Salt Mineralogy of Hanford High-Level Nuclear Waste Staged for

Jul 11, 2013 - in two of these Hanford staged feed tanks (Tanks AP-103 and AP-108) by ion ... The Hanford Site near Richland, Washington, United State...
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Salt Mineralogy of Hanford High-Level Nuclear Waste Staged for Treatment Jacob G. Reynolds,* Gary A. Cooke, Daniel L. Herting, and R. Wade Warrant Washington River Protection Solutions, LLC, P.O. Box 850, Richland, Washington, United States S Supporting Information *

ABSTRACT: The Hanford site near Richland Washington, United States, is staging some of its 56 million gallons of high-level waste for treatment. Limited understanding of solids precipitated from the concentrated electrolyte solutions complicates the development of waste treatment options. The implications for the precipitation of these solids are that overlying liquids are saturated in these salts, which may cause unwanted precipitation during downstream processing. This study characterizes the salts in two of these Hanford staged feed tanks (Tanks AP-103 and AP-108) by ion chromatography (for anions), X-ray diffraction, polarized light microscopy, and scanning electron microscopy with energy dispersive X-ray spectroscopy. The results show that Na2CO3·H2O (thermonatrite) was the most prevalent salt in one tank, whereas the double salt Na7F(PO4)2·19H2O (natrophosphate) was the dominant salt in the other. Natrophosphate occurred in both tanks as octahedrons in sizes ranging from 10 μm to a millimeter in diameter. Natroxalate (Na2C2O4) and kogarkoite (NaFSO4) were also common phases observed. Waste processing planners should recognize that these salts may have to be dissolved prior to treatment, and solutions saturated with these salts may complicate downstream processing.

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INTRODUCTION The Hanford Site near Richland, Washington, United States, has 56 million gallons of high level nuclear waste left over from

models. Some solubility data have been developed to parametrize these models.7−9 The models will eventually have to be validated against real waste data, an effort that has already started.10 Thus, identifying the major salt minerals that are likely to change phase during waste processing is important to this endeavor. Salts that precipitate during feed staging are important examples of such species. The Hanford tank farm is currently staging feed to support a waste vitrification facility, or an alternative treatment option still being considered. Much of the liquids are being staged in eight one-million gallon tanks in the AP Farm complex. These liquids are being evaporated up to a target density of 1420 kg/m3 (1.42 g/mL) (to save tank space) and then transferred to AP Farm. The tank farm operator is finding that salts precipitate from these wastes at these concentrations. Table 1 provides the current (as of this writing) densities of the liquids in these eight tanks, along with the identification of the presence or absence of a salt layer at the tank bottom, demonstrating that salts precipitated from the most concentrated solutions. The minerals in these saltcakes may be similar for all of the tanks. Even after these first eight tanks are emptied, other batches will be staged in this same manner, likely making the results of a mineralogical evaluation of these salts broadly applicable. Some of the waste in the staged feed is made from the dissolution of saltcakes in other tanks. These original saltcakes were created in the 1950s through the 1980s to save tank space by repeatedly evaporating the liquids. The waste was evaporated so that even very soluble phases such as sodium nitrate crystallized out. The tank farm operators deposited the

Table 1. Hanford AP Farm Densities and Saltcake Identifications AP Farm Tank Number

density (g/mL)

saltcake present?

101 102 103 104 105 106 107 108

1.40 1.37 1.38 1.31 1.40 1.21 1.21 1.42

yes11 yes12 yes13 yes14 yes15 no16 no17 yes18

plutonium production. Treatment options for this waste are currently being planned and evaluated. The waste consists of sludges, saltcakes, and supernatant liquids. The sludges are assumed to be metal (hydr)oxides of aluminum, iron, uranium, manganese, zirconium, chromium, and many other less prevalent metals.1 The saltcakes are assumed to be precipitated sodium salts of nitrate, nitrite, carbonate, phosphate, fluoride, sulfate, chloride, aluminates, oxalates and other trace constituents. The liquids are highly concentrated solutions containing these same electrolytes along with sodium hydroxide.1 Several recent expert review groups have noted that treatment flowsheet development is being impeded because of insufficient information about waste speciation.2−5 To address these limitations, thermodynamic models are being developed to predict solid-solubility in Hanford waste,6 and the appropriate solid phases need to be selected for inclusion in the © 2013 American Chemical Society

