W. R. Wilmarth, V. H. Dukes, J. T. Mills and F. F. Fondeur
Westinghouse Savannah River Company
Aiken, SC 29808

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Use of crystalline silicotitanate (CST) to remove cesium represents one of the alternatives identified for High Level Waste pretreatment at the Savannah River Site (SRS). Previous deployment of CST in the Department of Energy complex subjected the material to mildly caustic environments. Processing of SRS waste will expose CST to very alkaline solutions for extended period of time (typically 12 months in the proposed design).¹ Results of elevated temperature stability tests showed that silicon and one of the proprietary materials leached from the CST. UOP personnel indicated to SRS personnel that these materials exist in the sorbent in excess of required stoichiometry. The authors examined the pretreatment of CST with sodium hydroxide to remove these components prior to placing the CST in radioactive service. Additionally, researchers analyzed solids discovered in the feed line during a test by non-destructive techniques. These tests provide the following conclusions.

• Sodium hydroxide, under column flow conditions, will elute the excess materials of manufacture. Sodium hydroxide concentrations of 3 M effectively remove silicon and Proprietary Material 1. Lower concentrations prove less effective.
• Volumes of sodium hydroxide needed to remove the excess materials of manufacture proved excessive for practical implementation in a single-pass flow operation. Experimental data suggests greater than 6 million liters for 3 M sodium hydroxide in single pass flow (4 cm/min superficial velocity) for the proposed column design.
• The amount of Proprietary Material 2 and titanium that leached represents less than 1 wt % indicating little if any degradation of the IONSIV^Ò IE-911 framework.
• The tests showed no evidence of lead leaching from the sorbent as previously reported by other researchers.² The spectroscopy data shows overlap of emission lines for Proprietary Material 2, from the IONSIV^Ò IE-911 particle, and for lead. Hence, previous reports of lead leaching may represent errors from interpretation of the analytical spectra.

Crystalline Silicotitanate (CST) proved an effective inorganic ion exchange material for the removal of radio-cesium from caustic wastes at the Savannah River Site, the Idaho National Engineering and Environmental Laboratory, and the Oak Ridge National Laboratory.^3.4.5 These tests used an engineered form of CST distributed by UOP, Des Plains, Ill. Binding together the pure CST particles, IONSIV^Ò IE-910, produces this engineered form, IONSIV^Ò IE-911. The stability of these forms of silicotitanate proves important for the design of the pretreatment facility.

During the recent research conducted to aid in selection of the preferred pretreatment option for SRS, researchers reported a number of observations that indicated a potential instability of the engineered form of CST (IONSIV^Ò IE-911) in simulated SRS wastes. These observations included discoloration of the salt solution, formation of hazy colloidal precipitates, and column pluggage.⁶ Wilmarth⁷ and Taylor⁸ independently showed that materials leach from the IONSIV^Ò IE-911 and form precipitates during extended exposure to salt simulants.

Savannah River Site personnel held discussions with UOP representatives to understand the magnitude of these effects on the chemical stability of the IONSIV^Ò IE-911 sorbent. During these discussions, UOP revealed that they add silicon and one of the proprietary materials (referred to as PM1) during manufacture in excess of the required stoichiometry to form the sorbent. The UOP personnel suggested that the observations and experimental measurements show this excess material leaches upon exposure to simulated SRS wastes. High Level Waste Engineering requested that SRTC researchers examine a pretreatment method to remove these excess materials of manufacture.⁹ This report documents a series of tests planned to examine pretreatment with sodium hydroxide at various concentrations and flow rates.¹⁰

Personnel placed approximately 20 grams of the as-received IONSIV^Ò IE-911 (lot # 899902081000009) into a glass tube and washed to remove the CST fines. We then packed a small glass ion exchange column with the 20-grams in the manner previously described.¹¹ We prepared sodium hydroxide at three concentrations (0.5 M , 1.5 M and 3.0 M) and passed the liquid through ion exchange beds. Testing examined two superficial velocities with the 3 M NaOH solution, 4.1 cm/min and 2 cm/min. We submitted samples taken after the ion exchange sorbent bed for analysis of silicon, titanium, lead, Proprietary Material 1 (PM1), and Proprietary Material 2 (PM2) content. In reference to the proprietary materials, PM1 refers to the material integral to the IONSIV^Ò IE-910 and PM2 refers to material in the binder.

