Chromium is found in soils and waters from natural sources and anthropogenic activities. It occurs in the environment in two main valence states; hexavalent Cr(VI) and trivalent Cr(III). Within the range of pH and redox potential commonly found in soils and water, there are two trivalent forms of Cr (the Cr3+ cation and the CrO2- anion) and two hexavalent forms (the Cr2O72- and CrO42- anions) (Bartlett and Kimble 1976). Cr(III) is an essential human and animal nutrient at levels of 50 to 200 micrograms/day (e.g., Mertz 1969, Jones 1990), but in its hexavalent oxidation state becomes toxic and suspected carcinogen at concentration exceeding 50 micrograms/ml in drinkable waters (Ilton 1999). The parameters controlling the Cr input and output to an ecosystem and Cr oxidation states are a function of the geological environment and depends on a number of physical and chemical reactions, in addition to biological factors which are active in the ecosystem. In general, the delicate balance of the chemistry of a natural geological and biological system may be strongly affected by heavy element loading resulting from human activities. Cr has been extensively mined and deposed in the biosphere due to anthropogenic activity, exceeding in many cases the contribution from natural sources (e.g., Nriagu 1984, Adriano 1986). Cr has the property to dissolve easily in nonoxidizing mineral acids but to be resistant to attack by oxidizing acids, finding large applications in metallurgical and refractory industries and in the chemical industries, where it is used as a tanning agent pigment. Furthermore, Cr compounds are widely used in the manufacture of ceramics, catalyst, wood preservatives, metal finishing, corrosion control, magnetic tapes, etc. (Nriagu 1988). Even if Cr may occasionally be abundant in the Earth crust relatively to other transition elements, with concentrations reaching some weight percent in soils derived from ophiolitic complexes or from shales or phosphorites, it is evident that a number of human activities have altered the natural cycle and abundance of this element. Cr(VI) is partially converted to Cr(III) in the human body by fluids such as gastric juice, epitelial lining fluids of the respiratory tract or blood. Secondary reduction also occurs at the cellular level. Thus, at low levels of exposure, hexavalent Cr ions are reduced before the 6+ ions can interact with d.n.a., unless the dose is sufficient to overwhelm the body's reduction capacity. The excess of Cr can induce d.n.a. damage (Singh et alii 1998, Jones 1990). The fate of Cr in the environment depends on its chemical form: Cr(VI) compounds are very soluble and mobile compared with the sparingly soluble trivalent Cr species. Recent studies (Ellis et alii 2002) have underlined a Cr isotopic fractionation during the Cr(VI) to Cr(III) conversion, opening the application of the Cr stableisotope systematics as a sensitive indicator to estimate the reduction rate of the toxic hexavalent Cr to Cr(III). 2. Cr oxidation States and isotopic Fractionation Under the redox and pH conditions usually found in nature, Cr(VI) is removed from the solution as Cr(OH)3, or in the presence of acqueous Fe(II) (Pettine et alii 1998) or Fe-bearing minerals in suspension. Nevertheless, it has to be considered that in a natural environment there are many other complexing agents in addition to H2O and OH- which may dictate Cr speciation. For example, Cr(III) shows the tendency to form hexacoordinate octahedral complexes with many ligands including organic ligands containing oxygen, nitrogen, sulphur (Saleh et alii 1989). It is also worth noting that the redox potential of the Cr(VI)/Cr(III) couple is high, increasing the tendency to Cr reduction, and few oxidants are present in natural systems which can oxidize Cr(III) to Cr(VI) ( Johnson and Xyla 1991). Within the normal pH range of natural waters, the expected forms of Cr(VI) oxyanions are CrO42-, HCrO4- and Cr2O72- in a variety of compounds, generally soluble and easily mobile in the environment. Cr(VI) is reduced to Cr(III) by a number of electron donors, as already stressed. Preliminary data on a limited number of specimen indicate that industrial chromium sources would be isotopically similar to the Bulk Earth.