Received: Revised: Accepted: Published: 9741

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salt slurry in a tank, then drained out the residual liquid through a salt screen, and then ran the collected liquid through an evaporator once again to crystallize out even more salt.19,20 Therefore, there is a large amount of solid salts in other tanks at Hanford besides just the staged feed tanks. Some limited mineralogical analysis of these historic saltcakes and extensive chemical analysis of them are available.21,22 These studies show that the bulk of the solids in these older saltcakes are highly soluble salts like sodium nitrate. The mineralogy of the salts in the staged feed is important because the overlying liquid is likely saturated with these phases. Saturated liquids are problematic because small changes in the chemistry or temperature can cause unwanted precipitation in downstream processes such as ion exchange columns or evaporators.23,24 The present study was undertaken to understand the mineralogy of the salts that precipitated from the staged feed.

unaided eye, and some of these particulates were collected with tweezers for separate mineralogical analysis. X-ray diffraction analysis was conducted on air-dried sample material after vacuum filtration in an attempt to remove the remaining interstitial liquid. When dry, the XRD aliquot was crushed in a mortar and pestle, mixed with a small amount of collodion and packed into the shallow well of a quartz plate sample holder. The XRD analysis was performed on a Rigaku Miniflex X-ray diffractometer equipped with a variable slit and utilizing copper K-alpha X-rays. The samples were scanned between 5 and 70 degrees two-theta. The software used for pattern matching was JADA version 7.5 by Material Data Inc. with International Centre for Diffraction Data Pattern database ICDD PDF-4 + 2008 release. This software suggests potential combinations of phases based on the experimentally observed patterns, as well as the major chemical constituents (determined by anion analysis). The phases observed by PLM and SEM-EDS (see below) were screened against the patterns in this database, to ensure that the diffraction patterns were consistent with the presence of those phases, and residual XRD reflections were compared to that of other common phases in the Hanford waste to ensure there were no other likely phases present. For PLM analysis, each sample was stirred to disperse the particulate and drops of liquid containing some of the particulate were placed on a glass microscope slide. A glass coverslip was pressed into place, and the slide was transferred to the microscope stage. The microscopy was performed on a Nikon Eclipse E600 polarized light optical microscope. For a more detailed description for how PLM is performed on Hanford waste, see Herting.25 Another aliquot of the dispersed particulate was smeared onto a 0.45-μm-pore-sized polycarbonate filter, and as much of the dissolved solids and water as possible was removed by vacuum filtration. After being dried, a section of the filter was cut and fixed to a carbon planchet attached to an aluminum SEM stub. This specimen was coated with a thin conductive layer of carbon by evaporative deposition. The SEM used was a ASPEX PSEM (model II) equipped with a Noran light element energy dispersive spectrometer (EDS) capable of detecting carbon and heavier elements.

