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Cr-isotopes: analytical methods, spike calculations and application to Environmental Sciences (Articolo in rivista)
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- Cr-isotopes: analytical methods, spike calculations and application to Environmental Sciences (Articolo in rivista) (literal)
- Anno
- 2007-01-01T00:00:00+01:00 (literal)
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- Petrini R; Cavazzini G.; Slejko F. (literal)
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- Petrini R. - Dipartimento Scienze della Terra Universita' Trieste
Cavazzini G. - Istituto geoscienze e georisorse CNR
Slejko F. - Dipartimento Scienze della Terra Universita' Trieste (literal)
- Titolo
- Cr-isotopes: analytical methods, spike calculations and application to Environmental Sciences (literal)
- 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. (literal)
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