ALTERATION OF KAOLINITE TO CANCRINITE AND SODALITE BY SIMULATED HANFORD TANK WASTE AND ITS IMPACT ON CESIUM RETENTION

摘要

Caustic nuclear wastes have leaked from tanks at the US Department of Energy's Hanford site in Washington State (USA) causing hundreds of thousands of gallons of waste fluids to migrate into the underlying sediments. In this study, four simulant tank waste (STW) solutions, which are high in NaOH (1.4 and 2.8 mol/kg), NaNO_3 (3.7 mol/kg) and NaAlO_2 (0.125 and 0.25 mol/kg), were prepared and reacted with reference kaolinite KGa-1 and KGa-2 at 50 and 80℃ for up to 2 months. The structure and morphology of the resulting products were characterized using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. The products were also examined for cation exchange and Cs~+ sorption as a function of ionic strength and types of cations in the background solutions. Cancrinite and sodalite were the only new minerals observed in all of the conditions tested in this experiment. Two major chemical processes were involved in the reactions: dissolution of kaolinite and precipitation of cancrinite and sodalite. Increasing NaOH concentration and temperature, and decreasing NaAlO_2 concentration increased the transformation rate. Both cancrinite and sodalite appeared stable thcrmodynamically under the experimental conditions. The newly formed feldspathoids were vulnerable to acid attack and pronounced dissolution occurred at pH below 5.5. Cancrinite and sodalite can incorporate NaNO_3 ion pairs in their cages or channels. Sodium in cancrinite and sodalite was readily exchangeable by K~+, but less easily by Cs~+ or Ca~(2+). The feldspathoid products sorb nearly an order of magnitude more Cs~+ than the unaltered kaolinite. The Cs adsorption is reduced by competing cations in the background solutions. At low ionic strength (0.01 M NaNO_3 or 0.005 M Ca(NO_3)_2), Ca~(2+) was more competitive than Na~+. When the concentration of the background solution was increased 10 times, Na~+ was more competitive than Ca~(2+).