ALTERATION OF KAOLINITE TO CANCRINITE AND SODALITE BY SIMULATED HANFORD TANK WASTE AND ITS IMPACT ON CESIUM RETENTION H ONGTING Z HAO,Y OUJUN D ENG,J AMES B. H ARSH ,M ARKUS F LURY* AND J EFFREY S. B OYLE Department of Crop and Soil Sciences, Center for Multiphase Environmental Research, Washington State University, Pullman, WA 99164, USA Abstract —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 80ºC 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 thermodynamically 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+ . Key Words —Cancrinite, Cation Exchange, Cesium Sorption, Feldspathoid, Hanford Waste Tanks, Kaolinite, Mineral Stability, Mineral Transformation, Sodalite. INTRODUCTION Many single-shell tanks of the 177 nuclear waste tanks located at the US Department of Energy’s Hanford Reservation near Richland, WA (USA) have leaked, allowing 0.6 –1.4 million gallons of high-level nuclear waste fluids to migrate into the underlying coarse- textured, relatively unweathered sediments (Hanlon, 1996; Gephart and Lundgren, 1998). Tank sludge, arising from the Pu production and extraction procedures at the Hanford site, is chemically very complex and of extreme chemical conditions. In most cases, tank sludge has very high NaNO 3 (up to ~8 M), NaOH (pH >13), and aluminate concentrations (>0.1 M) (Serne et al. , 1998). The temperature in several tanks rose to >100ºC during storage and was reported as high as 160ºC in one tank (Pruess et al. , 2002). The temperature in the immediate neighborhood sediments was estimated as high as 120ºC, and in the sediments 20 m below the tanks could be as high as 70ºC (Pruess et al. , 2002). Reaction of the leaked waste solution with the underlying soil and sedimentary matrix will potentially result in dissolution of native minerals and subsequent formation of new secondary mineral phases. These chemical reactions can alter mineral surface properties, as well as the porosity and flow paths of the surrounding porous media. The physicochemical and mineralogical alterations of the sediment matrix could together lead to significant changes in the fate and transport of tank contaminants. Recent studies have shown that cancrinite, a feldspathoid, was formed after reacting Hanford sediments or quartz with tank simulant solutions at 60–90ºC (Nyman et al. , 2000; Bickmore et al. , 2001). We have found another feldspathoid, sodalite, in addition to cancrinite, which formed upon reacting the Hanford sediments with tank simulant for five weeks at 68ºC (our unpublished data). To the best of our knowledge, the retention properties of these materials for radionuclides have not yet been addressed. Quartz, feldspar, hornblende, mica, chlorite, illite, kaolinite, smectite and calcite are the most common minerals in Hanford sediments (Serne et al. , 1998). We found that these minerals have different reactivities toward the simulant solutions (our unpublished data). Even though kaolinite is not the most abundant clay mineral in Hanford sediments, here we report kaolinite alteration results because this mineral had shown the fastest alteration rate and the final precipitates are common products in the reactions with Hanford sediments. Clays and Clay Minerals, Vol. 52, No. 1, 1–13, 2004. Copyright # 2004, The Clay Minerals Society 1 * E-mail address of corresponding author: flury@mail.wsu.edu DOI: 10.1346/CCMN.2004.0520101