Silicon Dioxide

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Silicon dioxide is the main component of beach sand and is commonly known as quartz in its pure crystalline form. For industrial purposes the amorphous (non-crystalline form) silicon dioxide or silica is of greater importance. Amorphous silica can be found in a range of products including varnishes, glues and paints. It is also used in foods and in dietary supplements sold in drug stores and pharmacies.

How can I come into contact with this material?

Dietary uptake of small amounts of silicon is important for the human body as silicon is one of the ultra-trace elements. Amorphous silicon dioxide is also being used as a food additive (labelled E551) or as a component in medicinal clay whereby it could enter the human body via the gastro-intestinal tract. Dermal uptake of silicon dioxide particles derived from contact with paints, inks or adhesives is highly unlikely. Quartz, the crystalline form of silicon dioxide, can be inhaled as fine dust that is released from underground mining activities when digging for ore, coal and other minerals. Since the overall amount of silica in the environment is very high it is difficult to distinguish between naturally occurring silica and industrially generated silicon dioxide.

 

Is there any risk from this material to humans and the environment?

Nanoscaled silicon dioxide occurs almost exclusively in its unstructured amorphous form which so far hasn’t shown any negative characteristics in all performed experiments from animal to environmental studies. Silicon is an essential ultra-trace element for the human body and silicon dioxide in its amorphous form is considered to be non-hazardous. On the other hand the crystalline version of silicon dioxide is known to be harmful to humans: those who are permanently exposed to quartz dust, e.g. at their work place below ground, carry a high risk for chronic lung diseases (e.g. silicosis) and other pathological changes in the lung.

 

Conclusion

Humans come into contact with silicon dioxide particles on a daily basis: at the beach, in food products, like paints, glues and many more and this also includes silicon dioxide nanoparticles. The amorphous form of silicon dioxide is considered to be non-hazardous whereas the crystalline form has been shown to cause severe lung toxicity both in animals and humans.

 

By the way…

  • Silicon dioxide is a constituent of many food supplements.
  • Some plants and animals store silica internally in order to make themselves harder.
  • Without silicon, there would be no computer chips or solar panels on the roof.

 

Silicon dioxide (SiO2) is the most common silicon compound and a major constituent of the Earth’s crust.

Properties and Applications

Silicon dioxide is a very hard substance that is resistant against chemicals and alteration. Both crystalline and amorphous SiO2 are nearly insoluble in water and in acids. In aqueous suspensions, however, the very fine-grained forms of the amorphous type slowly transform into silicic acid SiO2 x n H2O. At 25 °C and a pH-value of 7 (neutral), approximately 0,12g SiO2 per liter water (120ppm) dissolve that way [6,7]. The dissolution rate of amorphous SiO2 is about 10 times higher than that of quartz. Particularly amorphous SiO2 can be dissolved by aqueous alkaline substances. Being resistant against other acids, SiO2 corrodes when exposed to hydrofluoric acid.

Natural silicon dioxide is an important base material for the glass industry, the optical industry and building industry. Quartz glasses provide the basis for manufacturing lenses and other optical components as well as temperature-resistant equipment for the chemical industry. Different kinds of SiO2 are used for manufacturing concrete and other building materials. In addition, SiO2 are used as filters and desiccants.

Synthetic amorphous SiO2 are used as fillers in plastics, rubber, dyes, and adhesives and serve as adsorbents or trickling agents. They improve the hardness and scratch resistance of surface coatings and varnishes. Although SiO2 has a lower hardness than aluminum oxide, which is used alternatively, clear varnishes that contain nanostructured SiO2 have a much better transparency.

Nano-SiO2 is used increasingly for tire manufacturing. Adding amorphous SiO2 as fillers in addition to Carbon Black, the tire roll resistance is reduced, and gasoline consumption decreases by up to five percent. Since CO2 emissions are reduced that way, this is not only easy on the wallet but also on the environment [5].

