Mary E. Barr

Optical spectroscopic studies of the sorption of UO2+2 species on a reference smectite

Abstract The speciation of UO 2+ 2 (uranyl) on a reference smectite (SAz-1 from Cheto, Arizona, USA) has been investigated by electronic emission and Raman vibrational spectroscopies. The spectroscopic studies have been done on uranyl-bearing clays prepared from aqueous solutions of uranyl nitrate in the pH range from ~2.5 to 7 and high initial ionic strength (~0.1–0.3 M). The uranyl loading levels in these samples ranged from ~0.1% to ~53% of the reported cation exchange capacity (~1.2 meq/g). Vibronically resolved emission spectra have been obtained for all samples. These spectra vary significantly in intensity and band-shape as a function of uranyl concentration in the clays and the equilibrium pH of the solutions from which the clays were prepared. For most clay samples the measured emission spectrum is a composite of spectra from multiple uranyl emitters. At the lowest loading levels a uranyl sorption complex with an apparent vibronic spacing of ~750 cm −1 dominates the spectra. At intermediate loading levels an additional uranyl sorption complex also having an apparent vibronic spacing of ~ 750 cm −1 is present at an approximately constant concentration ratio to the species in the most dilute samples. At the highest loading levels, a uranyl sorption complex with a vibronic spacing of ~ 850 cm −1 dominates the spectra. Raman spectra have been obtained for the more concentrated uranyl/clay samples. Two distinct bands (855 cm −1 and 883 cm −1 ) are seen in the spectral region of the totally symmetric uranyl stretch. The 855 cm −1 band correlates with the dominant high-coverage species, while the 883 cm −1 band arises from an additional sorption complex. Comparison of these results with aqueous solution spectral data suggests that monomeric uranyl moieties are responsible for the observed spectral responses in the clay samples, and the multiple spectral components are a result of occupancy by these moieties in several structurally and/or energetically different sites within the clay. It is proposed that the uranyl species responsible for the dominant components in the emission spectra in the low and intermediate coverage clay sample are sorbed to amphoteric edge site(s). The uranyl species responsible for the dominant component in the emission spectrum and the 855 cm −1 Raman band in the high-coverage clays is proposed to be exchanged into the fixed charge site(s). The additional complex identified by the Raman band at 883 cm −1 is also proposed as a sorption complex at fixed-charge sites.

Actinide Binding and Solubilization by Microbial Siderophores

Accurate predictions of actinide and fission product migration in the geosphere are critically dependent on identification of the biological, chemical and physical processes which affect actinide mobility in soil and water. Siderophores are low molecular weight iron chelators produced by microbes in response to low availability of soluble iron. Because of the similarities between iron(III) and tetravalent actinides, and the prevalence of siderophore-producing microbes in soil, there is strong likelihood that siderophores may also bind actinides, thereby influencing their mobility in the environment. In order to begin to assess the potential importance of siderophore-mediated actinide mobility, we have determined rate constants for solubilization of hydrous plutonium oxide by the siderophores enterobactin and desferrioxamine Β and selected carboxylate, amino polycarboxylate, and catecholate ligands. The measured rate constants for solubilization of insoluble actinide oxides show that siderophores are extremely effective in solubilizing actinides; on a per molecule basis, enterobactin is ~ 1 0 3 times more effective than the other chelators tested in increasing the rate of solubilization of hydrous plutonium oxide. Notably, ferric-siderophore complexes are more effective in solubilizing actinide oxides than the siderophores in the absence of iron. These results suggest that siderophores have the potential to mobilize actinides in the environment.

Crystallographic, spectroscopic and theoretical studies of an electron-deiocalized Cu(1.5)–Cu(1.5) complex

The mixed-valence complex [Cu2L]3+1[L ={N[CH2CH2N(H)CH2CH2N(H)CH2CH2]3N]} has been shown from EPR, visible Spectroscopic, and single-crystal X-ray diffraction measurements to possess an unpaired electron delocalized over a short Cu(1.5)–Cu(1.5) bond in both the hydrated nitrate [2.364(2)A] and acetate [2.415(2)A] salts: Fenske–Hall MO analysis reveals direct σ-bonding interactions between the copper centers.

Americium Separations from Nitric Acid Process Effluent Streams

Plutonium recovery operations offer several points at which americium removal may be attempted, and we are evaluating two classes of materials targeted at different steps in the process. Extraction chromatography resin materials loaded with three different alkylcarbamoyl phosphinates and phosphine oxides are assessed for Am removal efficiency and Am/Fe selectivity from 1–7M nitric acid solutions. Commercial and experimental anion exchange resins are evaluated for total alpha-activity removal from post-evaporator solutions whose composition, relative to the original nitric acid effluent, is reduced in acid and greatly increased in total salt content. With both classes of materials, americium and/or total-alpha reduction is sufficient to meet regulatory requirements even under sub-optimal conditions. Batch distribution coefficients and column performance data are presented.

