Nanotechnology's potential role in nuclear waste management
(Nanowerk Spotlight) Back in 2005, the U.S. Environmental Protection Agency (EPA) released an overview document that discussed Emerging Nanotechnologies for Site Remediation and Wastewater Treatment (pdf download, 844 KB). The EPA followed this up in 2008 with a fact sheet that provided an updated overview on the use of nanotechnology in environmental remediation (Nanotechnology for Site Remediation Fact Sheet; pdf download, 1.7 MB). Both documents contain extensive descriptions of various nanomaterials, such as carbon nanotubes (CNTs), and their use with in potential environmental remediation applications, e.g. removal of heavy metal particles from wastewater).
"In the environmental field, the application of carbon nanotubes is regarded as extremely promising for the development of novel energy-storage techniques, sensors, and sorbent materials for myriad uses including waste management," Fabio Belloni tells Nanowerk. "That suggested to us that CNTs might also be relevant for potential applications of in a field which has keen environmental implications, namely nuclear waste management."
Belloni is a researcher at the European Commission's Joint Research Centre, Institute for Transuranium Elements (ITU), in Karlsruhe, Germany. He worked with colleagues at ITU (Vincenzo V. Rondinella, Paul Carbol, Thierry Wiss) and scientists from the Institute of Nuclear Sciences at Ege University in Izmir, Turkey (Ceren Kuütahyali) and the Instituto Tecnologie Avanzate in Trapani, Italy (Alfonso Mangione). These three institutions have initiated exploratory projects to identify and address research needs while studying the feasibility, reliability, and effectiveness of CNTs for nuclear waste management.
In a recent paper in Environmental Science & Technology, the scientists argue that the significance of the possible role of carbon nanotubes in treating and sequestering nuclear waste stems from a number of recent research results that specifically investigate the interaction between CNTs and actinides or lanthanides (Can carbon nanotubes play a role in the
field of nuclear waste management?).
The authors explain that nuclear waste management is a complex series of processes that basically follows two different strategies for waste disposal: "once-through, or geological disposal of the spent fuel after a few decades' long period of wet or dry storages, mainly applied in U.S., Canada, Sweden, and Finland; and closed cycle, or spent fuel interim storage, then reprocessing through aqueous dissolution (e.g., PUREX method), and conditioning of the resulting high-level waste in borosilicate glass for geological disposals, currently applied in France, U.K., Japan, China, India, and Russia."
Diagram of the nuclear fuel cycle, showing the various options: once-through; conventional closed cycle; advanced closed cycle with partitioning and transmutation. Resultant high-level waste streams are also indicated. (Reprinted with permission from American Chemical Society)
One of the required processes during the waste cycle is the removal of heavy metals, lanthanides and actinides from waste waters. Due to its selective adsorption, activated carbon is already used for the separation of metal ions from solutions in the nuclear
industry and adsorption of uranium from aqueous solution has also been successfully performed.
Other schemes of application envisage chemical impregnation of the activated carbon to increase sorption selectivity and removal efficiency, especially from acidic media.
This is where carbon nanomaterials come in. In principle, researchers believe that CNTs, buckypapers and other CNT composite materials could ensure better performances than activated carbon. "In addition to the greater efficiency possible with an ordered geometry, that CNTs can be functionalized as opposed to merely impregnated, promises to enhance their sorptive abilities beyond that of activated carbon," says Belloni. "Another unique property of open-ended CNTs is the ability to encapsulate a variety of substances. This is accomplished by a capillary action within the CNT structure; filling the tubes with large surface tension fluids can be accomplished using high pressure. Water and aqueous solutions, liquid fuels, biomolecules, polymers, and nanoparticles have already been captured in CNTs core."
While there have been several studies that suggest that multi-walled CNTs are promising candidates for the preconcentration and solidification of some lanthanides and actinides from large volumes of aqueous solution, as required for remediation purposes, nuclear waste poses one particular problem: Unlike conventional heavy metals, the inherent radioactivity of (most) nuclear waste introduces another
variable in sorptive material engineering.
"Radiation-induced damage suffered by CNTs when diluted in highly radioactive solutions or by the effect of retained radionuclides can affect their structural stability" says Belloni. "This can result in a long-term effect which is not easy to assess on the time scale typical of laboratory sorption experiments. More research needs to be done in this area to better assess these issues."
After an initial assessment, the team believes that CNTs can achieve a better adsorption capacity that currently used carbon materials, mainly due to stronger sorption bonding abilities and optimizable properties afforded through not just impregnation but also functionalization.
They point out that the additional potential for encapsulation and solidification could extend the utility of CNT materials beyond filtering. The wider spectrum of possible applications include treatment of liquid effluents from nuclear plants, environmental remediation of contaminated sites, waste immobilization for final disposal, utilization in the backfill material of waste repositories, or, in the form of buckypapers, as filters or electrodes for partitioning processes.
"We intend for our article to be a thought-provoking initiative to stimulate curiosity and interest and collect comments and viewpoints" says Belloni. "We believe that this topic merits more thorough debate within the scientific and industrial communities in order to, among other things, identify research needs, study the feasibility of possible applications, and consider environmental issues of contaminated CNTs."
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