Titanate nanotubes and nanofibers for radioactive waste clean-up in water

Radioactive cesium and iodine ions are products of uranium fission and can be easily dissolved in water during an accident at a nuclear reactor like the one in Fukushima earlier this year. The fear is that these fission products could get into the groundwater and could make their way into the food chain.

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. 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.

“Based on our earlier work, we have now demonstrated a potentially cost-effective method to remediate cesium and iodine ions from contaminated water by using the unique chemistry of titanate nanotubes and nanofibers to chemisorb these ions,” HuaiYong Zhu, a professor of chemistry at the Queensland University of Technology, tells Nanowerk.

The team, which reported their findings in the September 20, 2011 online edition of Angewandte Chemie International Edition (“Capture of Radioactive Cesium and Iodide Ions from Water by Using Titanate Nanofibers and Nanotubes”), also found that the new sorbents can not only take up these ions but efficiently trap them for safe disposal because of their unique structural and chemical features.

“The sorbents take up cesium ions via an exchange with sodium ions in the nanostructures; the rapid uptake of cesium ions eventually triggers a phase transition of the titanate and traps the cesium ions inside permanently for safe disposal,” explains Zhu. “This is because the fibers and tubes consist of negatively charged thin layers – as thin as two oxygen atoms – and phase transition of the layers with low rigidity can be readily induced.”

In order to capture and immobilize iodine ions from water, the researchers anchored silver oxide nanocrystals on the external surfaces of titanate nanotubes and nanofibers by chemical bonds owing to their crystallographic similarity. These composites can efficiently capture iodine ions forming silver iodine precipitate on the titanates.

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Water and Air Pollution Removal by Photocatalytic processes

Since now three decades, advanced oxidation technologies “in particular photocatalysis” gained much attention by Scientifics. In their point of view, to counter environmental pollutions, a simple and comprehensive photonic reaction system which converts the photon energy into the chemical energy of a redox system such as photocatalysis, would be helpful for the detoxification processes.

The activity of the group has started on the physical chemistry line, and then more interest has been given to transfer phenomena of matter and light in the view of the design of photocatalytic reactors.

The activity of the team is summarized here after:

Photocatalysis with suspended photocatalyst.

As the photocatalyst is usually found in powder, it can be used in suspension in the water to be treated, which allows a large interface area. Thus work has been done by using 1,2-dichloroethane as a model pollutant. Much attention has been given to the adsorption process. Both the kinetics and the equilibrium have been studied as well as the inhibiting effect of inorganic ions.

Immobilised photocatalyst.

Immobilised catalyst permits to avoid the separation process inherent to the use of suspended catalyst. In this second study, different types of immobilised catalysts have been prepared and studied: either by direct deposition of TiO 2 or by the “sol-gel” method with a precursor.

Light collection.

Most deposits scatter light on the one hand and are more efficient as a thin layer on the other hand. Then, a photocatalytic reactor has to be designed with a 3-dimensional geometry, taking into account the absorption and scattering.

Solar detoxification.

As the photocatalyst can be excited by near UV light, a solar process is feasible. Works has been carried in the laboratory using fluorescent UV lamps emitting at λ 400 nm.

Air treatment.

The work carried out on water treatment is being extended in the field of air treatment (VOCs, odours) using deposited photocatalyst either in tubular or annular reactor. Removal of low-ppm concentrations of acetaldehyde, a common indoor air pollutant, by photocatalysis is investigated in an annular photoreactor. Reactor design is performed with the dispersion model and residence time distribution simulation by CFD. No by-products are detected and complete carbon balance is achieved allowing assumption that removed acetaldehyde is well converted into carbon dioxide and water. Dependence of reaction rate on light intensity is studied; showing a first order tendency in the experimental conditions.

Photocatalytic reactor design.

The purpose of this task is to design more or less complex monolithic supports and coat them with TiO 2 Degussa P25 to be at the maximum of absorption. Several supports with different arrangement in space were first designed by stereolithography.

Since now three decades, advanced oxidation technologies “in particular photocatalysis” gained much attention by Scientifics. In their point of view, to counter environmental pollutions, a simple and comprehensive photonic reaction system which converts the photon energy into the chemical energy of a redox system such as photocatalysis, would be helpful for the detoxification processes.

The activity of the group has started on the physical chemistry line, and then more interest has been given to transfer phenomena of matter and light in the view of the design of photocatalytic reactors.

The activity of the team is summarized here after:

Photocatalysis with suspended photocatalyst.

As the photocatalyst is usually found in powder, it can be used in suspension in the water to be treated, which allows a large interface area. Thus work has been done by using 1,2-dichloroethane as a model pollutant. Much attention has been given to the adsorption process. Both the kinetics and the equilibrium have been studied as well as the inhibiting effect of inorganic ions.

Immobilised photocatalyst.

Immobilised catalyst permits to avoid the separation process inherent to the use of suspended catalyst. In this second study, different types of immobilised catalysts have been prepared and studied: either by direct deposition of TiO 2 or by the “sol-gel” method with a precursor.

Light collection.

Most deposits scatter light on the one hand and are more efficient as a thin layer on the other hand. Then, a photocatalytic reactor has to be designed with a 3-dimensional geometry, taking into account the absorption and scattering.

Solar detoxification.

As the photocatalyst can be excited by near UV light, a solar process is feasible. Works has been carried in the laboratory using fluorescent UV lamps emitting at λ 400 nm.

Air treatment.

The work carried out on water treatment is being extended in the field of air treatment (VOCs, odours) using deposited photocatalyst either in tubular or annular reactor. Removal of low-ppm concentrations of acetaldehyde, a common indoor air pollutant, by photocatalysis is investigated in an annular photoreactor. Reactor design is performed with the dispersion model and residence time distribution simulation by CFD. No by-products are detected and complete carbon balance is achieved allowing assumption that removed acetaldehyde is well converted into carbon dioxide and water. Dependence of reaction rate on light intensity is studied; showing a first order tendency in the experimental conditions.

Photocatalytic reactor design.

The purpose of this task is to design more or less complex monolithic supports and coat them with TiO 2 Degussa P25 to be at the maximum of absorption. Several supports with different arrangement in space were first designed by stereolithography.

Further reading