Silicon

Typical wastewaters

• reverse osmosis concentrate monosilicic acid H₄SiO₄ concentrated past the 120 to 150 mg/L as SiO₂ solubility ceiling, polymerising to colloidal silica and metal silicates; up to 340 mg/L reported with inhibition and dispersion
• demineraliser anion resin regenerant silicate in warm spent caustic; silica is the last anion loaded and the first to leak from strong base resin
• semiconductor oxide chemical mechanical polishing (CMP) wastewater colloidal silica abrasive particles, destabilised and settled above 520 nm mean size by electrocoagulation
• geothermal brine (separated water for reinjection) monomeric silicic acid about 420 ppm SiO₂ at Dieng, polymerising to scale; seeded precipitation at pH 7 and 40 C
• cooling tower blowdown dissolved silica Si(OH)₄ with calcium and magnesium; 0.4 to 0.5 mol silica removed per mol iron or aluminium in electrocoagulation
• solar panel manufacturing wastewater silicate ion together with germanium ion in actual solar panel wastewater

1 · Identity

Symbol, number

Si, 14

Oxidation states in water

+4 only, as monosilicic acid Si(OH)₄ (written H₄SiO₄), its anion H₃SiO₄⁻ above pH 9.9, polysilicic acids, colloidal and particulate silica, and metal silicates; analysts report all of it as SiO₂.

Note

The element entry gives the crust, the furnace and the properties narrative already says silica travels as dissolved silica and silicate and that sand is the filtration medium. This chapter is the scaling story: the 120 to 150 mg/L solubility ceiling and what it does to membranes.

2 · Occurrence in water

Natural sources

Weathering of silicate minerals releases monosilicic acid to every natural water; groundwater in volcanic and geothermal terrain carries the most. Diatoms strip it from surface water in spring (the element entry).

Anthropogenic sources

Semiconductor chemical mechanical polishing wastewater and glass making carry colloidal silica; reverse osmosis concentrate and cooling tower blowdown concentrate natural silica several fold; silicate corrosion inhibitors and detergents (water glass) add silicate to sewage. No concentrations were read for these streams.

3 · Speciation

Below pH 9 dissolved silica is the uncharged monomer H₄SiO₄, which is why it passes anion exchangers weakly, is barely rejected by charge and cannot be precipitated as a simple salt. Its first pKa is 9.9, so only above pH 10 does the silicate anion dominate and solubility climb steeply. When the monomer exceeds about 2 mmol/L (120 to 150 mg/L as SiO₂ at 25 C) it polymerises to polysilicic acid and colloidal silica, fastest at neutral to slightly alkaline pH and slowest above 9.5 or below 6.5; calcium and above all magnesium accelerate polymerisation and precipitate metal silicates. Analysts distinguish reactive (molybdate) silica from total silica, the difference being the colloidal fraction.

Solubility

Crystalline silica (quartz) about 5 to 6 mg/L; amorphous silica 120 to 150 mg/L at 25 C, roughly constant up to pH 9 and rising dramatically above pH 10; solubility rises with temperature and falls with the presence of calcium, magnesium and some salts. Australian guidance quotes 100 to 140 mg/L for amorphous silica and about 6 mg/L for crystalline.

Hydrolysis

Monosilicic acid is a very weak acid, pKa 9.9 for the first dissociation; local pH rise inside a strong base anion resin is what lets it exchange at all.

Complexation

Silicate binds aluminium and iron hydroxide surfaces and competes with arsenate and fluoride on activated alumina and iron media (EPA arsenic manual, via the arsenic chapter); it forms hydroxyaluminosilicates in alum treated water.

Precipitates

Amorphous silica, magnesium silicate, calcium silicate, iron and aluminium silicates; silica coprecipitates on Mg(OH)₂ in lime softening.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Use plastic bottles, never glass, for silica; do not acidify below pH 2 for reactive silica or polymerised silica will not depolymerise in time; analyse promptly because supersaturated samples polymerise in the bottle and reactive silica falls. Report as SiO₂ or as Si and say which (SiO₂ is 2.14 times Si).