Amorphous silicon dioxides have been used for more than four decades as food additives (E551). They can be added to certain powdery foods such as table salt, seasonings, dietary supplements, and dry foods [3] to avoid clogging. Moreover, they are permitted for use as carrier substances in emulsifying agents, colorings, and flavors [9]. According to the EU Rules on Organic Farming[11], SiO2 additives are also approved for use in biological food. Since silicon dioxide can neither be absorbed nor salvaged by the human organism, it is excreted in its unchanged form. Amorphous SiO2 particles have been approved for use as food additives since they were first tested more than 40 years ago. Since the particle size and structure have remained unchanged, these substances are not considered products of modern nanotechnology [4].

Highly disperse (nanoscale) amorphous SiO2 are also contained in diverse pharmaceutical products such as tablets, suppositories, gels, and creams. The properties of the approved additives are laid down in the European Pharmacopoeia [10].

Moreover, amorphous silicon dioxide nanoparticles are used as water repellents for cotton in the textile industry and as abrasives in the electronics industry[8].

Silicon dioxid is not self-inflammable as nanometer-sized powder. Also as a mixture with air (dust) under the influence of an ignition source, it is not inflammable, so there is no possibility of a dust explosion.

Occurrence and Production

Image of an Opal. © K. Luginsland, TECHNOSEUM Mannheim.

It occurs in nature in the crystalline (mostly quartz) and amorphous forms. Being a major constituent of sand, it is found in numerous types of rock and occurs, in addition, in precious stones and gemstones such as rock crystal.

The so-called amorphous non-crystalline silicon dioxides may be of biological origin or are formed by nature whenever rock is subjected to high temperatures (volcanoes, meteorite impacts, lightning strokes, geysers). Opals, which are very popular due to their ”opalescent“ colors, are an amorphous form of SiO2.

High quantities of amorphous SiO2 are produced at a large scale through precipitation or in oxyhydrogen flames. The latter product is often referred to as pyrogenic SiO2 or pyrogenic silicic acid. Pyrogenic SiO2 occurs as powder that consists of primary particles sized 5-50nm and forms solid aggregates above 100 nm (150-200nm). The powders are characterized by high specific surface areas (above 50m²/g).

Literature

  1. Roempp Online (DE): Silizium (last access date: Jun 2010).
  2. Wikipedia (EN): Silicon Dioxide (last access date: Jun 2010).
  3. Zusatzstoffe-online.de:Siliziumdioxid(last access date: Jun 2010).
  4. NanoTrust Dossier No.004en (May 2008). Nanoparticles and nanostructured materials in the food industry, NanoTrust,Institute of Technology Assessment (ITA), Vienna.
  5. Hessen-Nanotech NEWS 4/2006. Nano-Produktion – Herstellung von und mit Nanotechnologie, Band 9, 01.09.2006.
  6. Amjad, Z (1998). Water soluble polymers: solution properties and applications, Kluwer Academic Publishers, New York, ISBN 0-306-45931-0.
  7. Iler, RK (1979). The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica, John Wiley Sons,ISBN 978-0471024040.
  8. Som, C et al. (Mar 2010). Nanomaterialien in Textilien: Umwelt-, Gesundheits- und Sicherheits-Aspekte, Fokus: synthetische Nanopartikel. Empa und TVS Textilverband Schweiz, St. Gallen 2010. (in German).
  9. Zusatzstoff-Zulassungsverordnung (ZZulV) (1998). Verordnung über die Zulassung von Zusatzstoffen zu Lebensmitteln zu technologischen Zwecken.gesetze-im-internet.de (last access date: Mar 2010). (in German)
  10. Europäisches Arzneibuch (Pharmacopoea Europaea) (2008), 6. Ausgabe, Grundwerk, Deutscher Apotherker Verlag Stuttgart. ISBN 978-3769253832. (in German).
  11. European Council Regulation (EC) No 834/2007 (28.06.2007). On organic production and labelling of organic products and repealing Regulation (EEC) No 2092/91.

Humans are constantly in contact with amorphous silica, as it is contained in many foodstuffs as a filler and flow aid and is approved as a food additive. In very high doses, silicon dioxide triggers inflammatory reactions, whereas treatment with low doses does not cause toxicity.

General Risks - Epidemiology

Being non-toxic, the amorphous type is contained in cosmetics and pharmaceuticals. Moreover, it is used in food processing (for example for clarification of beer) and is added to tooth pastes as abrasive agent. It is due to clogging that the nanoscale of the amorphous particles becomes irrelevant.

