Jess M. Cleveland

Speciation patterns of actinides in natural waters: a laboratory investigation

The results of laboratory investigations of actinide speciation in ground waters of varying compositions are summarized, re-evaluated and further interpreted, with the inclusion of additional data. Actinide speciation (solubility, oxidation state distributions) in these water samples exhibits great variability at 10−10 to 10−8 M total actinide concentrations. Americium solubility appears to be controlled by the formation of radiocolloids. neptunium is predictably oxidized and soluble in these water samples, with only a few exceptions. Plutonium exhibits widely varying oxidation state distributions—and hence solubility— despite the presence of 6 mg liter−1 dissolved oxygen in all experiments. Plutonium solubility is enhanced by carbonate and fluoride but is diminished by sulfate in reducing waters. In general, these results are compatible with the limited information available for actinide speciation from previous in-situ investigations but they emphasize the paucity of accurate thermodynamic data to describe these systems.

Plutonium Speciation in Water from Mono Lake, California

The solubility of plutonium in Mono Lake water is enhanced by the presence of large concentrations of indigenous carbonate ions and moderate concentrations of fluoride ions. In spite of the complex chemical composition of this water, only a few ions govern the behavior of plutonium, as demonstrated by the fact that it was possible to duplicate plutonium speciation in a synthetic water containing only the principal components of Mono Lake water.

Investigation of solubilization of plutonium and americium in soil by natural humic compounds

An investigation was conducted under simulated natural conditions to determine the solubilization of plutonium and americium in contaminated soil by humic compounds. The results indicate that the solubilization of plutonium and americium by fulvic acid--the more soluble humic compound--is slight. In no instance did the plutonium and americium concentrations in solution approach the drinking water limits. What little solubilization that occurs is probably due to complex formation and/or colloid peptization. The resulting solutions are unstable, and over a period of a few days, most of the plutonium and americium precipitate, either as a result of colloid coagulation or hydrolysis. Two other factors appear to limit the solubilization of these elements: only a relatively small fraction of the plutonium and americium seems to be readily soluble, and there is apparently a low saturation value for these elements in both water and fulvic acid solution.

Neptunium and americium speciation in selected basalt, granite, shale, and tuff ground waters

Neptunium and americium are relatively insoluble in ground waters containing high sulfate concentrations, particularly at 90�C. The insoluble neptunium species is Np(IV); hence reducing waters should enhance its formation. Americium can exist only in the trivalent state under these conditions, and its solubility also should be representative of that of curium.

Plutonium Speciation in Selected Basalt, Granite, Shale, and Tuff Groundwaters

The speciation of added plutonium was determined in ground waters of widely varying composition from four rock types: basalt, granite, shale, and tuff. Plutonium was soluble in the basalt water, which contained a high fluoride ion concentration, probably as a result of its stabilization in solution as fluorocomplexes, primarily of plutonium (IV); it was insoluble, however, in the shale ground water, which contained the highest concentration of sulfate ion. Results were intermediate in the granite and tuff ground waters. In synthetic waters containing significant concentrations of both sulfate and fluoride, the effects were smaller and less predictable. Dissolved oxygen, ionic strength, carbonate ion concentration, and pH had little effect on plutonium speciation in the ranges encountered in this study. However, other ionic species not involved in this study may also influence plutonium speciation. The results have potential significance in establishing site-selection criteria for radioactive waste repositories. All other factors being equal, host rocks whose associated waters contain low concentrations of free fluoride ion should be preferable for the location of waste repositories.

Characterization of plutonium in ground water near the idaho chemical processing plant

Plutonium is present in very low concentrations in ground water near the disposal well at the Idaho Chemical processing plant but was not detected in waters at greater distances. Because of the absence of strong complexing agents, the plutonium is present as an uncomplexed (perhaps hydrolyzed) tetravalent species, which is readily precipitated or sorbed by basalt or sediments along the ground-water flow path.