Pure silver is nearly white, lustrous, soft, very ductile, malleable, it is an excellent conductor of heat and electricity. It is not a chemically active metal, but it is attacked by nitric acid (forming the nitrate) and by hot concentrated sulfuric acid. It has the highest electrical conductivity of all metals, but its greater cost has prevented it from being widely used for electrical purposes (lenntech 2012).
Silver is almost always monovalent in its compounds, but an oxide, a fluoride, and a sulfide of divalent silver are known. It does not oxidize in air but reacts with the hydrogen sulfide present in the air, forming silver sulfide (tarnish). This is why silver objects need regular cleaning. Silver is stable in water.
The principal use of silver is as a precious metal and its halide salts, especially silver nitrate, are also widely used in photography. The major outlets are photography, the electrical and electronic industries and for domestic uses as cutlery, jewellery and mirrors. Both colour and black and white images have relied on silver since the early days of photography: siver bromide and silver iodide are sensitive to light. When light strikes a film coated with one of these compounds, some of the silver ions revert to the metal in tiny nuclei and the film is developed with a reducing agent which causes more silver to deposit on these nuclei. When the negative has the desired intensity, the uneffected silver bromide or iodide is removed by dissoving in a fixing agent, leaving the image behind. Silver is also employed in the electrical industry: printed circuits are made using silver paints, and computer keyboards use silver electrical contacts.
Silver's catalytic properties make it ideal for use as a catalyst in oxidation reactions. Other applications are in dentistry and in high-capacity zinc long-life batteries.
Silver in the environment
Silver levels in soil are not usually high except in mineral-rich areas when they can sometimes be as much as 44 ppm. Plants can absorb silver and measured levels come in the range 0.03-0.5 ppm.
Metallic silver occurs naturally as crystals, but more generally as a compact mass; there are small deposits in Norway, Germany and Mexico. The chief silver ores are acanthite mined in Mexico, Bolivia and Honduras, and stephanite, mined in Canada. However silver is mostly obtained as a byproduct in the refining of other metals.
World production of newly mined silver is around 17.000 tonnes per year, of which only about a quarter comes from silver mines. The rest is a byproduct of refining other metals.
Health effects of silver
Soluble silver salts, specially AgNO3, are lethal in concentrations of up to 2g (0.070 oz). Silver compounds can be slowly absorbed by body tissues, with the consequent bluish or blackish skin pigmentation (argiria).
Eye contact: may cause severe corneal injury if liquid comes in contact with the eyes. Skin contact: may cause skin irritation. Repeated and prolonged contact with skin may cause allergic dermatitis. Inhalation hazards: exposure to high concentrations of vapors may cause dizziness, breathing difficulty, headaches or respiratory irritation. Extremely high concentrations may cause drowsiness, staggering, confusion, unconsciousness, coma or death.
Liquid or vapor may be irritating to skin, eyes, throat, or lungs. Intentional misuse by deliberately concentrating and inhaling the contents of this product can be harmful or fatal.
Ingestion hazards: moderately toxic. May cause stomach discomfort, nausea, vomiting, diarrhea, and narcosis. Aspiration of material into lungs if swallowed or if vomiting occurs can cause chemical pneumonitis which can be fatal.
Target organ: chronic overexposure to a component or components in this material has been found to cause the following effects in laboratory animals:
- Kidney damage
- Eye damage
- Lung damage
- Liver damage
- Brain damage
- Cardiac abnormalities
- Repeated and prolonged overexposure to silver solvents may cause permanent brain and nervous system damage
- Repeated breathing or skin contact of methyl ethyl ketone may increase the potency of neuro-toxins such as hexane if exposures occur at the same time.
- 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.
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. Dissolved in water silver mainly occurs as Ag+ (aq), and in seawater as AgCl2- (aq).
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.
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