|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.
In what way and in what form does chromium react with water?
Elementary chromium does not react with water at room temperature.
Solubility of chromium and chromium compounds
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 (VI) oxide is an example of an excellently water soluble chromium compounds, solubility = 1680 g/L.
Why is chromium present in water?
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.
The metal industry mainly discharged trivalent chromium. Hexavalent chromium in industrial wastewaters mainly originates from tanning and painting. Chromium compounds are applied as pigments, and 90% of the leather is tanned by means of chromium compounds. Wastewater usually contains about 5 ppm of chromium. Chromium may be applied as a catalyser, in wood impregnation, in audio and video production and in lasers. Chromite is the starting product for inflammable material and chemical production.
Chromium may be present in domestic waste from various synthetic materials. Through waste incineration it may spread to the environment when protection is insufficient.
In nuclear fission the 51Cr isotope is released, and this can be applied for medical purposes.
What are the environmental effects of chromium in water?
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 water pollution is not regarded one of the main and most severe environmental problems, although discharging chromium polluted untreated wastewater in rivers has caused environmental disasters in the past.
Chromium (III) oxides are only slightly water soluble, therefore concentrations in natural waters are limited. Cr3+ ions are rarely present at pH values over 5, because hydrated chromium oxide (Cr(OH)3) is hardly water soluble.
Chromium (VI) compounds are stable under aerobic conditions, but are reduced to chromium (III) compounds under anaerobic conditions. The reverse process is another possibility in an oxidizing environment. Chromium is largely bound to floating particles in water.
The LC50 value for chromium in sea fish lies between 7 and 400 ppm, for daphnia at 0.01-0.26 ppm, and for algae at 0.032-6.4 ppm.
Chromium (VI) compounds are divided up in water hazard class 3, and are considered very toxic.
Chromium phytotoxicity is undetermined. At concentrations of between 500 and 6000 ppm in soils, plants were not damaged. Lime or phosphate in soils may further decrease chromium susceptibility. Air-dried soil generally contains 2-100 ppm of chromium. Chromium solubility in soil water is lower than that of other potentially toxic metals. This explains the relatively low plant uptake. Under normal conditions plants contain approximately 0.02-1 ppm chromium (dry mass), although values may increase to 14 ppm. In mosses and lichens, relatively high chromium concentrations can be found.
Chromium (VI) compounds are toxic at low concentrations for both plants and animals. The mechanism of toxicity is pH dependent. These compounds are more mobile in soils than chromium (III) compounds, but are usually reduced to chromium (III) compounds within a short period of time, reducing mobility. Soluble chromates are converted to insoluble chromium (III) salts and consequently, availability for plants decreases. This mechanism protects the food chain from high amounts of chromium. Chromate mobility in soils depends on both soil pH and soil sorption capacity, and on temperature. The guideline for chromium is agricultural soils is approximately 100 ppm.
Chromium naturally has four stable isotopes. There are eight instable isotopes. The 51Cr, which is applied for diagnosis purposes, has an average degree of radioactivity.
What are the health effects of chromium in water?
The human body contains approximately 0.03 ppm of chromium. Daily intake strongly depends upon feed levels, and is usually approximately 15-200 μg, but may be as high as 1 mg. Chromium uptake is 0.5-1%, in other words very small. The placenta is the organ with the highest chromium amounts.
Trivalent chromium is an essential trace element for humans. Together with insulin it removes glucose from blood, and it also plays a vital role in fat metabolism. Chromium deficits may enhance diabetes symptoms. Chromium can also be found in RNA. Chromium deficits are very rare, and chromium feed supplements are not often applied.
Chromium (III) toxicity is unlikely, at least when it is taken up through food and drinking water. It may even improve health, and cure neuropathy and encephalopathy.
Hexavalent chromium is known for its negative health and environmental impact, and its extreme toxicity. It causes allergic and asthmatic reactions, is carcinogenic and is 1000 times as toxic as trivalent chromium. Health effects related to hexavalent chromium exposure include diarrhoea, stomach and intestinal bleedings, cramps, and liver and kidney damage. Hexavalent chromium is mutagenic. Toxic effects may be passed on to children through the placenta.
Chromium (VI) oxide is a strong oxidant. Upon dissolution chromium acid is formed, which corrodes the organs. It may cause cramps and paralysis. The lethal dose is approximately 1-2 g. Most countries apply a legal limit of 50 ppb chromium in drinking water.
A professional illness in chromium industries is chromium sores upon skin contact with chromates. Chromium trioxide dust uptake in the workplace may cause cancer, and damage the respirational tract.
Which water purification technologies can be applied to remove chromium from water?
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.
Chromium removal from water is optional, because both ion exchangers and active carbon can be applied for this purpose.
Chromium (III) may precipitate as hydroxide. Coagulation is not a very effective mechanism of chromium (VI) removal. When iron sulphate is applied chromium (VI) may be reduced to chromium (III) by means of iron ions, and it can than be removed. This method is however very unusual in drinking water preparation.
Literature and the other elements and their interaction with water