Uranium mining and nuclear power generation often incur leakage of radioactive ions into the environment thus posing a threat to health. Now, as the demand for nuclear fuel continues to increase, scientists are searching for materials that can absorb radioactive pollution before it causes harm. Huai Yong Zhu at Queensland University of Technology in Brisbane and co-workers1 have found a promising absorbent material in the form of ‘nanofibres’ made from titanate.

In order to remove radioactive ions from water, materials must exchange positively charged ions, or cations. Natural clays and zeolites are often used for this purpose, but recently, researchers have started making more efficient synthetic absorbents.

“The synthetic exchangers are far superior to the natural materials for selective removal of the radioactive cations,” says Zhu. Zhu and co-workers mixed sodium hydroxide with a titanium compound to produce their new titanate nanofibres. The fibres had a layered structure formed by negatively-charged zigzag chains of titanium oxide, with exchangeable, positively-charged sodium ions sandwiched between the layers.

Fig. 1: Structure of titanate nanofibres made for removing radioactive ions from water. Heating and water treatment lead to a tightly packed layer structure (top right) with sodium ions (red dots) in between. When the nanofibres adsorb the radioactive pollutant ions (black dots in bottom image), some sodium ions are released and the fibre structure collapses, trapping the radioactive ions inside.Copyright © Huai Yong Zhu 2008

The absorption properties of the nanofibres were tested by suspending them with ions of strontium and barium—non-toxic ions that behave similarly to radioactive ions such as radium. The tests showed the fibres to have a high capacity for absorbing the pollutant ions by exchanging them with sodium ions, but not as high as was predicted by theory. This was because when the concentration of ions in the fibres reached a certain saturation point, the crystal structure of the fibres changed dramatically. The spacing between the layers decreased and the layer shape was deformed, so that the pollutant ions could not escape from the nanofibres (Fig. 1). The fibres were then easily removed and buried.

“The structure collapse can lock the ions into the fibres,” says Zhu. “Furthermore, the titanate materials are stable against radiation, chemicals and thermal changes, and so have potential as adsorbents for radioactive ions.”

The nanofibres can be made cheaply from abundant raw materials. Zhu and co-workers believe that many other layered nanoparticles can act as ‘intelligent’ absorbents in this way, and they plan to test a number of similar materials soon.