Chromium cycle

The chromium cycle is the biogeochemical cycle of chromium through the atmosphere, hydrosphere, biosphere and lithosphere.
Biogeochemical cycle.
Terrestrial weathering and river transport.
Chromium has two oxidation states: trivalent chromium, Cr(III), and hexavalent chromium, Cr(IV). Trivalent chromium adsorbs highly onto particles, whereas hexavalent chromium is highly toxic and soluble, making it a toxic contaminant in environmental systems. Chromium commonly exists as highly insoluble trivalent chromium, such as chromite, in soil and rocks. Terrestrial weathering could cause trivalent chromium to be oxidized by manganese oxides to hexavalent chromium, which is then cycled to the ocean through rivers. Estuaries release particulate chromium into rivers, increasing the dissolved fluxes of chromium to the ocean.
Oceanic cycling.
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. 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: oxic sediments in pelagic zones, hypoxic sediments in continental margins, anoxic or sulfidic sediments in basins or fjords with permanently anoxic or sulfidic bottom waters, and marine carbonates.
Influence from other biogeochemical cycles.
Manganese (III) can oxidize Cr(III) to Cr(IV) when complexed with organic ligands. This causes contaminant mobilization of Cr(IV), and also reduces Mn(III) to Mn(II), which can then be oxidized back to Mn(III) by oxygen.
Methods for chromium tracking.
Isotopic fractionation of chromium has become a valuable tool for monitoring environmental chromium contamination through recent advancements in mass spectrometry. Isotope fractionation during river transport is determined by local redox conditions based on dissolved organic matter in rivers.