Silicon is the most abundant electropositive element in The Earth’s crust. It’s a metalloid with a marked metallic luster and very brittle. It is usually tetravalent in its compounds, although sometimes its bivalent, and it’s purely electropositive in its chemical behaviour. Moreover, pentacoordinated and hexacoordinated silicon compounds are also known. 

Natural silicon contains 92.2% of the isotope 28, 4.7% of silicon 29 and 3.1% of silicon 30. Apart from those stable natural isotopes, various radiactive artificial isotopes are known. Elemental silicon has the physical properties of metalloids, similar to the ones or germanium, situated under it in the group IV of the periodic table. Silicon is an intrinsic semiconductor in it’s purest form, although the intensity of its semiconduction is highly increased by introducing small quantities of impurities. Silicon is similar to metals in its chemical behaviour.

It’s almost as electropositive as tin and much more positive than germanium or lead. According to this metallic character, it forms tetrapositive ions and various covalent compounds; it appears as a negative ion only in a few silicides and as a positive constituent of oxyacids or complex anions.

It forms various series of hydrides, various halides (many of which contain silicon-silicon bounds) and many series of compounds which contain oxygen, which can have ionic or covalent properties.

Applications

Silicon is the principal component of glass, cement, ceramics, most semiconductor devices, and silicones, the latter a plastic substance often confused with silicon. Silicon is also an important constituent of some steels and a major ingredient in bricks. It is a refractory material used in making enamels and pottery.

Elemental raw silicon and its intermetallic compounds are used as alloy integrals to provide more resistance to the aluminum, magnesium, copper and other metals. Metallurgic silicon with 98-99% purity is used as raw material in the manufacture of organosilicic and silicon resins, seals and oils. Silicon chips are used in integrated circuits. Photovoltaic cells for direct conversion of solar energy use thin cut slices of simple silicon crystals of electronic grade. Silicon dioxide is used as raw material to produce elemental silicon and silicon carbide. Big silicon crystals are used for piezoelectric glasses. Melted quartz sands are transformed in silicon glasses which are used in laboratories and chemical plants, as well as in electric insulators. A colloidal dispersion of silicon in water is used as a coating agent and as ingredient for certain enamels.

It is known that silicon forms compounds with 64 out of the 96 stable elements and possibly form silicides with other 18 elements. Appart from metallic silicides, which are used in big quantities in metallurgy, it forms important commonly used compounds with hydrogen, carbon, halogens, nitrogen, oxygen and sulphur. Moreover, many useful organosilicic by-products.

Silicon in the environment

Silicon is found in many dioxide forms and in uncountable variations from the natural silicates. 

The silicon is much more abundant than any other element, apart from the oxygen. It constitutes 27,72% of the solid Earth’s crust, while the oxygen constitutes 46,6%, and the next element after silicon, aluminium, is found in a 8,13%.

Sand is used as source of the silicon produced commercially. A few siliate minerals are mined, e.g. talc and mica. Other mined silicates are feldspars, nephenile, olivine, vermiculite, perlite, kaolinite, etc. At the other extreme there are forms of silica so rare that they are desirable for this reason alone: gemstone opal, agate and rhinestone.

Health effects of silicon

Silicon concentrates in no particular organ of the body but is found mainly in in connective tissues and skin. Silicon is non-toxic as the element and in all its natural forms, namely silica and silicates, which are the most abundant.

Elemental silicon is an inert material, which appears to lack the property of causing fibrosis in lung tissue. However, slight pulmonary lesions have been reported in laboratory animals from intratracheal injections of silicon dust. Silicon dust has little adverse affect on lungs and does not appear to produce significant organic disease or toxic effects when exposures are kept beneath exposure limits. Silicon may cause chronic respiratory effects. Crystalline silica (silicon dioxide) is a potent respiratory hazard. However, the likelihood of crystalline silica generation during normal processing is very remote. LD50 (oral)- 3160 mg/kg. (LD50: Lethal dose 50. Single dose of a substance that causes the death of 50% of an animal population from exposure to the substance by any route other than inhalation. Usually expressed as milligrams or grams of material per kilogram of animal weight.)

