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Posted: Aug 13, 2008
Nanotechnology solution for radioactive waste cleanup
(Nanowerk Spotlight) Radioactive material is toxic because it creates ions – by stripping away electrons from atoms – when it reacts with biological molecules. These ions can form free radicals, which damage proteins, membranes, and nucleic acids. Free radicals damage components of the cells' membranes, proteins or genetic material by "oxidizing" them – the same chemical reaction that causes iron to rust. This is called "oxidative stress". Many forms of cancer are thought to be the result of reactions between free radicals and DNA, resulting in mutations that can adversely affect the cell cycle and potentially lead to malignancy.
Nanotechnology has provided numerous constructs that reduce oxidative damage in engineering applications with great efficiency (see our Spotlight "Radical nanotechnology - how medicine can learn from materials science"). As a new research report shows, nanotechnology applications could also help to remediate radioactive contamination at the source, by removing radioactive ions from the environment.
Environmental contamination with radioactive ions that originate from the processing of uranium or the leakage of nuclear reactors is a potential serious health threat because it can leach into groundwater and contaminate drinking water supplies for large population areas. The key issue in developing technologies for the removal of radioactive ions from the environment – mainly from wastewater – and their subsequent safe disposal is to devise materials which are able to absorb radioactive ions irreversibly, selectively, efficiently, and in large quantities from contaminated water.
"Natural inorganic cation exchange materials, such as clays and zeolites, have been extensively studied and used in the removal of radioactive ions from water via ion exchange and are subsequently disposed of in a safe way" Dr. Huai Yong Zhu explains to Nanowerk. "However, synthetic inorganic cation exchange materials – such as synthetic micas, g-zirconium phosphate, niobate molecular sieves, and titanate – have been found to be far superior to natural materials in terms of selectivity for the removal of radioactive cations from water. Radioactive cations are preferentially exchanged with sodium ions or protons in the synthetic material. More importantly, a structural collapse of the exchange materials occurs after the ion exchange proceeds to a certain extent, thereby forming a stable solid with the radioactive cations being permanently trapped inside. Hence, the immobilized radioactive cations can be disposed safely."
Zhu, an Associate Professor in the School of Physical & Chemical Sciences at the Queensland University of Technology in Brisbane, Australia, points out that this phenomenon – that the uptake of large, radioactive cations eventually triggers the trapping of the cations – by itself represents a desirable property for any material to be used in decontamination of water from radioactive cations.
"Generally, ion exchange materials exhibiting a layered structure are less stable than those with 3D crystal structures and the collapse of the layers can take place under moderate conditions" says Zhu. "Then again, it has also been found that nanoparticles of inorganic solids readily react with other species or are quickly converted to other crystal phases under moderate conditions, and thus are substantially less stable than the corresponding bulk material."
Based on this, Zhu and his colleagues from Queensland University of Technology and Dr. Xue Ping Gao from the Institute of New Energy Material Chemistry at Nankai University in Tianjin, PR China, focused their search for potential candidates for intelligent absorbents on nanoparticles of inorganic ion exchange materials with a layered structure.
Scheme for the removal of the radioactive ions through ion exchange and structure deformation. (Image: Dr. Zhu)
"The novelty of our project is that the adsorption of bivalent toxic radioactive cations by the nanofibers finally induces structure collapse and deformation of the nanofibers, which permanently locks in the toxic radioactive cations," Zhu explains. "The permanent entrapment prevents the radioactive cations to be released from the adsorbents and assures that they can be safely disposed. Furthermore, the titanate nanofiber can selectively remove the radioactive ions in the presence of plentiful competitive ions."
It has been know for years that titanate solids possess a layered structure and exchangeable sodium ions. Also, titanate materials are stable to radiation, chemicals, and thermal as well as mechanical stress, so that they make an ideal carrier for radioactive ions.
Zhu points out that titanate nanofibers have a much larger capacity to take up the bivalent toxic radioactive cations, and they can do it much faster than other materials. "The most important finding by us is the nanofibers can trap the toxic radioactive cations permanently" he says.
Transmission electron microscopy image of the titanate nanofibers. (Image: Dr. Zhu)
The property of these nanofibers to permanently trapping radioactive cations makes them an ideal absorbent to remove them from contaminated water, while the used sorbents can be disposed safely without having to risk a release of the absorbed cations from the absorbents which may cause secondary contamination.
The scientists also mention that the titanate nanofibers possess a number of additional advantages for practical application in the removal of radioactive cations:
The synthesis of phase pure titanate nanofibers with a small and uniform particle size can be achieved in large quantities from abundant raw materials, such as titanium dioxide minerals, using simple and cost-effective processes.
The fabrication costs of nanofibers are much lower than that of titanate whiskers which are prepared by pyro-synthesis at high temperatures.
Titanate nanofiber sorbents can be readily dispersed into solutions because the fibers do not aggregate as much as clays and zeolites.
Moreover, the absorbents can be separated from any liquid after the absorption simply by filtration, sedimentation, or centrifugation due to their fibril morphology.
Absorption of radioactive ions by the fibers is very prompt. The amount absorbed within the first 24 hours is approximately 80% of the final equilibrium capacity.
Zhu and his team are now working on developing inorganic adsorbents with meta-stable structures for irreversible ion exchange. Such a structure will be useful for removal toxic ions in water. They are also making efforts to improve the selectivity and increase the capacity of the materials so that they can be used for treatment of radioactive wastes on an industrial scale and produce waste solids suitable for long-term storage and disposal.