In contrast, inhalation of the (microscale) crystalline type, i.e. quartz, causes silicosis (also referred to as quartz pneumoconiosis or grinder’s disease). Silicosis is a pathological alteration of the lung which is caused by long-time inhalation of quartz dust particles. Being in the group of pneumoconioses, it is considered an occupational disease that is subject to compensation. Suitable protective equipment must be worn therefore when working with quartz dust. Download-Link for Merck Material Safety Data Sheet "Quartz", date 25.08.2006. (PDF-Document, 23 KB).

Studies on Living Organisms – in vivo

Studies on laboratory rats of different ages inhaling silicon dioxide particles revealed that in spite of identical treatment, old rats reacted more sensitively than the young or adult ones. The SiO2 particles were observed to affect the lungs and the hearts of the rats [1].

In another study dedicated to a direct comparison between crystalline and amorphous silicon dioxides, rats were treated with the substances to analyze the inflammatory reactions in the lung as well as other effects (e.g. genotoxicity ). It is beyond dispute that the crystalline type (quartz) triggers severe inflammation which does not heal and has serious consequences. High doses of amorphous SiO2, on the other hand, were found to trigger short-time inflammation without any further effects after healing [2]. These results were proved by further studies which attribute the effects of the amorphous type to crystalline impurities.

As for the in vitro studies, dosages in vivo are decisive if one takes into account that very high doses may cause an overload effect that can trigger, for example, fibroses [3].

Literature

  1. Chen, Z et al. (2008), Environ Sci Technol, 42(23): 8985-8992.
  2. Johnston, CJ et al. (2000), Toxicol Sci, 56(2): 405-413.
  3. Nishimori, H et al. (2009), Eur J Pharm Biopharm, 72(3): 626-629.

Studies Outside the Organism - in vitro

Silicon dioxides (SiO2) may be of the amorphous or crystalline type. Amorphous SiO2 are mostly contained in consumer products. While synthesized particles are also mostly of the amorphous kind, nanoscale crystalline SiO2 is obtained by grinding coarse quartz. According to the present state of knowledge, amorphous SiO2 nanoparticles are generally rather considered harmless [2].

However, different studies have observed crystalline SiO2 to exert obvious effects and to cause damage, for example, to the DNA of cells. In cell culture, various cell types exhibit cell-toxic reactions (for example reduced cell health, defective cell membranes, or even cell death) after administration of very high, unrealistic doses of amorphous SiO2 particles. The higher the dose and the smaller the particles, the stronger the effect.

Further studies prove that relevant doses of silicon dioxide exert no significant impacts and are not toxic [7,8]. A relevant dose describes a concentration/dose which may as well be reached in the living organism and is thus not absolutely improbable. Inflammatory markers were detected only upon application of extremely high particle doses. The particles were observed to accumulate in vesicles of the cells but did not cause any other structural changes in the cells [8].

In addition to simple culture systems with only one cell line, complex so-called co-culture systems are used to better represent in vivo situations in the body through simulation of the interaction of the cells. With the co-cultures reacting more sensitively to SiO2 than the monocultures it is evident that communication between different cells may increase the respective effects [9, 10]. Low doses of SiO2 particles were found not to be toxic. This positive effect is exploited for in vivo gene transfer studies using SiO2 as transporters for introduction of genes into e.g., the lungs of mice [11].

Literature

  1. Chang, JS et al. (2007), Environ Sci Technol, 41(6): 2064-2068.
  2. Som, C et al. (Mar 2010). Nanomaterialien in Textilien: Umwelt-, Gesundheits- und Sicherheits-Aspekte, Fokus: synthetische Nanopartikel. Empa und TVS Textilverband Schweiz, St. Gallen 2010. (in German).
  3. Ye, Y et al. (2010), Toxicol In Vitro, 24(3): 751-758.
  4. Yang, H et al. (2008), Journal of Southeast University (Natural Science Edition), 2008-06.
  5. Yang, H et al. (2009), J Biomed Nanotechnol, 5(5): 528-535.
  6. Yang, H et al. (2010), J Nanosci Nanotechnol, 10(1): 561-568.
  7. Brunner, TJ et al. (2006), Environ Sci Technol, 40(14): 4374-4381.
  8. Peters, K et al. (2004), J Mater Sci Mater Med, 15(4): 321-325.
  9. Wottrich, R et al. (2004), Int J Hyg Environ Health, 207(4): 353-361.
  10. Mueller, L et al. (2010), J R Soc Interface, 7 Suppl 1(Suppl 1): S27-40.
  11. Kumar, MNVR et al. (2004), J Nanosci Nanotechnol, 4(7): 876-881.

