Dr. Bob Rundberg presented to the Mines chemistry department on radiochemistry. He works at the Los Alamos National Laboratory, where he has been involved with a number of important projects, including the Yucca Mountain nuclear storage facility. Yucca Mountain is a site in Nevada which was under consideration for a permanent repository for the most radioactive types of nuclear waste. Before it was ultimately rejected, many studies were done to assess the viability of the site. As a part of this investigation, Rundberg and his colleagues tested several natural metallic minerals, including goethite, hematite, and rutile, to see if and how efficiently they would complex with various radionuclides. If the waste in Yucca Mountain were to leak out into the surrounding rock, it would encounter minerals such as these; the affinity these minerals have for such radioactive compounds would in turn affect the compounds’ behavior, possibly retarding the transport of the radionuclides or changing their composition.
When a metal oxide’s surface complexes with another substance, the charge of the oxide changes. The charge of the oxide controls whether it attracts or repulses ions, or charged particles. The radionuclides dissociate into ions when dissolved in water, so if the oxides they pass by attract them, their transport will be slowed down, and vice versa. Through protonation—the addition of a proton to a molecule or the subtraction of an electron from it—a basic solution will give a metal oxide a positive surface charge. Through deprotonation an acid will do the opposite, giving a negative surface charge. For an example of how this will affect the transport of a radionuclide, take uranyl, one of several uranium ions. Uranyl has a positive charge, so a metal with a negative surface charge will attract it and, hopefully, keep it from percolating into the groundwater.
Rundberg and his colleagues devised a special method of titration to test just how the processes described above might proceed. This method allowed accurate pH measurements and excellent pH control, which were necessary when working with low concentrations of radionuclides. Previous methods were not as accurate, as the equipment would clog. Furthermore, the precise calculation of surface charge required a precise calculation of molecular surface area. In earlier experiments, molecules were treated as flat planes, giving erroneous results. Because the molecules in these tests tend to clump into colloids, Rundberg treated them as spherical instead. Though he acknowledged that this is still only an approximation, or a “spherical chicken”, as he joked, referring to scientists’ propensity to oversimplify things for the sake of calculation ease, it is a better approximation of a colloidal molecule than a plane, and likewise gave better, much more reasonable results. Finally, two types of scintillators were used to measure the sorption of the radionuclides onto the metals. A scintillator is a device which emits photons in response to radiation, which can then be detected and used to quantify the amount of radiation, and from there, the amount of radioactive material. Each type of scintillator measures a different type of radiation, allowing a multitude of radionuclides to be tested. Even with this much control on the precision of the experiments, the accuracy of Rundberg’s measurements broke down at very high sorption rates and very low concentration levels.
Rundberg went into great detail about the behavior he observed in his experiments and the mechanisms behind the results. All the radionuclides tested showed a steady increase in adsorption up to a high pH of about nine in the case of neptunyl, but other compounds had different results, after which the adsorption rate fell off. This curve, Rundberg found, closely approximated the hydrolysis curve for the metal involved. The relationship held true for all of the elements examined.
Rundberg observed another interesting result. Essentially, when a radionuclide adsorbed to the surface of the metal, it “kicked out” part of the resident material, thereby changing the metal’s surface charge. In the case of neptunyl, an ion of the man-made element neptunium, this process made sense—each neptunyl molecule adsorbed onto the metal displaced a single proton, equivalent to the neptunyl’s single positive charge. Not all of the radionuclides behaved as intuitively as neptunium, however. Uranyl, for instance, has a double positive charge, but when it adsorbed, it kicked off one and a half protons. Thorium, stable in a quadruple positive ion, displaces only three protons. For this reason, while neptunium does not alter the surface charge, the other radionuclides do, as the total charge they displace does not match their own charge. Some researchers have proposed “multidentate” charge geometries to explain this phenomenon, but Rundberg concluded that the strange results he obtained concerning charge instead indicate a problem in the way charge is modeled, something which future observers may be able to correct.