Screening Evaluation of Sodium Nonatitanate for Strontium
and Actinide Removal from Alkaline Salt Solution

D. T. Hobbs, M. S. Blume, and H. L. Thacker
Westinghouse Savannah River Company
Aiken, SC 29808

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1.0 Summary

The authors conducted screening strontium and actinides removal tests with sodium nonatitanate (ST) samples supplied by Honeywell. Physical and chemical characterization indicates that the three samples exhibited similar particle volume distributions, which prove larger than that measured for the reference monosodium titanate (MST) material. Strontium and actinide removal testing indicated that the samples exhibit lower removal capacities than MST. Removal rates appear similar after 24 hours. We recommend additional testing to measure removal kinetics during the first eight hours of contact between the solution and sorbent.

Review of the x-ray analyses for the ST by A. Clearfield suggests that the Honeywell samples represent a poor conversion of the sorbent to the desired structure and appear atypical of the material that the Honeywell production should yield. Hence, we also recommend that further testing of ST samples proceed only upon documented evidence that the new samples exhibit the structure expected for the synthesized sorbent.

Key Words: Salt Disposition, Plutonium, Uranium, Neptunium, HLW

The Savannah River Site (SRS) selected an amorphous sodium titanate material, referred to as monosodium titanate to remove strontium from supernatant high-level wastes as part of the In-Tank Precipitation (ITP) process.¹ This material is of a class of hydrous metal oxides originally developed by R. Dosch and coworkers at the Sandia National Laboratory in the 1970s.² Synthesis of these amorphous materials uses a sol-gel process that yields solids with a high surface area. Testing also indicated that these materials remove a number of other species from alkaline solution including the actinides.^2,3

Kilpatrick and Hobbs of the Savannah River Technology Center (SRTC) modified the synthesis of the MST, which provided a narrower particle size distribution and improvements in filtration and settling characteristics.⁴ Personnel provided this information to a number of vendors to prepare commercial quantities of the MST for the ITP operation. Two vendors, Optima Chemical Company and Boulder Scientific, successfully prepared materials that met purchase specifications for strontium removal capacity, particle size distribution, solids concentration and alcohol content.

Actinide removal characteristics of the MST came under increasing scrutiny in early 1990s to ensure that the MST would not sorb sufficient fissile isotopes from the waste to pose a nuclear criticality hazard.^5,6 Research also investigated whether the decontaminated liquid waste from the ITP process would meet Z-Area limits for total alpha activity.⁷ SRTC researchers conducted a number of tests to support these concerns. Results indicated that the MST effectively removes uranium and plutonium, but will not load sufficient quantities of fissile isotopes to pose a criticality concern.^8,9 None of this testing investigated the kinetics of the adsorption process.

The Salt Disposition Systems Engineering Team identified the adsorption kinetics of actinides and strontium onto MST as a technical risk for several of the processing alternatives selected for additional evaluation. They requested that the Savannah River Technology Center examine the adsorption kinetics of MST for several process alternatives.¹⁰ The first studies examined the extent and rate of adsorption of strontium, uranium, neptunium and plutonium as a function of temperature, monosodium titanate concentration, and the concentrations of sodium and adsorbing species (Sr, Pu, Np and U). Additionally, comparison tests in the design of the experiments assessed the effects of mixing, sludge solids and the presence of sodium tetraphenylborate solids. Preliminary¹¹ and final¹² reports documented findings of this testing. Analysis of the data indicated the need to perform additional kinetic testing with radioactive SRS tank waste and with simulants at lower ionic strength and MST concentrations.

The subsequent radioactive waste tests utilized a composite material prepared from archive samples from over twenty SRS tanks. Results indicated that the extent and rate of strontium, plutonium, neptunium and uranium removal with MST in radioactive waste agree with those previously measured with simulants.¹³ Additional tests with simulated waste solutions measured the extent and rate of strontium, plutonium, neptunium and uranium removal at 25°C in the presence of 0.2 and 0.4 g/L MST at 4.5 and 7.5 M sodium concentration.¹⁴

Flowsheet calculations indicate that the rate of actinide removal is a key variable in sizing equipment for the salt processing alternatives.¹⁵ Filtration of MST and sludge mixtures exhibits reduced filtration fluxes compared to mixtures containing MST, sludge and tetraphenylborate solids.¹⁶ Production of clarified feed solution occurs in the current Alpha Sorption process about one-third of the time. This, combined with the low filtration fluxes, results in notably larger equipment sizes for the Non-Eluatable Ion Exchange (N-IX), Caustic Side Solvent Extraction (CSSE) and Direct Grout (DG) processes than that necessary in the Small Tank Tetraphenylborate Precipitation (STTP) process. Thus, the Salt Disposition Systems Engineering Team requested that SRTC evaluate the commercially available alternate materials to MST.

