Easy catch: Ion exchange traps radioisotopes in titanate-based nanofibers

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).