The Analytical Development Section used Inductively Coupled Plasma – Emission Spectroscopy¹² to analyze for concentration. Elution profiles are shown with the abscissa in units of column volumes. As defined herein, one column volume is the volume that the IONSIV^Ò IE-911 sorbent occupies (~18 mL). Therefore, at the two flow rates the column volumes per hour equaled 24 for the superficial velocity of 4.1 cm/min and 11.8 for 2 cm/min. Solids identification came from Scanning Electron Microscopy.⁹ The analyses used routine quality assurance protocols. Personnel recorded data gathered during these experiments in accordance with Procedure 4.10¹³ of the L1 Manual, SRTC Procedures Manual. The laboratory notebook, WSRC-NB-99-00254, provides lifetime storage as a record.

Figure 1 shows the elution curves for silicon, titanium, and the two proprietary materials for the 3 M sodium hydroxide wash performed at a superficial velocity of 4.1 cm/min. Table 1 quantifies the amount of material leached for each experiment as discussed later in this report. Each of the components appeared to leach and reach maximums by approximately 300 column volumes. Titanium and Proprietary Material 2, associated the binder, reached maximum leach concentrations of 15-20 mg/L. However, Proprietary Material 1 reached a maximum measured concentration of ~ 150 mg/L which dropped rapidly (< 250 column volumes) with leaching extending over 1000 column volumes. The silicon behavior, likewise, reaches a maximum early. However, the concentration hovers around 15-20 mg/L for the entire experiment (> 2000 column volumes). Lastly, we analyzed the samples for lead concentration. In each of the test samples, the lead concentration proved less than the detection limit of 0.5 mg/L. Previous observations of lead probably resulted from misinterpretation for analytical overlap of emission bands of the elements of interest.

The results from the washing test with 3 M sodium hydroxide at the planned flowsheet superficial velocity (4.1 cm/min) indicate a very large quantity of sodium hydroxide needed to remove the excess materials of manufacture in a single-pass treatment. However, current research does not provide a firm understanding of the permissible leach rates for excess silicon, titanium or proprietary materials. A limited leach rate may not result in any deterioration in column operation, either during routine operations or during periods of interrupted flow.

As a first approximation, one might guess that washing to remove these elements to well below their maximum concentrations might suffice. If true, then Figure 1 indicates the wash would require 250 column volumes of the caustic solution. This corresponds to a generation of over 1.75 million gallons of 3 M hydroxide to treat the ion exchange column train (i.e., three columns with a combined volume of 30,000 L).

Figure 2 portrays the elution profiles for the various components of the IONSIV^Ò IE-911 as measured during a washing test using 3 M sodium hydroxide but at a lower superficial velocity (2 cm/min). This lower superficial velocity effectively increases the residence time of the sodium hydroxide in the column. We anticipated eluting the excess materials of manufacture using a smaller amount of sodium hydroxide.

The elution profiles for titanium and the Proprietary Material 2 appear very similar for the lower superficial velocity when compared to the previous column experiment. Both sets of results indicate little removal of these materials. This finding agrees with batch leach tests that showed little to no leaching of these materials from the IONSIV^Ò IE-911.¹⁴ The behavior of Si and Proprietary Material 1 also appear similar at the two flow rates. Maximum concentrations of Si and PM1 occurred after 100 hours of 3 M sodium hydroxide flow. The magnitude of the concentrations reduced significantly to ~ 40 mg/L for silicon and ~ 80mg/L for PM1. Note that the relatively few samples taken over this interval limits the definition of the peaks. The elution tales of the Si and PM1 profiles appear elongated with silicon concentrations above 20 mg/L at 500 column volumes.

