Chromium (Cr) and water
Chromium and water: reaction mechanisms, environmental impact and health effects
|Seawater chromium content varies strongly, and is usually between 0.2 and 0.6 ppb. |
Rivers contain approximately 1 ppb of chromium, although strongly increased concentrations are possible, for example 5-20 ppb in the River Rhine, and 10-40 ppb in the River Elbe in 1988.
Phytoplankton contains approximately 4 ppm chromium, sea fish contain between 0.03 and 2 ppm, and oyster tissue contains approximately 0.7 ppm (all values dry mass). Phytoplankton has a bio concentration factor of approximately 104 in seawater.
In dissolved form chromium is present as either anionic trivalent Cr(OH)3 or as hexavalent CrO42-. The amount of dissolved Cr3+ ions is relatively low, because these form stable complexes.
Oxidation ranks from Cr(II) to Cr(VI). In natural waters trivalent chromium is most abundant.
Elementary chromium does not react with water at room temperature.
Many chromium compounds are relatively water insoluble. Chromium (III) compounds are water insoluble because these are largely bound to floating particles in water. Chromium (III) oxide and chromium (III) hydroxide are the only water soluble compounds.
Chromium does not occur freely in nature. The main chromium mineral is chromite. As was mentioned earlier, chromium compounds can be found in waters only in trace amounts. The element and its compounds can be discharged in surface water through various industries. It is applied for example for metal surface refinery and in alloys. Stainless steel consists of 12-15% chromium. Chromium metal is applied worldwide in amounts of approximately 20,000 tons per year. It may be polished and it does not oxidize when it comes in contact with air.
Chromium is a dietary requirement for a number of organisms. This however only applies to trivalent chromium. Hexavalent chromium is very toxic to flora and fauna.
Chromium has a large influence upon drinking water quality. It cannot normally be found in groundwater and surface water in considerable concentrations. Specific removal in sewage water treatment is therefore unusual.