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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).

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

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.

Solubility of silver and silver 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.

Why is silver present in water?

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 compounds play an important role in photo and film production, and are applied in developing chemicals. It serves as a catalyser in many chemical processes. Silver oxides are applied in battery production.
Colouring agents for food stuffs, preservatives and disinfectants may contain silver. Silver is added to the atmosphere as AgI to prevent hail. It is generally a by-product of metal refinery, and may be recycled.
The 110mAg isotope is applied in nuclear physics.

What are the environmental effects of silver in water?

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.
Plants may absorb silver, although it has no biological use. Values of between 0.03 and 0.5 ppm (dry mass) were measured in the past. Fungi and green algae may even obtain a silver content of 200 ppm (dry mass).
Soils do not contain great amounts of silver. However, areas rich in minerals may contain higher amounts. In mining areas soil silver amounts of up to 44 ppm were found. In normal air-dried soil concentrations do not exceed 100 ppb.
In water silver and silver compounds are toxic to micro organisms. Fish contain approximately 11 ppm of silver. Silver toxicity to fish is reduced by water. Depending on water hardness, the lethal concentration for freshwater fish lies between 4 and 280 ppm. Freshwater plants tolerate between 30 and 7,500 ppb silver, depending on the species. The lethal concentration for daphnia is approximately 0.25 ppb, and for amphipods at 4,500 ppb.
Naturally occurring silver concentrations in soil and surface water do not normally cause any environmental problems.
LD50 values were determined for various silver compounds. For silver oxide the LD50 for rats at oral intake is 2820 mg/kg, and for silver nitrate the LD50 for mice at oral intake is 50 mg/kg. For dogs 2.3 g of silver nitrate is lethal. Silver difluoride is extremely toxic, and it is also excellently water soluble. Silver toxicity has a very broad spectrum.
Silver is not known to be carcinogenic. However, when it is directly implanted under the skin of animals it can cause cancer.
Silver has two stable and twenty four instable (radioactive) isotopes.

What are the health effects of silver in water?

Silver is not a dietary requirement for humans. The body of an adult contains approximately 2 mg of silver. Our daily intake of silver is 20-80 μg, of which approximately 10% is absorbed. These amounts are not health threatening. In larger amounts, some silver compounds may be toxic, because silver ions have a high affinity for sulphur hydryl and amino groups, and therefore complexation with amino acids, nucleic acids and other compounds occurs in the body. We known the mechanism of toxicity, so we also known a number of detoxification methods. The toxic mechanism is relatively small at oral uptake, because of the low absorption capacity of the body for silver.
Silver that ends up in the body is generally deposited in connective tissue, skin and eyes and causes a gray to black colouring. Within 50 years, one is able to accumulate approximately 9 mg of silver.
The drinking water guideline for silver is 0.05 mg/L, if a guideline is fixed at all. This is mainly because silver may bind to sulphur in food in boiling water. Silver oxide is harmful upon swallowing, because it irritates the eyes, respiratory tract and skin. Silver nitrate is much more harmful, because it is a strong oxidant. It causes corrosion and at oral uptake it leads to vomiting, dizziness and diarrhoea. At silver salt uptake the body may protect itself by converting them to insoluble silver chlorides.
Silver is a bactericide, and may therefore be applied in water disinfection.

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

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.
Silver is applied in water purification for swimming pool water disinfection. Only small amounts are applied that are not a health hazard.

Literature and the other elements and their interaction with water

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