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MATERIALS AND METHODS Tanks AP-108 and AP-103 were core sampled using a push mode core sampling device. The cores were transported to the 222-S Laboratory on the Hanford Site in steel containers for shielding. In the laboratory, the cores were extruded and the 19-in. long and one-inch wide core segments separated into borosilicate glass jars. The liquids were allowed to drain from the core segments onto a tray. The bottom three 19″ segments of the tank AP-108 core samples contained solids. The very bottom segment was divided into top and bottom halves because a small residual sludge layer was suspected to be present in this tank prior to the precipitation of salt when the tank was filled with concentrated liquid. Only one segment from the AP-103 core sample contained solids; the rest containing liquid. The top and bottom of this solids-containing AP-103 segment had very clear differences in granular appearance, so the AP-103 segment was also divided into top and bottom portions. The core segments containing only liquids were combined for each tank and analyzed for cations by inductively coupled plasma atomic emissions spectroscopy (ICP-AES), for anions by ion chromatography (IC), and for total inorganic carbon by acid volatilization with Coulombic detection. Hydroxide was measured in the liquid samples by titration with nitric acid after phosphate and carbonate are precipitated from the sample with barium. Each segment or half segment containing salt was centrifuged, and the liquid that was decanted off the top of the centrifuge cone was analyzed in the same manner as the supernatant liquid samples. Subsamples of the centrifuged salt segments were dissolved in water, and the resulting product was analyzed for anions by ion chromatography. Duplicate samples of the top solid segments were analyzed, and a single sample was analyzed for the rest of the segments. The density of the liquid samples was measured by pipetting a known mass of liquid into a volumetric flask. The density of the salt slurry samples was measured by placing a known mass of slurry in a volume calibrated centrifuge cone. The slurry was centrifuged to provide a flat sample surface so that the accurate height in the cone could be determined. All solid-containing segments or half segments were investigated by X-ray diffraction (XRD), polarized light optical microscopy (PLM), and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). Some large particulates in the AP-103 solids could easily be seen with the

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RESULTS Mineralogical Analysis Overview. This section describes the results of the mineralogical analysis, but some discussion of Table 2. Average Liquid Anion Concentrations anion

AP-103 Liquid (grams/L)

AP-108 (grams/L)

chloride fluoride nitrate nitrite oxalate phosphate sulfate total inorganic carbon

5.06 0.21 163 89.5 0.31 1.50 4.43 9.89

4.53 0.33 186 91.3 0.53 0.80 2.19 5.40

the advantages and disadvantages of the three mineralogical methods is useful for interpreting these results, prior to the phase identification below. The anion analysis provides the major anions in the sample (sodium being the dominant cation). These anions (along with SEM and PLM results) are then used to narrow down searches in XRD pattern matching 9742

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preparation minimized these artifacts, but as discussed later, did not eliminate them from some samples. PLM samples are not dried prior to analysis, so do not suffer from the artifacts of precipitation during drying. PLM is used to probe for solids observed by XRD to ensure that they are not XRD sample handling artifacts. NaNO3 and NaNO2, for instance, can be easily identified by their morphology coupled with the birefringence characteristics.25 PLM can also identify solid phases that are large but too rare to be observed in the XRD pattern. The crystal morphologies of particulate were noted in the PLM. Subsequently, the particles with the same morphology were analyzed by EDS in the SEM to determine the elemental composition, which can help identify the salt. However, analysis by EDS does not easily distinguish between some phases, such as sodium carbonate and natroxalate because they have the same elements, nor does EDS identify the number of waters of hydration. The numbers of waters of hydration are therefore determined by XRD and/or PLM inferred from the crystal structure or optical properties. Table 2 contains the average concentrations of the anions measured in this study in the liquid phase. Table 3 provides the mass fraction of the anions in both the solid phase and liquid phase, calculated with the measured bulk densities and anion concentrations. The standard deviation of each anion was within 30% of the mean for most samples. One exception was sulfate in the AP-103 centrifuged solids, where the differences occur because of a much lower sulfate concentration in the top half of the segment (see section on kogarkoite). The other

Table 3. Anion Mass Fractions in Solid and Liquid Phases AP-103 anion chloride fluoride nitrate nitrite oxalate phosphate sulfate total inorganic carbon

