phase uranium were identified in the saturated layer of source zone-1. Al-hydroxide coatings on feldspar and quartz, and 2. ferric hydroxide coatings intermixed with clay. The two separate sinks for uranium in the two locations under the former tailings area in the source zone would exhibit different mobilization mechanisms and require different management strategies. Thus, identifying major sinks for uranium in natural sediment is essential for risk assessment. Uranium coprecipitated into Al hydroxides is relatively stable against dissolution and insensitive to concentration-driven desorption. Meanwhile, the highly sorptive ferric hydroxide coatings increase the overall sorption capacity of the host rock and sorb a significant amount of uranium. Eventually, the concentration-driven desorption process might release considerable amounts of uranium from the solid phase. However, the uranium release from Al hydroxide coating is hypothesized to be slower than from the ferric hydroxide coating. These secondary sources of the former tailings area were not considered in the original transport model, which misled the uranium transport prediction (Dam et al. 2015). The dual hosts in the uranium-contaminated sediments increase complexity in the modeling of an already complex natural environment. Moreover, the association of uranium with tailings-derived Al and Fe precipitates is not well documented in modeling databases or literature. To our knowledge, Allard et al. (1999) is the only study that have analyzed the uranium mechanism in a quaternary system (U-Si-Al-Fe). Allard et al. (1999) showed that the sorption structure of uranium in Fe-rich gels was not the same as in the Si/Al-rich gels. Thus, there is a need to identify the elements associated with uranium at the field scale to avoid an incorrect correlation between uranium and other elements, which might lead to a misleading interpretation of uranium mobility.

The release of uranium from the source zone transport downward in the plume zone, which results in groundwater concentrations exceeding regulatory limits. Although uranium is found to be associated with stable apatite, the uranium-associated minerals that might significantly retard uranium transport were also not observed in this layer. Thus, minimum retardation of uranium is observed in the saturated layer of the plume zone, which aids in plume persistence.

This paper demonstrates the advancement of combining fission-track radiography with scanning electron microscopy and energy dispersive spectrometry and provides significant insights into directly identifying reservoirs of residual solid-phase uranium from a uranium milling operation. The application of fission-track radiography pinpointed low-concentration uranium in the sediments, which would be difficult to identify using traditional characterization methods. The microscopic characterization of the uranium-contaminated sediments delineated the significant reservoirs for residual uranium in the aquifer sediments of the source zone in the upgradient and plume zone in the downgradient. Collectively, the findings from our studies and those of others mentioned in literatures, indicate that the long-term association of uranium in former tailings areas likely results from the co-precipitation with Al hydroxide coating around feldspar and quartz; and sorption with ferric hydroxide coating interlayered with clay around feldspar. The study shows the comparative affinity of uranium in the presence of coatings, clay, and crystalline grains, where uranium shows more affinity towards the coatings. The coatings can modify the overall physical properties of the bulk rock, and these processes can have significant consequences on the bulk contaminant uptake properties of the soil/sediment. Thus, exact identification should be considered when developing models entailing adsorption-desorption processes in soils or weathered sediment environments. The data obtained from the methods applied in this study could be enhanced by combining other available spectroscopic chemical speciation studies. However, the direct identification of major sinks for sediment-bound uranium from this study will aid in building a refined conceptual model for uranium mobilization from secondary sources and help develop remediation strategies in field-scale applications.

Funding

Financial support was provided by the 2022 Geological Society of America Graduate Student Research Grant program and the U.S. Department of Energy Office of Legacy Management through contract number 89303020DLM000001, task order number 89303022FLM400039.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.
The authors have no competing interests to declare that are relevant to the content of this article.
All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
The authors have no financial or proprietary interests in any material discussed in this article.

Author Contributions

Rakiba Sultana did spectroscopic lab data acquisition and data analysis. Wrote the manuscript

Martin A Dangelmayr reviewed and edited the manuscript critically.

Charles J Paradis reviewed and edited the manuscript critically.

