Date of Award

5-2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Engineering and Science

Advisor

Fjeld, Robert A

Committee Member

Kaplan , Daniel I

Committee Member

Devol , Timothy A

Committee Member

Liu , Haibo

Abstract

Understanding the environmental behavior of plutonium (Pu) is essential for proper radioactive waste disposal or for remedial activities following an accidental release of Pu. The environmental behavior of Pu is influenced by physical, chemical, and biotic factors, such as the simultaneous existence of multiple Pu species, redox transformations at mineral surfaces, colloid formation, and the potential of microbes and plants to affect its sorption to soil. Plant Pu studies have been conducted for quantifying bioaccumulation or phytoremediation. Until now, experimental studies have not focused on the capacity of plants to affect the transport behavior and distribution of Pu in the subsurface.
This dissertation addressed the hypothesis that root uptake and transport in plants can influence the mobility of Pu in the vadose zone. The overarching goal was to provide experimental support for reactive transport modeling of root uptake and xylem transport and for a connection between Pu uptake and the plant's nutritional requirement for Fe. The objectives were to: (1) quantify complexed Pu retardation in graminaceous plants and to quantify complexed Pu sorption to plant xylem, (2) characterize the distribution and accumulation of complexed Pu in plants, and (3) compare correlations between plant uptake of complexed Pu and Fe. In addition, a couple of simple models for predicting Pu transport by roots were examined.
Bench scale experiments were conducted using corn (Zea mays) as a representative of the grass family. Corn was grown in 1L soil pots above 500 mL nutrient solution containers with the primary root inserted in solution. Growth conditions were 14/10 h day/night cycles (32/20 °C), 30-50% RH, and a photosynthetic flux of 1300-1500 micromol/ m2s. To commence exposure, an aliquot of Pu(DFOB) or Pu2DPTA3 or both Pu and 59Fe complexed with DFOB was added to the nutrient solution. Plants were 23 - 28 d old when sacrificed. Plutonium and 59Fe contents were determined by liquid scintillation analysis and stable element contents were determined by ICP-MS. Sorption tests were conducted with Pu as Pu(IV), Pu(DFOB), or Pu2(DTPA)3 and cellulose or xylem excised from cotton stem tissue.
The Pu plant transport velocities were 174 - 348 cm/h and water velocities were 300 - 800 cm/h. Thus the retardation factor of Pu in live plants was measured to be 1-5 and estimated to be 1-10 due to water velocity uncertainty. With respect to the second objective, analysis of the spatial distribution of Pu in corn indicated that discrimination occurs at the exodermis and in root tissues, most of the Pu in the plant was retained in the roots, and the fraction of Pu that entered the xylem was rapidly transported upward to the rest of the plant. An overall average of greater than 97% of the Pu was found in the roots with the remainder in the shoots. The maximum shoot activity fraction was four per cent for plants exposed for 10 d however steady state translocation was not attained. Profiles of Pu concentration versus shoot length showed that Pu concentrated in
the upper shoots. With respect to the third objective, several findings are of interest. The plant uptake of Pu remained unchanged for Fe: Pu ratios ranging from 0 - 2.2 x 105. Large changes in Fe concentrations did not inhibit or enhance plant uptake of Pu. Comparisons of the distribution profiles of Pu, 59Fe, stable Fe, and several other nutrient elements showed that Pu was distributed very much like 59Fe in the shoot. However six times as much Pu was found in the root than 59Fe, and 40% more 59Fe was found in the shoot than Pu. The shoot distribution data strongly suggest that upon entering the xylem, Pu and Fe are physiologically treated in a highly similar manner. Clearly, Pu is simultaneously taken up with Fe.
Using an instantaneous partitioning model, comparisons were remarkably consistent between the soil concentration data of the SRS lysimeters and predictions using concentration ratios derived from field studies involving different plants, soils, and experimental conditions. The steady-state advection model predicted Kd values for Pu and plant root zone soil that are much lower than batch sorption determinations. This is consistent with enhanced mobility of sorbed Pu by siderophores or other plant exudates.

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