Cages for nuclear waste |

Background

Meeting the growing energy demands of today’s world is a pressing global issue. With the soaring prices of oil and gas, dwindling fossil fuel reserves and pressure to tackle climate change, the government renewed its support for nuclear power in 2013. The UK government remains committed to nuclear power as an important part of the energy generation capacity over the next 30-50 years.

Nuclear power provides a virtually carbon-free way of producing a large quantity of electrical power; in a nuclear power generator, energy is released from the nucleus, usually of ²³⁵U, during the process of nuclear fission, which produces a vast amount of heat energy. This heat energy is then used to heat water, and produce steam, which turns a turbine to create electrical energy. The fission process is a chain reaction that produces radioactive waste products.

The need to safely manage and dispose of high- and intermediate-level wastes becomes more and more important, as we are facing an increase in our dependency on nuclear fuel. Researchers at the University of Birmingham carry out research in nuclear waste management and site decommissioning, alongside industrial partners and researchers from other Universities in the DISTINCTIVE consortium, funded by the EPSRC. To find out more about this work, click here.

Currently, there is a UK-Japan collaboration involving researchers from the University of Birmingham, including Dr Joe Hriljac from the School of Chemistry, helping with the Fukushima clean-up. The Fukushima Daiichi Nuclear Power Station was heavily damaged by the tsunami, following an off-coast earthquake in 2011. Following work to cool the reactors with seawater, water in the harbour and groundwater was contaminated with radioactive elements. Decontamination of this water is the focus of this research collaboration.

Zeolites are a type of crystalline solid material that occur both naturally, and can be made synthetically by a chemist in a lab. Zeolites are made up of SiO₄ and AlO₄ tetrahedra, which are connected together via their oxygen atoms. These link into a three-dimensional crystalline structure with molecule-sized tunnels and cavities, to build a cage-like structure:

tetrahedra_zeolite — The building blocks of 3D zeolite structures

Zeolites behave as cage structures on a tiny scale, which can be filled with ions. They have the empirical formula shown below; the cage framework, with an overall negative charge, is made out of the atoms shown in blue, and the cations (positively charged atoms) that fill the holes in the cage, are shown in red:

[(SiO₂)(AlO₂)_x]M_x/n ⁿ⁺. wH₂O

Zeolites are microporous, which means that they contain pores that are less than 2 nm in diameter). This means that they can adsorb molecules and ions that are smaller than the size of the pore openings.

ZeoliteA — The structure of Zeolite A (LTA structure), shown with empty cavities (left) and with sodium ions within the pore system (right), shown in red. Crystal structure downloaded from the IZA database.

If you wish to look at this structure go to the IZA database, following this link: in the ‘3D Drawing’ tab, click on the ‘LTA framework’ option to view to the cage structure. If you choose the ‘Si and O, wireframe’ option (in ‘Display Options), you can see the cage; each atom labelled as ‘Si’ in the structure is actually either Si or Al. You can rotate the structure and zoom in and out:

The ions that sit within the cavities of the structure are mobile, and can be exchanged quite readily:

ionexchange — zeolite cage structure (left), with mobile ions shown (second from left), and ion exchange (right)

You may have come across zeolites before; they are found in many washing powders in order to treat hard water, and are also used in water filter systems, to soften water, such as in dishwashers. Hard water contains Mg²⁺ and/or Ca²⁺ ions. These ions react with soap to form ‘scum’. In order to prevent the formation of ‘scum’ and allow the soap to lather, many washing powders contain zeolites to exchange the Mg²⁺ and Ca²⁺ ions with Na⁺ions, which do not prevent the soap from lathering. Hard water also produces solid deposits of calcium and magnesium salts, called ‘scale’ and can clog up pipes.

ionexchange2 — ‘Softening’ water with zeolites, to prevent ‘scum’ formation with soap

Zeolites are found in pet litter, in order to control odour; the porous crystalline structure of the zeolites helps by trapping unwanted liquids and odour molecules. Zeolites are also used as catalysts, for example in the ‘cracking’ process of alkenes. Again, it’s the porous structure of zeolites that proves important. The many pores in a zeolite’s open structure are like millions of tiny test tubes where atoms and molecules become trapped and chemical reactions readily take place. Since the pores in a particular zeolite are of a fixed size and shape, zeolite catalysts can work selectively on certain molecules.

