The role of secondary aluminosilicate minerals in technetium-99 immobilization in radioactive waste

Corroding waste tanks at select U.S. Department of Energy's nuclear waste facility have leaked highly alkaline tank waste solutions containing radionuclides and other contaminants into subsurface sediments. These tank wastes react with subsurface sediments to form secondary mineral phase(s) (feldspathoids), which may play key role in the transport of contaminants through the vadose zone and aquifers. Although transformation of secondary precipitates in subsurface sediments has been extensively studied, however, there is lack of knowledge about the role of feldspathoid selectivity in controlling the long-term fate and transport of key anionic radionuclides in the subsurface. The overarching objectives of this dissertation were to (1) determine secondary mineral transformation with aging time, alkalinity, anion identity and selectivity, and (2) quantify the competitive incorporation of ReO4- (a chemical analogue for Tc-99) into mineral phase(s) as a function of anion composition, size and selectivity under simulated waste leaks. The key results of this work showed that alkalinity, time and anion composition play important role in mineral transformation that control the mobility of key radionuclide species in the environment. Nitrate in 16 mol OH- /kg solution favored cancrinite nucleation while in 1 mol OH -/kg solution fostered mixed cancrinite/sodalite formation. The sequestering capacity of sodalite for ReO4- was ∼5 times higher than that of cancrinite. The immobilized ReO4- in the sodalite cages was not easily exchanged with other competing anions. Due to the less distortion to the beta-cage, sodalite displayed stronger preference for smaller competing anions relative to the larger ReO4 - anion. The selectivity of the mixed sodalite cage for ReO4 - was largely driven by the difference in anion radii (DIR) and increases in the following order: Cl-