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

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<p> 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 &sim;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^-&lt;NO₃^-&lt;MnO₄^- and CO₃^2-&lt;SO₄^2-&lt;WO₄^2- for the monovalent and divalent anions respectively. When the DIR between ReO₄^- and competing anions was less than &sim;12%, then ReO₄^- incorporation into sodalite was significant. The results imply that anion size is the major factor that determines sodalite anion compositions and that ReO₄^- is likely to serve as a suitable analogue for TcO₄^-</p>

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Identifer	oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:3640335
Date	24 October 2014
Creators	Dickson, Johnbull Otah
Publisher	Washington State University
Source Sets	ProQuest.com
Language	English
Detected Language	English
Type	thesis