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Ingested silica is essentially non toxic; the Australian health value rests on a rat NOAEL of 2,500 mg SiO₂/kg/day. The hazard of silicon is inhaled crystalline dust (silicosis), not water (the element entry).

Bioaccumulation

Diatoms and plants take up silicic acid to build silica frustules and phytoliths; no accumulation concern in animals is reported in the sources read.

Ecotoxicity

No US EPA aquatic life criterion; dissolved silica is a nutrient that limits diatom growth in lakes and coastal water rather than a toxicant.

Flags

• The 1 to 40 mg/L natural water range and the solubility figures are from a 2020 review; the Australian guideline gives 100 to 140 mg/L for amorphous silica solubility, the review 120 to 150 mg/L.
• The seawater figure is a single PubChem abundance figure expressed as Si.
• The boiler and turbine deposit statement is qualitative and unsourced this session.
• The magnesium silicate equation is the simplest stoichiometry for the softening mechanism the review describes; the review prints no equation.
• The activated silica coagulant aid is cited to MWH chapter 9 from memory.
• The 65 to 70 percent recovery cap for a 40 mg/L feed is arithmetic on the 120 to 150 mg/L ceiling, not a source figure.
• Australian guideline values are quoted because no WHO, EU or US value exists; they are national.

Gaps

• No source read gives silica in municipal wastewater, cooling tower blowdown or semiconductor wastewater as numbers.
• Boiler water silica limits and steam carryover figures were not read.
• Sodium silicate as corrosion inhibitor and sequestrant is not covered because no read source describes it.
• The Standard Methods 4500-SiO₂ detection limit was not read; the ISO 16264 range stands in.
• The ICP-MS interference at mass 28 and the reasons silicon is absent from EPA 200.8 were not read.
• Textbook citations (MWH chapters 9 and 17) are from memory of the chapters.
• The magnesium silicate scale and anion exchange stoichiometries are written here from the mechanisms the review describes; it prints neither equation.
• The second dissociation constant of silicic acid was not read, so no pK₂ is quoted.

Sources

Identity

Name and symbol

Silicon, Si

Atomic number

14 protons

Position

group 14 · period 3 · p-block · metalloid

CAS number

7440-21-3

Atomic structure

Atomic mass

28.0855 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p²
[Ne] 3s²³p²

Electrons per shell

2, 8, 4

Valence electrons

4 outer shell

Physical properties

State at room temperature

Solid

Melting point

1,687 K (1,413.85 °C)

Boiling point

3,538 K (3,264.85 °C)

Density

2.3296 g/cm3

Appearance

crystalline, reflective with bluish-tinged faces

Thermal conductivity

149 W/(m·K)

Electrical resistivity

2.33 Ω·m at 20 °C

Electrical conductivity

4.29e-1 S/m

Crystal structure

diamond cubic

Molar heat capacity

19.789 J/(mol·K)

Chemical properties

Oxidation states

+4, +2, -4

Electronegativity

1.9 (Pauling Scale)

Ionisation energy

8.152 eV
1st 786.5, 2nd 1,577.1, 3rd 3,231.6 kJ/mol

Electron affinity

1.385 eV

Atomic radius

empirical 111, covalent 111, van der Waals 210 pm

Ionic radius

Si⁴⁺ 40 pm

Reactivity

A group 14 metalloid with a strong affinity for oxygen; a continuous silica film makes the solid relatively inert, though halogens and hot alkali attack it, and molten silicon reacts with almost every container material.

