Water-soluble hexavalent compounds

The level of concern for health hazards frequently depends on the chemical form of the metal compounds. This is primarily because the critical influence of chemical form on the uptake of inhaled, ingested, or skin-deposited materials, and can result in substantially different exposure limits for different compounds. For example, the ACGIH TWA TLV for exposure to chromium compounds is most restrictive for insoluble hexavalent chromium compounds (0.01 mg/m ), which are poorly removed from lung following inhalation less restrictive for water-soluble hexavalent compounds (0.05 mg/m ), which are more readily absorbed into the bloodstream following inhalation and least restrictive for the less-reactive trivalent chromium compounds and chromium metal (0.5 mg/m ) (11). Chromic acid and chromate aerosols can be caustic to mucous membranes, and some water-insoluble chrome IV compounds are carcinogens. In addition,... 

Toxicology. The water-soluble hexavalent chromium compounds such as chromic acid mist and certain chromate dusts are severe irritants of the nasopharynx, larynx, lungs, and skin exposure to certain hexavalent chromium compounds, mainly water insoluble, appears to be related to an increased risk of lung cancer. 

Trivalent chromium compounds, except for acetate, nitrate, and chromium(III) chloride-hexahydrate salts, are generally insoluble in water. Some hexavalent compounds, such as chromium trioxide (or chromic acid) and the ammonium and alkali metal (e.g., sodium, potassium) salts of chromic acid are readily soluble in water. The alkaline metal (e.g., calcium, strontium) salts of chromic acid are less soluble in water. The zinc and lead salts of chromic acid are practically insoluble in cold water. Chromium(VI) compounds are reduced to chromium(III) in the presence of oxidizable organic matter. However, in natural waters where there is a low concentration of reducing materials, chromium(VI) compounds are more stable (EPA 1984a). For more information on the physical and chemical properties of chromium, see Chapter 3. 

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 +,... 

Hexavalent chromium compounds exposure is by inhalation. Water soluble hexavalent chromium compounds (such as chromic acid mists and chromate dusts) are severe irritants (lungs and skin), whereas water insoluble ones are confirmed A1 carcinogenic agents to humans [19]. 

The potential source of water-soluble hexavalent chromate [Cr(VI)] in cement is the trivalent [Cr(III)] compound (Cr203) in the raw materials from which it is produced. Cr(III) compounds are oxidised to Cr(VI) compounds (CrO ) when heated in the kilns to temperatures of approximately 1400 °C. The content of chromium in cement is determined by the presence of chromium compounds in the raw materials, by the kiln lining and by chromium steel abrasion during the... 

According to the synthetic strategy just described, a series of water-soluble carbosi-lane dendrimers were made, bearing peripheral galabiose trimers interspaced by 29 A, adjusted to the binding-site distances (Fig. 75).432 The binding affinity to SLT-I and SLT-II B subunits were determined by Scatchard plot analysis. Hexavalent compound 610 showed KD values of 1.3 and 1.6 /M for SLT-I and SLT-II,... 

Hexavalent chromium (Cr+6) is the most biologically active chromium chemical species, although little is known about the properties of organochromium compounds, water-soluble species, or their interactions in complex mixtures... 

Speciation and solubility of chromium in wetlands and aquatic systems is governed by the competition among chromium oxidation states, adsorption/desorption mechanism, and soil/sediment redox-pH conditions. Chromium (VI) is reduced to chromium (HI) at approximately +350 mV in soils and sediment. Reduced Cr(III) can be rapidly oxidized to the tetravalent chromate and dichromate forms by manganese compounds. Cr(III) is much less soluble in natural system than the hexavalent form and has a much lower toxicity. Chromium is less likely to be a problem in wetlands than in nonwetlands because the reducing conditions cause its reduction or conversion to the more insoluble Cr(III) form. This is depicted in Figure 12.15, which shows changes in water-soluble chromium as affected by the soil redox potential. 

In groundwater, hexavalent chromium tends to be mobile due to the lack of solubility constraints and the low adsorption of Cr anion species by metal oxides in neutral to alkaline waters. Above pH 8.5, no Cr" adsorption occurs in groundwater Cr+ adsorption increases with decreasing pH. Trivalent chromium species tend to be relatively immobile in most groundwaters because of the precipitation of low-solubility Cr+ compounds above pH 4 and high adsorption of the Cr+ ion by soil clay below pH 4. 

To produce the desired hexavalent, water-soluble chromate chemicals, the ore is bail-milled to less than 100 mesh, mixed with soda (Na2C03) and lime, and roasted in rotary kilns at 1100-1150°C in air. While the mixture does not fuse, the molten soda ash reacts with the chromite to form water-soluble sodium chromate. The lime reacts with any aluminum impurities in the ore to form water-soluble compounds. The resulting yellow mixture is then leached with water and filtered the aluminum impurities are precipitated as aluminum hydroxide the solution is then acidified and concentrated to yield sodium chromate. From the sodium chromate, chromium can be obtained by electrolytic reduction or reduction with carbon. 

Certain hexavalent chromium compounds have been demonstrated to be carcinogenic on the basis of epidemiological investigations on workers and experimental studies in animals. In general, these compounds tend to be of low solubility in water and thus are subdivided into two subgroups ... 

In laboratory investigations, hexavalent chromium compounds, such as chromic oxide (CTO3), are often used as the source of chromium, because of their high solubilities in water. Ammonium chromate or ammonium dichromate can also be used, whereby the NH4+ ion is lost during calcination. However, sodium or potassium chromates or dichromates are unsuitable because they leave alkali metal ions on the catalyst that can promote sintering. Other hexavalent chromium compounds that have been used in nonaqueous environments include chromyl chloride and even organic chromates such as bis(f-butyl) chromate. Early Phillips commercial catalysts used aqueous CrOs as the source. 

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