Aluminium hydroxide, Al(OH)3, archaically called hydrate of alumina or alumina trihydrate,
is found in nature as the mineral gibbsite 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 forms gels, which is the basis for application of aluminium salts as flocculants
in water purification. This gel crystallizes with time. Aluminium hydroxide gels can be
dehydrated 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.
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. Gibbsite is also known as hydrargillite, named
after the Greek words for water and clay. 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. In 1957 a symposium on alumina
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 their crystallographic 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.
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.
Aluminium hydroxide is amphoteric. It dissolves in acid, forming [Al(H2O)6]3+ or its hydrolysis
products. It also dissolves in strong alkali, forming [Al(OH)4]−.
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: Gibbsite
Bayerite Nordstrandite
Doyleite All polymorphs are composed of octahedral
layers of aluminium hydroxide molecules 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 and the surface of the minerals the substance forms on. Under
most conditions gibbsite is the most chemically stable form of aluminium hydroxide. All forms
of Al(OH)3 crystals are hexagonal. Production
Virtually all the aluminium hydroxide used commercially is manufactured by the Bayer
process 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. The aluminium
oxide that is produced can be converted to aluminium hydroxide through reaction with
water. Uses
Annual production is some 100 million tonnes, over 90% of which is converted to aluminium
oxide that is used in the manufacture of aluminium metal.
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.
Fire retardant Aluminium hydroxide also finds use as a fire
retardant filler for polymer applications in a similar way to magnesium hydroxide and
mixtures of huntite and hydromagnesite. 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.
Pharmaceutical This compound is used as an antacid under
names such as Alu-Cap, Aludrox, Gaviscon or Pepsamar. The hydroxide reacts with excess
acid in the stomach, reducing its acidity. 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 laxative effects. This compound is also used to control
phosphate levels in the blood of people suffering from kidney failure.
Precipitated aluminium hydroxide is included as an adjuvant in some vaccines. 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. The aluminium hydroxide causes adsorption of antigens
made of proteins, which slows the release of the antigen from the injection site, as
well as causing a nonspecific irritation to the immune system. Vaccine formulations containing
aluminium hydroxide stimulates the immune system by inducing the release of uric acid,
an immunological danger signal. This strongly attracts certain types of monocytes which
differentiate into dendritic cells. The dendritic cells pick up the antigen, carry it to lymph
nodes, and stimulate T cells and B cells. 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 immune responses, important for protection
against many pathogens, nor is it useful when the antigen is peptide-based.
Potential adverse effects In the 1960s and 1970s it was speculated that
aluminium was related to various neurological disorders including Alzheimer's disease. Since
then, multiple epidemiological studies have found no connection between exposure to aluminium
and neurological disorders. The pathological persistence of aluminium
hydroxide used in some vaccines has also been associated with macrophagic myofasciitis,
a rare muscle disease. References
External links International Chemical Safety Card 0373
"Some properties of aluminum hydroxide precipitated in the presence of clays", Soil Research Institute,
R C Turner, Department of Agriculture, Ottawa Effect of aging on properties of polynuclear
hydroxyaluminum cations A second species of polynuclear hydroxyaluminum
cation, its formation and some of its properties