Silver (Ag) and water
Silver and water: reaction mechanisms, environmental impact and health effects
|Seawater contains approximately 2-100 ppt of silver, and the surface concentration may be even lower. River water generally contains approximately 0.3-1 ppb of silver. The phytoplankton concentration is 0.1-1 ppm (dry mass), leading to a 104-105 bio concentration factor in seawater. In oyster tissue concentrations of approximately 890 ppm (dry mass) were found. |
Dissolved in water silver mainly occurs as Ag+ (aq), and in seawater as AgCl2- (aq).
Silver does not react with pure water. Is is stable in both water and air. Moreover, it is acid and base resistant, but it corrodes when it comes in contact with sulphur compounds.
Under normal conditions silver is water insoluble. This also applies to a number of silver compounds, such as silver sulphide. Some other sulphur compounds are more or less water soluble. For example, silver chloride has a water solubility of 0.1 mg/L, maximum. Silver nitrate has a water solubility of 2450 g/L. Silver fluorides are generally water soluble, but other silver halogens are not.
Silver mainly occurs in argenite and stephanite, from which it is released through weathering. In soils it is mainly present in sulphide minerals. Naturally occurring pure silver is extremely rare and is probably formes through the following reaction mechanism:
3 Ag2S + 2 H2O -> 6 Ag + 2 H2S + SO2
Besides gold, silver is the most bendable of all metals. It is known for its high thermal and electrical conductivity, its reflective power and its white colour. It is applied for example in copper, nickel and tungsten alloys. Amalgam is a silver alloy with a high mercury content. In electronics, silver is applied for outlets. Commonly known are applications in jewellery, coins and cutlery. Objects are often provided with a silver layer, including mirrors.
Silver is not a dietary requirement for organisms. It may even be lethal to bacteria, and it inhibits fungi reproduction. This is mainly caused by Ag+ ions. At oral silver uptake by warm-blooded organisms, about 10% is absorbed. Mammal flesh contains approximately 4-24 ppb (dry mass) of silver. Mammals take up silver mainly through plant feed.
Ionic silver may be removed from water by ion exchange. Some silver compounds may precipitate by coagulation. Two other efficient methods include active carbon filtration and sand filtration.