Silicon crystalline irritates the skin and eyes on contact. Inhalation will cause irritation to the lungs and mucus membrane. Irritation to the eyes will cause watering and redness. Reddening, scaling, and itching are characteristics of skin inflammation.

Lung cancer is associated with occupational exposures to crystalline silica specifically quartz and cristobalite. An exposure-response relationship has been reported in studies of miners, diatomaceous earth workers, granite workers, pottery workers, refractory brick workers, and other workers 

Several epidemiological studies have reported statistically significant numbers of excess deaths or cases of immunologic disorders and autoimmune diseases in silica-exposed workers. These diseases and disorders include scleroderma, rheumatoid arthritis, systemic lupus erythematosus, and sarcoidosis.

Recent epidemiological studies have reported statistically significant associations of occupational exposure to crystalline silica with renal diseases and subclinical renal changes

Crystalline silica may affect the immune system, leading to mycobacterial infections (tuberculous and nontuberculous) or fungal, especially in workers with silicosis

Occupational exposure to breathable crystalline silica is associated with bronchitis, chronic obstructive pulmonary disease (COPD) and emphysema. Some epidemiologic studies suggest that these health effects may be less frequent or absent in nonsmokers.

Effects of silicon on the environment 

No negative environmental effects have been reported.

Silicon and water: reaction mechanisms, environmental impact and health effects

Silicon is the most abundant element on earth after oxygen. Large amounts of silicon can be found in various minerals and it is abundant in oceans and nearly all other waters as silicic acid. In the surface layers of oceans silicon concentrations are 30 ppb, whereas deeper water layers may contain 2 ppm silicon. Rivers generally contain 4 ppm silicon. Silicon is usually not ionized when dissolved; it is present as ortho silicic acid (H₄SiO₄ or Si(OH)₄). These compounds are the result of slow dissolution of silica in water. Rivers transport large amounts of silicon to sea. Most likely, less than 20% of dissolved silicon is removed from rivers by means of biological or chemical transformation processes.

In what way and in what form does silicon react with water?

Silicon is never found in nature in free form. In crystallized form it is only reactive under conditions of extremely high temperatures. Water and water vapor probably have little influence upon silicon solubility, because a protective surface layer of silicon dioxide is rapidly formed.
There are many examples of silicon compounds reacting with water. Silicon tetra fluoride reacts with water to hydrogen fluoride. Silicon tetra chloride reacts with water quite violently. Silicides of the first and second group are generally more reactive than transitory metals. Typical reaction products include hydrogen and/ or silanes (SiH₄), for example Na₂Si + 3H₂O -> Na₂SiO₃ + 3H₂.

Solubility of silicon and silicon compounds

Silicon compounds differ in water solubility.
Silicon oxide is relatively water insoluble compared to other minerals. Upon dissolution the following equilibrium is formulated:

SiO₂(s) + 2 H₂O(l) <-> H₄SiO₄(s)

This balance contains silicic acid, a weak acid that also forms during silicon mineral hydrolysis:

H₄SiO₄(s) + H₂O(l) <-> H₃O+(aq) + H₃SiO₄^-(aq)

Silicon dioxide has a water solubility of 0.12 g/L, whereas for example silicon carbide is water insoluble.

Why is silicon present in water?