AP-108

wt % of liquid

wt % of solid

wt % of liquid

wt % of solid

0.37 0.02 11.86 6.51 0.02 0.11 0.32 0.72

0.39 1.91 6.04 3.26 3.31 14.90 0.92 0.58

0.32 0.02 13.14 6.43 0.04 0.06 0.15 0.38

0.31 0.41 9.54 4.63 0.88 0.75 1.56 2.59

software, which drastically reduces the likelihood of misidentification of the salt by XRD. Thus, XRD is a relatively conclusive method for identifying the mineral forms of the salts. However, XRD only recognizes crystalline phases. Amorphous phases are not expected in appreciable concentrations in saltcakes because salts are usually crystalline. A disadvantage of XRD is that the samples have to be dried prior to analysis. These solids are immersed in highly concentrated electrolyte solutions (8.1 M Na for AP-103 and 8.8 M Na for AP-108). Consequently, substantial amounts of salts precipitated as the sample dried, producing solids that were not present prior to sample preparation. These precipitated salts thus create artifacts in the XRD patterns. Vacuum filtration of the samples prior to XRD sample

Figure 1. XRD pattern for top segment of AP-108 saltcake. 9743

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Figure 2. XRD pattern for middle segment of AP-108 saltcake.

exceptions are in tank AP-108, where phosphate, fluoride, and sulfate also had large standard deviations because of heterogeneity within the saltcake (see the later discussion on natrophosphate and kogarkoite). The mass fractions reported for the centrifuged solids represent both the amount of anion in the in-tank solid phase plus the quantity in the interstitial liquid of the centrifuged samples. Thus, some of the anions reported for the centrifuged solids in Table 3 are actually dissolved in the interstitial liquid. The relative amount of anions in the solid phase can be evaluated by comparing the weight percent of each anion in the solid phase with the weight percent in the liquid phase. The weight percent of nitrate and nitrite is much higher in the liquid phase than the centrifuged solids for both tanks (comparing columns in Table 3), indicating that nitrate and nitrite are primarily dissolved. This is consistent with the known high solubility of these two electrolytes.26 In contrast, the weight percent of inorganic carbon, phosphate, sulfate, oxalate, and fluoride are much higher in the centrifuged solids than in the liquids, indicating that these anions are likely enriched in the real solids (Table 3). These results indicate that the salts in these tanks likely contain inorganic carbon, phosphate, oxalate, and fluoride anions. The XRD pattern for the top two AP-108 segments of salt are shown in Figures 1 and 2, respectively. The XRD pattern for the top and bottom halves of the bottom AP-108 segment are shown in Figures 3 and 4, respectively. Figures 5 and 6 contain the XRD pattern for the top and bottom halves of the AP-103 saltcake segment sample. These figures show the salts

that were thus identified by the XRD pattern matching software; this is discussed in more detail in the following sections. The following subsections discuss the evidence for the salt minerals identified (real or artifact). Representative SEM images of major phases identified are included here, but many other SEM and PLM images are included in the Supporting Information, including full color photos from the polarized light microscope. Solid Phase Identification. Sodium nitrate (NaNO3) and Sodium nitrite (NaNO2). Both NaNO3 and NaNO2 were found by XRD in all four AP-108 samples and the white particulate that was subsampled by tweezers from the AP-103 segment (Figures 1 through 4, see electronic attachment), but these phases are believed to be artifacts of evaporation of the samples prior to XRD analysis. The amount of NaNO3 and NaNO2 found in the XRD samples would have been readily identifiable by PLM if they had been present in the original samples. NaNO3 was only seen in the PLM samples on the edge of the coverslip where there was some evaporation during sample handling. The interstitial liquid has high concentrations of both sodium nitrate and nitrite (see Table 3), so these two salts would be expected to precipitate upon evaporation of the samples as part of the preparation of the XRD samples. Vacuum filtration was more effective at removing the interstitial liquid in the AP-103 samples, as evidenced by the fact that NaNO3 and NaNO2 were not found in the AP-103 XRD pattern (Figures 5 and 6). This provides good evidence that none of the other minerals identified by XRD in the AP-103 samples are handling 9744

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Figure 3. XRD Pattern for the top half of the bottom segment of AP-108 saltcake.

