Environ. Sci. Technol. 2004, 38, 4432-4438
Precipitation of Nitrate-Cancrinite in Hanford Tank Sludge E. C. BUCK* AND B. K. MCNAMARA Pacific Northwest National Laboratory, P.O. Box 999, MSIN P7-27, Richland, Washington 99352
The chemistry of underground storage tanks containing high-level waste at the Hanford Site in Washington State is an area of continued research interest. Thermodynamic models have predicted the formation of analcime and clinoptilolite in Hanford tanks, rather than cancrinite; however, these predictions were based on carbonate-cancrinite. We report the first observation of a nitrate-cancrinite [possibly Na8(K,Cs)(AlSiO4)6(NO3)2‚nH2O] extracted from a Hanford tank 241-AP-101 sample that was evaporated to 6, 8, and 10 M NaOH concentrations. The nitrate-cancrinite phase formed spherical aggregates (4 µm in diameter) that consisted of platy hexagonal crystals (∼0.2 µm thick). Cesium-137 was concentrated in these aluminosilicate structures. These phases possessed a morphology identical to that of nitrate-cancrinite synthesized using simulant tests of nonradioactive tank waste, supporting the contention that it is possible to develop nonradioactive artificial sludges. This investigation points to the continued importance of understanding the solubility of NO3-cancrinite and related phases. Knowledge of the detailed structure of actual phases in the tank waste helps with thermodynamic modeling of tank conditions and waste processing.
Introduction The U.S. Department of Energy’s (DOE’s) plan for the disposal of ∼200000 m3 of tank waste at the Hanford Site in Washington State is a tremendous environmental problem (1). The Hanford tank wastes are chemically complex, having been generated during implementation of three major plutonium separation processes and other operations at the site. The DOE plans to remediate the Hanford tank farms by retrieving, pretreating, immobilizing, and disposing of the wastes. The tank wastes will be partitioned into high-level waste (HLW) and low-activity waste (LAW) fractions. The LAW will be processed to remove 137Cs and 99Tc, and immobilized, possibly in a glass matrix. The HLW will be immobilized in a borosilicate glass matrix and cast into stainless steel canisters, which will then be disposed of at the proposed geologic repository at Yucca Mountain, NV. Because of the expected high cost of HLW immobilization and geologic disposal, pretreatment processes will be implemented to reduce the volume of immobilized high-level waste (IHLW). Removing aluminum, phosphorus, sulfur, and chromium from the HLW solids is essential to reducing the IHLW volume. Effective treatment, separations, and disposal of tank waste will depend on the types, concentrations, and forms of chemicals in the waste. To understand these factors in relation to how the waste will behave as it is retrieved and processed, information is needed on the physical and * Corresponding author phone: (509)376-7101; fax: (509)376-9781; e-mail:
[email protected]. 4432
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 16, 2004
TABLE 1. Estimate of Major Components in Tank 241-AP-101 (3) species
content (%)
Al Ca Cl CO32Cr F Fe K Na
2.51 0.01 0.04 7.02 0.04 0.56
