Chromate conversion coating

Abstract: Chromium is a highly toxic non-essential metal for microorganisms and plants. Due to its widespread industrial use, chromium (Cr) has become a serious pollutant in diverse environmental settings. The hexavalent form of the metal, Cr(VI), is considered a more toxic species than the relatively innocuous and less mobile Cr(III) form. The presence of Cr in the environment has selected microbial and plant variants able to tolerate high levels of Cr compounds. The diverse Cr-resistance mechanisms displayed by microorganisms, and probably by plants, include biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate efflux. Some of these systems have been proposed as potential biotechnological tools for the bioremediation of Cr pollution. In this review we summarize the interactions of bacteria, algae, fungi and plants with Cr and its compounds.

Abstract: Experimental measurements of reaction stoichiometries indicate that optimum conditions for chromate removal from oxygenated, 25/sup 0/C aqueous wastes by reduction with ferrous ion occur at pH less than 10.0 in solutions containing the following anions: ClO/sub 4//sup -/, Cl/sup -/, F/sup -/, NO/sub 3//sup -/, and PO/sub 4//sup 3 -/. At pH greater than 10.0 and at lower pH in solutions that contain greater than 10/sup -4/ M total phosphate, chromate is less efficiently reduced by ferrous ions as evidenced by nonstoichiometric reduction. Nonstoichiometric reduction of the chromate by the ferrous ion is caused by the competitive oxidation of the ferrous ions by dissolved oxygen. Depending on pH and solution composition, the reduction of chromate by ferrous ions causes rapid precipitation of (Cr/sub x/Fe/sub 1-x/)(OH)/sub 3/(s). The solubility of this solid limits Cr(III) concentrations, between pH 5.0 and pH 11.0, to less than the drinking water limit of 10/sup -6/ M.

Abstract: The toxicity and carcinogenicity of hexavalent chromium (Cr) in animal and human models are reviewed. The focus of this review is not on the well-established fact that hexavalent Cr compounds of low and high water solubility can induce respiratory cancers, but rather this review addresses other types of cancers induced by exposure to hexavalent Cr compounds. Additionally, non-cancer endpoints are also discussed with documentation of human and animal studies showing non-cancer health effects of hexavalent Cr exposure on the respiratory system, GI system, immune system, liver, and kidney. There is an emerging understanding that because hexavalent chromate is isostructural with phosphate and sulfate, it is readily taken up by the G.I. tract and penetrates to many tissues and organs throughout the body. This is supported by animal studies and experiments using human volunteers. From the epidemiological studies, there is suggestive evidence that hexavalent Cr causes increased risk of bone, prostate, lymphomas, Hodgkins, leukemia, stomach, genital, renal, and bladder cancer, reflecting the ability of hexavalent chromate to penetrate all tissues in the body. A high accumulation of Cr(III) in all tissues and organs is a strong indication of the wide toxic potential of exposure to soluble hexavalent Cr in the drinking water and in the ambient environment.