Chromium cycle

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)2O4, or FeO·Cr2O3), a mixed oxide mineral of the spinel group resembling magnetite (Fe3O4, Fe(II)Fe(III)2O4, or FeO·Fe2O3).

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.

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.

Chromium cycles through the Earth atmosphere , soil , freshwater bodies, rivers , oceans , crust and mantle . The arrows indicate fluxes given in gigagrams (1,000 metric tons ) of chromium per year. The corresponding stocks (inventories) indicate reservoirs of chromium given in gigagrams of chromium.