Raymond H. Johnson did fission-track radiography lab data acquisition and data analysis, and supervised the manuscript

Allard T, Ildefonse P, Beaucaire C, Calas G (1999) Structural chemistry of uranium associated with Si, Al, Fe gels in a granitic uranium mine. Chemical Geology 158, 81–103. https://doi.org/10.1016/S0009-2541(99)00025-X
Arai Y, Marcus MA, Tamura N, Davis JA, Zachara JM (2007) Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site. Environ. Sci. Technol. 41, 4633–4639. https://doi.org/10.1021/es062196u
Arias M (1995) Effects of Iron and Aluminium Oxides on the Colloidal and Surface Properties of Kaolin. Clays and Clay Minerals 43, 406–416. https://doi.org/10.1346/CCMN.1995.0430403
Bachmaf S, Merkel, BJ (2011) Sorption of uranium(VI) at the clay mineral–water interface. Environ Earth Sci 63, 925–934. https://doi.org/10.1007/s12665-010-0761-6
Campbell KM, Kukkadapu RK, Qafoku NP, Peacock AD, Lesher E, Williams KH, Bargar JR, Wilkins MJ, Figueroa L, Ranville J, Davis JA, Long PE (2012) Geochemical, mineralogical and microbiological characteristics of sediment from a naturally reduced zone in a uranium-contaminated aquifer. Applied Geochemistry 27, 1499–1511. https://doi.org/10.1016/j.apgeochem.2012.04.013
Catalano JG, Heald SM, Zachara JM, Brown GE (2004) Spectroscopic and Diffraction Study of Uranium Speciation in Contaminated Vadose Zone Sediments from the Hanford Site, Washington State. Environ. Sci. Technol. 38, 2822–2828. https://doi.org/10.1021/es049963e
Charlet L, Bosbach D, Peretyashko T (2002) Natural attenuation of TCE, As, Hg linked to the heterogeneous oxidation of Fe(II): an AFM study. Chemical Geology 190, 303–319. https://doi.org/10.1016/S0009-2541(02)00122-5
Dam WL, Campbell S, Johnson RH, Looney BB, Denham ME, Eddy-Dilek CA, Babits SJ (2015) Refining the site conceptual model at a former uranium mill site in Riverton, Wyoming, USA. Environ Earth Sci 74, 7255–7265. https://doi.org/10.1007/s12665-015-4706-y
Dangelmayr MA, Reimus PW, Wasserman NL, Punsal JJ, Johnson RH, Clay JT, Stone JJ (2017) Laboratory column experiments and transport modeling to evaluate retardation of uranium in an aquifer downgradient of a uranium in-situ recovery site. Applied Geochemistry 80, 1–13. https://doi.org/10.1016/j.apgeochem.2017.02.018
Davis JA, Meece DE, Kohler M, Curtis GP (2004) Approaches to surface complexation modeling of Uranium(VI) adsorption on aquifer sediments. Geochimica et Cosmochimica Acta 68, 3621–3641. https://doi.org/10.1016/j.gca.2004.03.003
Duff MC, Amrhein C (1996) Uranium(VI) Adsorption on Goethite and Soil in Carbonate Solutions. Soil Science Society of America Journal 60, 1393–1400. https://doi.org/10.2136/sssaj1996.03615995006000050014x
Duff MC, Coughlin JU, Hunter DB (2002) Uranium co-precipitation with iron oxide minerals. Geochimica et Cosmochimica Acta 66, 3533–3547. https://doi.org/10.1016/S0016-7037(02)00953-5
Dumitru TA, Stockli DF (1998) Technical note, in: van den Haute, P., de Corte, F. (Eds.), Advances in Fission-Track Geochronology. Springer Netherlands, Dordrecht, pp. 325–330. https://doi.org/10.1007/978-94-015-9133-1_21
Fenton BR, Waite TD (1996) A Kinetic Study of Cation Release from a Mixed Mineral Assemblage: Implications for Determination of Uranium Uptake. Radiochimica Acta 74, 251–256. https://doi.org/10.1524/ract.1996.74.special-issue.251
Gallagher K, Brown R, Johnson C (1998) FISSION TRACK ANALYSIS AND ITS APPLICATIONS TO GEOLOGICAL PROBLEMS. Annu. Rev. Earth Planet. Sci. 26, 519–572. https://doi.org/10.1146/annurev.earth.26.1.519
Goldberg S (1989) Interaction of aluminum and iron oxides and clay minerals and their effect on soil physical properties: A review. Communications in Soil Science and Plant Analysis 20, 1181–1207. https://doi.org/10.1080/00103629009368144
Hurford AJ, Carter A (1991) The role of fission track dating in discrimination of provenance. SP 57, 67–78. https://doi.org/10.1144/GSL.SP.1991.057.01.07
Johnson RH, Hall SM, Tigar AD (2021) Using Fission-Track Radiography Coupled with Scanning Electron Microscopy for Efficient Identification of Solid-Phase Uranium Mineralogy at a Former Uranium Pilot Mill (Grand Junction, Colorado). Geosciences 11, 294. https://doi.org/10.3390/geosciences11070294
