Sue B. Clark

Uptake of natural and anthropogenic actinides in vegetable crops grown on a contaminated lake bed

Abstract Activity concentrations and plant/soil concentration ratios (CRs) of 239,240 Pu, 241 Am, 244 Cm, 232 Th, and 238 U were determined for three vegetable crops grown on an exposed, contaminated lake bed of a former reactor cooling reservoir in South Carolina, USA. The crops included greens and tubers of turnips ( Brassica rapa var. white-globe), bush beans ( Phaseolus vulgaris ), and husks and kernels of sweet corn ( Zea mays var. silver queen). Although all plots were fertilized, some received K 2 SO 4 , while others received no K 2 SO 4 . The K 2 SO 4 fertilizer treatment generally lowered activity concentrations for 241 Am, 244 Cm, 232 Th and 238 U, but differences were statistically significant for 241 Am and 244 Cm only. Highly significant differences occurred in activity concentrations among actinides and among crops. In general, turnip greens exhibited the highest uptake for each of the actinides measured, while corn kernels had the least. For turnip greens, geometric mean CRs ranged from 2.3×10 -3 for 239,240 Pu to 5.3×10 -2 for 241 Am (no K 2 SO 4 fertilizer). For corn kernels, geometric mean CRs ranged from 2.1×10 -5 for 239,240 Pu and 232 Th to 1.5×10 -3 for 244 Cm (no K fertilizer). In general, CRs across all crops for the actinides were in the order: 244 Cm> 241 Am> 238 U> 232 Th > 239,240 Pu. Lifetime health risks from consuming crops contaminated with anthropogenic actinides were similar to the risks from naturally occurring actinides in the same crops (total ∼2×10 -6 ); however, these risks were only ∼0.3% of the risk from consuming the same crops contaminated with 137 Cs.

A cryogenic fluorescence spectroscopic study of uranyl carbonate, phosphate and oxyhydroxide minerals

Abstract In this work we applied time-resolved laser-induced fluorescence spectroscopy (TRLIF) at both room temperature (RT) and near liquid-helium temperature (6 K) to characterize a series of natural and synthetic minerals of uranium carbonate, phosphate and oxyhydroxides including rutherfordine, zellerite, liebigite, phosphuranylite, meta-autunite, meta-torbernite, uranyl phosphate, sodium-uranyl-phosphate, becquerelite, schoepite, meta-schoepite, dehydrated schoepite and compreignacite, and have compared the spectral characteristics among these minerals as well as our previously published data on uranyl silicates. For the carbonate minerals, the fluorescence spectra of rutherfordine showed significant difference from those of zellerite and liebigite. The fluorescence spectra of the phosphate minerals closely resemble each other despite the differences in their composition and structure. For all uranium oxyhydroxides, the fluorescence spectra are largely red-shifted as compared to those of the uranium carbonates and phosphates and their vibronic bands are broad and less resolved at RT. The enhanced spectra resolution at 6 K allows more accurate determination of the fluorescence band origin and offers a complemental method to measure the O=U=O symmetrical stretch frequency, ν1, from the spacings of the vibronic bands of the fluorescence spectra. The average ν1 values appear to be inversely correlated with the average pKa values of the anions.

The impact of mineralogy in the U(VI)—Ca—PO4 system on the environmental availability of uranium

Kinetic dissolution studies were conducted on four prominent U-Ca-PO4 minerals (metaschoepite, becquerelite, chernikovite and metaautunite). Synthetic samples were contacted with four extractants (acetic acid, deionized water, EDTA and sodium bicarbonate) at room temperature at two concentrations, 100 mM and 1 mM. Dissolution progress was monitored by periodic sampling for dissolved U, and dissolution rates were obtained from fits to a three term exponential model. Significant variations were observed in the rate and extent of dissolution among the mineralsexamined. The uranyl phosphates chernikovite and metaautunite proved resistant to dissolution in non-carbonate systems, with dissolution half-times of days to weeks in 100 mM systems and weeks to years in 1 mM systems. In contrast, the uranyl oxide hydrates schoepite and becquerelite were solubilized over much shorter time scales. While 100 mM bicarbonate was successful in dissolving U in all forms, dissolution rates varied among the four minerals. Overall, EDTA was the least sensitive to a 100 to 1 mM drop in its concentration in its solubilization of all four mineral phases, underscoring the importance of organic complexation for the environmental mobility of uranium.