Zeolites and other cage-like materials can be used to trap all sorts of ions, including radioactive ions; researchers at the University of Birmingham study the way in which zeolites and other porous crystalline structures can exchange ions with radioactive ions, in order to clean up nuclear waste.

The experiment

A pdf version of this experiment is available here. Please ensure you refer to the safety card. Details for teachers or technicians can be found here.

AIM

To observe ions exchanging from solution into the zeolite cavities of molecular sieves

YOU WILL NEED:

glass conical flasks
2% w/v copper(II) sulfate (aq) solution
0.1% w/v citric acid (aq) solution
universal indicator
Zeolite A powder (4 Å molecular sieve powder)
measuring cylinder
stirring rod
plastic Pasteur pipette
test tubes / vials
spatula
filter paper
weighing boats

PROCEDURE:

Part 1: Ion exchange of Cu²⁺ ions

Measure out 50 mL of copper sulfate solution into two glass conical flasks, labelled A and B. Keep flask A as a control solution. Add 1 g of zeolite powder to solution B, and swirl the solution, continually for 5 min. Leave the solution to settle and move on to part 2.

Part 2: Ion exchange of H⁺ ions

Measure out 100 mL of citric acid solution into a beaker, and add 0.5 mL of universal indicator into the solution. Stir the solution, and note the colour and pH of this solution. Divide the solution into two glass conical flasks, labelled C and D. Keep flask C as a control solution, and add 1 g of zeolite powder to the solution, and swirl for 1 min. Note the colour change that you observe. Leave the solution to settle for 5 min.

Observations:

Without disturbing the settled solutions (B and D) note what you observe in each case. Carefully remove 2 mL of solution A and solution B (without disturbing the solid at the bottom) into separate test tubes / vials, and compare the colours. Finally, scoop out some of the solid from the bottom of solution B, onto some filter paper, and observe its colour.

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Questions:

Look at solutions A and B. When copper(II) ions are present in water, they have a blue colour. Compare the colour of the solutions taken from A and B – what do you notice?
Looking at the zeolite that has settled in solution B, what colour is this? What does this tell you about where the copper ions have gone from the solution?
What has replaced the Cu²⁺ ions from the zeolite, in this ion exchange experiment?
Looking now at solutions C and D – what is the pH of solution C? What is the pH of solution D? pH is a measure of H⁺ concentration, as the pH gets lower, the concentration of H⁺ increases. Which solution contains more H⁺?
Can you explain what has happened to the solution in terms of ion exchange with the zeolite?
We have zeolites in the water filters in our dishwashers to ‘soften water’. As the hard water runs through the filter, Ca²⁺ and Mg ²⁺ ions exchange with Na⁺ ions in the filter. Why do you think we have to add dishwasher salt (NaCl) to the filter every so often?
How do you think zeolites could be used to clean up nuclear waste in contaminated water? Once the zeolites have swapped all their Na⁺ ions with radioactive ions, what do you think is the main problem that we need to overcome?

In the research lab

Researchers at the University of Birmingham use zeolite-type structures of porous materials to exchange ions with radioactive ions that are found in nuclear waste. Researchers, including Dr Tzu-Yu Chen, in the group of Dr Joe Hriljac, have recently looked at the ion-exchange of radioactive isotopes of cesium using cage-type crystalline structures, similar to zeolites. Radioactive cesium is produced during the uranium fission in nuclear reactors, and is also the main medium-term health risk remaining from the Fukushima accident.

Ion exchange is performed by passing contaminated ground water, containing radioactive ions, through columns containing porous zeolite-type structures. Once used, and full of radioactive ions, these columns are then classified as high-level radioative waste, and need to be safely stored; the ions are mobile and the exchange is reversible – so once trapped inside the cage-structures – how do we stop them leaching out? Researchers in Dr Joe Hriljac’s lab investigate how the chemical structure of the cage-like materials can be changed using heat and pressure, to lock the ions in place permanently, using a process called ‘hot isostatic pressing’:

isostatic pressing
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