As observed in Figure 7, the thermonatrite in these samples are platy, which impacted the crystal orientation on the XRD slide. The plates were strongly oriented with the b-axis perpendicular to the slide surface. This enhanced the XRD reflections located at 16.91, 34.17, and 53.23 two-theta (Figures 1 through 4) and thus are much larger than they are in the standards in the XRD software database. These reflections are consistent with other platy thermonatrite samples synthesized in this laboratory and analyzed in the same manner as the samples analyzed in Figures 1 through 4 (see Figure 3.5.1−7 in ref 21). Table 3 indicates that inorganic carbon is 2.59 wt % of the solid phase, but Na2CO3·H2O weighs about 10 times more than carbon (molecular weight of 124 g/mol versus atomic weight of 12 g/mol). Thus, Na2CO3·H2O is approximately 25% of the sample mass (including the mass of the interstitial liquid), which is qualitatively consistent with the quantity of these particles observed by PLM. Thus, XRD, PLM, SEM-EDS and anion analysis are all consistent with thermonatrite being the most prevalent mineral in the tank AP-108 saltcake. Natroxalate (Na2C2O4) can have a similar morphology as thermonatrite,25 and oxalate does appear to be enriched in the solid slurry compared to the liquid phase (Table 3). Thus, it is likely that a small fraction of these particles are natroxalate. However, natroxalate was not observed in any AP-108 sample XRD pattern, so very few of these particles would be natroxalate. In contrast to the AP-108 saltcake, Na2CO3·H2O was not the most prevalent component in the AP-103 saltcake, though it

artifacts because nitrate and nitrite were at higher concentrations in the interstitial liquid than any of the anions in the minerals identified. Sodium Carbonate Monohydrate (Na2CO3·H2O) and Sodium Oxalate (Na2C2O4). Na2CO3·H2O (thermonatrite) is the major component of all four AP-108 segment samples. It occurs as tapered laths or prisms in a broad size range from several micrometers long up to a millimeter or more. With PLM, the optical properties of these particles are seen to be consistent with Na2CO3·H2O mineral standards.25 The most common particles observed in the SEM were sodium and carbon-bearing particles with the same morphology as those seen with PLM (see Supporting Information), indicating that this same phase is being observed by both techniques. Figure 7 shows an example of a SEM image of a thermonatrite crystal from tank AP-108, showing that Na, C, and O are the only SEM-EDS observable elements of appreciable concentration in these particles. Thermonatrite was observed by both XRD and by PLM in large concentrations, indicating that it is not an artifact of evaporating the samples for XRD. The concentration of total inorganic carbon in the AP-108 liquid is 0.45 mol/L, so a small amount of carbonate would have precipitated upon evaporation of the XRD samples, but the amount of inorganic carbon in the interstitial liquid is small compared to the amount of nitrate and nitrite (Table 2). Given that thermonatrite has a higher concentration in the solid samples than NaNO3 and NaNO2, we can conclude that most of the thermonatrite in the sample is not an artifact of evaporation of the sample for XRD. 9745

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Figure 4. XRD pattern for the bottom half of the bottom segment of AP-108 saltcake.

