The Removal of Strontium from Simulated Hanford Tank Wastes Containing Complexants

Two inorganic ion-exchange materials, a sodium nonatitanate and a sodium titanosilicate, were evaluated for the removal of strontium from two simulated Hanford tank wastes, both of which contained substantial amounts of complexing agents. In simulant 101-SY, both exchangers gave distribution coefficients (K ds) < 220 mL/g at a volume-to-mass ratio of 200. However, in a second simulant, 107-AN, the titanosilicate gave a K d of 2240 mL/g while the nonatitanate gave a similar K d to the value obtained in the 101-SY simulant. The reason for the difference in behavior was determined to be the concentration of calcium in the waste simulants. A high calcium concentration (as found in 107-AN) resulted in strontium, previously chelated by EDTA and other complexants, being released into solution and absorbed by the titanosilicate. Consequently, by adding sufficient calcium to 101-SY simulant to saturate the EDTA present, it proved possible to improve the strontium K ds for the titanosilicate from 215 mL/g to in excess of 8000 mL/g. The titanate exhibited a high selectivity for calcium in comparison to the titanosilicate, and thus the high concentrations of calcium in the waste simulants competed with the strontium ions for the available ion-exchange sites and resulted in low Sr K ds. As expected, K ds for the titanate showed little improvement, and the addition of calcium only resulted in a modest increase from 185 to 395 mL/g. Waste generation is minimal, so the addition of calcium to the tank wastes to facilitate the removal of strontium by ion exchange is an economical approach to the remediation of complexant-bearing Hanford tank wastes.