We experienced difficulty with the feed pump during this experiment after processing ~800 column volumes. The feed actually stopped for approximately 12 hours as best we could determine. We resumed flow to the column and observed increased silicon and PM1 concentrations over the next 200 column volumes. These increased concentrations reflect the results of batch leaching during the flow outage. Si and PM1 concentrations reached ~35 mg/L. These results suggest that the elution tales observed may have continued for many more column volumes (i.e., 12 hours of flow at 2.1 cm/min superficial velocity would have required additional column volumes).

Figure 3 contains the elemental concentration for the species of interest measured during the pretreatment with 1.5 M sodium hydroxide conducted with a superficial velocity of 4.1 cm/min. This experiment tested the effect of lowering the hydroxide concentration. The results from this column run appear almost identical to the 3 M experiment. Concentrations of Ti and PM2 remain very low. However, the concentration of Si and PM1 rise to very nearly the same concentration as in the 3 M sodium hydroxide wash. Again, maximum concentrations occurred within the first 100 column volumes with the elution tales of the major Si and PM1 stretching past 200 column volumes. In this experiment, a stoppage of flow occurred near 350 column volumes. We observed the same behavior (i.e., a static leach) as noted in the previous experiment. The overall results still indicate significant volumes of sodium hydroxide needed to remove the excess materials of manufacture.

Lastly, we conducted a column wash experiment using a very dilute sodium hydroxide feed (0.5 M). We reasoned that if similar profiles occurred then perhaps the process could use the sodium hydroxide as dilution water for subsequent processing following a filtration step to remove precipitated compounds. Figure 4 contains the result of this test.

The lower hydroxide concentration appeared less effective in leaching the sorbent. The elution maximum concentration peak lasted over a larger number of column volumes with the peak occurring significantly later. The elution profile spread considerably. Of particular importance, the elution profile of PM1 reaches a maximum at > 100 column volumes whereas at higher sodium hydroxide concentration the maximum occurred before processing 100 column volumes. The elution tale for PM1 remains above 10 mg/L at 350 column volumes. The silicon profile appears to oscillate around 10-15 mg/L over the entire experiment (~1000 column volumes). This behavior for these two materials indicates the removal and reuse of the dilute wash water appear impractical if implemented in the manner tested.

Table 1 contains the calculated amounts of each component of interest removed from the IONSIV^Ò IE-911 during each of the column washing experiments. We estimated the amounts of each component by integrating the area under each of the elution curves using the Peak FitÔ program. We estimated the amount of material leached to determine if the framework of the IONSIV^Ò IE-911 had experienced chemical attack from the caustic solution. The elements of interest come from three sources and include loosely adhered material, materials in excess of stoichiometric requirements and the sorbent framework. Discussions with the vendor indicate the loosely adhered material only represents ~ 1 wt %. Additionally the vendor revealed the average amount of excess materials. Therefore, one can compare the amount leached to these known quantities.

The amount of silicon lost in each of these experiments proved less than the expected 4 wt % excess added during manufacturing. The excess component, PM1, also leached in lesser amounts than present in excess. Based on these analyses, the majority of the leached element occurred during the initial peak. Because the titanium did not leach to any degree and the Ti to either Si or PM1 ratio measured less than 1, the data indicates the material leached early in the column runs represents excess material (e.g., Si and PM1). Lastly, increasing the residence time in the column (i.e., reducing the superficial velocity) slightly reduced the washing effectiveness.

The stability of IONSIV^Ò IE-911 in the highly alkaline environment of the SRS supernate proves important for understanding processing lifetime and downstream effects of leached components. Results of the stability tests indicate that silicon and Proprietary Material 1 leach from the IONSIV^Ò IE-911 along with minor amounts of titanium and Proprietary Material 2.^7,8 We completed testing to help assess the option of washing these excess materials from the IONSIV^Ò IE-911 using sodium hydroxide prior to loading the ion exchange columns. The sodium hydroxide treatment can remove the excess materials of manufacture. For example, using 3 M sodium hydroxide, we removed more than 75% of the excess Si.

