Chemical degradation of an ion exchange resin processing salt solutions

This paper describes the results from an investigation into the chemical degradation of an organic ion exchange resin, SuperLig® 644, over 25 repeated cycles separating cesium from an alkaline solution of sodium salts with subsequent elution. Battelle Pacific Northwest Division (PNWD) tested the resin with a salt solution simulating the radioactive wastes currently stored at Hanford, Washington, USA generated from plutonium production activities. Battelle PNWD used a simulated waste consisting predominantly of nitrate, nitrite and hydroxide salts of sodium for a sodium concentration of ∼120 g/L. We tested the resin in a column with a bed volume of ∼10 mL. Battelle PNWD linked probes positioned on the column feed and effluent lines to a spectrometer that analyzed for dissolved oxygen. A cycle test commenced with converting the resin to the sodium form by pumping 0.25 M sodium hydroxide solution through the bed. Battelle PNWD then processed the simulated waste followed by column rinses with 0.1 M sodium hydroxide solution and de-ionized water. We then eluted the resin with 0.5 M nitric acid solution and the cycle finished with a de-ionized water rinse. The resin bed lost 34% of its mass over the 25 cycles of operation. Some resin appeared to dissolve in the 0.25 M sodium hydroxide regeneration solution since we observed the regeneration effluents of every cycle to be colored deep red-brown. Some regeneration effluents also contained resin fines that settled after ∼1 day. In addition, examination of the resin showed the used resin to have apparently higher porosity than the fresh material. The resin appeared to lose approximately 60% of its effective capacity for cesium over the course of the 25 cycles. However, ∼34% of the resin mass was also lost over this period. Battelle PNWD hypothesizes the difference of 26% to represent a loss in capacity and is presumably due to chemical alteration of the ion exchange sites. In addition, though the weight of resin decreased with each cycle, Battelle PNWD used the same volume of eluant to elute the resin in each cycle, indicating a reduction in elution efficiency. Battelle PNWD assumed resin oxidation to be the major degradation mechanism since oxygen dissolved in the feedstock was consumed upon processing.