Silicon dioxide is a naturally occurring compound and crystalline silicon dioxide is the major constituent of sand. In case of environmental exposure, it is therefore difficult to distinguish between naturally occurring and technically produced nanoforms of silicon dioxide.

SandDue to the broad spectrum of applications and production volumes, an environmental exposure to silicon dioxide nanoparticle is very probable. However, to date, there only exist predicted environmental concentrations (PEC) of technically produced silicon dioxide nanoparticles (see cross cutting topic - Estimating the occurrence of nanomaterials in the environment).

Based on those theoretical models, industrially produced silica nanoparticles in Europe are most likely to be found in sediments, less in soils, air and water. The predicted concentrations are, however, far below those predicted to be harmful for environmental organisms [1].

Literature

  1. Wang, Y et al. (2016), Environ Sci Technol, 545-546: 67-76.

Amorphous silica can be absorbed via the lung by inhalation or with food via the gastrointestinal tract.

Uptake via the Gastro-Intestinal Tract

In vitro studies on stomach and intestinal cells show that the health of isolated cells can only be damaged by ultrahigh particle concentrations [1]. Amorphous silicon dioxide is excreted undigested because of its poor solubility.

 

Literature

  1. Chang, JS et al. (2007), Environ Sci Technol 41(6): 2064-2068.

Uptake via the Skin – Dermal Uptake

In vitro studies on skin cells have shown that the health of cells is the worse the smaller the particles and the higher the dosage administered. With no knowledge available so far on the interaction between particles and cells, possible effects on the skin remain to be investigated [1].

Literaturearrow down

  1. Yang, X et al. (2010), Part Fibre Toxicol, 7(1)

Uptake via the Lung – Inhalation

Inhalation of crystalline silicon dioxide causes silicosis which is also referred to as quartz pneumoconiosis or grinder’s disease. Silicosis is a pathological alteration of the lung caused by long-time inhalation of quartz dust particles. According to the International Agency for Research on Cancer (IARC) respirable quartz dust is considered carcinogenic [1]. Suitable protective equipment must be worn therefore when working with quartz dust. Download-Link for Merck Material Safety Data Sheet "Quartz", date 25.08.2006. (PDF-Document, 23 KB).

In vivo studies of mice that were injected with silicon dioxide have shown that 70 nm of amorphous SiO2 particles put a strain on the liver while leaving the lung unaffected.

In another study dedicated to a direct comparison between crystalline and amorphous silicon dioxides, rats were made to inhale both crystalline and amorphous SiO2 over a three-month period to analyze inflammatory reactions in the lung. While the crystalline type was observed to cause severe inflammation which did not heal, high doses of the amorphous type triggered short-time inflammation without any other effects after healing [2].

 

Literature

  1. International Agency for Research on Cancer (IARC) (1997). IARC Monograph on the Evaluation of carcinogenic risks to humans, No.68: Silica.
  2. Johnston, CJ et al. (2000), Toxicol Sci, 56(2): 405-413.

Silicon dioxide occurs in two different crystal structures, amorphous and crystalline. The structure has to be considered, as it may influence the effect on environmental organisms. Most results are available for amorphous silicon dioxide nanoparticles. These nanoparticles were only harmful to environmental organisms in very high, non-environmentally relevant doses.

 

Icon bacteria

Towards bacteria and microbial soil communities, different sizes of silicon dioxide nanoparticles exert little toxicity. Due to the photocatalytic activity of some nanoforms of silicon dioxide, effects on bacteria are greater under illumination compared to exposure in the dark. Coarser silicon dioxide particles are considered completely non-toxic towards bacteria. Some studies even show a beneficial effect of silica nanoparticles on bacterial growth. Contrary, the bacteria in activated sludge are inhibited by silicon dioxide nanoparticles (see cross cutting topic - Nanomaterials in waste water treatment) [1,15-19].

