Cesium Ion Exchange Loading Kinetics Testing with SRF Resin

Abstract

Ion exchange using the Spherical Resorcinol-Formaldehyde (SRF) resin has been selected by the U.S. Department of Energy's Office of River Protection for use in the Pretreatment Facility of the Hanford Tank Waste Treatment and Immobilization Plant (WTP) and for potential application in an at-tank deployment for removing ¹³⁷Cs. Recent proposed changes to the WTP ion exchange process baseline indicate that loading may include a broader range of sodium molarities (2 to 8 M) due to caustic leaching and higher temperatures (50°C) to alleviate post-filtration precipitation issues prior to reaching the ion exchange columns. Therefore, it is important to understand the behavior of SRF resin performance under the conditions expected with the new equipment and process changes.

This research examined the impact of linear load velocity (4, 6, 8 cm/min), initial sodium concentration (2, 5, 8 M), initial sodium-to-cesium ratio (1.4E + 05, 2.1E + 05, 2.8E + 05 mol/mol), initial sodium-to-hydroxide ratio (2.0, 3.0, 4.0 mol/mol), and resin degradation during extended solution flow using elevated temperature (45°, 50°, 55°, 60°, 65°, 75°C). Testing was performed using a ∼2 mL column packed with SRF resin with feed flowing through it in an up-flow pattern. Samples were taken at set intervals and the data analyzed to help understand the impact of these conditions on the SRF resin performance.

It was found that the loading kinetics were not significantly impacted by the sodium concentration over the range tested. However, the loading kinetics were impacted by the linear load velocity. These results indicated that at the test temperature, the adsorption of cesium is strongly dependent on mass transfer through the film and not significantly impacted by interparticle diffusion. Testing for extended times at elevated temperatures showed that the resin does degrade and loading capacity is reduced at and above 45°C. Above 60°C the resin appears to not load at all.

Keywords:

• Cs
• ion exchange
• kinetics
• SRF resin

Notes

^(a)Resin loading required approximately 10 h to achieve kinetic equilibrium. Samples were collected periodically to define the cesium uptake curve (e.g., 0, 6, 12, 18, 24, 36, 48, 60, 80, 120, 180, 240, 600 min).

^(b)The 10 hr kinetics tests were completed at temperatures specified for each run. Six extended duration (720 h) solution flow tests were completed at 45°, 50°, 55°, 60°, 65° and 75°C.

^(c)The total simulant solution volume was varied to target an expected loading of 0.52 C/C₀.

^(d)BV = bed volume. Elution with 0.25 M HNO₃ began after feed displacement (7.5 BV 0.1 M NaOH) and water rinse (7.5 BV DI Water) solutions had been passed through the column. A single elution composite sample was collected and analyzed.

^(e)The actual flow velocity was limited by the pump configuration and was determined at the time of experimentation and was estimated to be approximately 0.08 cm/min.

^(f)Each test series (e.g., A, B, C, D, E, F, and G) represented an independent ion exchange column that was loaded and eluted one or moretimes using the conditions listed. The exact bed volume (BV) of the ion exchange resin depended upon final construction of the apparatusbut was ∼1 to 2 mL.

^(g)The final column (Test 1-D-1) served as a comparison to prior kinetics testing (Nash et al. 2006).

^(h)The flow rate for elution was to be approximately 2.8 BV/h. The flow rate was approximately 5 mL/h and the equivalent BV/h varied slightly with the exact bed volume for each column.

^(a)1st Position:Initial Na M (2 = 2.00, 5 = 5.00, 8 = 8.00 M Na) 2nd Position: Initial Cs M (A = 1.4E-05, B = 2.4E-05, C = 2.9E-05, D = 3.8E-05, E = 5.8E-05) 3rd Position: NaOH M Before Gibbsite (05 = 0.50, 10 = 1.00, 17 = 1.67, 20 = 2.00, 40 = 4.00).

^(b)Sodium varied as per experimental design [Na]_Total = 2, 5, or 8 M.

^(c)Aluminum varied at 90% solubility as calculated from hydroxide per Li, et al (2005).

^(d)Hydroxide varied per experimental design [OH]_Total = 0.50, 1.00, 1.67, 2.00, 4.00 M.

^(e)Chloride varied depending upon NaNO₃ added to keep sodium at prescribed level.