A Study of Metal-Humate Interactions Using Cation Exchange

Complexes of different strengths between metal cations (Eu and UO!) and humate are distinguished by a cation exchange method using a radioactive tracer technique. The equilibrium and kinetics of the distribution between two binding modes (strong and weak) are dependent on the pH of the system, cation loading and the nature of metal cations. An interactive binding mechanism involving the mode of binding of metal ions and the conformational changes of polyelectrolytes is discussed in terms of polyelectrolyte theory.

Complexation of Uranium(VI) by Gluconate in Acidic Solutions : a Thermodynamic Study with Structural Analysis

Within the pC(H) range of 2.5 to 4.2, gluconate forms three uranyl complexes UO(2)(GH(4))(+), UO(2)(GH(3))(aq), and UO(2)(GH(3))(GH(4))(-), through the following reactions: (1) UO(2)(2+) + GH(4)(-) = UO(2)(GH(4))(+), (2) UO(2)(2+) + GH(4)(-) = UO(2)(GH(3))(aq) + H(+), and (3) UO(2)(2+) + 2GH(4)(-) = UO(2)(GH(3))(GH(4))(-) + H(+). Complexes were inferred from potentiometric, calorimetric, NMR, and EXAFS studies. Correspondingly, the stability constants and enthalpies were determined to be log beta(1) = 2.2 +/- 0.3 and DeltaH(1) = 7.5 +/- 1.3 kJ mol(-1) for reaction (1), log beta(2) = -(0.38 +/- 0.05) and DeltaH(2) = 15.4 +/- 0.3 kJ mol(-1) for reaction (2), and log beta(3) = 1.3 +/- 0.2 and DeltaH(3) = 14.6 +/- 0.3 kJ mol(-1) for reaction (3), at I = 1.0 M NaClO(4) and t = 25 degrees C. The UO(2)(GH(4))(+) complex forms through the bidentate carboxylate binding to U(VI). In the UO(2)(GH(3))(aq) complex, hydroxyl-deprotonated gluconate (GH(3)(2-)) coordinates to U(VI) through the five-membered ring chelation. For the UO(2)(GH(3))(GH(4))(-) complex, multiple coordination modes are suggested. These results are discussed in the context of trivalent and pentavalent actinide complexation by gluconate.

Microscale characterization of uranium(VI) silicate solids and associated neptunium(V)

Summary The uranium(VI) silicate phases uranophane, Ca[(UO2)(SiO3OH)]2·5H2O, and sodium boltwoodite, Na[(UO2)(SiO3OH)]·1.5H2O, were synthesized in the presence of small, variable quantities (0.5–2.0 mol % relative to U) of pentavalent neptunium (Np(V), as NpO2+), to investigate the nature of its association with these U(VI) solid phases. Solids were characterized by X-ray powder diffraction (XRD), gamma spectrometry (GS), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) with electron energy-loss spectroscopy (EELS). Neptunium concentration was determined in the bulk solid phases by GS and was found to range from 780–15800 μg/g. In some cases, Np distributions between the aqueous and solid phases were monitored, and 78–97% of the initial Np was associated with the isolated solid. Characterization of individual crystallites by TEM/EELS suggests the Np is associated with the U(VI) phase. No discrete Np phases, such as Np oxides, were observed. Because the U(VI) silicates are believed to be important solubility-controlling solids on a geologic timescale, these results suggest that the partitioning of the minor actinides to these solids must be considered when assessing the performance of a waste repository for spent nuclear fuel.

Optimization and characterization of a sulfate based electrodeposition method for alpha-spectroscopy of actinide elements using chemometric analysis

Alpha-spectrometric measurements using Si detectors is the standard method for the determination of alpha emitting actinide elements. This method requires the preparation of sources for analysis which do not degrade the energy spectrum of the emitted alpha particles via sample self-absorption. A variety of methods for the electrodeposition of actinides have been reported in the literature, many of which require long deposition times and lack reproducibility. A sulfate based method has been evaluated for the preparation of these sources using chemometric analysis to optimize the method and evaluate several variables and their interactions with the goal to achieve high yield source preparation in 1 hour or less. Typical resolution for this method is 30 keV or less with recoveries approaching unity.

