Archive for ion exchange
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The release of radioactive materials after the recent tsunami destruction of the Fukshima-Dai nuclear power plant has reignited public awareness to the problem of capturing nuclear waste on a grand scale. Less dramatically, but more common and just as important to control, are small scale leaks from nuclear power plants and radioactive waste generated by medical tests and research. Capturing and containing radioactive stuff is no small challenge, but a new study shows that small—nanoscale small—may be the way to go.
Researchers at Queensland University of Technology in Australia, in collaboration with a group at Penn State University, may have found a cheap, effective, nonreversible way of capturing radioactive cesium and iodine wastes using titanate-base nanofibers and nanotubes.
A multi-institution, international team led by QUT professor, Huaiyong Zhu, has published a paper demonstrating that Na2Ti3O7 nanofibers and nanotubes can effectively capture and store Cs+ and I- ions. Sodium titanate has the advantage of being easy and economical to synthesize through hydrothermal processes.
Cesium isotopes can be captured by inorganic cation exchange with materials like silico-titanates, zeolites, clay minerals, layered zirconium phosphates and layered sulfides. These materials are able to withstand high radiation levels and high temperatures, and they are blessed with a high ion-exchange capacity. Unfortunately, the ion-exchange process is reversible, which means radioactive ions can be released back into solution when exposed to water.
Sodium titanate has a layered structure where TiO6 octahedra form the basic structural units, with Na+ ion between the layers, and the radioactive 137Cs+ isotope is captured in a simple ion exchange.
The team compared the chemisorption properties of two forms of sodium titanate: nanofibers and nanotubes. The nanotubes had a greater absorptive capacity and were able to remove to remove about 80 percent of the ions from solutions compared to only about 36 percent ion removal by the nanofibers.
During uptake, the nanofiber morphology is maintained, but if enough Cs+ ions are absorbed, the titanate layers deform. When a large concentration of Cs+ ions is absorbed, a phase transformation occurs and creates microporous tunnels in the layered structure. The diameter of the tunnels is narrower than the diameter of the cesium ion, thus immobilizing the ion and rendering the exchange irreversible.
In contrast, Cs+ uptake by nanotubes results a significant change in the aspect ratio: They become more squat and wide. The layered structure of the tubes remains (there is no mention of a phase change), but the interlayer space expands, which may swell the nanotubes.
Nanofibers and nanotubes absorb iodine through a different mechanism. Because I- is an anion, a direct ion exchange with sodium is impossible. By coating sodium titanate nanofibers and nanotubes with nanoparticles of silver oxide (Ag2O), iodine ions can be stuffed into the nanofibers or nanotubes by means of several chemical reactions involving intermediate compounds, hydration and dehydration. The chemistries and crystallographies involved combine to provide excellent absorption properties. Follow-up tests showed that the leaching rate of iodine back into solution was very low. Thus, Ag2O-coated titanate nanofibers and nanotubes also show promise as effective candidates for capturing radioactive iodine.
In a press release from QUT, paper co-author Zhu says, “One gram of the nanofibers can effectively purify at least one ton of polluted water.” If so, the material should be an easy and cost-effective addition to the toolkits at facilities working with or managing radioactive wastes that contain cesium and iodine isotopes.
(The paper does not address disposal of the nanofibers/nanotubes after the ion-exchange capture is completed.)
In the US, Zhu collaborated with ACerS member Sridhar Komarneni, a professor at Penn State University.
The paper is “Capture of Radioactive Cesium and Iodide Ions from Water Using Titanate Nanofibers and Nanotubes,” Angewandte Chemie International Edition (doi: 10.1002/anie201103286).
Although it seems that the decision actually was made several weeks ago, news is just now starting to bubble up about how officials managing the development and construction of the new 1 World Trade Center building in New York City have axed plans to sheath the first section of the new structure in large and special prismatic glass panels.
This part of the construction project had been in the works for many months, and a story in the the New York Times reports that production was underway and that $10 million had already been spent on the panels before the plug was pulled.
The NYT story provides many of the details, but allow me to summarize: The 13′4″ x 4″ (I haven’t been able to determine the thickness) panels were designed by the architects and engineers to be made of ultraclear (low-iron) tempered glass that, once formed, would have unique wedges cut into them to provide prism-like optical effects. After heat strengthening, several panels would be laminated together and, according to the architect’s (SOM) website, attached to an aluminum screen framework. Two thousand of the resultant five-foot thick units would have been required.
The region of the building where the panels were to be attached has been plagued with problems. The panels were an aesthetic afterthought added when critics complained that the architectural concept for the base appeared to be a large, concrete or stone bunker. The glass panels were meant to provide a colorful and more inviting entry level. But, a furor ensued when planners announce that the panels and structural assemblies would be made in China. Eventually, WTC officials backtracked (the NYT reports the decision was based on production inabilities, not politics) and gave PPG a contract to make the glass using its Starphire composition (with Chinese companies and others doing the post-glassmaking work).
As far as I know, Glass Magazine, on May 4, was the first media outlet to catch wind of the problems with making the panels.
It appears that the problem with the panels is linked to glass-strenthening issues, a topic covered by many speakers at the Glass & Optical Materials Division meeting that just finished in Savannah, Ga. Several of those in attendance at the GOMD event also turned out to be aware of the 1 WTC project, but not surprised by reports that “the glass panels tended to bow after they were cut and tempered, which interfered with the lamination process. The ridges cut into the glass also proved to be too brittle and broke into large pieces, rather than tiny pellets.”
The experts I spoke with said that it makes sense that that tempering proved to be problematic. Quality tempering on even small pieces can be difficult, and tempering large structural glass pieces requires rare skills. These experts said that when the cutting to create the prism wedges is added as a prior step, consistent tempering would be even more difficult and fraught with quality control problems.
From a technical viewpoint, the tempering difficulties arise because the wedges create distinct differences in panel thickness. When heated to begin tempering, it would be difficult to maintain consistent temperature depth profiles on all of the surfaces. Then, when the quenching temperature is reached and sudden cooling is required — an extremely critical step to attain the strength-adding surface compressive stress — the same temperature management problems are going to be present.
So, at this point, it appears that at least one Chinese manufacturer and PPG failed to figure out a cost-effective way to make the panels without excessive production failures.
Nevertheless, SOM says on its website, “The prismatic glass was developed over the course of four years in conjunction with some of the world’s top glass manufacturers, who worked with the architects to achieve the desired visual qualities and to produce the innovative prismatic glass.” Neither PPG nor SOM, to my knowledge, have issued a response to the latest developments.
Tempering isn’t the only way to strengthen glass. Ion exchange (chemical treatment) would have been another option and it is impossible to imagine that planners didn’t considered it at some point. But ion-exchange techniques tend to have higher costs associated with them, and there aren’t a lot of experts or facilities to treat 13-foot panels. Thus, my guess is that several overly optimistic participants thought they were making the correct business decision to take the tempering route and not the ion-exchange path.
After burning through $10 million and incurring another PR black eye, I am sure that’s a decision they now regret. Pennywise-and-pound-foolish, and all that …