The chromium cycle is the biogeochemical cycle of chromium through the atmosphere, hydrosphere, biosphere and lithosphere. [1] [2] [3] [4]
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Biogeochemical cycles |
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Chromium has two common
oxidation states relevant for environmental conditions:
trivalent chromium, Cr(III) (reduced form), and hexavalent chromium, Cr(VI) (most oxidized form). The poorly
soluble trivalent chromium
cation (Cr3+
) strongly
adsorbs onto
clay particles and
particulate organic matter, whereas the highly
toxic and
carcinogenic hexavalent chromate
anion (CrO2−
4) is soluble and non-sorbed, making it a toxic
contaminant in environmental systems. Chromium commonly exists in
soil and
rocks as highly insoluble trivalent chromium, such as
chromite (Fe(II)Cr(III)
2O
4, or FeO·Cr
2O
3), a
mixed oxide mineral of the
spinel group resembling
magnetite (Fe
3O
4, Fe(II)Fe(III)
2O
4, or FeO·Fe
2O
3). Terrestrial
weathering could cause trivalent chromium to be
oxidized by
manganese oxides to hexavalent chromium, which is then solubilized and cycled to the
ocean through
rivers.
Estuaries release particulate chromium from rivers to the
sea, increasing the dissolved fluxes of chromium to the ocean.
[1]
Soluble
hexavalent chromium is the most common type of chromium in
oceans, where over 70% of dissolved chromium in the ocean is found in
oxyanions such as
chromate (CrO2−
4).
Soluble
trivalent chromium is also found in the oceans where
complexation with
organic
ligands occurs. Chromium is estimated to have a
residence time of 6,300 years in the oceans. Hexavalent chromium is
reduced to trivalent chromium in
oxygen minimum zones or at the surface of the ocean by divalent
iron and organic ligands. There are four sinks of chromium from the oceans: (1)
oxic sediments in
pelagic zones, (2)
hypoxic sediments in
continental margins, (3)
anoxic or
sulfidic sediments in
basins or
fjords with permanently
anoxic or sulfidic (
euxinic) bottom waters, and (4) marine
carbonates.
[1]
Manganese (III) can oxidize Cr(III) to Cr(VI) when complexed with organic ligands. [5] This causes contaminant mobilization of Cr(VI), and also reduces Mn(III) to Mn(II), which can then be oxidized back to Mn(III) by oxygen. [5]
Isotopic fractionation of chromium has become a valuable tool for monitoring environmental chromium contamination through recent advancements in mass spectrometry. [1] Isotope fractionation during river transport is determined by local redox conditions based on dissolved organic matter in rivers. [1]