Icon algae

Green algae are able to take up very small silica nanoparticles (5 nm), whereas larger particles are not able to cross the algae cell wall. Yet, particles are found to adhere to the surface of algae causing a reduction in algae growth, which can be traced back to shading effects. This effect, however, only occurs at very high and not environmentally relevant concentrations and can be reversed by natural organic matter, probably by preventing the particle attachment to algae [2-4,6].

Icon mussel

Towards blood cells of mussels, silicon dioxide nanoparticles exert no acute toxicity but caused signs of inflammation as well as reactive oxygen production (ROS) [5].

Icon Nematode

Nematodes exposed to silicon dioxide nanoparticles show ROS formation, causing impaired mobility and reproduction [14].

Icon fishWhen tested for an application as a drug carrier, small amounts of silica nanoparticles injected into zebrafish embryos don't cause any negative effects. However, in other zebrafish studies, silica nanoparticles cause developmental and behavioural effects. In grass carp, nanosized silicon dioxide cause a change in the blood composition. Different fish cell lines representing different organs are more sensitive to silica nanoparticles if they originated from a tissue getting directly in contact with the environment, such as skin or gills [8-13].

Icon insect

Upon exposure to silica nanoparticles, fruit flies show evidences for genetic damage. Silicon dioxide nanoparticles administered via the food cause slight intestinal cell damage to bumble bees and impaired their reproduction [7,20].

Icon plantArabidopsis plants showed a slower growth and a reduced content of green leaf dye after exposure to silica nanoparticles via the roots. This is attributed to a reduced nutrient uptake due to the binding the nutrients to silicon dioxide nanoparticles. In other plants such as maize, rice, wheat, lupine, pumpkin and reed however, no toxic effects from nanoscale silicon dioxide are observed. Sometimes even the germination of seeds and the growth of seedlings is favoured [15,19,21-26].

The majority of the observed effects of silicon dioxide nanoparticles on environmental organisms were caused by very high and non-environmentally relevant concentrations. Therefore, according to current knowledge, environmental organisms are not endangered by silicon dioxide nanoparticles.

Literature

  1. Adams, LK et al. (2006), Water Res, 40(19): 3527-3532.
  2. Van Hoecke, K et al. (2008), Environ Toxicol Chem, 27(9): 1948-1957.
  3. Fujiwara, K et al. (2008), J Environ Sci Health A Tox Hazard Subst Environ Eng, 43(10): 1167-1173.
  4. Wei, C et al. (2010), J Environ Sci (China), 22(1): 155-160.
  5. Canesi, L et al. (2010), Aquat Toxicol, 96(2): 151-158.
  6. Van Hoecke, K et al. (2011), Environ Int, 37:1118-1125.
  7. Mommaerts, V et al. (2012), Nanotoxicology, 6(5):554-561.
  8. Sharif, F et al. (2012), Int J Nanomed, 7:1875-1890.
  9. Duan, J et al. (2013), Biomaterials, 34:5853-5862.
  10. Duan, J et al. (2013), PLOSone, 8(9):e74606.
  11. Pham, DH et al. (2016), Sci Rep, 6:37145.
  12. Krishna Priya, K et al. (2015), Ecotoxicol Environ Safe, 120:295-302.
  13. Vo, NTK et al. (2014), In Vitro Cell Div Biol Anim, 50(5):427-438.
  14. Wu, Q et al. (2013), Chemosphere, 90:1123-1131.
  15. Rangaraj, S et al. (2014), Biotechnol Appl Biochem, 61(6):668-675.
  16. Chai, H et al. (2015), Bull Environ Contam Toxicol 94:490-495.
  17. Sibag, M et al. (2015), J Hazard Mater, 283:841-846.
  18. McGee, CF et al. (2017), Ecotoxicol, 26:449-458.
  19. Karunakaran, G et al. (2013),IET NAnobiotechnol, 7(3): 70-77.
  20. Demir, E et al. (2015),J Hazard Mater, 283:260-266.
  21. Slomberg, DL and Schoenfisch, MH (2012), Environ Sci Technol, 46:10246´7-10254.
  22. Yang, Z et al. (2015), Int J Environ Res Public Health, 12:15100-15109.
  23. Koce, JD et al. (2014), Environ Toxicol Chem, 33(4):858-867.
  24. Siddiqui, MH et al. (2014), Environ Toxicol Chem, 33(11):2429-2437.
  25. Sun, D et al. (2016), Chemosphere, 152:81-91.
  26. Schaller, J et al. (2013), Sci Total Environ, 442:6-9.