Jové Colón CF, Sanpawanichakit C, Xu H, Cygan RT, Davis JA, Meece DM, Hervig RL (2006) A Combined Analytical Study to Characterize Uranium Soil and Sediment Contamination: The Case of the Naturita UMTRA Site and the Role of Grain Coatings (No. NUREG/CR-6898). Rockville, MD.
Luo W, Kelly SD, Kemner KM, Watson D, Zhou J, Jardine PM, Gu B (2009) Sequestering Uranium and Technetium through Co-Precipitation with Aluminum in a Contaminated Acidic Environment. Environ. Sci. Technol. 43, 7516–7522. https://doi.org/10.1021/es900731a
Merritt RC (1971) The extractive metallurgy of uranium. Colorado School of Mines Research Institute.
Narasimhan TN, White AF, Tokunaga T (1986) Groundwater contamination from an inactive uranium mill tailings pile: 2. Application of a dynamic mixing model. Water Resour. Res. 22, 1820–1834. https://doi.org/10.1029/WR022i013p01820
Noubactep C, Schöner A, Dienemann H, Sauter M (2005) Release of coprecipitated uranium from iron oxides. J Radioanal Nucl Chem 267, 21–27. https://doi.org/10.1007/s10967-006-0004-1
Rakovan J, Reeder RJ, Elzinga EJ, Cherniak DJ, Tait CD, Morris DE (2002) Structural Characterization of U(VI) in Apatite by X-ray Absorption Spectroscopy. Environ. Sci. Technol. 36, 3114–3117. https://doi.org/10.1021/es015874f
Singer DM, Zachara JM, Brown Jr GE (2009) Uranium Speciation As a Function of Depth in Contaminated Hanford Sediments - A Micro-XRF, Micro-XRD, and Micro- And Bulk-XAFS Study. Environ. Sci. Technol. 43, 630–636. https://doi.org/10.1021/es8021045
Stubbs JE, Veblen LA, Elbert DC, Zachara JM, Davis JA, Veblen DR (2009) Newly recognized hosts for uranium in the Hanford Site vadose zone. Geochimica et Cosmochimica Acta 73, 1563–1576. https://doi.org/10.1016/j.gca.2008.12.004
U.S. Department of Energy (2016) 2015 Advanced Site Investigation and Monitoring Report Riverton, Wyoming, Processing Site September 2016 (No. S–14148, 1351628). https://doi.org/10.2172/1351628
U.S. Department of Energy LM (2014) Evaluation of Mineral Deposits Along the Little Wind River, Riverton, Wyoming, Processing Site
U.S. Environmental Protection Agency (2014) Test Methods for Evaluating Solid Waste: Physical/Chemical Methods, SW-846 Update 5. Washington, DC, July
Walter M, Arnold T, Reich T, Bernhard G (2003) Sorption of Uranium (VI) onto Ferric Oxides in Sulfate-Rich Acid Waters. Environ. Sci. Technol. 37, 2898–2904. https://doi.org/10.1021/es025749j
White AF, Delany JM, Narasimhan TN, Smith A (1984) Groundwater contamination from an inactive uranium mill tailings pile: 1. Application of a chemical mixing model. Water Resour. Res. 20, 1743–1752. https://doi.org/10.1029/WR020i011p01743
Wolf RA, Farley KA, Silver LT (1997) Assessment of (U-Th)/He thermochronometry: The low-temperature history of the San Jacinto mountains, California. Geol 25, 65. https://doi.org/10.1130/0091-7613(1997)025<0065:AOUTHT>2.3.CO;2
Wollenberg HA (1971) FISSION-TRACK RADIOGRAPHY OF URANIUM AND THORIUM IN RADIOACTIVE MINERALS. Proceedings LBL-754 of the 4th International Symposium on Geochemical Exploration 347–357.
Zielinski RA, Budahn JR (1998) Radionuclides in fly ash and bottom ash: improved characterization based on radiography and low energy gamma-ray spectrometry. Fuel 77, 259–267. https://doi.org/10.1016/S0016-2361(97)00194-4

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Combining Fission-Track Radiography and Scanning Electron Microscopy to Elucidate Uranium Mobility Controls

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Abstract

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Introduction

Materials And Methods

Site Description

Sample Collection and Uranium Extraction

Thin Section Preparation

Fission-Track Radiography

SEM-EDS Analysis

Results And Discussions

Macroscale Solid-Phase Uranium Associations

Microscale Uranium Associations: Uranium Hosts

Conclusion

Declarations

References

Additional Declarations

Supplementary Files

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