Zeolites: Opening Doors and Windows to Possibilities

According to Encyclopedia Britannica, zeolites are hydrated aluminosilicates that are characterized by their tetrahedral cage-like framework. These molecules are known for their characteristic ion exchange and adsorption properties. The image below from a 2010 publication of Inorganic Chemistry displays several examples of zeolites to show the microporous structure.1  The specific zeolites displayed are a few that are have current industrial applications. According to the source, the lines of the various structures displayed below are representative of oxygen atoms, and the corners are representative of either aluminum or silicon atoms.

The specific structural characteristics determine the function and reactive behavior of each zeolite.1 One main use of these molecules is for separation. The size of the pores of structure, the size of the substrate, and the differing size of the rest of the mixture all play a role in the desired separation. Molecules that are small enough to enter the zeolite structure do so and react with the zeolite, whereas, any molecule too large is kept outside the structure.
            Besides the aforementioned applications, zeolites are being used for numerous practical applications, ranging from water filters such as for swimming pools to catalyst-preparation for petroleum refining. Students who have taken organic chemistry classes may recall using molecular sieves to extract water content. Molecular sieves are an example of zeolites. NASA  briefly explains the use of zeolites for the refinement of petroleum, adding to the varying list of the uses of zeolites.         A space application of these molecules includes the absorption and separation of CO2 and water for portable life support systems. Other general applications of these molecules that were mentioned in Britannica were synthetic blood clotting for medical uses and filtration for the purpose of controlling pollution.
Over the past years, zeolites have been gaining attention for their abilities to pull metal ions and other toxins from water. As described in a 1999 publication  of the Proceedings of the National Academy of Science (PNAS), zeolites have the ability to lose and gain water, and exchange extra-framework cations without changing their overall structure.2  The pores of these structures have water molecules which form hydration spheres around exchangeable cations. When the water molecules are removed by heating, smaller molecules can enter the pores and interact while large molecules of a mixture are excluded. The weakly-bonded cations of the extra-framework can be easily exchanged by ions of the solution in which the zeolite is placed. The end result of such an interaction, for example, is that the once free metal ions of a filtered medium are no longer reactive ions in solution.
The chemical capabilities of these molecules open up ideas for numerous beneficial applications, one being the handling of nuclear wastes. Nuclear energy has long been known for immense amounts of energy that can be produced in comparison to traditional methods of energy production. Those against nuclear power plants and the use of nuclear energy as a major energy provider for our societies stand on the harmful and lethal effects of the waste produced as their support. Opposition for nuclear energy looks at the dangers of the waste produced, and the current lack of an ideal disposal method thereof.
In a 2011 publication of Journal of Radioanalytical and Nuclear Chemistry, Akbar Malekpour et al cited that cobalt (II) and nickel (II) are two of the heavy-metals that are commonly found in nuclear waste.3 These heavy metals carry significantly harmful effects to both the environment and human health. This research group was successfully able to use clinoptilolite, a naturally occurring zeolite, to remove these target metals from radioactive wastewater. To broaden the horizon, they also used a chemically modified version of the same zeolite to run the tests. Results showed that the synthetic zeolite adsorbed the metal ions more than the naturally occurring form. Similarly, Sharma et al outlines the successful extraction of thallium (IV) and europium (III) using nanocrystalline mordentite (MOR) in their 2011 publication of Journal of Colloid and Interface Science. Just as the focus metal ions of the previous study, thallium (IV) and europium (III) are of metals that are found in the contents of nuclear wastes.4 The article provided research data demonstrating the high success rate of their synthetic MOR zeolite in extracting the metals from solutions. 
Many more scientific papers can be found regarding the successful uses of various zeolites in extraction of metals and numerous other toxins from such mediums as wastewater and nuclear waste. Mumpton mentioned several incidents where zeolites were employed to treat sites where nuclear waste unexpectedly leaked into the environment, Chernobyl being his key example. Various papers were cited that outline the successful use of zeolites in several ways, such as extracting 137Cs from the soil around the location, and reduction of sorption of the toxin by animals that ate effected food through the use of zeolite supplements. These examples show the use of zeolites in the aftermath of nuclear disasters, but, as previously mentioned, they are also effective in handling wastes prior to such disaster. These studies provide promising support that nuclear energy may become a possible option for safe energy production.
The controversy stands in the question of what happens after the harmful materials have been extracted from the nuclear waste. It comes back to the main point against nuclear energy, the radiation. Radiation has detrimental effects on its surrounding environment for vast distance and the living beings within the exposed environment. Though the harmful components can now be successfully extracted, they remain radioactive. So the questions that arise are what can be done with the “saturated” zeolites, and do zeolites truly provide the answer? Several possibilities have been proposed on protocol to deal with the saturated zeolites. Most of these involve simply removing the material as far away as possible from living beings by transforming them into cement, glass, or ceramic that is “stored indefinetly”.2 Though this may seem as just the lesser of two evils, I would still say that zeolites are an answer. Based on the evidence shown, it can be seen that there is still a journey left and questions, such as that of radiation exposure, that need to be answered before nuclear energy production is further pursued, but zeolites do open one of the closed doors.

Cited References

1 Smeets, P.J.; Woertink, J. S; Sels, B. F.; Solomon, E. L.; Schoonheydt, R. A.; “Transition-Metal Ions in Zeolites: Coordination and Activation of Oxygen” Inorganic Chemistry. 2010, 49, 3575-3583.
2 Mumpton, F. A.; “La roca magica: Uses of natural zeolites in agriculture and industry” Proceedings of the National Academy of Science. 1999, 96 (7), 3463-3470.
3 Malekpour, Akbar; Edrisi, Mohammad; Hajialigol, Saeed; Shirzadi, Shamsollah; “Solid phase extraction-inductively coupled plasma spectroscopy for adsorption of Co(II) and Ni(II) from radioactive wastewaters by natural and modified zeolites.” J. Radioanal. Nucl. Chem. 2011, 288, 663-669.
4 Sharma, Pankaj; Tomar, Radha; “Sorption behavior of nanocrystalline MOR type zeolite for Th(IV) and Eu (III) removal from aqueous waste by batch treatment.” J. Colloid Interface Sci. 2011, 362, 144-156.