appears to be present in AP-103 samples, as identified by XRD. Natroxalate is also present and is the primary phase associated with the “white chunks” found in the bottom half of the segment (XRD pattern found in Supporting Information). While PLM and SEM specimen preparations from AP-103 contain many particles exhibiting the “classic” tapered lath or prism crystal habit of thermonatrite, there appear to be a number of particles with a morphology that is transitional to classic needle morphology of natroxalate. Precise discrimination between these two phases can be difficult with PLM and SEM when the morphologies of the particles are intermediate between the lath- or blade-like morphology of the carbonate and the needle- or lath-like habit of the oxalate. Under the PLM, both phases are uniaxial negative and have high birefringence.25 In the SEM, the EDS spectrum of the two phases is nearly identical. The XRD results suggest that most of these particles in the AP-103 samples are natroxalate rather than thermonatrite. The anion results are also consistent with the enrichment of oxalate and a lower enrichment of inorganic carbon content in the AP-103 centrifuged solids (Table 3), supporting the conclusion that these are primarily natroxalate particles rather than thermonatrite. Figure 8 shows an example of a SEM image of lath-like natroxalate crystals. Sodium Fluoride Phosphate Hydrate [Na7F(PO4)2·19H2O]. Na7F(PO4)2·19H2O (natrophosphate) was observed in both AP-103 solid half-segment samples, and appears to be the most prevalent solid phase in the AP-103 saltcake. The PLM and XRD estimate the concentration of the fluoride-phosphate phase at greater than 50% in both halves of the AP-103

segment. Natrophosphate occurs as large octahedrons from about 10 μm to greater than 1 mm in size. Figure 9 shows a SEM image of an example natrophosphate octahedron from the bottom half of the AP-103 segment. Natrophosphate was observed in all four AP-108 samples. However, only PLM analysis detected this phase in the middle segment. In AP-108, this phase occurs as large octahedrons from about 10 μm to greater than 1 mm in size. Frequently, natrophosphate is observed in cemented sheets or patches in this tank. The XRD peak intensities indicate that natrophosphate is a minor phase in the AP-108 samples. However, the coarse and cemented nature of this phase may have led to its partial exclusion from the SEM and XRD subsamples,27 because it appeared more abundant in the PLM preparations. This bias may have been present in all four AP-108 samples and would cause the natrophosphate component to be underestimated by XRD and SEM. Either way, however, natrophosphate was not the most abundant phase in the AP108 saltcake, as it was for the AP-103 saltcake. The best quantification of the relative abundance is using the anion analysis (Table 3), where the relatively lower phosphate mass fraction corroborates that this is a less important phase than thermonatrite in tank AP-108. These results also indicate that phosphate was much more abundant in top and bottom segments than the middle segment (the measured concentrations were 16000, 1100, and 6100 μg/g in top, middle, and bottom segments, respectively). Sodium Fluoride Sulfate (Na3FSO4). Na3FSO4 (kogarkoite) occurs in minor amounts in all four AP-108 samples. It is found 9746

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Figure 5. XRD pattern for top half of AP-103 saltcake segment.

as hexagonal plates ranging in size from 20 to 70 μm. Kogarkoite also makes up the individual laths in patches of radial aggregates that were seen in all four samples. Figure 10 is an example image of a patch of kogarkoite crystals found in the AP-108 solids. The two different morphologies may indicate two different times of formation or a different chemical environment during precipitation of kogarkoite. The finegrained aggregates of kogarkoite appear to be more common in the top two AP-108 segments than in the bottom segment. The anion results indicate that the kogarkoite concentration decreased with depth. The fluoride concentration decreased from 7600 μg/g in the top segment to 1000 μg/g in the bottom half of the bottom segment, while sulfate decreased from 26000 μg/g in the top segment to 3950 μg/g in the bottom half of the bottom segment. Kogarkoite in trace amounts was identified in the XRD analysis of the bottom half of the AP-103 sample. This phase was not observed in either the PLM or SEM samples. However, the anion analysis (Table 3) shows sulfate to be somewhat enriched in the centrifuged solid relative to the liquid, corroborating the XRD pattern and indicating that Na3FSO4 is likely not an XRD-sample evaporation artifact. The concentration of sulfate was nearly 30 times higher in the bottom half of the saltcake than the top half (26000 μg/g in bottom half versus 875 μg/g in the top half), consistent with the XRD evidence. Miscellaneous Trace Phases. Miscellaneous trace phases are often observed during the SEM analysis. Small and/or opaque particles are not identifiable in PLM analysis, and the XRD

