17-كتالوج اجهزة معمل تحاليل المياه والهندسة الصحية النظرى(المعادن الثقيلة)

Chemical parameters of water
Heavy metals
Definition of a Heavy Metal

There are 35 metals that concern us because of occupational or residential exposure; 23 of these are the heavy elements or "heavy metals":
antimony, arsenic, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, gold, iron, lead, manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium, vanadium, and zinc
"Heavy metals" are chemical elements with a specific gravity that is at least 5 times the specific gravity of water. The specific gravity of water is 1 at 4°C (39°F). Simply stated, specific gravity is a measure of density of a given amount of a solid substance when it is compared to an equal amount of water. Some well-known toxic metallic elements with a specific gravity that is 5 or more times that of water are arsenic, 5.7; cadmium, 8.65; iron, 7.9; lead, 11.34; and mercury, 13.546 .
Environmental Impact: Metal ions are dissolved in groundwater and surface water when the water comes in contact with rock or soil containing the metals, usually in the form of metal salts. Metals can also enter with discharges from sewage treatment plants, industrial plants, and other sources. The metals most often found in the highest concentrations in natural waters are calcium and magnesium. These are usually associated with the carbonate anion (CO32-) and come from the dissolution of limestone rock. As mentioned under the discussion of hardness, the higher the concentration of these metal ions, the harder the water; however, in some waters other metals can contribute to hardness. Calcium and magnesium are non-toxic and normally absorbed by living organisms more readily than the other metals. Therefore, if the water is hard, the toxicity of a given concentration of a toxic metal is reduced. Conversely, in soft, acidic water the same concentrations of metals may be more toxic.
High pH in a stream can cause precipitation of metal salts which makes them temporarily unavailable. Because of this relationship of toxicity to hardness, Warm Water Aquatic Habitat Criteria for metals are calculated by a rather complex mathematical formula employing the natural log of the hardness. As hardness increases, the allowable concentration increases. The metal criteria in this manual were calculated based on a hardness of 100 mg/L. If the hardness values in the test results vary much from 100 mg/L, the criteria can be recalculated. Even though metal concentrations may be very low (below a toxic level), aquatic organisms can bioaccumulate (or concentrate) certain metals (for example, mercury, lead, and cadmium). If more is absorbed than excreted, the levels can then build up over time to a toxic level.

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