Cr-isotopes: analytical methods, spike calculations and application to Environmental Sciences
Cavazzini G;
2007
Abstract
Chromium is found in soils and waters from natural sources and anthropogenic activities. It occurs in the environment in two main valence states; hexavalent Cr(VI) and trivalent Cr(III). Within the range of pH and redox potential commonly found in soils and water, there are two trivalent forms of Cr (the Cr3+ cation and the CrO2- anion) and two hexavalent forms (the Cr2O72- and CrO42- anions) (Bartlett and Kimble 1976). Cr(III) is an essential human and animal nutrient at levels of 50 to 200 micrograms/day (e.g., Mertz 1969, Jones 1990), but in its hexavalent oxidation state becomes toxic and suspected carcinogen at concentration exceeding 50 micrograms/ml in drinkable waters (Ilton 1999). The parameters controlling the Cr input and output to an ecosystem and Cr oxidation states are a function of the geological environment and depends on a number of physical and chemical reactions, in addition to biological factors which are active in the ecosystem. In general, the delicate balance of the chemistry of a natural geological and biological system may be strongly affected by heavy element loading resulting from human activities. Cr has been extensively mined and deposed in the biosphere due to anthropogenic activity, exceeding in many cases the contribution from natural sources (e.g., Nriagu 1984, Adriano 1986). Cr has the property to dissolve easily in nonoxidizing mineral acids but to be resistant to attack by oxidizing acids, finding large applications in metallurgical and refractory industries and in the chemical industries, where it is used as a tanning agent pigment. Furthermore, Cr compounds are widely used in the manufacture of ceramics, catalyst, wood preservatives, metal finishing, corrosion control, magnetic tapes, etc. (Nriagu 1988). Even if Cr may occasionally be abundant in the Earth crust relatively to other transition elements, with concentrations reaching some weight percent in soils derived from ophiolitic complexes or from shales or phosphorites, it is evident that a number of human activities have altered the natural cycle and abundance of this element. Cr(VI) is partially converted to Cr(III) in the human body by fluids such as gastric juice, epitelial lining fluids of the respiratory tract or blood. Secondary reduction also occurs at the cellular level. Thus, at low levels of exposure, hexavalent Cr ions are reduced before the 6+ ions can interact with d.n.a., unless the dose is sufficient to overwhelm the body's reduction capacity. The excess of Cr can induce d.n.a. damage (Singh et alii 1998, Jones 1990). The fate of Cr in the environment depends on its chemical form: Cr(VI) compounds are very soluble and mobile compared with the sparingly soluble trivalent Cr species. Recent studies (Ellis et alii 2002) have underlined a Cr isotopic fractionation during the Cr(VI) to Cr(III) conversion, opening the application of the Cr stableisotope systematics as a sensitive indicator to estimate the reduction rate of the toxic hexavalent Cr to Cr(III). 2. Cr oxidation States and isotopic Fractionation Under the redox and pH conditions usually found in nature, Cr(VI) is removed from the solution as Cr(OH)3, or in the presence of acqueous Fe(II) (Pettine et alii 1998) or Fe-bearing minerals in suspension. Nevertheless, it has to be considered that in a natural environment there are many other complexing agents in addition to H2O and OH- which may dictate Cr speciation. For example, Cr(III) shows the tendency to form hexacoordinate octahedral complexes with many ligands including organic ligands containing oxygen, nitrogen, sulphur (Saleh et alii 1989). It is also worth noting that the redox potential of the Cr(VI)/Cr(III) couple is high, increasing the tendency to Cr reduction, and few oxidants are present in natural systems which can oxidize Cr(III) to Cr(VI) ( Johnson and Xyla 1991). Within the normal pH range of natural waters, the expected forms of Cr(VI) oxyanions are CrO42-, HCrO4- and Cr2O72- in a variety of compounds, generally soluble and easily mobile in the environment. Cr(VI) is reduced to Cr(III) by a number of electron donors, as already stressed. Preliminary data on a limited number of specimen indicate that industrial chromium sources would be isotopically similar to the Bulk Earth.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
