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.