As was explained earlier, silicon is part of various minerals, from which it may be released during weathering processes. It is also released under water during volcanic activity. Water in interspaces of marine sediments contains more silicon than the sea surface. The present current causes silicon to flow from sediments to seawater. Antarctic weathering also releases silicon. Silicon is removed from waters naturally, through plankton fixation, sediment settling, or reactions of dissolved silicon with clay minerals (reverse weathering).
Sand is the primary substance for commercially produced silicon. Minerals such as talc, mica, feldspar, nepheline, olivine, vermiculite, perlite and kaolinite also contain silicon. Gemstones such as opal and amethyst also contain silicon.
Construction processes silicon compounds in sand and cement, a calcium silicate. Glass and porcelain production is based on sand.
Silicon is applied as an aid in steel, chemical and electron industries, where it is processed under high temperatures. Steel and other alloys are eventually processed to for example engine blocks or cylinder heads.
Industrially significant silicon compounds are rubber- or resin-like compounds, which are generally water resistant and also withstand oxidation processes and chemical weathering. These are applied as lubricants under high temperatures, as a sealing kit for windows, roofs and pipes, in rubber hoses and in plastic parts for car engines. Silicon oils are applied in cosmetics, and for textile impregnation. In microchips this element is a semi conductor, as it is in transistors and other electronic parts.
Solar panels consist of n-semi conductors of silicon and arsenic and p-semi conductors of silicon and boron. It occurs in elementary form in optic lenses and prisms for infrared light.
Silicon carbide is nearly as hard as diamond and is applied as an abrasive. Quartz crystals that exist naturally and are produced chemically have the characteristic of vibrating in very exact frequencies, when they come in contact with electricity. This may be applied in watches, radios and televisions. Alkali silicones are added to cleansing agents, glue and bleaching agents for textiles.
Zeolites are silicones that are applied as foam regulators in detergents. These directly influence water quality. Other silicon compounds may be applied as absorbents.

What are the environmental effects of silicon in water?

Silicon dioxide is a dietary requirement for various organisms. The mechanism of intake is currently unclear. Diatoms and sea sponges apply silicon for skeleton strengthening. Small hairs on nettles also consist of silicon. Chickens and rats require silicon for bone development. It is very likely that silicon is a dietary requirement for humans, as the skin and connective tissue contains significant amounts of this element.
Silicon is also essential for plant growth. Various plant species contain about 200-62,000 ppm (dry mass) of silicon. Plants such as dandelions and bamboo contain silicon in stems and leaves, increasing stability.
Silicon is generally harmless when present in water, because it is naturally present in large amounts. Abnormally high concentrations might limit algal growth. Water organisms may be affected by zeolite, a phosphate replacement in detergents.
Silicon has three natural isotopes that are all non-radioactive. We now know of the existence of seven instable isotopes.

What are the health effects of silicon in water?

The human body contains a total amount of 1 g of silicon, which decreases at a later age. For a number of organisms silicon is a dietary requirement, and consequently it is considered a dietary requirement for humans, as well. Organisms mainly require silicon for bone development, whereas the element is found mostly in skin and connective tissue. Daily intake may vary between 20 and 1200 mg, and is mostly met by eating grains. Shortages are unknown.
All naturally occurring types of silicon, sand and silicon compounds are non-toxic. Elementary silicon has no clear mechanisms of toxicity. High concentrations of soluble silicon compounds may disturb phosphorilation. A number of silicon compounds have a fiber-like texture and are carcinogenic, for example asbestos. Fine particles of silicon compounds may cause silicosis, a typical profession related illness of for example mine workers or stone grinders. Pulmonary alveoluses harden and their flexibility decreases. This results in shortness of breath, panting and couching. Only inhalation of silicon particles may cause these effects.
Silicon breast implants may cause auto immune disorders and even cancer. There is however no scientific prove to substantiate these claims. Silicon is present in stomach tablets to treat colic and intestinal gases.
A number of silicon compounds, such as silicon halogens, are corrosive and extremely toxic. Silicon tetra chloride is an eye irritant, and may also cause breathing problems and skin irritation.
In drinking water only silicic acid is present, which is relatively safe.

Which water purification technologies can be applied to remove silicon from water?

Silicon is mainly present in drinking water as silicic acid, and needs not be removed with regard to human health.
Silicon compounds may be applied as absorbents in water purification.