Aluminum (Al) and water
Aluminum and water: reaction mechanisms, environmental impact and health effects
|The amount of aluminum in seawater varies between approximately 0.013 and 5 ppb. The Atlantic Ocean is known to contain more aluminum than the Pacific Ocean. River water generally contains about 400 ppb of aluminum. |
Aluminum mainly occurs as Al3+ (aq) under acidic conditions, and as Al(OH)4- (aq) under neutral to alkalic conditions. Other forms include AlOH2+ (aq) en Al(OH)3 (aq).
Aluminum metal rapidly develops a thin layer of aluminum oxide of a few millimeters that prevents the metal from reacting with water. When this layer is corroded a reaction develops, releasing highly flammable hydrogen gas.
Al3+(aq) + 6H2O(l) <-> [Al(H2O)6]3+ (aq)
The most abundant aluminum compounds are aluminum oxide and aluminum hydroxide, and these are water insoluble.
Aluminum forms during mineral weathering of feldspars, such as and orthoclase, anorthite, albite, micas and bauxite, and subsequently ends up in clay minerals. A number of gemstones contain aluminum, examples are ruby and sapphire.
Aluminum may negatively affect terrestrial and aquatic life in different ways. Regular aluminum concentrations in groundwater are about 0.4 ppm, because it is present in soils as water insoluble hydroxide. At pH values below 4.5 solubility rapidly increases, causing aluminum concentrations to rise above 5 ppm. This may also occur at very high pH values.
The total aluminum concentration in the human body is approximately 9 ppm (dry mass). In some organs, specifically the spleen, kidneys and lung, concentrations up to 100 ppm (dry mass) may be present. Daily aluminum intake is approximately 5 mg, of which only a small fraction is absorbed. This leads to relatively low acute toxicity. Absorption is about 10 μg per day. These amounts are considered harmless to humans. Silicon may decrease aluminum uptake. However, once the element is taken up in the body it is not easily removed.
Aluminum may be removed from water by means of ion exchange or coagulation/ flocculation. Aluminum salts are applied in water treatment for precipitation reactions. Adding aluminum sulphate and lime to water causes aluminum hydroxide formation, which leads to settling of pollutants. Hydroxide is water insoluble, therefore only 0.05 ppm dissolved aluminum remains. This is below the legal limit for drinking water of the World Health Organization (WHO), of 0.2 ppm aluminum.