Silver is a disinfectant for non-spore forming bacteria at concentrations about 1000 times lower than the levels at which it is toxic to mammalian life. This extreme mammalian-to-bacterial toxicity differential is the definition of an oligodynamic material. The low concentration necessary for oligodynamic activity allows silver or one of its insoluble salts to be used indefinitely in contact with sterile liquids without silver levels building up to concentrations harmful to people.
The biological effects of silver are apparently due to reversible bonds with enzymes and other active molecules on the surface of cells. Due to its sulphydryl binding propensity, biologically-available silver disrupts membranes, disables proteins and inhibits enzymes. The ionic form of silver is necessary for biological activity and the lipid phase of the membrane appears to be important in adsorbing silver ions to living cells. The active sites on enzymes which are affected by silver are apparently the electron-rich functional groups such as-SH groups.
Silver combines with plasma proteins, is removed by the liver and over 90% is eliminated in the bile; most of this in the feces with very little in the urine. That silver which is not excreted is deposited in the skin and mucous tissues. Tissue deposition of silver results from precipitation of insoluble salts such as silver chloride and silver phosphate. These may be transformed to soluble silver sulphide albuminates and bind with amino or carboxyl groups in proteins and nucleic acids. They may also be oxidized to metallic silver by ascorbic acid or catecholamines.
The discoloured skin in argyric patients exposed to ultraviolet radiation is likely caused by photoreduction of silver chloride to metallic silver, which is then oxidized to black silver sulphide and bound by tissues. If the diet is high in selenium, the silver sulphide is converted to silver selenide which may result in higher silver deposition rates than with silver sulphide.
Silver is tightly bound by sewage sludge and elevated levels of silver are often associated with sewage outfalls receiving minimal treatment. In the absence of sewage, silver associates with iron oxides and humic substances. The relative bioavailability of either silver-inorganic complexes or silver-organic complexes appears to depend on the individual compounds. Silver-inorganic complexes are probably the most common in the marine environment. Silver-chlorides are generally not bioavailable except for silver chloride. Silver-iron oxides or silver-magnesium complexes in sediments increase the availability to bottom feeding organisms. Activated sludge organisms may bioaccumulate silver at 100 times the concentration in the effluent.
Silver has low toxicity to vertebrate animals and is eliminated rapidly when ingested orally. It is not a cumulative toxin. Since surface waters in Canada generally contain low levels of silver and there are few data on chronic silver toxicity to animals, no criteria seem to be justified at this time for wildlife, livestock or laboratory animal drinking water. Wildlife, free range and confined livestock and laboratory animals can safely drink water which meets the aquatic life criteria.
Maximum and 30-day average criteria have been set despite the paucity of data for marine fish since fish are not sensitive to silver at concentrations over 10 times the levels that affect most invertebrates and algae. Criteria are set to protect the most sensitive lifestage of the most sensitive species. The literature indicates that the most sensitive organisms are phytoplankton and the embryonic and larval stages of invertebrates. Since the sensitivity of invertebrates and phytoplankton to silver is much greater than that of fish, and because a great deal of literature is available concerning their sensitivies, the requirements for marine fish data were considered to be superfluous and were waived.
The life stage, size of the organisms, length of time of exposure, species sensitivity and salinity all contribute to the variation in toxicity reported in the literature. The relative proportion of the free silver+ ion to the total silver content in the oceans is a function of the salinity. In the ocean, most of the silver at any one time is present as chloride complexes; of these complexes only the mono-chloro complex silver chloride is biologically available. At a salinity of 25 parts per thousand, which is almost the natural concentration in the sea, only about 1 part in 16 000 parts of the silver would exist as the free ion.
This lack of free silver ion is generally not accounted for in the literature where most values are given as total silver and the actual toxic forms of the silver would be a considerably smaller value. This factor accounts for much of the wide variation in toxicity values reported in the literature, for while salinity is generally reported, it is rarely considered as a factor affecting toxicity. The total silver measurements reflect the worst-case scenario, which should be considered when setting any water quality criteria in order to protect the most sensitive life stages of the most sensitive species. This worst-case scenario is particularly important for estuaries, where salinity can fluctuate quite dramatically.
Silver is one of the most toxic of the heavy metals to freshwater micro-organisms. Water hardness, length of exposure, size of the organism and life stage of the organism all affect the toxicity values. Reports of the validity of static versus flow-through tests within the literature are variable; however static tests with renewal of test water appear to be as accurate as flow-through bioassays. Invertebrates and embryos of fish are generally much more sensitive than juvenile and adult fish.
The effect of speciation on the acute and chronic toxicity of silver was compared using the fathead minnow as the test organism. Silver sulfide, silver thiosulfate and silver chloride were compared to the silver ion, added as silver nitrate. The tests were flow-through in soft water at 25C. Silver chloride was found to be 300 times less toxic, silver sulfide was 15,000 times less toxic, and silver thiosulfate was 17,500 times less toxic than silver nitrate.
Most existing silver criteria, objectives or regulated amounts are not based on the free ionic monovalent ion, which is acutely toxic to aquatic life. Instead they are based on total silver which includes the metal, complexes and precipitates, all of which are very much less toxic than the monovalent ion. Thus, these existing regulations and criteria are often overprotective. A method of measuring the biologically-available forms of silver is needed so that the criteria and the risk are correlated.
Regulations should reflect the appropriate risk but the problem is that there is no monovalent ion specific measurement. In addition, some non biologically-available silver may be in forms that are in equilibrium with monovalent silver and thus much of the silver pool becomes ultimately available as the monovalent silver is taken up. Benthic organisms will take up some insoluble forms of silver as they graze, and thus more than just the monovalent form is available to them. Therefore, total silver is recommended.
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P. D. Warrington, Ph.D.
Water Quality Branch
Environmental Protection Department
Ministry of Environment, Lands and Parks