This report describes results from screening tests evaluating strontium and actinide removal characteristics of a sodium titanate material developed by Clearfield and coworkers at Texas A&M University and offered commercially by Honeywell. Sodium nonatitanate may exhibit improved actinide removal kinetics and filtration characteristics compared to MST and thus merit testing.^17,18 At the request of SRTC, Honeywell provided samples having a larger particle than that of MST. The larger particle size may exhibit improved crossflow filter performance.

Honeywell Performance Polymers and Chemicals (Morristown, NJ) provided three samples of sodium nonatitanate (ST) to SRTC for testing, which arrived identified as ST-0073A, ST-003B and ST-01520. We analyzed the samples for particle size distribution, powder x-ray diffraction (XRD) pattern, particle morphology and elemental composition. Particle size measurements featured a Microtrac Model SRA150 instrument suspending the samples in an alkaline salt solution having the same salt composition used in the subsequent strontium and actinide removal screening tests. Qualitative elemental analyses resulted from the analysis of the x-rays generated during scanning electron microscopic examination.

Researchers utilized the same simulated waste solution previously tested to determine the performance of MST in removing strontium, plutonium, neptunium and uranium from a salt solution 5.6M in sodium. Prior to testing with the ST, we filtered the solution through a membrane filter having a pore size of 0.45 m m to remove any undissolved solids that had formed upon standing during the five weeks since the original preparation of the solution. Table I provides the composition of the simulant.

Strontium and actinide removal testing utilized the same experimental method previously reported using the MST sorbent.¹⁹ We added approximately 0.030 grams of the ST to 75 mL of the salt solution equilibrated at 25°C. This quantity of ST provides the equivalent titanium content obtained upon addition of the current baseline material, MST, at 0.4 g/L. Researchers pulled samples from the test bottles after 5, 24, 48, 144, 170 and 244 hours of contact at 25°C.

4.1 Physical Characterization

Characterization of the samples included particle size, scanning electron microscopy (SEM), x-ray diffraction (XRD) and thermogravimetric analysis. Based on the volume distribution data, samples ST-0073A and ST-0073B appear nearly identical in particle size. The third sample, ST-01520 contains a wider distribution of particle sizes including much larger particles, but the overall distribution does not significantly differ from the first two samples. Figure 1 presents a plot of the particle volume distribution for each sample in addition to that of several MST materials.

The SEM analysis reveals similar spherical morphology for the ST materials compared to the MST (see Figure 2). Unlike MST, the ST samples also contain irregular shaped particles. The irregular shaped particles may reflect additional processing steps employed in the manufacture of the ST, but not used with MST (e.g., drying, grinding and sieving).

Figure 1. Particle Volume Distribution Data for ST and MST Samples

Figure 2. Scanning Electron Micrographs of Sodium Nonatitanate
and Monosodium Titanate Materials

The XRD patterns for the three ST samples indicated similar degree of crystallinity for all three materials (see Figure 3). Peaks identified as thermonatrite (Na₂CO₃^.xH₂O) occurred in samples ST-0073A and ST-0073B. Thermal analysis indicated broad exotherms attributed to loss of water centered at about 75°C and weak endotherms above 500°C attributed to either phase transformations or volatilization of sodium oxide.

Figure 3. X-Ray Diffraction Patterns of Sodium Nonatitanate Samples

4.2 Strontium and Actinide Removal

We evaluated strontium and actinide removal with ST samples ST-0073A and ST-0073B from a 5.6M sodium salt solution previously used in testing with MST.¹⁹ The quantity of ST added to the test provided the equivalent titanium content as provided by 0.4 g/L MST.

