Cancrinite and sodalite formation in the presence of cesium, potassium, magnesium, calcium and strontium in Hanford tank waste simulants
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High-level radioactive tank waste solutions that have leaked into the subsurface at the US Department of Energy Hanford Site, Washington, are chemically complex. Here, the effect of five cations, Cs+, K+, Sr2+, Ca2+ and Mg2+, on mineral formation and transformation pathways under conditions mimicking Hanford tank leaks is investigated. Sodium silicate was used to represent the dissolved silicate from sediments. The silicate was added into a series of simulants that contained 0.5 M aluminate, 1 M or 16 M NaOH, and the NO3- salts of the cations. The precipitates were monitored by X-ray diffraction, scanning electron microscopy, and X-ray energy dispersive spectroscopy. In the 1 M NaOH simulants, low concentration of Cs+ (<100 mM) did not affect the formation of lepispheric cancrinite and sodalite, whereas only highly crystalline cancrinite formed when Cs+ concentration was 250 mM. An unidentified feldspathoid or zeolite intermediate phase was observed in the presence of high concentrations of Cs+ (500 mM). The presence of K+ did not alter, but slowed, the formation of cancrinite and sodalite. The presence of divalent cations led to the formation of metastable or stable silicates, aluminates, hydroxides, or aluminosilicates. The formation of these intermediate phases slowed the formation of cancrinite and sodalite by consuming OH-, silicate, or aluminate. Compared with the concentrations used in this study, the concentrations of radioactive Cs+ and Sr2+ in the tank solutions are much lower and divalent cations (Ca2+ and Mg2+) released from sediments likely precipitate out as hydroxides, silicates or aluminates; therefore, the authors do not expect that the presence of these monovalent and divalent cations significantly affect the formation of cancrinite and sodalite in the sediments underneath the leaking waste tanks.
