Zeolites

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Zeolites are important materials for numerous industrial and consumer products. They have been used intensively for several decades. Zeolites are very porous, as their skeletal structure contains innumerable pores and channels ranging in size from nano to micrometers. Their structure resembles a sponge with many (micro or nano) holes, but they are not flexible. They belong to the very few material classes that are considered nanomaterials not only because they form nanoscale particles, but also because if they have nanopores. They are of particular interest as catalysts in chemical processes, as liquids can move freely through the pores and chemical reactions can take place on their walls. They can also be used for filtration: because of the nanopores, even very small substances can be separated, which is then called nanofiltration.

Further areas of application are for example: ion exchange, water softener in detergents, cat litter, fertilizer additive, self-cooling beer keg.

cat stepping into a box with cat litter © Foto-Laupheim / fotolia.com

How could I get in touch with it?

By using zeolites as fertilizer additives, they could enter the food chain. Likewise, swallowing cannot be excluded if the water in swimming ponds is cleaned with the aid of zeolite powder. In addition, zeolite-based dietary supplements are offered time and again, although scientific proof of their effectiveness is lacking. Since it is also used as cat litter, in particular these animals repeatedly come into contact with zeolites, but also the people who clean the litter box.

How dangerous is the material for people and the environment?

Zeolites are poorly soluble in water, i.e. they are excreted unchanged when digested. However, no scientific studies have been conducted to date on the safety of the dietary supplements; these preparations are not approved as drugs in Germany.

So far there is no data available on how zeolites behave in the environment. Here it is generally difficult to differentiate naturally occurring zeolites from the technically produced zeolites. It is currently assumed that around 150 types of zeolite are produced industrially. Only some of them consist of nanoparticles. Since zeolites are used as phosphate substitutes in detergents, it cannot be excluded that the zeolites are released into the environment via the wastewater. Zeolites are converted into other compounds under natural conditions; in addition, zeolites also decompose in wastewater treatment plants. This decomposition releases components that contain aluminum and silicon. These components are also non-toxic.

Conclusion

Commercially available zeolites are not dangerous; they are excreted by the body.

By the way…

Zeolites also have an antiseptic effect due to their mineral structure. This is particularly beneficial for animal health when using cat litter.

Properties and Applications

Structure of ZeolithA ©natros/fotolia.com

Structure of ZeolithA ©natros/fotolia.com

Zeolites were discovered in 1756 by Axel Frederick Cronstedt. They consist of the basic building blocks silicate, aluminate and phosphate. They belong to the group of aluminosilicates. The aluminium, silicon and phosphorus atoms are connected via oxygen atoms in all three spatial directions. The result is a highly porous structure with many pores and channels[1]. Depending on the type of channel system, zeolites are divided into fiber, leaf and cube zeolites. The applications of zeolites are very often based on their ability to absorb other substances inside. Either to release previously introduced substances (fertilizing plants with potassium) or to absorb – in most cases unwanted - substances as in cat litter, where odour molecules are bound.

Due to the micro to nanoscale cavities and channels, zeolites have a large internal surface area, which is why they are important in industry as catalysts in chemical processes. Since the zeolite lattice framework is composed of negatively charged anions, zeolites can absorb positively charged ions. Zeolites are therefore also used as ion exchangers, filters for the separation of chemical substances or as water softeners in detergents (as polyphosphate substitutes) and for flue gas desulphurization. Zeolites are suitable for removing ammonia from soil and water and for purifying radioactively contaminated water. Zeolites serve as filters for dirt in swimming pools and as odour filters in cat litter.

Nanoscale zeolite powders are offered as part of commercial products to keep the water of aquariums and fish ponds clean. In Fukushima, zeolites are used to decontaminate the sea and soil from radioactive contaminants. Due to their low density of 2-2.5 g/cm³ they are used as fillers for plastics, rubber, paper or asphalt. They are also used as additives for adhesives.

In agriculture, zeolites are used as additives to fertilizers to supply the plant with nutrients such as potassium and nitrogen as required.
Zeolites are used in cosmetics to bind moisture or neutralize odors. It is claimed that nano-zeolites as dietary supplements should improve the absorption of minerals in the body or remove toxins such as heavy metals or radicals. However, there are no medical studies available and thus the application of zeolites is discussed very controversially.

Although zeolites have been known for a long time, they have only been used intensively in recent decades, and new applications are increasingly being developed.