Plutonium(IV) sorption by soluble anion-exchange polymers

Soluble anion-exchange polymers have been designed, synthesized, and evaluated for their ability to take up Pu(IV) from nitric acid solutions. These polymers, based on linear poly(4-vinylpyridine) (PVP) and polyethyleneimine (PEI) are soluble in aqueous and strong acid solutions. Weak-base sites on the polymers are protonated under experimental conditions, and, in most cases, have been converted by alkylation to form mono- and bifunctional strong-base anion-exchange sites. Distribution of Pu(IV) onto these polymers was determined by comparing visible spectroscopic data in the presence and absence of the soluble polymer. Overall plutonium affinity for the anion-exchange sites in the soluble materials is found to be much lower than for comparable solid resins, but the distribution behavior follows similar trends in that bifunctionalized materials are superior to monofunctionalized and a five-atom “spacer” between the two cationic sites is superior to other spacer lengths.

AMERICIUM SEPARATIONS FROM HIGH-SALT SOLUTIONS USING ANION EXCHANGE

The aging of the US nuclear stockpile presents a number of challenges, including the increasing radioactivity of plutonium residues due to the ingrowth of 241Am from the β-decay of 241Pu. We investigated parameters that affect the sorption of Am onto anion-exchange resins from concentrated effluents derived from nitric acid processing of plutonium residues. These postevaporator wastes are nearly saturated solutions of acidic nitrate salts, and americium removal is complicated by physical factors, such as solution viscosity and particulates, as well as by the presence of large quantities of competing metals and acid. Single- and double-contact batch distribution coefficients for americium and neodymium from simple and complex surrogate solutions are presented. Varied parameters include the nitrate salt concentration and composition and the nitric acid concentration. We find that under these extremely concentrated conditions, Am(III) removal efficiencies can surpass 50% per contact. Distribution coefficients for both neodymium and americium are insensitive to solution acidity and appear to be driven primarily by low water activities of the solutions.

New bifunctional anion-exchange resins for nuclear waste treatment

Additional1 bifunctional anion-exchange resins have been designed, synthesized and evaluated for their ability to take up Pu(IV) from nitric acid solutions. Bifunctionality is achieved by adding a second anion-exchange site to the pyridine nitrogen (also an anion-exchange site) of the base poly(4-vinylpyridine) resin. Previous work focused on the effect of varying the chemical properties of the added site along with the length of an alkylene ‘spacer’ between the two sites. Here we examine four new 3- and 4-picolyl derivatives which maintain more rigidly defined geometries between the two nitrogen cationic sites. These materials, which have the two anion-exchange sites separated by three and four carbons, respectively, exhibit lower overall Pu(IV) distribution coefficients than the corresponding N-alkylenepyridium derivatieves with more flexible spacers. Methylation of the second pyridium site results in a ca. 20% increase in the Pu(IV) distribution coefficients.

Sorption of Pu(IV) from nitric acid by bifunctional anion-exchange resins

Anion exchange is attractive for separating plutonium because the Pu(IV) nitrate complex is very strongly sorbed and few other metal ions form competing anionic nitrate complexes. The major disadvantage of this process has been the unusually slow rate at which the Pu(IV) nitrate complex is sorbed by the resin. The paper summarizes the concept of bifunctional anion-exchange resins, proposed mechanism for Pu(IV) sorption, synthesis of the alkylating agent, calculation of K{sub d} values from Pu(IV) sorption results, and conclusions from the study of Pu(IV) sorption from 7M nitric acid by macroporous anion-exchange resins including level of crosslinking, level of alkylation, length of spacer, and bifunctional vs. monofunctional anion-exchange resins.

Washing of Rocky Flats Combustible Residues (Conducted March - May 1995)

The scope of this project is to determine the feasibility of washing plutonium-containing combustible residues using ultrasonic disruption as a method for dislodging particulate. Removal of plutonium particulate and, to a lesser extent, solubilized plutonium from the organic substrate should substantially reduce potential fire, explosion or radioactive release hazards due to radiolytic hydrogen generation or high flammability. Tests were conducted on polypropylene filters which were used as pre-filters in the rich-residue ion-exchange process at the Los Alamos Plutonium Facility. These filters are similar to the Ful-Flo{reg_sign} cartridges used at Rocky Flats that make up a substantial fraction of the combustible residues with the highest hazard rating. Batch experiments were run on crushed filter material in order to determine the amount of Pu removed by stirring, stirring and sonication, and stirring and sonication with the introduction of Pu-chelating water-soluble polymers or surfactants. Significantly more Pu is removed using sonication and sonication with chelators than is removed with mechanical stirring alone.

New anion-exchange polymers for improved separations

Objective is to improve the understanding of how the structure of a new class of anion-exchange polymers controls the binding of anionic actinide complexes from solution. This is needed to develop practical separation systems that will reduce the cost of actinide processing operations within the DOE complex. In addition anion exchange is widely used in industry. Several new series of bifunctional anion- exchange polymers have been designed, synthesized, and tested for removing Pu(IV), Am(III), and U(VI) from nitric acid. The polymers contain a pyridinium site derived from the host poly(4-vinylpyridine) and a second cationic site attached through a chain of 2 to 6 methylene groups. The new polymers removed Pu four to ten times more efficiently than the best commercial materials.