Chromium

Chromium is known as an inert metal most familiar for its use for decorative and protective plating, but also with other industrial applications. It is an essential element in trace amounts, being a component of an enzyme involved in sugar metabolism, but it is also one that can cause serious health effects in some forms. The two common oxidation states environmentally are Cr(III) and Cr(VI). Chemically, Cr(III) behaves somewhat like Fe(III) in that it is readily hydrolyzed to an insoluble hydroxide. In this form it is comparatively harmless as long as it remains immobilized. Because of its electronic configuration, compounds of Cr(III) tend to be chemically inert—that is, reactions such as exchange of ligands tend to be quite slow.

One widespread use of Cr(III) salts is in leather tanning; hides are soaked in chromium solution to chemically alter components of them to produce durability. Considerable amounts of chromium are released in tannery wastewater (about 0.4 kg of Cr per 100 kg of hides), and where this is untreated, river and soil pollution results. This is a serious problem in parts of India, for example, where tanning is a major industry.

Chromium(VI) compounds are encountered as oxo species, which are good oxidizing agents. One such application is treating steel to resist corrosion.

Solutions of the chromate ion, CrO4", are effective in "passivating" steel, presumably by reacting to form a protective oxide layer on the surface. Wastes from such operations are a prime source of chromium pollution. In acid, chromate is in equilibrium with dichromate:

Chromium(VI), a possible carcinogen, is highly toxic (USPH limit for chromium in water, 0.05 mg/liter), causing skin lesions upon excessive exposure. Chromate dusts have been associated with lung cancer.

Although organic materials may reduce Cr(VI) to the less toxic Cr(III), redox potentials predict that Cr(III) may be oxidized to Cr(VI) by oxygen in well-aerated systems. This, and other possible reactions, make soils contaminated by Cr(III) quite hazardous. Stabilization of Cr(III)—for example, by converting it to the very insoluble and unreactive sulfide—can reduce this hazard.

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