with water

Does not react with water; it dissolves readily in hot aqueous alkali to silicates and hydrogen: $\ce{Si + 2 NaOH + H2O -> Na2SiO3 + 2 H2}$

with oxygen, air

No measurable reaction with air below 900 C because of the silica film; above about 950 C the oxide grows rapidly, and at 1400 C nitrogen also reacts to give silicon nitrides: $\ce{Si + O2 -> SiO2}$

with acids

Does not react with most aqueous acids, hydrofluoric acid excepted; mixtures of hydrofluoric with nitric acid or chlorine oxidise it to hexafluorosilicates.

with halogens

Fluorine attacks silicon vigorously at room temperature, chlorine at about 300 C, bromine and iodine at about 500 C, giving the tetrahalides $\ce{SiX4: Si + 2 Cl2 -> SiCl4}$

Typical compounds

• SiO₂ silicon dioxide silica; sand, quartz, flint, opal; glass
• SiC silicon carbide carborundum, one of the most important abrasives
• SiCl₄ silicon tetrachloride volatile tetrahalide; iridises glass
• SiHCl₃ trichlorosilane decomposed in hydrogen to hyperpure silicon
• Na₂SiO₃ sodium silicate water glass; detergents and adhesives
• (CH₃)₂SiCl₂ dimethyldichlorosilane hydrolysed and condensed to silicones

Occurrence, production and use

Crustal abundance

2.82×105 milligrams per kilogram

Oceanic abundance

2.2 milligrams per liter

Occurrence and sources

Silicon is present in the sun and stars and is a principal component of a class of meteorites known as aerolites. It is also a component of tektites, a natural glass of uncertain origin.

Silicon makes up 25.7% of the earth's crust, by weight, and is the second most abundant element, being exceeded only by oxygen. Silicon is not found free in nature, but occurs chiefly as the oxide and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, clay, mica, etc. are but a few of the numerous silicate minerals.

Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. The Czochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process.

• dissolved in seawater about 2.2 mg/L; crustal estimate 282,000 mg/kg (Jefferson Lab figures via PubChem)
• quartz, quartzite, silica sand and sandstone worldwide; quartzite is the raw material for silicon metal and ferrosilicon; quartz-rich sand for glass, foundry, filtration and hydraulic fracturing
• silicate minerals: feldspar, clay, mica, hornblende, asbestos, granite the bulk of the crust

Extraction, production

Carbothermic reduction of quartzite in an electric furnace to silicon metal (above 98 percent) or ferrosilicon

$\ce{SiO2 (s) + 2 C (s) -> Si (l) + 2 CO (g)}$

Silica and carbon are heated in an electric furnace with carbon electrodes; ferrosilicon, more than half of world silicon output, goes to steel and foundries, silicon metal to aluminium alloys and silicones (RSC, LANL, USGS printed pp. 160 to 161). No equation is printed in the sources.

Polysilicon by thermal decomposition of ultrapure trichlorosilane in hydrogen, then Czochralski crystal growth

$\ce{SiHCl3 (g) + H2 (g) -> Si (s) + 3 HCl (g)}$

Semiconductor and solar grades; four US producers in 2024; silicon metal was added to the EU Critical Raw Materials Act as a strategic raw material in 2024 (USGS). Described in words only.

Direct synthesis of methylchlorosilanes for silicones

$\ce{Si + 2 CH3Cl -> (CH3)2SiCl2}$

Copper-catalysed reaction of silicon metal above 98 percent purity with methyl chloride, yield typically above 78 percent (SIC BREF, PDF p236); the dimethyldichlorosilane is then hydrolysed and condensed to silicone oils, rubbers and resins.

Uses

Two allotropes of silicon exist at room temperature: amorphous and crystalline. Amorphous appears as a brown powder while crystalline silicon has a metallic luster and a grayish color. Single crystals of crystalline silicon can be grown with a process known as the Czochralski process. These crystals, when doped with elements such as boron, gallium, germanium, phosphorus or arsenic, are used in the manufacture of solid-state electronic devices, such as transistors, solar cells, rectifiers and microchips.

