Silicon (Si) and water
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. |
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.
Silicon compounds differ in water solubility.
SiO2(s) + 2 H2O(l) <-> H4SiO4(s)
H4SiO4(s) + H2O(l) <-> H3O+(aq) + H3SiO4-(aq)
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).
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.
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.
Silicon is mainly present in drinking water as silicic acid, and needs not be removed with regard to human health.