In her lecture Waging Chemical Warfare on Hazardous Waste: Green Chemistry at St. Mary’s College of Maryland on Apr. 7, Chemistry professor Leah Eller discussed the dangers of radioactive waste and current methods being designed to prevent its further exposure to the environment.
Eller presented her talk last Wednesday to an audience of professors, students, and community members in Schaefer Hall as the second-to-last lecture of the Natural Science and Mathematics Colloquium Series. Beginning after an introduction from Department Chair of Chemistry Andrew Koch, she began with the basics of hazardous waste: what it looks like, and its potential for environmental impact.
“The majority of radioactive wastes look like any other compounds,” said Eller. “The only difference is that certain elements are of an atypical isotope that exhibit radiation.”
During World War II, radioactive materials were disposed of by being dumped down a drain called a “hot drain” (next to an ordinary drain) that would travel through pipes throughout the facility into an underground collection tank for storage. The idea was that everything but the radiation would evaporate over time, leaving only the hazardous material away from the environment and people.
Most of these sites, however, were near sources of moving water, as these tanks were normally used to store waste from nuclear power plants that use the water as a coolant. Included in this description is the Hanford site near the Columbia River in Washington, being the most well-known site for radioactive waste disposal. The tanks hold two-thirds of the world’s radioactive waste in single-walled stainless steel.
The waste itself rests in the tank based on density, with the heavier “sludge”, or solid waste of (mostly) uranium-235 and plutonium 239 below a lighter, highly-radioactive level of radioisotopes and salt cakes, followed by a lower-radioactivity level of molecules that have come in contact with radioisotopes, followed by an evaporating layer on top.
A problem occurred when, to combat the buildup of acid solvent in the tanks, too much sodium hydroxide (abbreviated NaOH) was dumped into the tanks, creating salt cakes (since acid and bases mix to create salt and water) but also an overall basic environment (in terms of pH or acidity) that began corroding the stainless steel. Without a second reinforcing wall that now surrounds most tanks, these vessels began leaking radioactive waste into the environment, a leak that continues to head towards the Columbia River.
The idea behind removal of these wastes is the separation of each individual layer for specific treatment. Given that evaporation would leave salt cakes and the radioactive “sludge” layers, adding an aqueous solution to the tanks would dissolve the salt cakes and leave the sludge behind, which could be treated with concrete to prevent further reactivity.
The problem lies in the following step for the aqueous solution. While condensing the materials into glass rods using borosilicate would allow for easy transport of the radioisotopes and other salt materials, sulfate anions (abbreviated SO42-) also exist in the salt cakes, which greatly slows the vitrification (glass-forming) process. One solution to this problem would involve barium precipitation. “Barium is a +2 cation, and sulfate is a -2 anion,” said Eller. “These things come together and crash out of water, to be separated from the solvent.” The problem lies in how expensive barium is to purchase, as is adding much more borosilicate to combat the slowing-down of vitrification.
The remaining solution, extracting sulfate from the solution, is still an unsuccessful attempt. The difficulty in this process is explained by the Hofmeister Series, which classifies an ion’s ability to alter the layer-forming properties of water in solution. Nitrate, another anion, more readily extracts into organic solvent (separable from the aqueous layer) than does sulfate, hindering the chance of extracting enough sulfate out of solution.
“If I could put any agent in that can help to move ions into the organic phase,” said Eller, “guess which one it’s going to choose: nitrate.”
After a discussion of static and dynamic equilibrium, Eller discussed her use of radioactive sulfate (containing the radioisotope of sulfur, sulfur-35) to measure certain extracting agents’ affinity for removing sulfate from the aqueous solution and into a removable organic layer. However, finding an agent that binds to sulfate selectively requires an agent that is shaped in such a way that the non-spherical anion will fit into the agent, which is difficult to construct. While the chemical Aliquat-336 aided in the extraction, the agent was not a fast and effective one.
Not only shape-specific, the agent must also not dissolve in water, be dissolvable in organic solvents, and be stable, cheap, and easy to synthesize.
Eller’s research continued with the use of a chemical that fit these descriptions: cyclopyrrole, which could cheaply be made within one synthesizing step. While able to trap sulfate, cyclopyrrole was a slow agent with poor organic solubility. Modifying the structure to increase organic solubility did not aid in these kinetics, until Aliquat-336 was added alongside the agent. Using both allowed for a process with fast kinetics.
However, a problem still remained: pyrrole needed to be essentially recycled for continuous use, which was prevented by its non-basic structure and de-protonated (hydrogen atom lacking) state. The use of the basic trioctylamine instead of Aliquat-336 aided in this procedure, but resulted in slow kinetics of recycling that further resulted in lower extraction efficiencies of sulfate.
While research continues in this area, Eller concluded her talk by mentioning other projects being conducted in her research lab that involve green chemistry (environmentally-friendly methods of conducting reactions and syntheses).
“I thought that Dr. Eller gave an interesting lecture,” said senior Thomas Montgomery following the lecture, “which gave new insight into practical real world chemistry.”
“I did not realize that there was an environmental problem when it came to disposing radioactive waste,” said sophomore Gabrielle Cantor. “I enjoyed how [Dr. Eller] gave a general background of the history of the problem and then went into the specifics and how her research has related to these specifics.”
The next NS&M lecture, titled Animal Welfare and Modern Zoo Design: Designing with the Animal in Mind, will be presented by Don Moore, from the Washington, D.C. National Zoo, on Apr. 14. at 4 p.m.