Chemical Evolution of Leaked High-Level Liquid Wastes in Hanford Soils
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Highly soluble radionuclides likely to travel in contaminant plumes include 795e, Tc and '33Cs. The upper limits of the radionuclides in the solutions are approximately ' Cs, 20 ppm 99Tc, and up to 70 ppm 798e.[9] The phases which form as a result 5.7 ppm 33 of solution interactions with neutralizing media such as CO 2 or silicates must be identified in order to determine the location, speciation and phase association of these contaminants within the Vadose zone. Further, changes in soil permability as a result of alteration and precipitation processes must be assessed with regard to their impact on the flow paths of past or future leaks. These questions were addressed by initiating a suite of scoping studies intended to simulate the various changes that would occur as a leaked fluid would (DSSF-7) migrated thorugh the vadose zone toward the water table. 99
EXPERIMENT Soil-Solution Reactions Soils used in this study were obtained from the Hanford Reservation and are typical of the sandy surface cover deposited by the last great Lake Missoula flood. The fluid used in these experiments ("DSSF-7 ") was developed to be similar to that originating from rinsing operations associated with tank 101AW [3] . The major components of the DSSF-7 simulant solution, the radionuclide analogues, and the concentrations of these species are compiled in Table I. The nonradioactive analogues ' Cs and 798e, respectively. The include Re for 99Tc, and nonradioactive Cs and Se for 33 soils were exposed to DSSF-7 solutions for 1 - 2 weeks at 90' and 200 'C in Parr reaction vessels and for 82 days at room temperature, 60', and 90' C in polyethylene bottles. After completing the 90' and 200 'C experiments the liquid phase was isolated from the solid phase by filtration. Concentrations of Cs, Se and Re in both the solid phases and liquid phases were determined by ICP-Mass Spectrometry. The 82 day experimental fluids did not contain radionuclide surrogates but were used to identify solid alteration products and observe textural features reaulting from the treatment (i.e. grain cementation and zeolite morphologies). £Qz-Solution Reactions Two experiments were also performed to study uptake of CO 2by the DSSF-7 test solution. First, an aliquot of DSSF-7 solution was left stirring at room temperature in contact with the air to verify that eventually even this dilute source of CO 2 could neutralize enough hydroxide to initiate precipitation of a solid phase. Second, an accelerated experiment was executed by bubbling pure CO 2 gas through DSSF-7 solution at room temperature. There was enough of this second precipitate that it could be isolated by filtration and analyzed by X-ray diffraction (phase identification) and ICP-MS (radionuclide surrogate content). The remaining DSSF-7 solution was also analyzed by ICP-Mass Spectrometry for Se, Cs and Re.
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Table I. Component
Composition of DSSF-7 Simulant Solution
Concentration in solution *Cs 100 ppm *Se 100 ppm *Re 100 ppm NaNO3 1.162 mol/L KNO3 0.196 mol/L KOH 0.749 mol/L Na 2SO 4 0.008 mol/L * Ra
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