Tailored Ion Exchange Resins for Combined Cesium and Strontium Removal from Soluble SRP High-Level Waste
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TAILORED ION EXCHANGE RESINS FOR COMBINED CESIUM AND STRONTIUM REMOVAL FROM SOLUBLE SRP HIGH-LEVEL WASTE
MARTHA A. EBRA, RICHARD M. WALLACE, DARRELL D. WALKER, AND ROICE A. WILLE E. 1. du Pont de Nemours & Co., Savannah River Laboratory, Aiken, South Carolina, USA 29808
ABSTRACT Novel organic resins that achieve high selectivities They for both cesium and strontium have been synthesized. are condensation polymers of resorcinol and formaldehyde with attached chelating groups. Their column performance compares favorably with that of commercially available resins for either cesium or strontium removal. By combining Cs+ and Sr2+ removal in the same bed, these resins can significantly reduce the size and complexity of proposed facilities for processing defense high-level waste.
INTRODUCTION Cesium-137 and strontium-90 are the major radioactive components in the soluble fraction of alkaline defense waste from the Savannah River Plant (SRP). In the reference process for immobilizing SRP waste, cation exchange is being considered to isolate and concentrate these radionuclides before incorporating them in borosilicate glass. At this time, the preferred cation exchange resins are Duolite® CS-100 (Diamond Shamrock Cotp.) for cesium removal and Amberlite® IRC-718 (Rohm and Haas Co.) for strontium removal. The novel bifunctional resins that have been synthesized show high selectivities for both cesium and strontium. These bifunctional resins would obviate separate cesium and strontium removal beds in the reference Defense Waste Processing Facility. These new resins are condensation polymers of resorcinol and formaldehyde with chelating functionalities. Of the chelating groups studied, iminodiacetic acid (IDA) and catechol appear to be the most effective for strontium removal. Cesium selectivities of these bifunctional resins are 5 to 10 times higher than those of CS-100. Strontium selectivities are comparable to those of IRC718. Total sodium capacity of these resins, which needs to be minimized for adequate interfacing with immobilization processes, is only slightly higher than that of the reference candidate resins. The column performance of the new resins is good. This paper describes the syntheses, properties, and elution characteristics of these bifunctional resins. DISCUSSION Syntheses Work at SRL has shown that phenolic ion exchange resins have a high selectivity for cesium even in the presence of large excesses of sodium [1,2,3]. This selectivity, highly dependent on alkalinity, decreases significantly below pH 12. The pH dependence strongly indicates that the hydroxyl group of the phenolic resins determines the cesium selectivity. To further increase the cesium selectivity, resorcinol (Figure 1), with two hydroxyl groups, rather than phenol, was used to form condensation polymers with formaldehyde [4).
634 The current reference process for strontium-90 removal uses a chelating ion exchange resin (IRC-718). The chelating group present in IRC-718 resins is iminodiacetic acid (Figure 2). Amines such as IDA
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