luminium hydroxide, Al(OH)3, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three, much more rare polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO(OH), and aluminium oxide, Al2O3, differing only by loss of water. These compounds together are the major components of the aluminium ore bauxite. Freshly precipitated aluminium hydroxide formsgels, which is the basis for application of aluminium salts asflocculants in water purification. This gel crystallizes with time. Aluminium hydroxide gels can be dehydrated (e.g., using water-miscible non-aqueous solvents like ethanol) to form an amorphous aluminium hydroxide powder, which is readily soluble in acids. Aluminium hydroxide powder which has been heated to an elevated temperature under carefully controlled conditions is known as activated alumina and is used as a desiccant, an adsorbent, in gas purification, as a Claus catalyst support, water purification, and an adsorbent for the catalyst during the manufacture of polyethylene by the Sclairtech process.
[edit]Nomenclature
The naming for the different forms of aluminium hydroxide is ambiguous and there is no universal standard. All four polymorphisms have a chemical composition of aluminium trihydroxide (an aluminium atom attached to three hydroxide groups).[3]
Gibbsite is also known as hydrargillite, named after the Greek words for water (hydra) and clay (argylles). The first compound named hydrargillite was thought to be aluminium hydroxide, but was later found to be aluminium phosphate; despite this, both gibbsite and hydrargillite are used to refer to the same polymorphism of aluminium hydroxide, with gibbsite used most commonly in the United States and hydrargillite used more often in Europe. In 1930 it was referred to as α-alumina trihydrate to contrast it with bayerite which was called β-alumina trihydrate (the alpha and beta designations were used to differentiate the more- and less-common forms respectively). In 1957 a symposia onalumina nomenclature attempted to develop a universal standard, resulting in gibbsite being designated γ-Al(OH)3 and bayerite becoming α-Al(OH)3 and nordstrandite being designated Al(OH)3. Based on theircrystallographic properties, a suggested nomenclature and designation is for gibbsite to be α-Al(OH)3, bayerite to be designated β-Al(OH)3 and both nordstrandite and doyleite are designated Al(OH)3. Under this designation, the α and β prefixes refer to hexagonal, close-packed structures and altered or dehydrated polymorphisms respectively, with no differentiation between nordstrandiate and doyleite.[3]
[edit]Properties
Gibbsite has a typical metal hydroxide structure with hydrogen bonds. It is built up of double layers of hydroxyl groups with aluminium ions occupying two-thirds of the octahedral holes between the two layers.[4]
Aluminium hydroxide is amphoteric. It dissolves in acid, forming Al(H2O)63+ (hexaaquaaluminium(3+)) or its hydrolysis products. It also dissolves in strong alkali, forming Al(OH)4- (tetrahydroxidoaluminate(1-)).
[edit]Polymorphism
Four polymorphs of aluminium hydroxide exist, all based on the common combination of one aluminium atom and three hydroxide molecules into different crystaline arrangements that determine the appearance and properties of the compound. The four combinations are:[3]
All polymorphs are composed of octahedral layers of aluminium hydroxidemolecules with the aluminium atom in the centre and the hydroxyl groups on the sides, with hydrogen bonds holding the layers together. The polymorphisms vary in how the layers stack together, with the arrangements of the molecules and layers determined by the acidity, presence of ions (including salt) and the surface of the minerals the substance forms on. Under most conditions gibbsite is the mostchemically stable form of aluminium hydroxide. All forms of Al(OH)3crystals are hexagonal.[3]
[edit]Production
Virtually all the aluminium hydroxide used commercially is manufactured by the Bayer process[5] which involves dissolving bauxite in sodium hydroxide at temperatures up to 270°C. The remaining solids, which is a red mud, is separated and aluminium oxide is precipitated from the remaining solution. This red mud is damaging to the environment and highly toxic. It is usually stored in large artificial lakes, this is what led to the Ajka alumina plant accident in 2010 in Hungary, killing nine people and injuring 122. The dam holding back the red mud burst allowing it to contaminate large areas of land and waterways.[6] The aluminium oxide that is produced can be converted to aluminium hydroxide through reaction with water.
Annual production is some 100 million tonnes,[citation needed] over 90% of which is converted to aluminium oxide[citation needed] (alumina) that is used in the manufacture of aluminium metal.[citation needed]
The major other uses of aluminium hydroxide is as a feedstock for the manufacture of other aluminium compounds: specialty calcined aluminas, aluminium sulfate, polyaluminium chloride, aluminium chloride, zeolites, sodium aluminate, activated alumina, aluminium nitrate.[citation needed]
[edit]Fire retardant
Aluminium hydroxide also finds use as a fire retardant filler for polymer applications in a similar way tomagnesium hydroxide and mixtures of huntite and hydromagnesite.[7][8][9][10][11] It decomposes at about 180 °C, absorbing a considerable amount of heat in the process and giving off water vapour. In addition to behaving as a fire retardant, it is very effective as a smoke suppressant in a wide range of polymers, most especially in polyesters, acrylics, ethylene vinyl acetate, epoxies, PVC and rubber.
[edit]Pharmaceutical
This compound is used as an antacid under names such as Alu-Cap, Aludrox or Pepsamar. The hydroxide reacts with excess acid in the stomach, reducing its acidity.[12] This decrease of acidity of the contents of the stomach may in turn help to relieve the symptoms of ulcers, heartburn or dyspepsia. It can also cause constipation and is therefore often used with magnesium hydroxide or magnesium carbonate, which have counterbalancing laxativeeffects. This compound is also used to control phosphate (phosphorus) levels in the blood of people suffering from kidney failure.
Precipitated aluminium hydroxide is included as an adjuvant in some vaccines (e.g. anthrax vaccine). One of the well-known brands of aluminium hydroxide adjuvant is Alhydrogel, made by Brenntag. Since it absorbs protein well, it also functions to stabilize vaccines by preventing the proteins in the vaccine from precipitating or sticking to the walls of the container during storage. Aluminium hydroxide is often mis-called "alum" even by researchers; however, "alum" properly refers to aluminium potassium sulfate (alum).[citation needed] The aluminium hydroxide causes adsorption of antigens made of proteins, which slows the release of the antigen from the injection site (the "depot effect"), as well as causing a nonspecific irritation to the immune system.[13]Vaccine formulations containing aluminium hydroxide stimulates the immune system by inducing the release ofuric acid, an immunological danger signal. This strongly attracts certain types of monocytes which differentiate into dendritic cells. The DCs pick up the antigen, carry it to lymph nodes, and stimulate T cellsand B cells.[14] It appears to contribute to induction of a good Th2 response, so is useful for immunizing against pathogens that are blocked by antibodies. However, it has little capacity to stimulate cellular (Th1) immune responses, important for protection against many pathogens,[15] nor is it useful when the antigen ispeptide-based.[13]
[edit]Potential adverse effects
In the 60's and 70's it was speculated that aluminium was related to various neurological disorders includingAlzheimer's disease.[16][17] Since then, multiple epidemiological studies have found no connection between exposure to aluminium and neurological disorders.[18][19][20]
The pathological persistence of aluminium hydroxide used in some vaccines has also been associated withmacrophagic myofasciitis,[21] a rare muscle disease.