Zeolites are nanometer sized powders; they are not self-igniting. Even when igniting a fine dispersed mixture of zeolites with air, there is no possibility of a dust explosion. Zeolites are not flammable.

Occurence and Production

Naturally occurring zeolite in Iceland (Photo: C. Steinbach)

Zeolites are found in nature mainly in volcanic rocks and in the surroundings of hot springs. Of the approximately 60 zeolites occurring on earth as minerals, nine are mined and technically usable. They have different pore sizes and shapes and are loaded with different alkali or alkaline earth ions depending on their area of origin. The water content also varies.

One example of naturally occurring zeolites is the sponge-like faujasite. The industrially produced zeolites X and Y have a comparable structure. Faujasite is not toxic, but is even used in medicine. However, the naturally occurring, rare, fibrous erionite is known to be carcinogenic - for understandable reasons no applications are known in Europe.

Zeolite structures that do not occur naturally are chemically produced. The starting materials used are scaffolding materials such as silica, boric acid or aluminium hydroxide. These reaction mixtures are heated in a furnace until they crystallize. Depending on the composition of the reaction mixture, stirring speed and crystallization temperature, different types of zeolites can be produced. In the manufacturing process, organic molecules are often used as "templates" around which the zeolite framework crystallizes. Usually particles or flakes with dimensions in the range of a few micrometers are produced.

Particles with dimensions of 200 nm - 500 nm are offered as "nano-zeolites". The production of even finer and therefore even more active zeolites is an issue worldwide. Particles with a diameter in the range of 50 nm can already be generated. The Korean KAIST Institute reports on the production of thin flakes with a thickness of a few nm that form larger, very porous particles.


Literature

  1. Osterhoff, C (200). Untersuchung der Kristallinität oberflächennaher Bereiche mikroporöser Materialien mittels NMR-Spektroskopie, Dissertation, Ruhr-University Bochum. (PDF-Document, in German)

At the moment, there are hardly any results on the toxicity of synthetically produced nanozeolites from studies on humans or animals.

General Hazard

In biomedical applications, nanozeolites are being tested for usage in contrast agents for diagnostic imaging (CT, MRI), bone or tissue growth scaffolds, as well as carrier materials for various pharmaceutical products. As drug carriers, the pore size of the zeolite nanoparticles can be optimised for the later application or the active substance to be transported. These applications are currently being intensively researched and tested in cell culture and animal experiments [1-4].

Faujasite, a naturally occurring member of the zeolite family, is classified as non-critical or nontoxic. It is therefore often used in medicine as a carrier material or due to its good biocompatibility and high porosity as wound dressing materials [5,6].

Erionite, another naturally occurring member of the zeolite family, however causes causes astbest-like effects due to its fibrous structure and was therefore classified as carcinogenic in humans by the International Agency for Research on Cancer (IARC) [7,8].

In summary, synthetically produced nanozeolites are being tested as substrates for biomedical applications because of their good biocompatibility


Literature

  1. Georgieva, V et al. (2016). Micropor Mesopor Mat, 232 256-263.
  2. Kaur, B et al. (2015). Colloids Surf B Biointerfaces, 135 201-208.
  3. Costa, R et al. (2013). J Mater Sci Mater Med, 24(2): 395-403.
  4. Vilaca, N et al. (2013). Colloids Surf B Biointerfaces, 112 237-244.
  5. Ninan, N et al. (2014). Colloids Surf B Biointerfaces, 115 244-252.
  6. Ninan, N et al. (2013). ACS Appl Mater Interfaces, 5(21): 11194-11206.

Studies on Living Organisms – in vivo

At the moment, nanozeolites are being tested as potential new materials for medical applications. In a study on mice, the compatibility of nanomaterial-equipped special membranes was tested. These membranes can serve as scaffolding material in the body for bone or tissue growth. This combination of membrane and nanoscale zeolites stimulated cell growth in animal experiments and caused neither inflammation nor toxic effects [1]. However, for future applications in humans, further studies need to be carried out.


