After reading this article you will learn about:- 1. Ores of Chromium 2. Sources of Chromium 3. Toxicity.
Ores of Chromium:
The major Chromium ore is Chromite, FeOCr2O3, from which Ferro-Chrom alloys and Chromium metal are obtained. Chromium is used in tanning industry and in corrosion protection in steel. Treatment of steel with CrO4–2 may form an oxide film on the metal rendering it resistant to corrosion. There are two chromium ionic forms; hexavalent and trivalent forms.
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Hexavalent Chromium (VI) has two main oxy-anion forms CrO4–2 and Cr2O7–2 which are involved in the equilibrium:
Chromium (VI) is toxic but its lifetime in the aqueous environment is limited as it is reduced to Chromium (III) by organic material.
In fact, Chromium (VI) is used for the estimation of total organic Carbon in environmental studies of materials such as soil and water according to the reactions:
The Chromium (III) state, which is said to be non-toxic (though this may be because of its low concentrations in water), is mobilised in acid as the Cr+3 hydrated ion, while, in alkaline conditions low soluble Cr(OH)3 is produced. The hydroxy species of Cr(III) dominates in natural water systems
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The recommended limit for Cr in potable water is 50 µg/1. A conceptual model depicting potential processes affecting inorganic and organic chromium in the aqueous environment is shown in Fig. 25.7.
Sources of Chromium:
There are various sources of chromium entry into the human system:
1. Drinking water:
Normally contains very low concentrations of Cr (i.e., 5 µg/1 or less). Cr+3 rarely occurs in drinking water that is chlorinated. It is assumed that most waterborne Cr Ts in the hexavalent form.
2. Food:
Foodstuffs vary considerably in chromium content which ranges from 20 to 590 µg/kg. Some seafood’s appear to contain elevated levels of 0.02 – 0.21 µg/kg. Wines containing ‘Cr’ and concentrations up to 60 µg/litre have been reported.
3. Air:
There is only a limited amount of information on the levels of chromium in the air. Cr concentrations in air in towns are about 0.02 µg/m3. Most of the Cr in the air will be in the form of fine particles of which, perhaps, one half of those inhaled could become deposited in the respiratory tract.
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4. Smoking:
Cigarettes contain traces of chromium—a value of 1.4 µg per cigarette have been reported and some of this will be inhaled and absorbed. The chromium retained in the lung by smoking 20 cigarettes per day would not exceed a few micrograms per day.
5. Occupational Exposure:
Levels of airborne chromium in a number of industrial situations, particularly in plating plants and where welding occurs, can be very much higher than in the ambient environment. Concentrations as high as hundreds of micrograms per cubic meter of air have been reported.
6. Ingestion of Dirt, Dust etc.:
Little is known about the levels of ‘Cr’ in dust but, in general, it is unlikely to be a very important source of exposure even where young children have the opportunity to ingest dirt and dust. Chromium is absorbed through both the gastrointestinal and respiratory tracts.
The amount absorbed differs in each system and depends on the form of chromium. Trivalent ‘Cr’ is an essential form of the element for human beings. Hexavalent is toxic. From 0.1% to 1.2% of trivalent ‘Cr’ salts are absorbed whereas 25% of Glucose Tolerance Factor (GTF), a chromium complex necessary for normal glucose tolerance is absorbed.
The largest stores of ‘Cr’ in man are in skin, muscle, and fat tissue levels and are a function of sex age, and geographical location. Chromium is excreted slowly—mainly in the urine but also in the feces. The overall mobilisation of chromium is shown in Fig. 25.8.
Toxicity of Chromium:
At sub lethal level, chromium has a number of metabolic effects. Some of these effects are:
The level of sugar in the blood is critical to the functioning of body tissues, especially the brain .This blood sugar level stays remarkably constant under normal conditions.
Many factors help to control the amount of sugar in blood: one is the compound called Insulin. Insulin is secreted by the pancreas and works as a control in lowering the blood sugar level. When the pancreas fails to secrete enough insulin the disease called diabetes mellitus results.
Chromium plays a role in lowering the blood sugar level in the’ body by increasing effectiveness of insulin. This may explain why chromium deficiency produces symptoms similar to diabetes mellitus.
A series of relevant experiments has shown that diets supplemented with inorganic chromium or high chromium brewers’ yeast can significantly lower total serum cholesterol in human subjects. In addition to brewers’ yeast, whole grain cereals and liver are good sources of chromium.
Hexavalent chromium at 10 mg/kg of body weight will result in liver necrosis, nephritis and death in man; lower doses will cause irritation of the gastrointestinal mucosa. Hexavalent chromium in high doses has been implicated as the cause of digestive tract cancer in man and there is firm evidence that there is an increased risk of lung cancer for workers who are exposed to high level of Cr.
Prostate cancer and maxillary sinus cancers have been reported in workers involved in Cr-planting.
Other health effects related to industrial exposures have been reported, e.g. hexavalent chromium can produce cutaneous and nasal mucous membrane ulcers and mermatitis (from skin contact). Higher concentration of chromium, however, cause much more deteriorous effects viz. genetic damage and carcinogenesis.