Mineral formation during simulated leaks of Hanford waste tanks

Highly-alkaline waste solutions have leaked from underground tanks at the US DOE Hanford Site, Washington, causing mineral dissolution and re-precipitation upon contact with subsurface sediments. The main mineral precipitation and transformation pathways were studied in solutions mimicking tank leak conditions at the US DOE Hanford Site. In batch experiments, Si-rich solutions, representing dissolved silicate minerals, were mixed with caustic tank simulants. The tank wastes encompass a large range of chemical compositions. The effect of the following factors on mineral transformations were investigated: temperature (22, 50 and 80 °C), concentration of NaOH (from 0 to 16 M), 6 types of common inorganic anions in the tank supernatant, concentration of NaNO3 (the most abundant electrolyte in the tanks), and the Si/Al ratio in the starting solutions. Precipitates were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy. A general mineral transformation pathway was observed: poorly crystalline aluminosilicate → Linde Type A (LTA) zeolite → cancrinite/sodalite. Cancrinite and sodalite were the two stable mineral phases. The concentration of NaOH and the type of anion played the determinative roles in mineral formation and transformation. Increasing NaOH concentration and temperature favored the formation of cancrinite and sodalite. Cancrinite formed in the presence of NO3- or SO42-; sodalite formed in the presence of Cl− or NO2-. The experiments indicate that (1) NaOH is a mineralization agent in the mineral transformation and the anions served as templates in the formation of cancrinite and sodalite by forming ion-pairs with Na+ and (2) cancrinite and sodalite with various morphologies and crystallinity should form in the contaminated sediments.