Kinetic And Mechanistic Studies Of Reduction Of The Cr(â…¥) By Hydroxyl Compounds

Chromium, one of heavy metals, is a major pollutant in soil and wastewater. The main sources of chromium pollution are smelting, electroplating, mining, leather tanning, as well as industrial discharge of chromium-containing waste gas, wastewater and waste residue. Of the two environmentally available forms of chromium, hexavalent and trivalent, the hexavalent form has been demonstrated to be associated with the toxic parameters and classified as human carcinogen and mutagen. The hexavalent ion has been shown to enter the cell through general anion channels and bind to cellular components, causing disruptions in biochemical pathways. Cr(â…¥) complexes, which can lead to DNA damage, plasmid cleavage, and protein cleavage, are mutagenic in bacterial and mammalian cells, while most of the Cr(â…¢) complexes are shown to be nonmutagenic. Chromium, as an essential trace metal nutrient, plays an essential role in maintaining normal lipid, protein and carbohydrate metabolism. Cr(â…¢) also improves lipid profile and glucose tolerance by increasing the level of high density lipoprotein cholesterol and decreasing total serum cholesterol.Hexavalent chromium can be converted to trivalent chromium under particularly reducing condition, by which environmental pollution can be reduced. The reduction of Cr(â…¥) by 1,2-butanediol,1,3-butanediol and lactic acid was studied. Oxidation products have been identified and characterized by chemical methods and various physicochemical techniques such as HPLC and IR spectroscopy. The oxidation kinetics is followed by conventional spectrophotometry in acidic medium over a range of temperature. Through analysis of the oxidation products and kinetic data, coherent and consistent mechanistic pictures have been depicted. The activation parameters for the rate determining step were also calculated.The reduction of Cr(â…¥) by 1,3-butanediol is accelerated when ethylenediamine-tetraacetic acid(EDTA) or 2,2'-bipyridyl (bipy) acts as catalyst. In presence of the catalyst, the uncatalysed and catalysed reactions proceed simultaneously, so that a mechanism of parallel reaction was proposed, which is consistent with the experimental phenomena.From the environmental point of view knowledge of Cr(â…¥) reduction mechanism is essential not only for tracing Cr(â…¥) fate in the environment but also for understanding its role in the oxidation of some organic compounds. Form the macrosopical experimental phenomena, we can explain the microstructure of the complex, and conjecture the stability and stucyure of the transition state. It is very significative for the development of theory of coordination chemistry.