Silicon dioxide (SiO2), silicon's most common compound, is the most abundant compound in the earth's crust. It commonly takes the form of ordinary sand, but also exists as quartz, rock crystal, amethyst, agate, flint, jasper and opal. Silicon dioxide is extensively used in the manufacture of glass and bricks. Silica gel, a colloidal form of silicon dioxide, easily absorbs moisture and is used as a desiccant.

Silicon forms other useful compounds. Silicon carbide (SiC) is nearly as hard as diamond and is used as an abrasive. Sodium silicate (Na2SiO3), also known as water glass, is used in the production of soaps, adhesives and as an egg preservative. Silicon tetrachloride (SiCl4) is used to create smoke screens. Silicon is also an important ingredient in silicone, a class of material that is used for such things as lubricants, polishing agents, electrical insulators and medical implants.

Silicon is one of man's most useful elements. In the form of sand and clay it is used to make concrete and brick; it is a useful refractory material for high-temperature work, and in the form of silicates it is used in making enamels, pottery, etc. Silica, as sand, is a principal ingredient of glass, one of the most inexpensive of materials with excellent mechanical, optical, thermal, and electrical properties. Glass can be made in a very great variety of shapes, and is used as containers, window glass, insulators, and thousands of other uses. Silicon tetrachloride can be used as iridize glass.

Hyperpure silicon can be doped with boron, gallium, phosphorus, or arsenic to produce silicon for use in transistors, solar cells, rectifiers, and other solid-state devices which are used extensively in the electronics and space-age industries.

Hydrogenated amorphous silicon has shown promise in producing economical cells for converting solar energy into electricity.

Silicon is important to plant and animal life. Diatoms in both fresh and salt water extract Silica from the water to build their cell walls. Silica is present in the ashes of plants and in the human skeleton. Silicon is an important ingredient in steel; silicon carbide is one of the most important abrasives and has been used in lasers to produce coherent light of 4560 A.

Silcones are important products of silicon. They may be prepared by hydrolyzing a silicon organic chloride, such as dimethyl silicon chloride. Hydrolysis and condensation of various substituted chlorosilanes can be used to produce a very great number of polymeric products, or silicones, ranging from liquids to hard, glasslike solids with many useful properties.

• Iron, steel and foundries: ferrosilicon to deoxidise steel and for transformer and dynamo plates; engine blocks, cylinder heads and machine tools ferrosilicon was more than 50 percent of world silicon production on a silicon-content basis in 2024, consumed mainly by the ferrous foundry and steel industries
• Aluminium alloys: aluminium-silicon casting alloys; producers of aluminium alloys are the main consumers of silicon metal
• Chemicals: silicones: oils for lubricants and cosmetics, rubbers for sealants; silicon carbide abrasives; silicon tetrachloride
• Electronics and solar: hyperpure doped silicon for transistors, rectifiers and solar cells; a Georgia plant began making silicon solar modules in April 2024 and a Washington polysilicon plant restarted
• Glass, ceramics and construction: sand as the principal ingredient of glass; sand and clay for concrete, cement and brick; silicates in pottery, enamels and refractories glassmaking sand 7 percent of US industrial sand in 2024
• Oil and gas: hydraulic-fracturing proppant and well-packing sand about 83 percent of US industrial sand tonnage in 2024
• Water treatment: filtration sand and gravel

Safety, toxicity

Miners, stonecutters, and others engaged in work where siliceous dust is breathed into large quantities often develop a serious lung disease known as silicosis.

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H320 Causes eye irritation Serious eye damage/eye irritation

Discovery and name

Discovered by

Jöns Jacob Berzelius

Discovered

1823

First isolated

not in sources

Named by

Thomas Thomson

Origin of the name

after Latin silex or silicis, meaning 'flint'

Crystalline silicon has a metallic luster and grayish color. Silicon is a relatively inert element, but it is attacked by halogens and dilute alkali. Most acids, except hydrofluoric, do not affect it. Elemental silicon transmits more than 95% of all wavelengths of infrared, from 1.3 to 6.y micro-m.