Cages to trap metals


Most of the elements in the periodic table are metals. Metals have fascinating properties and are essential for our buildings, technological devices and even in healthcare. Some metals are essential for our bodies to function properly, e.g. iron in haemoglobin to transport oxygen around our body.

image of some metal engine parts

Exercise 1: Choose a metallic element from the periodic table. Try and pick something you are unfamiliar with. Look it up on the internet and find out some of the applications it is used in.

However, if certain metals get into our water supply they could cause problems. Some metals are toxic and can cause damage to our bodies.

image of water running through hands

Metals can enter waterways in many different ways. Some leach (dissolve) into ground water from the minerals that exist in the Earth’s crust. An example of this is arsenic, which is present in the groundwater in many countries and has caused many illnesses and deaths. Metals can also enter waterways from industry or from consumers. This could be through lack of regulation, poor implementation of regulation or from accidents. For example, the Fukushima Daiichi Nuclear Power Station in Japan was damaged by the 2011 earthquake and tsunami. Following work to cool the reactors with seawater, water in the harbour and groundwater was contaminated with radioactive elements.

One way to remove metals from water is to use adsorbents. These can include activated carbons or zeolites

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 silicon and aluminium atoms which are connected together via their oxygen atoms. These link into a three-dimensional structure with molecule-sized tunnels and cavities, to build a cage-like structure:

The building blocks of 3D zeolite structures

Zeolites behave as cage structures on a tiny scale, which can be filled with ions. They are microporous, which means that they contain pores that are less than 2 nm in diameter. This means atoms and molecules can ‘stick’ to the surface inside the pores (they can adsorb them). The cage has an overall negative charge, with positively charged atoms and molecules filling the holes.

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 (with 3D model here).

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

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

Zeolites 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’ and can produce solid deposits of calcium and magnesium salts (‘scale’) which can clog pipes. 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.

‘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 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.

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The experiment


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


  • 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


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.


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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  1. 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?
  2. 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?
  3. What has replaced the Cu2+ ions from the zeolite, in this ion exchange experiment?
  4. 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+?
  5. Can you explain what has happened to the solution in terms of ion exchange with the zeolite?
  6. 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?
  7. 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?

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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 radioactive 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

To find out more, you can read this open-access paper by clicking here.

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