analysis is not capable of detecting noncrystalline phases or trace phases (generally < 5%). The SEM detection of trace particulate is enhanced in backscatter mode where particles with higher atomic weight elements appear brighter. The SEM surveys of these specimens in backscatter mode show a very sparse scattering of high atomic weight particles in samples from both tanks. Most of these are either micrometer-sized uranium-rich particles, or larger iron-rich particles and patches. Amber-colored particles observed occasionally in the PLM analysis may be iron-rich. Not enough uranium-rich or iron-rich particulate occurs in these samples to allow phase identification by XRD. Trace amounts of an aluminum-rich phase were also observed in the SEM analysis in tank AP-108. Some trace aluminosilicate phases were observed in tank AP-103 with the SEM, and have been previously observed in other tank waste samples.28,29 The electronic attachment has additional SEM and PLM images of some of these trace phases.

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DISCUSSION These results indicate the most prevalent solids were thermonatrite in AP-108 and natrophosphate in AP-103. Natrophosphate was observed in both tanks, indicating that both tanks were saturated with this solid. Both natrophosphate and thermonatrite have been reported previously in Hanford tank waste.21,22,29 Other major salts observed, such as kogarkoite and natroxalate, could conceivably be more prevalent in other AP Farm saltcakes if the relative inventory of sulfate or oxalate is larger than in tanks AP-103 and AP-108. 9747

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Figure 6. XRD pattern for bottom half of AP-103 saltcake segment.

Figure 7. From the top segment of the tank AP-108 solids, SEM image of sodium carbonate with EDS spectrum taken from area marked with a cross (+).

All of the phases identified here have been previously identified in other Hanford tanks.21,22,29 The saltcakes that have been investigated previously have been created from repeated extensive evaporation of tank waste liquids,21 so the mineralogy was dominated by highly soluble salts such as sodium nitrate and sodium nitrite. The phases identified here also occur in those saltcakes,21,22 but they are diluted out by the larger abundance of sodium nitrate and sodium nitrite. Therefore, the salt minerals identified in the present study are less soluble. The fact that natrophosphate and thermonatrite have also been identified as components in sludge layers in Hanford waste,29

(layers that have much lower salt concentrations than saltcakes), supports this conclusion. A thermodynamic model was used to evaluate the solubility of the wastes. The model Environmental Simulation Program (version 8.3) by OLI Incorporated (Morristown, N.J., U.S.A.) was used to calculate the saturation indices of the major phases in the tank liquids. A saturation index of 1 or greater indicates the phase is supersaturated and an index below 1 indicates the system is unsaturated with the phase (according to the model). Given that this is a commercial product, the thermodynamic data used by the model is proprietary, but the liquid phase activities are calculated within the software by the mixed9748

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Figure 8. From the top half of the tank AP-103 solids, SEM image, and EDS spectrum from spot marked with a cross (+). Cluster of small lath-like crystals with morphology and EDS spectrum consistent with sodium oxalate.

Figure 9. From the bottom half of the tank AP-103 solids, SEM image and EDS spectrum from spot marked with a cross (+): large, euhedral sodium fluoride phosphate hydrate crystal.

Figure 10. From the middle segment of the tank AP-108 solids, SEM image of sodium fluoride sulfate aggregates with EDS spectrum taken from area marked with a cross (+).

solvent electrolyte model.30 While all aspects of this model have not been tested against Hanford waste, this software has been used to successfully model at least some tank waste processes successfully.31 The saturation indices calculated are shown in Table 4. The model indicates that both tanks are supersaturated with respect to natroxalate (Table 4). Tank AP-108 is supersaturated with respect to thermonatrite, the most prevalent phase in this tank. This model indicates that both tanks were supersaturated with respect to gibbsite [Al(OH3)]

even though gibbsite was not observed in samples from either tank. This is consistent with previous studies that indicate that Hanford tank waste exhibits unexplained high gibbsite solubility.10 In contrast, natrophosphate was predicted to be significantly unsaturated in both tanks despite the fact it was found in both tanks. This likely occurs because natrophosphate solubility is difficult to model, and there are substantial discrepancies in the available experimental data for this species.32 9749