Numerous studies show an uptake of silica particles in cells.

Behaviour of Uptake in somatic cells

During in vitro studies, quartz particles are taken up into the cytoplasm of the cells through endocytosis and are enclosed by membranes. No particles were detected in cell compartments such as the cytoblast (which contains the DNA) [1]. Moreover, amorphous particles were observed to accumulate in the cells in vesicles (which, among other things, serve to digest molecules in the cell) but were found not to cause other structural changes in the cell [2]. The amorphous particles are recognized as foreign matter.

Literature

  1. Li, H et al. (2007), Mutat Res, 617(1-2): 46-57.
  2. Peters, K et al. (2004), J Mater Sci Mater Med, 15(4): 321-325.

Behaviour at the Blood-Brain Barrier

From this, it can be inferred that the blood-brain barrier is not passed [1]. Further studies are dedicated to investigating this phenomenon.

 

Literature arrow down

  1. Nishimori, H et al. (2009), Eur J Pharm Biopharm, 72(3): 626-629.

Silicon dioxide nanoparticles are stable in aqueous solutions compared to other particles which can affect their behaviour, e.g. in sewage treatment plants. In addition, silicon dioxide nanoparticles can bind other (harmful) chemicals already present in the environment.

Due to their negative surface charge under natural conditions, silicon dioxide nanoparticles are very stable in aqueous solutions compared to other particles. That is also why, in contrast to many other synthetic nanoparticles (e.g. Titanium dioxide, fullerenes), silicon dioxide nanoparticles do not bind humic acids even though other constituents of the natural organic matter can absorb to the surface. It was also found that in sandy soils smaller silica particles are less mobile than larger ones [1-3,6,8].

Kläranlage © Mariusz Szczygie / fotolia.com

In wastewater treatment plants, it is assumed for the purification of nanoparticle-containing effluents that a high salt content of the water will result in rapid agglomeration. This will lead to sedimentation of the particles and consequent removal of these particles from the water. However, this assumption does not apply to silicon dioxide nanoparticles due to their stability even in the presence of salt. Therefore, for this type of particle (and potentially other particle types), the wastewater cleaning procedure should include an additional filtration step [4]. (see cross cutting article – nanomaterials in the wastewater treatment plant)

Silicon dioxide nanoparticles are able to bind aromatic hydrocarbons such as phenanthrene and naphthalene [5]. The binding strength is dependent on the solutions’ pH. Silicon dioxide nanoparticles can also bind dichlorophen leading to an acceleration of degradation of this chemical [7]. Accordingly, silicon dioxide nanoparticles may alter the availability of contaminants for environmental organisms.

Silicon dioxide nanoparticles have a low tendency for agglomeration and sedimentation in aqueous solutions. They can bind various chemicals. This may have an impact on the effects of these chemicals on animals and plants.

Literature

  1. Yang, K et al. (2009), Langmuir, 25(6): 3571-3576.
  2. Zhang, Y et al. (2009), Water Res, 43(17): 4249-4257.
  3. Considine, RF et al. (2005), Aust J Chem, 58(12): 837-844.
  4. Zhang, Y et al. (2008), Water Res, 42(8-9): 2204-2212.
  5. Fang, J et al. (2008), Langmuir, 24(19): 10929-10935.
  6. Xue, N et al. (2016), Environ Sci Pollut Res, 23:11835-11844.
  7. Escalada, JP et al. (2014), Water Res, 50:229-236.
  8. Wang, C et al. (2012), Environ Sci Technol, 46:7151-7158.

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