Thechromium cycle is thebiogeochemical cycle ofchromium through theatmosphere,hydrosphere,biosphere andlithosphere.[1][2][3][4]
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Chromium has two commonoxidation states relevant for environmental conditions:trivalent chromium, Cr(III) (reduced form), and hexavalent chromium, Cr(VI) (most oxidized form). The poorlysoluble trivalent chromiumcation (Cr3+
) stronglyadsorbs ontoclay particles andparticulate organic matter, whereas the highlytoxic andcarcinogenic hexavalent chromateanion (CrO2−
4) is soluble and non-sorbed, making it a toxiccontaminant in environmental systems. Chromium commonly exists insoil androcks as highly insoluble trivalent chromium, such aschromite (Fe(II)Cr(III)
2O
4, or FeO·Cr
2O
3), amixed oxide mineral of thespinel group resemblingmagnetite (Fe
3O
4,Fe(II)Fe(III)
2O
4, or FeO·Fe
2O
3). Terrestrialweathering could cause trivalent chromium to beoxidized bymanganese oxides to hexavalent chromium, which is then solubilized and cycled to theocean throughrivers.Estuaries release particulate chromium from rivers to thesea, increasing the dissolved fluxes of chromium to the ocean.[1]
Solublehexavalent chromium is the most common type of chromium inoceans, where over 70% of dissolved chromium in the ocean is found inoxyanions such aschromate (CrO2−
4).Solubletrivalent chromium is also found in the oceans wherecomplexation withorganicligands occurs. Chromium is estimated to have aresidence time of 6,300 years in the oceans. Hexavalent chromium isreduced to trivalent chromium inoxygen minimum zones or at the surface of the ocean by divalentiron and organic ligands. There are four sinks of chromium from the oceans: (1)oxic sediments inpelagic zones, (2)hypoxic sediments incontinental margins, (3)anoxic orsulfidic sediments inbasins orfjords with permanentlyanoxic or sulfidic (euxinic) bottom waters, and (4) marinecarbonates.[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) byoxygen.[5]
Isotopic fractionation of chromium has become a valuable tool for monitoring environmental chromium contamination through recent advancements inmass spectrometry.[1] Isotope fractionation during river transport is determined by localredox conditions based ondissolved organic matter in rivers.[1]
