For example, in slightly acidic to alkaline environments, Fe(III) is fairly insoluble and precipitates as a solid phase (hydrous ferric oxide) that has a large adsorption capacity for metal ions. In contrast, Fe(II) is fairly soluble and its oxides have a much lower adsorption capacity. As the Fe(III) solid phase is reduced, not only is the Fe(II) brought into solution but so are any contaminants that may have been adsorbed onto it. Chromium is a naturally occurring element that primarily exists in three states Chromium-0, Chromium III and Chromium VI (Hexavalent Chromium)