We tested several concentrations of sodium hydroxide. We also varied the flow rate to conserve the amount of sodium hydroxide needed to remove the excess components and to understand the mass transfer restrictions to the leaching. The results show that a large amount of sodium hydroxide required to remove the excess components for washing performed in a single-pass flow operation. The amount of sodium hydroxide required to remove only the elution maxima approaches 200 column volumes or 1.75 million gallons for the three sequential ion exchange columns as currently configured in the process flowsheet. The components leached to the largest degree include silicon and one of the proprietary components. These results agree with the batch leach tests. Flowing over 2000 column volumes did not remove the manufacture’s expected excess. Therefore, downstream effects are not completely mitigated.

We analyzed many samples from the column effluents for the presence of lead. In no case did we identify lead as present above the detection limit. The authors suggest analytical overlap of emission lines as the most probable explanation of previously reported² measurements of lead.

The authors express appreciation to Frank Pennebaker for his analytical support.

References

1. H. H. Elder, et al., "Bases, Assumptions and Results of the Flowsheet Calculations of the Short List Alternatives," WSRC-RP-98-00168, October, 1998.
   
2. P. A. Taylor and C. H. Mattus, "Thermal and Chemical Stability of Crystalline Silicotitanate Sorbent," WSRC-TR-99-00364, Rev. 0, October 7, 1999.
   
3. D. W. Hendrickson, "Hanford Salt Cake Cesium Removal Using Crystalline Silicotitanate," SESC-EN-RPT-006, Rev. 0, SGN Eurisys Services Corporation, September 1997.
   
4. D. D. Walker, "Modeling of Crystalline Silicotitanate Ion Exchange Columns using Experimental Data from Simulated SRS Waste," WSRC-TR-98-00396, Rev. 0, January 6, 1999.
   
5. N. R. Mann, T. A. Todd, K. N. Brewer, D. J. Wood, T. J. Tranter, and P. A. Tullock, "Evaluation and Testing of IONSIV^Ò IE-911 for the Removal of Cesium-137 from INEEL Tank Waste and Dissolved Calcines," INEEL/EXT-99-00332, April 1999.
   
6. D. D. Walker, "Cesium Removal from Savannah River Site Radioactive Waste using Crystalline Silicotitanate," WSRC-TR-99-00308, September 16, 1999.
   
7. W. R. Wilmarth, T. Hang, J. T. Mills, V. H. Dukes and S. D. Fink, "The Effect of Pretreatment, Superficial Velocity, and Presence of Organic Constituents on IONSIV^Ò IE-911 Column Performance," WSRC-TR-99-00313, August 31, 1999.
   
8. P. A. Taylor and C. H. Mattus, "Thermal and Chemical Stability of Crystalline Silicotitanate Sorbent," WSRC-TR-99-00364, Rev. 0, October 7, 1999.
   
9. R. A. Jacobs, HLW Technical Task Request, "Post-precipitation in Simulants and Capacity/Kinetics of IE-911," HLW-SDT-TTR-99-37.1.
   
10. D. D. Walker, W. R. Wilmarth, F. F. Fondeur, and T. Hang, "Task Technical and Quality Assurance Plan for Non-Elutable Ion Exchange Process Waste Stability and IONSIV^Ò IE-911 Performance Tests," WSRC-RP-99-01079, Rev. 0, December 20, 1999.
    
11. W. R. Wilmarth, T. Hang, J. T. Mills, V. H. Dukes and S. D. Fink, "The Effect of Pretreatment, Superficial Velocity, and Presence of Organic Constituents on IONSIV^Ò IE-911 Column Performance," WSRC-TR-99-00313, August 31, 1999.
    
12. "Inductive Coupled Plasma - Emission Spectrometer for Aqueous Liquid Sample Analysis," Manual L16.1, Procedure ADS-1509, June 25, 1996.
    
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14. W. R. Wilmarth and D. D. Walker, "Stability of UOP IONSIV^Ò IE-911 in SRS simulated Salt Solution at Elevated Temperature and Subjected to Radiation Exposure," WSRC-TR-99-00374, September 15, 1999.