PRELIMINARY REPORT ON EVALUATION OF POTENTIAL ELUANTS FOR NON-ACID ELUTION OF CESIUM FROM RESORCINOL-FORMALDEHYDE RESIN

Abstract

Small-column ion exchange (SCIX) units installed in high-level waste tanks to remove Cs-137 from highly alkaline salt solutions are among the waste treatment plans in the DOE-complex. Spherical Resorcinol-Formaldehyde (sRF) is the ion exchange resin selected for use in the Hanford Waste Treatment and Immobilization Plant (WTP). It is also the primary ion exchange material under consideration for SCIX at the Hanford site. The elution step of the multi-step ion exchange process is typically done with 0.5 M nitric acid. An acid eluant is a potential hazard in the event of a spill, leak, etc. because the high-level waste tanks are made of carbon steel. Corrosion and associated structural damage may ensue. Studies are ongoing to explore non-acid elution as an alternative. Batch contact sorption equilibrium screening tests have been conducted with 36 potential non-acid eluants. The sorption tests involve equilibrating each cesium-containing eluant solution with the sRF resin for 48 hours at 25 C in a shaker oven. In the sorption tests, an eluant is deemed to have a high cesium elution potential if it minimizes cesium sorption onto the sRF resin. The top candidates (based on lowest cesium sorption distribution coefficients) include ammonium carbonate, ammonium carbonate/ammonium hydroxide, ammonium bicarbonate, rubidium carbonate, ammonium acetate, ammonium acetate/ammonium hydroxide, ammonium bicarbonate/ammonium hydroxide, calcium chloride, and magnesium chloride. The next phase of testing for this work will focus on the following down selected eluants: Ammonium carbonate, ammonium acetate, calcium acetate, magnesium acetate, nitric acid, and ammonium hydroxide. The next testing phase is a confirmation of the elution ability of the selected eluants. It will mimic a typical sRF cesium ion exchange process i.e., sorption or loading, caustic wash, water rinse, and elution via batch contact sorption and quasi column caustic wash/water rinse/elution. Due to corrosion concerns, calcium acetate and magnesium acetate will be tested instead of calcium chloride and magnesium chloride respectively. Nitric acid is for benchmarking since it is the baseline sRF eluant. The information at hand indicates ammonium hydroxide, while a weak base, may hold promise as an effective eluant. Hence, its inclusion among the eluants to be studied despite the fact that it was not tested as a stand-alone eluant earlier.