Literature

  1. Costa, R et al. (2013). J Mater Sci Mater Med, 24(2): 395-403.
  2. International Agency for Research on Cancer (Iarc); 1987; IARC MONOGRAPHS ON THE EVALUATION OF THE CARCINOGENIC RISK OF CHEMICALS TO HUMANS Silica and Sorne Silicates - VOLUME 42,
  3. International Agency for Research on Cancer (Iarc); 1987; Supplement 7 (1987) Overall Evaluations of Carcinogenicity: An Updating of IARC Monographs Volumes 1 to 42,

Studies Outside of Organisms - in vitro

Currently, there is little known on the effects of nanozeolites on humans. Therefore, different cell lines are used in the laboratory representing the respective uptake pathways to be examined (lung, skin, gastrointestinal tract, see section zeolites - uptake) or target organs (see article zeolites - behaviour inside the body). Only at very high concentrations, pure zeolite nanoparticles can cause toxic reactions in a variety of cell types. (lung, intestine, kidney, cervix, brain, phagocytes). Since nanozeolites tend to agglomerate in biological fluids, the observed toxic effects at high concentrations are likely to be due to side effects and not to the particles themselves [1-9].

In addition to concentration, size, surface type (with and without coating) as well as shape (round, cube-shaped) of the nanozeolites influence the toxicity of these nanoparticles. One study in lung cells showed the formation of reactive oxygen species and a change in the metabolic activity of the cells after the administration of high concentrations of zeolite nanoparticles. Damage to the genome also occurred, but was repaired in the case of the larger particles, while the effects remained permanent with the smaller particles [3,4].

The different studies show that pure zeolite nanoparticles negatively affect cell health only at very high concentrations. Due to their porous surface structure and filter function, they are currently being tested for use as scaffold material for biomedical applications. Comparative studies in animal experiments are few, which demonstrates a great need for further research.


Literature

  1. Thomassen, LC et al. (2012), Nanotoxicology, 6(5): 472-485.
  2. Petushkov, A et al. (2009), Chem Res Toxicol, 22(7): 1359-1368.
  3. Kihara, T et al. (2011), J Biosci Bioeng, 111(6): 725-730.
  4. Bhattacharya, K et al. (2012). Toxicol Lett, 215(3): 151-160.
  5. Li, Z et al. (2013). Small, 9(9-10): 1809-1820.
  6. Vilaca, N et al. (2013). Colloids Surf B Biointerfaces, 112 237-244.
  7. Laurent, S et al. (2013). Toxicology Research, 2(4): 270-279.
  8. Anfray, C et al. (2017). ACS Appl Mater Interfaces, 9(16): 13849-13854.
  9. Georgieva, V et al. (2016). Micropor Mesopor Mat 232 256-263.
Water purification filter

Water purification filter © ReginaQ / fotolia.com

Up-to-now there are no data on environmental exposure with nanozeolites. Zeolites occur naturally, but are also produced synthetically. Therefore, it is difficult to differentiate between naturally occurring zeolites and engineered zeolites.

For the use of zeolites in filter materials (e.g. for removal of drug residues or contaminants from wastewater), the release is expected to be very low, as zeolites are firmly embedded in the filter membranes.

The asbestos-like zeolite erionite occurs naturally in various regions of the world. Through destruction (natural weathering or man-caused) or on the application as a floor covering (for example in gravel pebbles) erionite can become airborne and released into the environment [1-3].


Literature

  1. Fruijtier-Polloth, C (2009), Arch Toxicol, 83(1): 23-35.
  2. Carbone, M et al. (2011), Proc Natl Acad Sci U S A, 108(33): 13618-13623.
  3. Ortega-Guerrero, MA et al. (2014), Environ Geochem Health, 36(3): 517-529.

Little information is available on the uptake of zeolites. Nanozeolites have been tested in the laboratory as carrier materials for cancer drugs.

Uptake via the Lung – Inhalation

In cell culture experiments with A549 lung epithelial cells, zeolite nanoparticles neither affected cellular growth nor triggered toxic effects. Only very high concentrations of nanozeolites were able to cause the formation of reactive oxygen species as well as alter the metabolic activity of the cells [1].

Literature

  1. Bhattacharya, K et al. (2012), Toxicol Lett, 215(3): 151-160.

 

Uptake via the Gastro-Intestinal Tract

The zeolite nanoparticles themselves were taken up by the investigated colon cancer cell lines, but caused no negative toxic effects. The pore size of the nanozeolites significantly influenced the loading rate, release and efficiency of the tested cancer drugs. For optimal results, the pore size of the zeolite nanoparticles should therefore be individually adjusted to the molecular size of the active ingredients [1].


