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 oxidizedbymanganese 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]
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]
^Rauch, Jason N.; Pacyna, Jozef M. (2009). "Earth's global Ag, Al, Cr, Cu, Fe, Ni, Pb, and Zn cycles". Global Biogeochemical Cycles. 23 (2): GB2001. doi:10.1029/2008GB003376.