dubia indicated that the LC50 was in the range of acute values reported earlier for similar species such as C. Results of acute tests of chromium (VI) on C. Drinking water levels are generally less than 2 parts per billion (ppb). In air, the concentrations generally range between 0.01 and 0.03 microgram per cubic meter (µg/m3). A yellow color is imparted to the water at about 1 mg/l Cr(VI). In contaminated areas, Cr(VI) concentrations are commonly 300 to 500 mg/l and have been reported to reach 14 g/l. Dissolved concentrations of total Cr in groundwater from natural processes are typically below 10mg/l. Shales, suspended river material, and soils with fine grain sizes have the highest concentrations, whereas granite, carbonates, and large grained (sandy) sediments have the lowest concentrations. soils ranges from 1 to 2 000 parts per million (ppm). The concentration of naturally occurring chromium in U.S. There is a wide natural variation in Cr concentrations in soils and rocks. In rocks and soil, Cr is a common trace mineral, found as amorphous Fe(III), Cr(III) hydroxides, eskolaite (Cr2O3(s)), and chromite (FeCr2O4(s)), all Cr(III) forms. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is any chemical compound that contains the element chromium in the +6 oxidation state (thus hexavalent). Īlthough chromium does bioaccumulate, it is not reported to undergo biomagnification in the food chain. Adsorption of Cr (VI) by clayey soil and natural aquifer materials is low to moderate under near neutral pH ranges commonly encountered in groundwater. However, in the presence of organic matter, ferrous iron (Fe II) and sulfide, Cr (VI) can be readily reduced to Cr(III) and immobilized. Under high Eh (oxidizing)Īnd alkaline (pH above 7) conditions, Cr(VI) can be predominant in groundwater. Physical, chemical and biological coefficientĬr (VI) is readily soluble in water. Cr(VI) is far more mobile than Cr(III) and more difficult to remove from water. The hexavalent (VI or chromate) is the second most stable state however, it only occurs naturally in rare minerals such as crocoite (PbCrO4). Ĭhromium compounds are most stable in the trivalent state under environmental conditions and occur in nature in ores, such as ferrochromite (FeCr2O4). Some hexavalent compounds, such as chromium(VI) oxide (or chromic acid), and the ammonium and alkali metal salts (e.g., sodium and potassium) of chromic acid are readily soluble in water. Hexavalent chromium can also be found in drinking water and public water systems In these situations the chromium is not originally hexavalent, but the high temperatures involved in the process result in oxidation that converts the chromium to a hexavalent state. Hexavalent chromium can be formed when performing "hot work" such as welding on stainless steel or melting chromium metal. Industrial uses of hexavalent chromium compounds include chromate pigments in dyes, paints, inks, and plastics chromates added as anticorrosive agents to paints, primers, and other surface coatings and chromic acid electroplated onto metal parts to provide a decorative or protective coating. Hexavalent chromium is used in textile dyes, wood preservation, anti-corrosion products, chromate conversion coatings, and a variety of niche uses. The relative amount of these two species depends on pH. The most common Cr(VI) forms are chromate (CrO42–), and hydrogen chromate (HCrO4–) also called bichromate. A monoatomic hexacation and a chromium cation
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