Figure 2 provides a plot of strontium concentration versus time for tests using ST and MST materials. Table II provides a listing of the distribution constants for strontium and actinides for the two ST samples at the reference MST material. The tests used a volume-to-solids ratio of 2500 mL/g.

Table II. Distribution Constants (K_d) for ST and MST Materials

Both of the sodium nonatitanate materials exhibited lower distribution constants for strontium and the actinides compared to the reference monosodium titanate material. Strontium removal from the 5.6M sodium salt solution satisifed the Z-Area limit for ⁹⁰Sr based on an average ⁹⁰Sr removal requirement.¹⁵ Insufficient samples during the early stages of the testing precludes direct comparison of the initial removal kinetics for ST and MST. However, the tests satisfied the Z-Area limit for strontium removal within five hours of contact indicating rapid strontium removal. Based on the strontium removal results, we conclude that the ST materials appear promising as an alternate material to MST for strontium removal.

Figure 4. Strontium Removal with Sodium Nonatitanate and Monosodium Titanate

As observed with strontium removal, the ST materials exhibited lower capacities for plutonium compared to MST based on the 24-hour distribution constants (see Table IV). Distribution constants for the two ST samples measured on average a factor of 6.4 less than that for the reference MST material. Based on an average alpha activity, plutonium removal with the ST samples failed to meet the Z-Area limit (see Figure 3). Unlike MST, plutonium removal appeared to reach equilibrium within 24 hours demonstrating faster sorption kinetics for the ST. Based these results, these ST samples do not exhibit sufficient capacity for plutonium removal.

Figure 5. Plutonium Removal with Sodium Nonatitanate and Monosodium Titanate

The amount of uranium removed with the two ST samples proved lower (1% to 5%) than that observed with MST (36%). Neptunium distribution constants measured an average factor of 3.0 lower with the ST samples compared to the MST reference material (see Table III). These values are similar to the differences in strontium distribution constants. Note that plutonium distribution constants are about a factor of 6.4 lower for ST compared to MST. The lower plutonium distribution constants relative to those for strontium and neptunium suggests the ST materials exhibit a lower affinity for plutonium than MST.

Based on the limited number of samples taken during testing, removal rates for neptunium appear similar for both the ST and MST materials. At the initial concentration tested, neither ST sample removed sufficient neptunium to achieve the Z-Area limit. Note, however, that the initial neptunium concentration equals the estimated maximum concentration expected for the process. At lower initial neptunium concentrations, the sorbent performance may satisfy the Z-Area limit.

Figure 6. Uranium Removal with Sodium Nonatitanate and Monosodium Titanate

Figure 7. Neptunium Removal with Sodium Nonatitanate and Monosodium Titanate

The authors conducted screening strontium and actinides removal tests with sodium nonatitanate samples supplied by Honeywell. Physical and chemical characterization indicates that the three samples exhibited similar particle volume distributions, which prove as expected larger than that measured for the reference monosodium titanate (MST) material. Strontium and actinide removal testing indicated that the samples exhibit lower removal capacities than MST. The lower capacity of the ST samples appears consistent with the larger particle size compared to that of the reference MST material. Removal rates appear similar after 24 hours. We recommend additional testing to measure removal kinetics during the first eight hours of contact between the solution and sorbent.

Review of the x-ray analyses for the ST by A. Clearfield suggests that the Honeywell samples represent a poor conversion of the sorbent to the desired structure and appear atypical of the material that the Honeywell production should yield. Hence, we also recommend that further testing of ST samples proceed only upon documented evidence that the new samples exhibit the structure expected for the synthesized sorbent.

This work used the following task plan.

This document provides the deliverable for the screening evaluation of sodium nonatitanate materials requested in the authorizing task request,

Notebooks WSRC-NB-2000-00063 and WSRC-NB-2000-00120 (D. T. Hobbs) contain the experimental data obtained from this work.

The authors thank S. Yates of Honeywell Performance Polymers and Chemicals for supplying the sodium nonatitanate samples, E. A. Kyser for supplying the actinide materials used in preparing the simulated waste solutions, and F. Fondeur for performing the thermal analyses. We also thank D. Diprete, W. Boyce, and other members of the Analytical Developmental Section of the SRTC for performing the many radiochemical analyses.

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