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was in both kogarkoite and natrophosphate, the concentration of fluoride influenced the solubility of both phases in these tanks, which means they could potentially compete with each other for fluoride. The dissolution of one salt would free fluoride for precipitation of additional quantities of the other double salt. Similarly, the hydrated salts such as Na7F(PO4)2·19H2O crystallize out water,34 which complicates the water mass balance calculations and moderates the change in dissolved sodium concentration during precipitation. An important conclusion of this study is that the solids in these tanks are not simple anhydrous sodium salts of the anions identified.

Table 4. Saturation Indices Calculated for Major Solid Phases Using Environmental Simulation Program (Version 8.3) solid

AP-103

AP-108

natroxalate (Na2C2O4) gibbsite (Al(OH)3) thermonatrite (Na2CO3·1H2O) kogarkoite (Na3FSO4) natrophosphate (Na7F(PO4)2·19H2O)

10.4 3.38 0.644 0.31 0.288

58.6 1.21 1.143 1.17 0.288

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The relative abundance of solid phases in the saltcakes did not change much with depth, at least qualitatively. The only substantial difference between top and bottom parts of the AP108 saltcake was the relative abundance of natrophosphate particles (as indicated by change in phosphate concentration), and a continuously decreasing concentration of kogarkoite with depth. In contrast, kogarkoite was observed in the bottom half of the saltcake from tank AP-103, but not the top half. This may indicate that the kogarkoite formed under different tank conditions than the conditions that deposited the overlying salt. In AP-103, thermonatrite was only observed in the bottom half of the saltcake; but it is not clear if it is because these salts were only present in one segment, or if it is related to the fact that they were only barely detectable in any segment. As noted in the experimental section, a small sludge layer was suspected by tank farms operations personnel to have been present in tank AP-103 prior to the precipitation of the salts. Sludges are usually dominated by metal (hydr)oxides, and only a trace amount of phases resembling metal (hydr)oxides were found in any layer in either tank. Therefore, this sludge layer appears to have not been present or to have been substantially diluted out by salt precipitation. The liquid phase of the waste is likely saturated with the solids observed in these saltcakes. Evaporation of these liquids or other changes in the chemistry may induce additional solids to precipitate. Thus, downstream processes should be wary of precipitation of the major salts identified in this study [Na2CO3·H2O, Na7F(PO4)2·19H2O, Na2C2O4, Na3FSO4], and potentially even the more minor phases. Indeed, previous researchers23,24 have reported the precipitation of natrophosphate and natroxalate in laboratory flowsheet tests of Hanford waste leaching. A thermodynamic-based solubility model is currently being developed to incorporate into the process flowsheet of the Hanford waste treatment mission.6 This model would be used in the future to predict solids that precipitate in staged feed. A complex numerical algorithm is needed to solve the set of equations,33 which can substantially impact the model time when modeling many years of operation. The algorithm can arrive at a solution considerably faster if fewer solid phases are included in the model. The thermodynamic model currently contains 21 solid phases, including all of the major solids found in the present study.6 Thus, the present study supports their selection of solids for inclusion in the thermodynamic model. Work is currently ongoing to validate the thermodynamic model against data for real Hanford waste.10 The results of the present study are a crucial input to that validation. This phase identification underscores the complexity of the chemistry of Hanford salt solutions. The anion analysis indicated that the saltcakes were enriched in oxalate, inorganic carbon, fluoride, sulfate, and phosphate. Fluoride, phosphate, and sulfate crystallized out as double salts. Given that fluoride

ASSOCIATED CONTENT

S Supporting Information *

SEM and PLM photos of the minerals identified in AP-103 and AP-108. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Industrial & Engineering Chemistry Research

Article

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