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Long-term desorption behavior of uranium and neptunium in heterogeneous volcanic tuff materials


Please use this identifier to cite or link to this item: http://hdl.handle.net/1928/10867

Long-term desorption behavior of uranium and neptunium in heterogeneous volcanic tuff materials

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Title: Long-term desorption behavior of uranium and neptunium in heterogeneous volcanic tuff materials
Author: Cynthia, Dean
Advisor(s): Fulghum, Julia
Committee Member(s): Atanassov, Plamen
Cabaniss, Stephen
Reimus, Paul
Department: University of New Mexico. Dept. of Chemical and Nuclear Engineering
Subject(s): Uranium
LC Subject(s): Radioisotopes--Migration
Thermal desorption
Transport theory
Volcanic ash, tuff, etc.--Nevada--Yucca Mountain
Degree Level: Doctoral
Abstract: Uranium and neptunium desorption were studied in long-term laboratory experiments using four well-characterized volcanic tuff cores collected from southeast of Yucca Mountain, Nevada. The objectives of the experiments were to 1. Demonstrate a methodology aimed at characterizing <italic>distributions<italic> of sorption parameters (attributes of multiple sorption sites) that can be applied to moderately-sorbing species in heterogeneous systems to provide more realistic reactive transport parameters and a more realistic approach to modeling transport in heterogeneous systems. 2. Focus on uranium and neptunium because of their high solubility, relatively weak sorption, and high contributions to predicted dose in Yucca Mountain performance assessments. Also, uranium is a contaminant of concern at many DOE legacy sites and uranium mining sites. 3. Focus on desorption measurements rather than classic emphasis on sorption measurements. Desorption measurements are the key to interrogating the desired multi-site distributions of sorption parameters. 4. Investigate the effects of mineralogy on the sorption/desorption of uranium and neptunium to obtain mechanistic insights into the measured distributions of sorption parameters. In the long term desorption experiments, the percentages of uranium and neptunium sorbed as a function of time to zeolitic and devitrified volcanic tuffs of varying mineralogy were determined. In addition, the desorbed activity as a function of time was fit using a multi-site, multi-rate model to demonstrate that different desorption rate constants ranging over several orders of magnitude exist for the desorption of uranium and neptunium from Yucca Mountain volcanic tuff. To evaluate the applicability of rate constants obtained from the long-term desorption experiments under more realistic flow conditions and with more realistic solid to solution ratios, scaled up experiments were conducted in which uranium and neptunium were eluted at different flow rates through columns packed with one of the volcanic tuffs used in the desorption experiments. The up-scaled column breakthrough curves and sorbed concentration profiles (obtained after the experiments were terminated) were fit using a multi-site, multi-rate advection-dispersion-reaction model. However, this model could not simultaneously provide a good description of both the column profiles and the breakthrough data (using the same rate constants) for either the uranium or neptunium columns. In general, the combined breakthrough and profile data suggest that while the majority of the radionuclide mass was strongly retarded (profile data), there was a minor fraction that was less strongly retarded (breakthrough data). The inability to simultaneously model these two fractions of radionuclide mass might be explained by (1) disequilibria of uranium and neptunium solution species and/or (2) a dual porosity flow regime within the up-scaled columns. To further investigate the possibility of multiple sorption sites for uranium in the volcanic tuff, the average local structural features of uranium freshly sorbed to and after one week of desorption from volcanic tuff was compared using Extended X-ray Adsorption Fine Structure (EXAFS) measurements. To complement the EXAFS, Electron Probe Microanalysis and x-ray mapping of similar samples was used to probe uranium spatial distributions and elemental associations within the volcanic tuff. The multiple methods employed in this study provide many more insights and more realistic parameterization of sorption and desorption than simple batch experiments. When coupled with knowledge of mineralogical and geochemical heterogeneities along groundwater flow paths, this multi-method approach should result in significant improvements to predictions of subsurface contaminant transport.
Graduation Date: May 2010
URI: http://hdl.handle.net/1928/10867

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