Thermodynamic model for the solubility of Cr(OH)3(am) in concentrated NaOH and NaOH-NaNO3 solutions

The main objective of this study was to develop a thermodynamic model for predicting Cr(III) behavior in concentrated NaOH and in mixed NaOH–NaNO3 solutions for application to developing effective caustic leaching strategies for high-level nuclear waste sludges. To meet this objective, the solubility of Cr(OH)3(am) was measured in 0.003 to 10.5 m NaOH, 3.0 m NaOH with NaNO3 varying from 0.1 to 7.5 m, and 4.6 m NaNO3 with NaOH varying from 0.1 to 3.5 m at room temperature (22 ± 2°C). A combination of techniques, X-ray absorption spectroscopy (XAS) and absorptive stripping voltammetry analyses, were used to determine the oxidation state and nature of aqueous Cr. A thermodynamic model, based on the Pitzer equations, was developed from the solubility measurements to account for dramatic increases in aqueous Cr with increases in NaOH concentration. The model includes only two aqueous Cr species, Cr(OH)4− and Cr2O2(OH)4− (although the possible presence of a small percentage of higher oligomers at >5.0 m NaOH cannot be discounted) and their ion–interaction parameters with Na+. The logarithms of the equilibrium constants for the reactions involving Cr(OH)4− [Cr(OH)3(am) + OH− ⇌ Cr(OH)4−] and Cr2O2(OH)42− [2Cr(OH)3(am) + 2OH− ⇌ Cr2O2(OH)42− + 2H2O] were determined to be −4.36 ± 0.24 and −5.24 ± 0.24, respectively. This model was further tested and provided close agreement between the observed Cr concentrations in equilibrium with Cr(OH)3(am) in mixed NaOH–NaNO3 solutions and with high-level tank sludges leached with and primarily containing NaOH as the major electrolyte.

Failure of ESI Spectra to Represent Metal-Complex Solution Composition: A Study of Lanthanide–Carboxylate Complexes

Electrospray ionization-mass spectrometry (ESI-MS) shows great promise as a rapid method to identify metal-ligand complexes in solution. However, its application for quantitative determination of the distribution of species present in complicated equilibria is still in its infancy, and a direct correlation between ions observed in the gas phase and species expected in solution must be made with caution. The present work focuses on a seemingly simple system; the complexation of lanthanide cations with the acetate ligand. Using a high resolution quadrupole time-of-flight mass spectrometer, ions created by electrospray of solutions containing trivalent neodymium and acetate were identified. The gas phase distribution of species was compared to the solution phase speciation predicted using thermodynamic complexation constants. Apparent gas phase speciation diagrams were constructed as a function of solution conditions and fragmentation potential. Despite the expected variability of metal-ligand complexes as solution conditions change, the observed gas phase speciation was independent of the metal to ligand ratio but dependent on the operating conditions of the ESI-MS.

Oligomerization of chromium(III) and its impact on the oxidation of chromium(III) by hydrogen peroxide in alkaline solutions

Monomeric, dimeric and trimeric chromium(III) species in solution were separated by ion exchange and characterized with UV/Vis absorption and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). The kinetics of the oxidation of the separated species by hydrogen peroxide in alkaline solutions were studied by conventional and stopped-flow UV/Vis absorption spectroscopy. Results indicate that the intensity of Cr–Cr scattering in the EXAFS spectra (dCr–Cr ∼ 2.99 A), a measure of the degree of oligomerization, increases as the solution alkalinity is increased. As the oligomerization proceeds, the rate of oxidation by hydrogen peroxide in alkaline solutions decreases in the order: monomer > dimer > trimer > aged/unseparated alkaline chromium(III) solution where higher oligomers dominate. The dominant redox pathway has an inverse order with respect to CNaOH. The data suggest that the rate-determining step involves the weakening of the bridging bonds in the oligomer and a concomitant release of one hydroxyl group from the chromium(III) moiety upon the attack by hydrogen peroxide.