Literature

  1. Vilaca, N et al. (2013), Colloids Surf B Biointerfaces, 112 237-244.
Amoeba

Amoeba ©Wire_man / fotolia.com

There are no ecotoxicological data on nanoscaled zeolites available. For zeolites, it is generally believed that they are insoluble, and upon internalisation into the body, they are excreted unchanged.

Upon disruption of zeolites, their basic building blocks aluminium and silicon are released, which are considered nontoxic. [1].

Due to the filter, effect toxic substances can be bound by zeolites. This effect is used to remove toxic cadmium from the water or make toxic ammonium less available for crustaceans. On the other side, zeolite particles can also slow down the degradation of pollutants by bacteria. Zeolites inhibit the growth of fungi and microorganisms, amoebas are not affected [2-6].


Literature

  1. Fruijtier-Polloth, C (2009), Arch Toxicol, 83(1): 23-35.
  2. Ghiasi, F et al. (2011), Asian J Anim Vet Adv, 6(6): 636-641.
  3. Burgess, RM et al. (2004), Arch Environ Contam Toxicol, 47(4): 440-447.
  4. Hrenovic, J et al. (2010), J Hazard Mater, 183(1-3): 655-663.
  5. Bautista-Toledo, MI et al. (2015), J Environ Manage, 156 81-88.
  6. Toledano-Magana, Y et al. (2015), Biomed Res Int, 2015 164980.

So far, there are only a few studies on the possible behaviour of synthetically produced nanozeolites in the body.

Behaviour inside the Body

Biomedical applications of nanozeolites as scaffolding material are a current research focus. A study on mice has investigated the suitability of nanomaterial-equipped membranes as scaffolding structures for bone or tissue growth. In animal experiments, the combination of membrane and zeolite nanoparticles stimulated cell growth as desired without causing toxic side effects or inflammatory reactions. [1]

 

Behaviour at the Blood-Brain Barrier

There are no data availabel whether nanozeolithes are able to cross the blood-brain barrier. Nanozeolites caused no toxic effects in laboratory experiments with different cell types of the brain (astrocytes, neurons).[2,3]

However, long-term effects as well as the effects on the whole body in particular for use as a substrate for drugs in biomedical applications have to be further investigated.


Literature

  1. Costa, R et al. (2013), J Mater Sci Mater Med, 24(2): 395-403.
  2. Georgieva, V et al. (2016). Micropor Mesopor Mat 232 256-263.
  3. Anfray, C et al. (2017), ACS Appl Mater Interfaces, 9(16): 13849-13854.

 

Behaviour of uptake in somatic cells

Investigations in the laboratory have shown that, depending on the surface charge of the zeolite nanoparticles and the studied cell line (e.g. intestine, lung, phagocytes), the nanomaterials are taken up into the cells by different routes. In HeLa cells, modified nanozeolites enter the cells via active processes, depending on their surface charge and agglomeration state. Positively charged and specially coated zeolite nanoparticles show a high uptake rate in contrast to uncoated or negatively charged nanozeolites. However, the exact uptake mechanisms for the individual zeolite classes are still unclear [1-4].

Due to their porous structure, zeolites can also act as filters in biological fluids. On top of the particle surface a layer of proteins and other constituents of the surrounding fluid form depending on the nature of the particle surface. This process not only influences the agglomeration behaviour of the particles, but also their availability and subsequent uptake into the cells. [5].

The uptake of zeolite nanoparticles in cells depends strongly on the cell type, the surface charge and the respective protein or lipid layer on the particle surface.


Literature

  1. Thomassen, LC et al. (2012), Nanotoxicology, 6(5): 472-485.
  2. Vilaca, N et al. (2013), Colloids Surf B Biointerfaces, 112 237-244.
  3. Bhattacharya, K et al. (2012), Toxicol Lett, 215(3): 151-160.
  4. Li, Z et al. (2013), Small, 9(9-10): 1809-1820.
  5. Laurent, S et al. (2013), Toxicology Research, 2(4): 270-279.

As an older work on zeolites shows, they are converted into other compounds under natural conditions. There is also a degradation of zeolites in wastewater treatment plants [1,2].


Literature

  1. Cook, TE et al. (1982), Environ Sci Technol, 16(6): 344-350.
  2. Fruijtier-Polloth, C (2009), Arch Toxicol, 83(1): 23-35.

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