PNNL

Abstract

High pH, high NaNO3 solutions with varying amounts of dissolved Al were reacted with quartz sand at 88.7 degrees Celcius in order to simulate possible reactions between leaked nuclear waste fluid and primary subsurface minerals at the U.S. Department of Energy's Hanford site in Washington. After 2-10 days, nitrate-cancrinite, a feldspathoid mineral with a zeolite-like crystal structure, precipitated onto the quartz surfaces, cementing the grains together. Estimates of the Keq for the precipitation reaction differ for solutions with 0.1 or 1.0 m OH- (log Keq = 30.4 +/- 0.8 and 36.2 +/-, respectively). It is inferred that the difference in solubility is attribute to more perfectly crystallinity (i.e., fewer stacking faults) in the higher-pH cancrinite structure. This hypothesis is supported by electron micrographs of crystal morphology and measured rates of Na volatilization under an electron beam. Precipitate crystallinity may be important for radionuclide mobility, because stacking faults in the cancrinite structure can negate its zeolitic cation exchange properties. The log of the precipitation rate depends lineraly on the activity of Al(OH)4-in solution. The evolution of Si concentration in experimental solutions was successfully modeled by considering the dependance of quartz dissolution rate on Al(OH)4- activity, cancrinite precipitation, and the reduction of reactive surface area of quartz due to coverage by cancrinite.