Hexavalent chromium salts

First, a screen based on physicochemical properties should be used. This usually means pH, but also should be extended to materials with high oxidation or reduction potentials (e.g., hexavalent chromium salts). 

Among all chromium compounds only the hexavalent salts are a prime health hazard. Cr + is more readily taken up by cells, than any other valence state of the metal. Occupational exposure to these compounds can produce skin ulceration, dermatitis, perforation of the nasal septa, and kidney damage. It can induce hypersensitivity reactions of the skin and renal tubular necrosis. Examples of hexavalent salts are the chromates and dichromates of sodium, potassium, and other metals. The water-soluble hexavalent chromium salts are absorbed into the bloodstream through inhalation. Many chromium(VI) compounds are carcinogenic, causing lung cancers in animals and humans. The carcinogenicity may be attributed to intracellular conversion of Cr + to Cr +,... 

Chromic acid, being a strong oxidizing agent, passivates a metal surface, so that a chromate film will not develop. To overcome this, activators such as sulfate, formates, and chlorides are added to the chromate bath to inhibit the passivation of the base metal. The primary components of chromate baths are chromic acid and hexavalent chromium salts. 

Chromium Chromate. Chromium chromate treatment baths are acidic and made up from sources of hexavalent chromium and complex fluoride, fluorosiHcate, fluorozirconate, fluorotitanate, and siHcofluorides. Optional additional components added to accelerate coating rate are free fluoride, ferricyanide, and other metal salts such as barium nitrate. Conversion coating on aluminum precedes by the following reactions (2,3,17) ... 

In removing excess free chlorine from municipal or industrial water and from wastewater, sodium sulfite competes with bisulfite or sulfur dioxide. Other commercial appHcations of sodium sulfite in wastewater treatment include the reduction of hexavalent chromium to the less toxic Cr " salts as well as the precipitation of silver and mercury. 

Clarke and Andrew have developed a similar solution further activated by addition of chloride ions which deposits more substantial films shown to contain hexavalent chromium. The films give appreciable protection against salt spray and tarnishing by sulphur dioxide. 

Spent pickle liquor is considered a hazardous waste (K062) because it contains considerable residual acidity and high concentrations of dissolved iron salts. For example, spent pickle liquor and waste acid from the production of stainless steel is considered hazardous. The hazardous constituents in K062 are lead, nickel, and hexavalent chromium. Waste pickle liquor sludge generated by lime stabilization of spent pickle liquor is not considered hazardous unless it exhibits one or more of the characteristics of hazardous waste. An estimated 6 million tons of spent pickle liquor are generated annually in the U.S.1... 

Chromate conversion coatings for aluminum are carried out in acidic solutions. These solutions usually contain one chromium salt, such as sodium chromate or chromic acid and a strong oxidizing agent such as hydrofluoric acid or nitric acid. The final film usually contains both products and reactants and water of hydration. Chromate films are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of accelerators such as cyanides, acetates, formates, sulfates, chlorides, fluorides, nitrates, phosphates, and sulfamates. 

This is a process mainly used in power plants for reduction of hexavalent chromium to bivalent chromium. Sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous salts are common reducing agents to be used in the process. A pH range of 2-3 should be controlled. The process efficiency of removal is about 99.7%. 

The Phillips Cr/silica polymerization catalyst is prepared by impregnating a chromium compound onto a wide pore silica and then calcining in oxygen to activate the catalyst. This leaves the chromium in the hexavalent state, monodispersed on the silica surface. Chromium trioxide (Cr03) has been impregnated mast commonly, but even a trivalent chromium salt can be used since oxidation to Cr(VI) occurs during calcining. 

Trivalent chromium compounds, with the exception of acetate, hexahydrate of chloride, and nitrate salts, are generally insoluble in water (Table 3-2). 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. The alkaline metal salts (e.g., calcium, strontium) of chromic acid are less soluble in water. The zinc and lead salts of chromic acid are practically insoluble in cold water (Table 3-2). The hexavalent chromium compounds are reduced to the trivalent form in the presence of oxidizable organic matter. However, in natural waters where there is a low concentration of reducing materials, hexavalent chromium compounds are more stable (EPA 1984a). 

Chromium. Chromium is not an easily analyzed element because three distinct standards are listed by OSHA (Tables IV and V ). One standard exists for hexavalent chromium, chromic acid and chromates, another standard for soluble chromium compounds and chromous salts, and another standard for insoluble chromium compounds and chromium metal. The permissible amount of chromium in air decreases as the oxidation state increases. The analysis of chromium is further complicated by the multiplicity of NIOSH methods for chromium compounds. Hexavalent chromium shall be collected on PVC filters, although a criteria document for chromic acid (14) specifies mixed cellulose ester filters. The analytical method described in the hexavalent... 

The most stable state of chromium is the +3 state compounds of hexavalent chromium are almost as good oxidizing agents as elemental chlorine, whereas compounds of Cr(II) ( chromous compounds) are potentiometrically more easily oxidized than cadmium metal. Divalent chromium, like Ag(II) and Au(III), may exist in equilibrium with aqueous media only as the cation of a relatively insoluble salt or in a slightly dissociated complex. However, solutions containing the blue Cr24 ion may be... 

In 1946, a maximum permissible concentration was added for hexavalent chromium. The use of the salts of barium, hexavalent chromium, heavy metal glucosides, and other substances were prohibited in water treatment processes. In addition, the 1946 standards authorized the use of the membrane filter procedure for bacteriological examination of water samples. 

Chromium in wastewaters is usually present in its most oxidized form, hexavalent chromium(VI). This is the form that is most toxic to humans. It is usually present as the chromate ion, HCrO, at pH between 1.5 and 4, and CrO above pH 4.0, or it may be present as dichromate, Cr O.. The dichromate is also in equilibrium with singly dissociated chromate. These salts are highly soluble. 

The only stable strontium compound known to be genotoxic is strontium chromate. Strontium chromate induced sister chromatid exchanges in Chinese hamster ovary cells in vitro (Venier et al. 1985). In the Ames test using the Salmonella typhimurium strain TA100, strontium chromate induced mutations in the presence, but not in the absence of S9 microsomes. The genotoxicity of strontium chromate is related to the ability of the hexavalent chromium ion to enter cells and become metabolized, forming a reactive DNA-adduct. Strontium only contributes to the solubility of the salt (Elias et al. 1989, 1991). 

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